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a NBS TECHNICAL NOTE 977

U. S. DEPARTMENT OF COMMERCE/National Bureau of Standards

Methods For Characterizing

^

Adobe Building IVIaterials

3

977
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress March 3, 1901. The

Bureau's overall goal is to strengthen and advance the Nation's science and technology and

facilitate their effective application for pubUc benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is performed by the National Measurement Laboratory, the National Engineering Laboratory,

and the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

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Agencies; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities^ — Radiation Research — Thermodynamics and

Molecular Science — Analytical Chemistry — Materials Science. THE NATIONAL ENGINEERING LABORATORY provides technology and technical

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in support of objectives in these efforts; builds and maintains competence in the necessary

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proposes new engineering practices; and develops and improves mechanisms to transfer

results of its research to the utlimate user. The Labo^tory consists of the following centers:

Applied Mathematics — Electronics and Electrical Engineering^ — Mechanical

Engineering and Process Technology^ — Building Technology — Fire Research —

Consumer Product Technology — Field Methods.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal Agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal Agencies; and

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The Institute consists of the following divisions:

Systems and Software — Computer Systems Engineering — Information Technology.

'Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234. 'Some divisions within the center are located at Boulder, Colorado, 80303.

The National Bureau of Standards was reorganized, effective April 9, 1978.
SATIONAIi BUREAtI OP STANDARDS

UBHARY

JW ^ 1 t978

Methods for Characterizing Adobe Building Materials '^"J"'

'

- — 1^^

James R. Clifton C.

^

and Paul Wencil Brown

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

Washington, D. C. 20234

and

Carl R. Robbins

Center for Materials Science

National Measurement Laboratory

National Bureau of Standards

Washington, D. C. 20234

Prepared for:

U. S. Department of the Interior National Park Service Washington, D. C. 20240

.»f °'' c„

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued June 1978
National Bureau of Standards Technical Note 977

Nat. Bur. Stand. (U. S.). Tech. Note 977. 59 pages (June 1978)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1978

For sale by the Superintendent of Documents, U.S. Government Printing OfBce, Washington, D.C. 20402

Price S2. 30—Stock No. 003-003-01940-2 (Add 25 percent additional for other than U.S. mailing)

.
TABLE OF CONTENTS

Page

LIST OF FIGURES iv

LIST OF TABLES v

ABSTRACT vi

1. INTRODUCTION 1

2. SELECTION OF SAMPLES ,- . 2

3. METHODS FOR THE ANALYSIS OF ADOBE 3 3.1 Color Determination 3 3.1.1 Description of Munsell Color Charts 6 3.1.2 Method of Making Color Determinations 6 3.1.3 Factors Affecting Color Matching 8 3.2 Determination of Soluble Salts 9 3.3 Determination of pH 10

3.3.1 Method of Measuring pH 10

3.3.2 Results of pH Measurements 11

3.4 Determination of Particle Size Distributions 12

3.4.1 Method for Determining Particle Size Distribution 12

3.4.1.1 Sample for Analysis'. 13

3.4.1.2 Dispersion of Sample and Separation of Organic Matter. . . 14 3.4.1.3 Partical Separation of Silt-Clay Fraction 15 3.4.1.4 Sieve Fractionation and Determination of Coarse Particles

.

15

3.4.1.5 Separation of Silt from Clay 17 3.4.2 Useful Information from Particle Size Determinations 18

3.5 Liquid and Plastic Limits 19

3.5.1 Determination of Liquid and Plastic Limits 20 3.5.2 Information from Liquid and Plastic Limit Determinations 22 3.6 Determination of Minerals in Adobe by X-ray Dif fractometry 27 3.6.1 Method of Obtaining and Interpreting Powder Diffraction Patterns. . 27 3.6.2 Bulk Analysis 28

3.6.3 Identification of Clay Minerals 30

3.7 Analysis of Adobe by Microscopy 31 3.7.1 Binocular Microscope 31 3.7.2 Polarizing Microscope 40

3.7.3 Scanning Electron Microscope 40

4. SUMMARY AND RECOMMENDATIONS 48

5. REFERENCES ^'^

APPENDIX - DESCRIPTION OF SOURCES OF ADOBE SAMPLES 52
LIST OF FIGURES

Page

Figure No.

1 Flow Chart for Analysis of Adobe 4

2 Munsell Soil Color Chart 7

3 Particle Settling Time 16

4 Mechanical Device Used for Measuring the Liquid Limit 21

5 Plot of a Liquid Limit Determination 23

6 Determination of Plastic Limit 24

7 Aluminum Holder for Preparing Powder Samples for X-ray Analysis 29

8 Microscopic View of Epoxy-Impregnated Adobe Specimen from Tumacacori . . 35

9 Gypsum in Pore Space of Adobe Specimen from Tumacacori 36

10 Microscopic View of Epoxy-Impregnated and Polished Section of Adobe

Sample from Tumacacori 37

11 Microscopic View of Epoxy-Impregnated and Polished Section of Adobe

Sample from Escalante , 38

12 Microscopic View of Epoxy-Impregnated and Polished Section of Adobe

Sample from Fort Bowie 39

13 Thin Section of Adobe Sample from Fort Bowie 41

14 Scanning Electron Micrograph of Adobe Sample from Tumacacori 43

15 Energy Dispersive X-ray Element Map of Adobe Sample from Tumacacori. . . 44

16 Energy Dispersive X-ray Spectrum of White Material Shown in Figure 15 . . 45

17 Energy Dispersive X-ray Spectrum of Salts Extracted from Adobe

Sample from Tumacacori 46

18 Microscopic View of Hygroscopic Salts Extracted from Adobe Sample

from Tumacacori 47
LIST OF TABLES

Table No.

Page

1 Selection of Methods 5

2 Color Determination of Adobe and Soil Samples 8

3 Results of pH Determinations 11

4 Measured Particle Size Distribution 19

5 Compilation of Liquid and Plastic Limits for Clays 25

6 Measured Liquid and Plastic Limits of Adobe and Soil Samples 25

7 Relationship between Total Expansion and Plasticity of Soils 27

8 X-ray Identification of Principal Clay Minerals 32

9 Temperatures Required in the Identification of Clay Minerals 33
Abstract

Methods are described for the characterization of those physical properties

and mineralogical features of adobe which appear to have the most significant

affect on the durability of adobe. These methods include determinations of

color, pH, soluble salts, particle size distribution, liquid and plastic limits,

and the X-ray "fingerprint" of adobe. In addition, methods are given for the

identification of the mineralogy of adobe soils and for the examination of the

microfabric of adobe.

Key words: Adobe building materials; adobe soil; color determination; microfabric analysis; mineralogical analysis; particle size distribution; pH;

physical properties; plastic and liquid limits; soluble salts.

VI
METHODS FOR CHARACTERIZING ADOBE BUILDING MATERIALS

by

James R. Clifton and Paul Wencil Brown

National Engineering Laboratory

and

Carl R. Robbins

National Materials Laboratory

1. INTRODUCTION

1 The need for standard test methods to characterize the important physical properties

and mineralogical features of adobe soils and adobe building materials was identified in a

review of the technology of preserving historic adobe structures [1] . The use of standard

test methods should assist the National Park Service in the assimilation and interpretation

of data from different sources. Furthermore, standard tests should form the basis for evaluating the effects of preservation materials and methods, and for selecting soils to be used

I for repairing or replacing deteriorated adobe.

The complete physical and mineralogical characterization of soils is a lengthy process

. because soils consist of a complex mixture of different mineral and organic substances.

However, by only considering the most important properties affecting the performance of a

i soil used as an adobe building material, the analysis is simplified. This was the approach

taken in selecting the test methods described in this report. These methods can form the

basis for a standard analysis of the important physical properties and mineralogical features

of adobe. Included are test methods which measure those properties of adobes which appear to

have the most significant impact on their durability, such as particle size distribution,

chemical and phase compositions and the response of adobes to moisture [1] . In addition,

methods are provided for the rapid comparison of the properties of repair materials with

those of the original adobe.

I Numbers in brackets refer to the literature references listed at the end of this paper.
Most of the tests can be carried out in a modestly-equipped laboratory and the results

are easily interpreted. However, the microscopic and X-ray powder diffraction methods

should be performed by an analyst experienced in their use. The test procedures given in

this report were either taken from existing standard test methods or were developed at the

National Bureau of Standards, specifically for the analysis of adobe. In either case, the

validity of the procedures was verified by using them to characterize the properties of

adobe and soil samples supplied by the National Park Service.

2. SELECTION OF SAMPLES

The usefulness of the information obtained from the analysis of an adobe specimen is

largely dependent on the selection of representative specimens. Samples should be carefully

chosen according to the objectives of the work and should be representative of the structure

at the sampling site. Often the size and number of samples which can be taken from historic

sites are limited. A wide range of compositional and microstructural features, which are

not visually apparent, may exist within a single adobe specimen. Such variations in the

features of an adobe are caused by the heterogeneous nature of soils. In addition, soil

could have been taken from several sites during the construction or repair of an adobe

structure. Because of these difficulties, either the analyst or personnel experienced in

sample selection should actively participate in the selection and removal of specimens.

The size and number of specimens which should be obtained depends on the particular

properties which will be studied. For example, the determination of the particle size

distribution (Section 3.4) may require a sample which weighs up to 500 grams, whereas only a

few grams of material may be needed for mineralogical analysis by X-ray powder diffractometry (Section 3.6). However, the analyst should be aware that the properties of a few

small samples may not be representative of those of the "average" adobe in the structure.

If carefully carried out, the comparison of analyses of several samples from each of several

locations randomly chosen, should provide an indication of the homogeneity of the adobe

material in the structure.

The position from which a sample was removed should be recorded and photographically

documented at time of removal. In addition, the physical properties of the sample should be

recorded including its size and shape, color (using Munsell color charts. Section 3.1),
moisture condition, the presence of any large aggregates, and the degree of consolidation

(e.g., a hard mass, powdery, etc.). The sample should be transferred to the laboratory in a

capped container that maintains its original moisture content.

3. METHODS FOR THE ANALYSIS OF ADOBE

A sequential analysis of adobe building materials, using the methods described in this

section, is illustrated in figure 1. This analysis is more complete than that normally

necessary. The analysis should be organized to provide the information that is needed in

the different aspects of adobe preservation, as indicated in table 1.

The testing procedures for the first five methods are given in detail because these

methods can be readily performed using simple laboratory equipment and provide the information

most often needed. The x-ray dif fractometry and microscopy techniques require more sophisticated equipment and expertise in interpreting the results • Procedures for

using them, therefore, is not given in detail. Some results are included under the individual test methods that are based on the characterization of samples taken from structures at

Escalante, Fort Bowie and Tumacacori, Arizona. These results are included to indicate the

type of information which can be obtained from the individual methods. A brief description

of the source of the samples is given in the Appendix.

3.1 Color Determination

The color of an adobe is largely determined by the mineralogy, amount, and particle

size of the various constituents. Therefore, color determination is a useful field method

for screening soils to be used in the repair of adobe structures. The final selection of

the soil to be used for repairs, however, should be made after its composition is more

completely characterized by such techniques as X-ray diffraction and particle size distribution analysis.

The color of adobe samples can be easily determined in the field using the methods

outlined in this section. The color of a sample should be determined and recorded while the

sample is being visually inspected in the field (figure 1).
ADOBE SAMPLE

Record: (1) Sampling Location

(2) Visual Inspectton of Sarmpl-e

(size, color, moisture condition

and ph-ysical condition) Trans fer t Lab

ture

oratory

BUIK ADOBE

1 Composition and Microstruc

i

Moisture Content

Soluble Salts

X-Ray Diffraction Optical Microscope

Scanning Electron Microscope

pH

i

Fractionation - Ar ^

MATERIAL SAND (.02-2 mm)

<425Mm ^ r GRAVEL (>2mm)

SILT AND

CLAY SIZE

1

Liquid Limits Determination of X-Ray Diffraction Amount of Scanning Electron Microscope (1)Sand (2) Gravel Determination of: i (1) Amount Mineral Identification of Organic Matter

(2) Amount of Magnetic Oxides (by X-Ray and Optical

Fractio natior^ Microscope) Particle Morphology

4 4.

CLAY SIZE SILT SIZE <2Mm >2/im- 20/im>

Determination of Amount of Clay

X-Ray Diffraction (type of clay)

(1) Before Treatment

(2) Glycolation

(3) Heated to 500°K

Scanning Electron Microscope

Determination of Amount of Silt. Mineral Identification (by X-Ray and optical

Microscope)

Figure 1. Flow chart for analysis of adobe.
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3.1.1 Description of Munsell Color Charts

Adobe and soil colors can be readily determined in the field by comparison with a color

chart. The collection of charts generally used with soils is the Munsell soil color charts

[2]. The soil collection is comprised of seven charts which display 196 different standard

color chips systematically arranged according to their Munsell notation. A combination of

three variables are used to describe colors. These variables are Hue, Value and Chroma.

The Hue notation of a color indicates its relation to red, yellow, green, blue, and purple;

the Value notation indicates its lightness; and the Chroma notation indicates its strength

(or departure from a neutral of the same lightness) . The notation of Hue used for soils is

the letter abbreviation of the colors of the rainbow (R for red, YR for yellow-red, and Y

for yellow) preceded by a numerical range of to 10. Within each letter range, the Hue

becomes more yellow and less red as the numbers increase. Value is given a numerical notation which ranges from 0, for absolute black, to 10, for absolute white. The notation for

Chroma is also numerical and ranges from (neutral grays) to 8 for soils, increasing in

equal intervals. The complete Munsell designation is given in the sequence Hue, Value/

Chroma.

Descriptive color names can also be obtained by using the Munsell Color Charts and

should be reported along with the Munsell notation of color, because each system conveys

valuable information. The color names convey a visual description of the color of soil

which can be visualized without consulting the color charts, while the Munsell notation

gives a more exact description of the color of a soil.

i(

3.1.2 Method of Making Color Determinations

The color names and the appropriate Munsell notation of an adobe or soil are obtained

by comparison with the color charts. Apertures between closely matching color chips (figure 2)

are placed over the sample thereby permitting direct visual comparison. Normally, the match

will not be perfect and the closest match should be selected. The color names and the

Munsell notations selected for materials from Escalante, Fort Bowie, and Tumacacori are

given in table 2.
Figure 2. Munsell soil color chart,
Table 2

Color Determination of Adobe and Soil Samples

Sample

Escalante Adobe

Fort Bowie Adobe

Tumacacori Adobe

Tumacacori Soil

Soil Color

Name

Pinkish gray

Yellowish brown

Dark brown

Dark grayish brown

Munsell

Color Notation

7.5 YR 7/2

10 YR 5/4

7.5 YR 4/2

10 YR 4/2

H

3.1.3 Factors Affecting Color Matching

An imperfect match may occur when comparing an original adobe with a soil or a freshly

prepared adobe of similar composition or even with materials taken from the same approximate

site from which the old adobe originated. For example, a perceptible difference was observed

in the color notation of the adobe and soil sample from the Tumacacori area (table 2)

.

Some of the causes for such mismatches include the traditional practice of adding straw and

small twigs to a soil during the preparation of an adobe, weathering of the adobe, and

differences in moisture contents between the adobe and soil. If possible, the soil or new

adobe should be compared with an unexposed portion of the original adobe by scraping or

rubbing off the exterior weathered adobe layers. It is advisable that the lighting conditions under which a color determination is made should be recorded and, if possible, duplicated

when making another determination. For example, if the color of an original adobe is determined in bright sunlight then the color of a freshly prepared adobe should also be determined

in bright sunlight.

The color of the adobe of an historic adobe structure may vary throughout the structure.

To obtain the best color match between the original and the repair adobe, a series of color

determinations should be made around the border of an area to be repaired. Occasionally,

an objectional color difference may exist between the original and the repair adobe, and a

coloring material used to obtain a closer color match. In such cases, the coloring material

should be carefully selected because of the possibility that upon exposure to sunlight its

color could change or fade.
3.2 Determination of Soluble Salts

The presence of water-soluble salts in adobe may have a significant impact on the durability of an adobe structure. The build-up of soluble salts, which occurs as the result of

the migration of groundwater into an adobe structure, and their subsequent crystallization

may result in surface deterioration [1] . The soluble salt content of a sample may also be

an important consideration in the determination of the particle size distribution. Because

the clay content of a sample is determined by the procedures given in section 3.4, as the

difference between the total sample weight and the combined weights of the gravel, sand,

silt and organic fractions, the soluble salt fraction is calculated as clay. Accordingly, a

high soluble salt fraction can cause significant error in this determination of clay content.

For example, the soluble salt content in one sample of adobe from Tumacacori was found to be

in excess of 4 percent of the sample weight. Therefore, corrections should be made for the

amounts of soluble salts when the clay content is calculated. The determination of soluble

salt content is straight-forward and can be completed within 72 hours using the following

procedure. '

A representative adobe sample is dried for 24 hours at 105 °C and weighed to the nearest

0.01 gram. In most cases a sample weight of about 200 g is adequate to yield a sufficient

amount of soluble salt for subsequent analysis, if desired. The sample is then placed in a

600 ml borosilicate glass filter funnel (Buchner type with a fritted filter disc) attached

to a 1000 ml filter flask. About 200 ml of distilled water are poured over the sample which

is then allowed to slake for a minimum of three hours. The sample is stirred from time to

time to aid in dispersion. Then, the water containing the dissolved salts is collected in

the filter flask by evacuating the flask with an aspirator. When water stops dripping from

the tail of the filter funnel, the aspirator is disconnected. Additional 200 mis of distilled

water is then poured over the sample, the slurry is stirred and the aspiration procedure is

repeated. This process is repeated four times altogether to remove the soluble salts.

After the sample has been washed four times, the soluble salt solution which has been

collected in the filter flask is transferred to a glass beaker which has been weighed to the

nearest 0.01 g. The free water is evaporated by warming the beaker on the hot plate, however,

the water should not be allowed to boil. Once the water has evaporated, the beaker is placed

in an oven at 105 °C for 24 hours to remove any residual moisture from the salts and then

reweighed to the nearest 0.01 g. The weight difference is the weight of the soluble salts

9
and this can be calculated as a percentage of the original sample weight. The elemental

composition of the soluble salts can be obtained by a variety of techniques including X-ray

powder diffraction, flame photometry, X-ray fluorescence or by atomic absorption [3].

3 .3 Determination of pH

The behavior of clay particles when wet is largely controlled by the exchangeable

cations of the clay and the pH of the clay-water system. For example, a low pH promotes

flocculation of the clay particles from suspension, while a high pH can lead to the formation

of a stable suspension or dispersion of clay particles [4] . Preservation treatments such as

electroosmosis , electrochemical hardening, and preservatives which can produce either basic

or acid solutions, may drastically change the pH of an adobe. In the cases of a dimensionally

unstable and deteriorating adobe, such a change in pH could improve its performance. However,

a change in pH could have an adverse effect on a stable adobe. In addition, pH measurements

can be used to determine if repair materials are compatible with the original adobe. It is

suggested that the pH of a repair adobe should be within + 2 pH units of the original adobe.

A pH difference of 2 units indicates a difference of a factor of 100 in the concentration of

hydrogen ions. Therefore pH measurements provide a rapid method for monitoring the effect

of preservation techniques on the properties of the adobe.

The pH of the bulk adobe samples should be measured, i.e., the unf ractionated material

(figure 1) . Portable pH meters with internally contained batteries are available so that

field measurements are possible.

3.3.1 Method of Measuring pH

The pH of a soil or adobe can be readily measured in an hour or two using an inexpensive

commercially available pH meter. Usually a saturated calomel electrode is used as the

reference electrode and a glass electrode as the indicating electrode.

The calibration of the pH meter and electrodes should be checked at least daily using

standard buffer solutions. Usually standard buffer solutions and calibration instructions

are included when a pH meter is purchased.

Several procedures have been developed for preparing a soil prior to making pH measurements [5] and the following procedure was found to be satisfactory and is recommended for

the analysis of adobe.

10
(1) A sample of 20 grams of the adobe or soil which passed through a No . 40

sieve is mixed with 40 ml of distilled water in a 50 ml beaker.

(2) The mixture is stirred well using a glass rod and then allowed to

stand for one hour.

(3) After one hour the electrodes are placed in the mixture and its pH

is measured.

3.3.2 Results of pH Measurements

The pH's of samples from Escalante, Fort Bowie and Tumacacori, measured by the methods

described herein, are in the range of 8 to 9 units (table 3), indicating that these are

slightly basic materials, i.e., the concentration of available hydroxide ions is greater

than that of the available hydrogen ions.

Table 3. Results of pH Determinations

Sample pH

Test ConditionsFort Bowie 8.72 8.18

Adobe

Escalante

Adobe 8.19 7.61

Tumacacori

Adobe 8.12 7.58

Soil , 8.25 7.77

Soil-Sand- 8.39 7.97

— Testing condition No. 1 (recommended method);

40 ml of distilled water mixed with 20 gm of

sample mixture, allowed to equilibrate for an

hour then pH measured. Condition No. 2; 30 ml of distilled water mixed with 20 gm

of sample mixture allowed to equilibrate for

24 hours before pH was measured.

2/ — One part soil was mixed with one part Ottawa

grade sand.

11
In another set of measurements the testing conditions were changed: 30 ml of distilled

water were mixed with 20 gm of the samples, and the mixture was allowed to stand for 24 hours.

The measured pH's were lower than those measured by the recommended procedure. This indicates the necessity of adhering to a standard procedure when characterizing adobes and

soils

.

3.4 Determination of Particle Size Distributions

The particle size distribution of an adobe is one of the main factors controlling its

durability [1, 6-9]. A good adobe will consist of between 60 and 80 percent sand, 20 to

40 percent silt and clay, with little or no gravel present. The clay and, to a lesser

extent, the silt portion of an adobe act as the binder forming a matrix in which the sand

particles are embedded. A large proportion of clay could cause excessive swelling in the

adobe through absorption of moisture by the clay and excessive shrinkage when the clay dries.

Clearly, knowledge of the particle size distribution of an historic adobe can provide

valuable insights to some of the problems which will be encountered in its preservation.

Additionally, in repairing deteriorated adobe, the repair material should have a particle

size distribution either close to that of the original adobe, or a distribution which is in

the range found for durable adobes. Therefore, the particle size distribution is one of the

most important properties of an adobe which should be characterized.

As indicated in the flow chart for the analysis of adobe (figure 1) , fractionation of

the adobe into different particle size fractions xs necessary to obtain the particle size

distribution and also to identify the mineralogy of the various fractions. The procedures

for fractionating adobe and for obtaining the particle size distribution are given in this

section.

3.4.1 Method for Determining Particle Size Distribution

Several methods are available for the determination of the particle size distribution

in soils. These include ASTM D1140, "Amount of >Iaterial in Soils Finer than the No. 200

Sieve," [10] and ASTM D422, "Particle Size Analysis of Soils" [11]. The present method is

a modification of these ASTM standard methods. The modifications simplify the analytical

procedure while permitting retention of the various size fractions for further analysis.

The method described herein can be used to separate the gravel, sand, silt and organic

12
fractions which are found in adobe. These size fractions are defined as follows:

gravel: particle diameters larger than 2 mm

sand: particle diameters between 20 microns (0.02 mm) and 2 mm

silt: particle diameters between 2 and 20 microns

clay: particle diameters less than 2 microns.

Separation of size fractions within the sand-gravel size ranges relies on sieving, while

separation of the clay and silt fractions from the coarser fractions, as well as from each

other, relies on particle sedimentation.

3.4.1.1 Sample for Analysis

The adobe sample selected for analysis should, if possible, be in the form of a well

consolidated agglomeration. Powdery material, which may collect at the bases of severely

eroded areas, is generally not representative of the structure. Similarly, consolidated

material which has been eroded by rain or flowing water should not be considered representative. In the former case, a sample would probably be deficient in coarse particles

while in the latter it may be deficient in the finer clay and silt fractions.

The mass of the sample to be analyzed is largely determined by the size of the largest

particles present. For example, ASTM D422 recommends the following sample sizes:

Nominal Diameter of

the Largest Particles

mm inches

9.5

19.0

25.4

38.1

(3/8)

(3/4)

(1)

(1-1/2)

Approximate Minimum

Sample Mass

grams

500

1000

2000

3000

However, from the standpoint of characterizing the particle size distribution in the size

range below about 9.5 mm, sample masses in this fraction need only range from 65 grams for

clay-silt soils to 115 grams for sandy soils . If the adobe to be analyzed is removed from an

historically significant structure, it is generally desirable to limit the mass of the

collected sample. Based on the analyses of adobe from three historical structures

(Tumacacori, Fort Bowie, and Escalante) , a sample mass between 150 and 300 grams was found

to be adequate.

13
3.4.1.2 Dispersion of Sample and Separation of Organic Material and Gravel

Once a suitable sample has been selected, it is crushed lightly using a mortar and a

rubber covered pestle, the sample is xcfeighed to the nearest 0.01 gram and dried at 105 °C

for about 24 hours. Then it is again weighed to the nearest 0.01 gram. The moisture content

of the sample is then calculated and expressed in percent based on the dry weight. After

weighing, the sample is transferred to a 600 ml high- form beaker, covered with about 200 ml

of a freshly prepared sodium hexametaphosphate solution, stirred until the soil is completely

wet, and allowed to stand overnight. The sodium hexametaphosphate solution is prepared by

adding 40 grams of sodium hexametaphosphate crystals to one liter of distilled or deionized

water. Distilled or deionized water should be used exclusively throughout this procedure.

The sodium hexametaphosphate is used to disperse or separate the clay particles. To ensure

complete dispersion of the adobe it is usually necessary to agitate the mixture of adobe and

dispersion solution by mechanical means. However, prior to this step it was found advisable

to hand stir the mixture using a stirring rod and then filter it through a number 8 or

number 10 sieve, collecting the filtrate. This removes the larger particles which are

retained on the sieve as well as most of the twigs and larger pieces of organic material

which may be present. Removal of the larger particles facilitates mechanical stirring. The

organic material retained on the sieve is removed with a tweezers from the gravel particles

after the fraction has been dried at 105 °C. The weight of the gravel is measured and

recorded or further fractioned using coarse sieves if desired. The organic fraction should

be retained for identification purposes.

If a significant amount of organic material is washed through the sieve (no. 8 or 10),

the majority of it may be retrieved when the contents of the pan are transferred back into

the 600 ml beaker by adding about 100 ml of water, stirring and allowing the sediment to

settle. The organic fraction, which is less dense than water, floats on the surface, and

can be removed with a tweezers or a tea strainer. This organic fraction may then be dried

and combined with the other organic fraction, weighed, and retained for further analysis.

Usually, most of the organic material is removed by this method. The unremoved portion is

usually sufficiently small so it does not introduce significant errors into the particle

size distribution analysis.

Prior to further separation of the sand, silt and clay size fractions it is necessary

to completely disperse the particles. ^s^ile ASTM D422 recommends the use of a high speed

14
blender, the use of a magnetic stirrer was found to be adequate. Dispersion was accomplished

by stirring the mixture at high speed for 15 minutes using a 5 cm long Teflon coated stirring

bar. The only disadvantage to this dispersion technique is that any magnetic fraction of

the sample will adhere to the stirring bar. However, this magnetic material is easily

rinsed from the bar with a wash bottle. Then the amount of magnetic material is estimated

by drying the material at 105 °C and weighing it.

3.4.1.3 Partial Separation of Silt-Clay Fraction

Partial separation of the silt-clay fraction may be accomplished by adding sufficient

water to cover the sample with a 8-10 cm high column of water, dispersing the sample by

magnetic stirring, allowing the solid material to settle for a prescribed period and then

siphoning the clay-silt fraction which remains in suspension into a large beaker. These

fractions should be retained for further separation. If this procedure is carried out 3 or

4 times prior to sieving, a substantial amount of the silt-clay fraction can be separated,

thus minimizing clogging of the finer sieves.

The appropriate settling time can be determined using the nomograph given in figure 3,

in which the settling time in cm per second is plotted against the particle diameter in microns

for various temperatures. For example, diameters of 20 microns or less, require a settling

time of 30 s per cm if the solution is at a temperature 18 °C. Thus if the column of water

is 10 cm high, the settling period should be 300 s or 5 minutes. This allows the particles

having diameters larger than 20 microns to settle out of the solutions. However, because

the particles with diameters smaller than 20 microns are initially relatively uniformly

distributed throughout the solution, those near the bottom of the column of water will settle

out before 300 seconds and will not be removed by siphoning. As a consequence it is necessary

to carry out this procedure a number of times, until little change in the color of the

solution is observed.

3.4.1.4 Sieve Fractionation and Determination of Coarse Particles

Once a portion of the silt-clay fraction has been separated from the larger particles,

the contents of the beaker are poured into a set of sieves for size fractionation of the

coarser material. The sieve sizes used are as follows:

15
outer scale 12,000

10,000

9,000

8,000

7,000

6,000

5,000

4,000 ^ 3,000

I <u-° 2,000

LUo 1.000 10 cc 900 9 800 8

CO 700 7 600 6 oC_3 UJ 500 5 COz 400 4

CD

CO

300

200

100

90

80

inner scale

10

PARTICLE SIZE l-IOO/i

TEMPERATURE 16°C-34°C

PARTICLE SP.GR 2.65

20

inner 30 40 50 60 80 100 scale 3 5 6 8 10 outer scale

PARTICLE DIAMETER IN MICRONS

Figure 3. Particle settling time. (Nomograph was developed by Tanner and Jackson [12].)

16

i
Sieve No. [13] Size of Opening In Sieve Screen

No. 16 118 mm

No. 30 600 ym

No. 50 300 vim No. 100 150 ym

No. 200 75 ym

No. 270 53 ym

No. 400 38 ym

A catch pan is placed beneath the No. 400 sieve. The material retained on the No. 16 sieve

is washed with water to remove any fine particles which are in turn washed on to the No. 30

sieve. The No. 16 sieve is then removed from the stack of sieves and placed in an oven at

105 °C for about an hour. The particles are then transferred to a weighing dish and weighed.

This procedure is repeated in turn until the contents retained on each sieve in the series

have been thoroughly rinsed, dried, and weighed. The material in the catch pan, which consists of the size fraction below 38 ym, is then transferred to a beaker. This material is

then stirred and fractionated by the previously described settling time technique to separate

them in IB 20 ym silt-clay fraction from the plus 20 ym minus 38 ym fraction. This minus 20 ym

fraction is then dried and the silt-clay fraction combined with that obtained previously

(section 3.4.1.3).

After the various sieve fractions have been dried they are weighed and the weight of each

size fraction recorded. Care should be taken to ensure that minimal material remains entrapped

in the mesh of each sieve. The weight of the clay fraction is calculated as the difference

between the total sample weight and the combined weights of the silt, sand, gravel and organic

fractions. As a consequence, particles that remain entrapped in the sieves are not weighed

as members of the various sieve size fractions and will be calculated as part of the clay

fraction. Most of the particles entrapped in the finer sieves may be removed by brushing

the inverted sieve with a stiff brush. Particles entrapped in coarser sieves may be removed

with a pin or tweezers.

3.4.1.5 Separation of Silt from Clay

Prior to separating the silt from the clay fraction, the remaining solution (from 3.4.1.3)

was usually left undisturbed for 2 to 3 days thereby allowing the fine clay particles to

settle. However, lengthy settling is necessary only if the analyst desires to retain the

clay fraction for additional analysis. After several days of settling, most of the solution

17
may be siphoned off and discarded with minimal loss of the clay fraction. This reduces the

volume of the solution, which may be as much as 4 liters, to a few hundred milliliters, and

also reduces the concentration of soluble salts (initially present in the adobe) and the

concentration of the sodium hexametaphosphate dispersant. It was found convenient to allow

the solution to settle sufficiently well so that after siphoning off the clear portion, the

remaining mixture could be transferred to a 600 ml high-form beaker.

Separation of the silt from clay can be carried out by the same procedure as was

described for separating the silt-clay fraction from the coarser particles. In this case,

however, because the particle diameters are smaller, longer settling times are required.

For example, the settling rate for a 2 micron clay particle in a solution at 18 °C is 3000

seconds per cm (figure 3). Thus, if the column of water above the sediment is 10 cm high,

the settling time is 8 hours 20 minutes. As a consequence, several days are required to

adequately separate the silt and clay fractions. When this separation is complete, the silt

is dried at 105 °C and weighed. The clay fraction is allowed to sit undisturbed until the

solution becomes clear. The solution is then siphoned off and the clay sediment is dried at

60 °C and retained for further analysis, such as identification of the clay mineralogy by Xray diffraction methods. The clay fraction should be dried at 60 °C rather than 105 °C to

avoid the possibility of irreversible dehydration of the clay minerals.

3.4.2 Useful Information from Particle Size Determinations

Two examples of useful information obtained from the determination of the particle size

distribution are provided by an analysis of adobes from Fort Bowie and Tumacacori. The

first example involves the adobe from Fort Bowie (table 4 ) . This adobe contains about 25

percent, by weight, of particles larger than 2.4 mm (gravel-sized particles) and about 60

percent of particles between 2.4 mm and 300 ym (coarse sand). As a consequence, when the

adobe walls become damp and the cohesiveness of the silt-clay binder is reduced, the gravel

and coarse sand particles tend to settle downward disrupting the matrix. This matrix can be

readily eroded by rain and accumulated run-off water, which probably accounts for the observed

rapid deterioration of the walls at Fort Bowie.

As a second example, a variety of adobe samples from the church at Tumacacori were

analyzed along with a soil sample obtained from the vicinity of the church. The range of

particle size distributions of the adobe are compared with the distribution of the soil in

18
table 4. The distribution for the adobe samples are in the range characteristic of durable

adobe [1]. However, the soil is rich in silt and clay. This soil is probably not dimensionally stable, i.e., it may expand when wet and shrink upon drying to a larger extent than the

adobes. The addition of 150 parts of sand, on a weight basis, to 100 parts of the soil will

increase the proportion of sand to about 70 percent and reduce the silt and clay content to

about 30 percent. This will produce an adobe material with a particle size distribution

in the approximate range of the adobe from Tumacacori.

Table 4. Measured Particle Size Distribution

Type of

Particle Amount (percent on weight basis)

Adobe from Adobe from , Soil from

Fort Bowie Tumacacori — Tumacacori Area

Gravel 25 5-8 2 Sand 59 65-80 24

Silt 9 8-12 26

Clay 7 8-12 46 — Range based on analyses of 3 different adobe samples from the

Tumacacori church.

3 .5 Liquid and Plastic Limits

The rheological behavior of a soil changes with a change in water content [14,15]. At

high water contents, soils are suspensions, with the flow properties of liquids. As the

water content decreases, the soil becomes consecutively pastelike, sticky, then plastic, and

finally at a low water content the soil has the properties of a solid. The physical state

of a soil at a given water content is termed its consistency, which is a measure of the

resistance of a soil to flow. Different soils usually have different consistencies at the

same water contents; therefore, the determination of consistency provides information about

both the constituents and the rheological properties of a soil or adobe.

The specific consistencies that are used for soil classification purposes are the

2/ liquid and plastic limits.— The liquid limit (LL) is the water content, in percent by

2/ — Sometimes called the Arterberg limits, in recognition of his early work on soil

consistency.

19
weight, at which a soil ceases to behave as a liquid and begins to exhibit the behavior of a

plastic. The plastic limit (PL) is the water content, in percent by weight, below which the

soil is not plastic and crumbles when worked. The plasticity index of a soil is also

sometimes given in reports, and is defined as the liquid limit minus the plastic limit. The

plasticity index largely depends on the amount of clay present; the strength of a soil

increases as the plasticity index increases. However, the tendency of a clay to expand when

wet and shrink when dried also increases as the plasticity index increases.

3.5.1 Determination of Liquid and Plastic Limits

Determination of the liquid and plastic limits requires only an hour or two of actual

work and the final results are obtained within 24 hours. One pound (454 grams) of bulk

adobe will normally be sufficient for the determinations. The determinations are made using

fractionated material (figure 1)

.

The liquid limit of a soil or adobe can be determined by following the methods given in

ASTM D423 [16]. The determination essentially consists of:

(1) Separating and collecting the portion of the sample passing a No . 40 (425 Mm) sieve.

About 100 grams of the fine material is required.

(2) Mixing this fine material with distilled water, if available.

(3) Placing this mixture in the cup of a mechanical device which jars the sample in

a reproducible way (figure 4)

.

(4) A groove of about 1/2 inch (1.27 cm) is made in the moist sample with a

standard grooving tool (figure 4)

.

(5) Then the cup is dropped repeatedly from a standard height (0.3937 in (exactly

1 cm) ) until the sample flows together in the groove. The liquid limit is

defined as the water content at which 25 of the standardized blows are required

to close the groove over a distance of about 1/2 in (1.27 cm). Because it

would be time consuming to add exactly the correct amount of water, several

additions of water are made, proceeding from the dryer to the wetter condition

of the soil. At each addition of water, the number of blows required to close

the groove is recorded. The purpose of this procedure is to obtain samples

of such consistency that the number of blows will be above and below 25. The

range of data should be between 10 to 40 blows. A best-fit straight line is

20
3

XT

•H

0)C

•H

)-l 3M

0)eo

13

0)

CUo

•H> U] Q) tHO

iH cn O MH

•H

O c

CO 4J o e

(-1

21
drawn through the data and the water contents at 25 blows is estimated

(fig. 5). The water contents are measured weighing the moist specimen then

drying it at 110 °C until at constant weight (overnight drying is usually

sufficient)

.

The plastic limit can be determined by the methods given in ASTM D424 [17] . In this

method, a wet sample is dried by being rolled back and forth over a ground glass plate or a

piece of absorbent paper until the sample crumbles at a diameter close to 1/8 in (3.2 mm)

(fig. 6). The rolling promotes the drying of the sample. The sample is weighed and then

oven dried at 110 °C until at constant weight. The water content of the sample is defined

as its plastic limit.

Both the liquid and plastic limits should be reported in percent, as calculated by the

formula:

weight of moisture ^ .

.

percent moisture content = -

° :—, ^ rr- x 100.

dry weight of soil

Practice is required to obtain reproducible results in measuring both the liquid and

plastic limits. With practice, standard deviations should be within 10 percent of the

average values [18-19]. Therefore, it is recommended that triplicate determinations be made

and the average and standard deviation of the data reported.

3.5.2 Information from Liquid and Plastic Limit Determinations

The liquid and plastic limits determined as described herein, can be useful in the

process of characterizing the properties of particles smaller than 425 pm. However, it is

the clay size particles i.e., those smaller than 2 pm which have the greatest effect on the

magnitude of these limits. The limits for some clays are listed in table 5. Clearly, a

clay which is predominantly montmorillonite can be distinguished from other clays such as

kaolinite, illite, chlorite on the basis of its liquid and plastic limits. The liquid and

plastic limits of some samples from Escalante, Fort Bowie and Tumacacori are given in

table 6. Based on these data it can be concluded that little, if any, montmorillonite is

present in these samples. Note that the adobe sample from Fort Bowie has a small plastic

index, which occurs with soils having small amounts of fine particles. Furthermore, the

22
C3

oC9

UJ5

10 20 25 30

NUMBER OF BLOWS

40 50

Figure 5. Plot of a liquid limit determination. The water content

at 25 blows is estimated by constructing a straight line

between data points denoted by the symbol A.

23
1

" -ll

r J^'

K.'

Wt "" "3^ /~ ' . r

•H -H e ^ C30

r-l .-H O .-I o

•H D. J-i

en CO 0)

CO tn M

o

en u

CO 4-1 e

CO

. M-l •P o

•H -H •H C H 0) CB

4-1 U C 4-1 •H O

4-) O

p. 4J

CO o

01

^ C

01 -H

4-1 cn

CO CO

o

S-i Co • D I

CO

Q) CM

S-i O)

4-1 M-l 01 0)

0)

00

24
Table 5. Compilation of Liquid and Plastic Limits for Clays [14,20]

Clay Liquid Limit Plastic Limit

(percent) (P ercent)

52 26

73 36

61 34

90 40

700 97

177 63

171 145

94 65

231 136

85 78

Kaolinite-Na

Kaolinite-K

Illite-Na

Illite-Ca

Montmorillonite-Na

Montmorillonite-Ca

Attapulgite

Halloysite

Allophane (undried)

Allophane (dried)

Chlorite 44-47 36-40

Table 6. Measured Liquid and Plastic

Limits of Adobe and Soil Samples —

Sample Liquid Limit (LL)

(percent)

Plastic Limit (PL)

(percent)

Plasticity Index (PI)

(percent)

II

Escalante

Adobe 32.9 21.8 11.1

Fort Bowie

Adobe

3/ Tumacacori —

Adobe T-011

Adobe T-012

Adobe T58

20.2

23.5

24.8

42.5

14.6

16.0

21.6

31.7

5.6

7.5

3.2

10.8

Soil 36.5 20.7 15.8

— For particles less than 425 ym.

2/ — PI = LL - PL.

3/ — Adobe specimens taken from different parts of the church at Tumacacori,

25
small plastic limit for this adobe indicates that it has poor cohesion and poor resistance

to cracking. Liquid and plastic limits are significantly affected by several factors [4,9]

including the presence of organic matter (straw, manure, etc.), type and amount of soluble

salts, and by the specific exchangeable cation of the clay (table 5). Therefore, while the

liquid and plastic limits give useful information that can be used to supplement the mineral

identification of clays obtained from X-ray diffraction and optical microscopy techniques,

3/ they do not provide definitive mineralogical identification.—

Important information concerning the dimensional response of a clay to moisture is provided by the value of the plasticity index [4,20]. Higher plasticity indices are accompanied

by larger expansions upon wetting and larger shrinkages upon drying. For example, a clay

with a plasticity index of 50 percent could have a total expansion, when saturated with water,

of between 33 and 40 percent, while a clay with a plastic limit of 10 percent may expand by

only 4 to 10 percent (table 7). Therefore, usually the closer that adobes match in their

plastic indices the more compatible are their dimensional responses to moisture. The soil

from Tumacacori has a plastic index of 15.8 percent (table 6), which is probably sufficiently

close to the plasticity indices of the adobe samples from Tumacacori to permit its use,

provided sand is added (see section 3.4.2). Of course, as close a match in plasticity

indices as possible would be desirable.

Another use of the liquid limit determination is to obtain an estimate of the amount of

mix water required to make good adobe brick, and adobe test specimens. Usually the amount

of required water will be less than the water contents at the liquid limit as the adobe will

contain particles larger than 425 ym. Particles larger than 425 urn absorb much less water

than clay-size particles and the reduction in mixing water was found to be somewhat proportional to the percentage of material larger than 425 um.

3/ — Except in the case of allophane clay minerals which are identified on the basis of the

effects of drying on their plastic limits and plastic indices. However, it is doubtful

that allophane clays will be found in abundance in the arid southwestern region of the

United States.

26
Table 7. Relationship between Total

Expansion and Plasticity of Soils [20]

Plasticity Index

of Soil— Total Expansion

(percent) (percent)

10 4.5 - 10.0

20 13.5-18.7

30 21.4 - 28.0

40 28.0 - 35.0

50 33.0 - 40.0

— For particles smaller than 2 ym.

3. 6 Determination of Minerals in Adobe by X-ray Dif fractometry

The mineral composition and particle size distribution of adobes and soils largely

controls their mechanical properties and durabilities. For example, an adobe containing

a montmorillonite type of clay would swell excessively when wet and shrink upon drying, which

would impair its durability. Therefore, such a material should not be used to replace or

repair deteriorated adobe. If possible, the material used to replace or repair the adobe of

historic structures should have the same mineral composition as the original adobe.

3.6.1 Method of Obtaining and Interpreting Powder Diffraction Patterns.

X-ray powder dif fractometry is a rapid method for identifying the mineralogy of the

constituents of adobe materials and soils. It is also useful in ascertaining if a soil

selected for repair purposes has the same mineralogical features as the original adobe.

Because the diffraction patterns reflect the types and amounts of the various minerals

present, they serve to characterize or "fingerprint" a particular adobe. Therefore, by comparing the pattern of the original adobe with that of the repair material the analyst can

ascertain if minor or gross differences exist, without identifying the mineralogical composition of the adobe.

The type of the X-ray powder analysis selected will depend on the information desired.

Usually the "fingerprint" will be sufficient for the analysis of adobes. In some cases, a

more detailed analysis in which the mineralogical composition is determined may be desired.

However, identification of the clay mineralogy can be difficult, even for an experienced

analyst. The procedure for making either a fingerprint or detailed analysis are discussed

27
to acquaint the preserver with these techniques and with the type of information they

provide.

It is assumed that the reader is familiar with the use of standard X-ray equipment.

The dif fractometric powder technique is recommended. Detailed discussions of theory and of

the equipment and its operation are given by Klug and Alexander [21] . Cu radiation is

commonly used.

In practice, a subsample of adobe is crushed and ground using a mortar and pestle.

Although an average particle size of less than 10 Mm is preferred for intensity measurements,

it may not be desirable to grind the material to that extent. Excessive grinding may destroy

the structure of layer silicates such as the micas and most of the clay minerals. In order

to avoid orientation of the crystallites and hence to obtain diffraction intensities more

proportional to the amount of a mineral present, a special aluminum specimen holder is used.

Figure 7 shows the manner in which the specimen is prepared by gently shaking the powder

specimen so that the powder falls into the cavity formed by the holder and a glass slide.

This technique is used by the Joint Committee on Powder Diffraction Standards (JCPDS)

Associates at the National Bureau of Standards and has proven very effective.

The diffraction angles (2 ) of the dif f ractometer traces may be converted to d_ spacings

o in Angstrom units by reference to tables such as those of Fang and Bloss [22] . The experimentally obtained d^ spacings are then compared with d^ spacings of known compounds until

identification is achieved. The Powder Diffraction File of the JCPDS is a standard reference

for powder diffraction analysis. A laboratory doing X-ray analysis should have the powder

diffraction file and appropriate search manuals.

3.6.2 Bulk Analysis

X-ray powder dif fractometry provides the most rapid method for the bulk characterization

of adobe materials. For example, the bulk X-ray analysis of several adobe specimens from

Tumacacori showed that all contain quartz, feldspar (both alkaline and calcic), biotite mica,

muscovite mica, amphibole, gypsum, rutile, hematite, illite clay, kaolinite clay and calcite.

In general, estimates of amounts of minerals present in a complex mixture on the basis of

diffraction intensities are semiquantitative at best because of differences in mass absorption

coefficients, degree of crystallinity , lack of random particle orientation, variation in

sample weights, degree of hydration and other variables. Quantitative analysis is sometimes

28
Figure 7. Aluminum holder for preparing powder

samples for X-ray analysis.

29
possible, however, and a detailed discussion of the subject is given in reference 21.

References 4 and 23 are particularly good sources of information on the properties

and identification of minerals in soils.

3.5.3 Identification of Clay Minerals

With exception of the clays, the minerals listed above are mainly primary minerals

formed during the cooling of a magma. Their resistance to chemical weathering is roughly

inversely proportional to the sequence in which they were formed, or the reverse of Bowen's

reaction series [4]. The clay minerals are one product of chemical weathering. In the

discussion which follows, the term "clay" will refer to the less than 2 vm size fraction of

the adobe and will therefore include both the clay minerals and clay-sized minerals. Before

analysis, the clay fraction (figure 1 and section 3.4) must first be separated from particles

larger than 2 ym by sedimentation.

Precise identification of clay minerals is a specialized and sometimes complex X-ray

powder diffraction problem. Therefore, it is recommended that standard texts be consulted when

attempting identification of clay minerals. A useful discussion of experimental methods

is given by Brindley in [24] . This book is also an excellent source of X-ray data on the

clay minerals and many of the common soil minerals, as is the text by Grim [25].

Brindley [24] points out that on the order of a dozen X-ray dif fractograms may be

required for the definitive identification of a particular clay sample, including patterns

of oriented, unoriented, glycolated and heated samples. Oriented sample mounts are prepared

by evaporation of a clay-water suspension on a Vycor glass slide [26] , or by deposition by

centrifugation on a porous ceramic plate [27]. Basal spacings are obtained from oriented

specimens and are most useful in clay identification. The specimen should be scanned from

60 to 2 degrees at a rate of 0.5 degrees per minute. A slower scanning rate may be found

desirable when a definitive identification is needed. Dif fractograms of both oriented and

unoriented specimens should be recorded after specimens have been air dried and then placed

in a desiccator for 24 hours. Ethylene glycol can enter between the layers of certain types

of clays, causing a controlled separation of the layers, which can be diagnostic. The

specimens are subjected to glycolation by heating them in a desiccator containing a beaker

of ethylene glycol. The desiccator should be placed in an oven at 60 °C and held for five

hours. If the clay has been deposited on porous tile by centrifugation, the glycol may be

30

I
applied directly to the sample surface. A 25 percent glycol-water solution is frequently

used. After treatment, the specimen is air dried and the X-ray diffraction pattern obtained.

As discussed by Brindley [24] and by Carroll [26] a variety of techniques are often

required for identification of the clay minerals. In addition to glycolation, a heat treatment of the clays may be necessary. Two very useful tables of data on clay minerals have

been compiled by Carroll. The first (table 8) permits a preliminary identification of most

clay minerals on the basis of their X-ray diffraction data. The second (table 9) gives

characteristic reactions and structural changes of clay minerals when heated in a muffle

furnace. By these methods, the predominant clay minerals of adobe materials may be identified. For example, the diagnostic method for montmorillonite clay is to record the basal d

o spacings (001), which are between 14 to 15 A for the montmorillonite group. If the clay

o does belong to the montmorillonite group, this spacing will increase to about 17 A upon

o treatment with glycol and collapse to 9 A when the sample is heated at 300 °C. It is

difficult to identify small amounts of montmorillionate because x-ray diffraction lines of

this clay group are weak.

3. 7 Analysis of Adobe by Microscopy

The binocular microscope, the polarizing microscope and the scanning electron microscope

can be used to analyze the microfabric of adobes and soils. The term "fabric" refers to the

arrangement of particles, particle groups and pores in an adobe or soil. The microfabric are

those features which require a microscope for study. This includes information on the shape,

packing, texture, and size distribution of particles. In addition, information on the type

and distribution of minerals can be obtained.

The information which can be obtained from use of each of the three microscopes by an

experienced petrographer is discussed in this section.

3.7.1 Binocular Microscope

The size, shape and arrangement of pores and voids, the presence of deposits in pores,

the size, shape and packing of coarse and fine aggregates, and the presence of organic

material such as twigs and straw are the main features that may be observed macroscopically

by a low-power binocular microscope using reflected light. Many of the fine details of the

microfabric of consolidated adobe specimens are obscured by the silt-clay matrix, and their

31
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32
Table 9. Temperatures Required in the Identification of Clay Minerals 1/

Mineral Temperature,

1 hour

Effect

Kaolinite, well-crystallized

Kaolinite, disordered

("fire-clay")

Dickite

Nacrite

Halloysite

Metahalloysite

Allophane

Mica, well-crystallized

Illite, and clay micas

Glauconite

Celadonite

Biotite

Vermiculite

Montmorillonite Group

Chlorite Group

Mg-chlorite

Fe-chlorite

Mixed-layer clays

Palygor skit e-Sepio lite

Group

Sepiolite

Palygorskite

575-625 °C

550-562 °C

665-700 °C 625-680 °C 125-160 °C 560-605 °C 125-150 °C 560-590 °C 140-180 °C

700 °C

125-250 °C 350-550 °C 700 °C 530-650 °C

500-600 °C 700 °C

< 300 °C

300 °C

600-800 °C

650 °C

500 °C

< 600 °C

< 200 °C

> 200 °C

400-440 °C

Replacement by amorphous metakaolin; no diffraction pattern

Similar to kaolinite

Similar to kaolinite

Similar to kaolinite

Water removed

Similar to kaolinite Water removed

Similar to kaolinite

Adsorbed water removed; amorphous

at all temperatures

Shows gradual loss in weight, but

does not break down below 700 °C; (001) spacings remain in diffractograms below 700 °C and up to 1000 °C Loss of hydroscopic water

Reverts to mica structure

Similar to mica

Loss of interlayer water; reverts

to mica structure

Similar to glauconite

Phlogopite is similar to muscovite

Biotite shows breakdown 700-1000 °C

o Wateroremoved in stages with 14 A

13.8 A 11.6 A 9 A. Initial (001)

spacing controlled by humidity

Original 15 A spacing disappears;

o 9 A spacing develops

Show gradual weight loss, but no structural change

14 A spacing is intensified; (004) at 3.54 not affected

o 14 A spacing less intense and may

become broad and diffuse

Varies with kinds of minerals present and amount of each ; check with

X-rays

Water occurs in channels in structure

Rapid dehydration

o Slow dehydration; spacing at 12 A above 350 °C becomes weak and diffuse;

a spacing at 9.8 A appears and spacing

at 7.6 A becomes more intense; re- crystallizes at 800 °C Rapid dehydration, but no change in structure 400 °C; 400 °C 10.5 A spacing becomes broad and diffuse; near 800 °C structure is destroyed — Compiled by Dorothy Carroll [26] 33
observation requires a binocular microscope with good resolving power.

The binocular microscope is a useful tool for adobe studies using specimens impregnated

with a material such as epoxy, sectioned and polished. Consolidated samples which can be

cut and polished are obtained by impregnation with an epoxy. Several commercially available

epoxies have given excellent results. Methods of sample impregnation have been described by

Morreland [28]. Sample preparation is also available from commercial laboratories at a

nominal cost.

Examination of the polished specimens in reflected light gives a wealth of information.

For example, many of the minerals and larger rock fragments may be identified and the size,

shape, packing and colors of the grains observed. In addition, the types of organic material

present, the residual structures of decomposed straw and twigs, the weathering of mineral

fragments, angularity of the coarser silt particles, and the presence of soluble salts

(precipitated as pore fillings after evaporation of pore water) are further examples of the

types of information that may be obtained by this simple technique. Impregnation with a

colored epoxy may be useful in studies of pore sizes and pore distributions. An alternative

to polishing sawed specimens is to simply spray their surfaces with a colorless acrylic.

This method frequently permits observation of greater detail and eliminates the danger of

removal of the softer matrix that may occur in the polishing process. The photographic

reproductions in figures 8-12 show the kinds of information which can be obtained with the

binocular microscope. Adobe specimens from the mission church at Tumacacori are shown in

figures 8-10. Figure 11 is a microscopic view of adobe from Escalante, and figure 12 shows

the aggregate and coarse matrix of an adobe specimen from Fort Bowie. Low magnifications have

been used (3-lOx) to show the general aspect of these materials in figures 8-10 although

magnifications of 80-lOOx are common with this type of microscope.

The binocular microscope is especially useful in assessing the effectiveness of the

size fractionation process. The effectiveness of the sample dispersion method should be

determined by examining the various size fractions microscopically, particularly for complete

separation of the clay-silt fraction. In addition, individual minerals and rock fragments

may be identified and studied in much more detail after being fractionated than was possible

by a study of polished sections. By utilization of the binocular microscope, an experienced

analyst should be able to obtain a fairly complete identification of the mineralogy of the

adobe (with the exception of the clay-silt fraction) , and an assessment of the homogeneity

of the material. „,34
0**«

?,^»

9f: •**%

"t

%-C^

' ^

^n^

VI... • ,1. ^

4.

Figure 8. Microscopic view of epoxy- impregnated adobe

specimen from Tumacacori. The specimen was viewed with a binocular microscope using reflected

light (magnification of 3X)

.

35
Figure 9. Gypsum in pore space of adobe specimen from

Tumacacori viewed with a binocular microscope

using reflected light (magnification of lOX)

.

36
> /rv»

#

X

^ii.

,^^

*>%.,

%h^

-"# %

f

Figure 10. Microscopic view of epoxy-impregnated and

polished section of adobe sample from

Tumacacori. The fabric of the sand and the coarse matrix are shown. The specimen was viewed with a binocular microscope using

reflected light (magnification of lOX)

.

37
^^

m

•

t':t

f

o

^f^F

m

Figure 11. Microscopic view of epoxy-impregnated and

polished section of adobe sample from Escalante.

The specimen was viewed with a binocular

microscope using reflected light

(magnification of lOX)

.

38
Figure 12. Microscopic view of epoxy-impregnated and polished

section of adobe sample from Fort Bowie. The

specimen was viewed with a minocular microscope

using reflected light (magnification of lOX)

.

39
3.7.2 Polarizing Microscope

Thin sections of adobe (standard thickness 30 microns) may be studied with a petrographic

microscope in transmitted polarized light. With a magnification capability of lOOOx, this

instrument permits observation of much finer microfabric detail than the binocular microscope.

Figure 13 is an example of a photomicrograph of a thin section of adobe taken in transmitted

polarized light at very low magnification (SOX) . Even at this low magnification considerable

detail is revealed by the contrasting interference colors, which will assist the microscopist

in selecting areas for examination at higher magnifications.

The polarizing microscope is commonly used to identify the mineral present in a soil.

All crystals have a characteristic index or indices of refraction which are unique for a given

compound and serve as a means of identifying it. In practice the unknown mineral, in powder

form, is immersed in a series of oils of known index of refraction until a match is observed

under the microscope. The index of the oil is then taken as the index, or one of the primary

indices, of the mineral. A crystal may have one, two or three primary indices of refraction

depending upon its symmetry. Classification of the crystal into one of three symmetry classes

is made, indices measured and reference tables listing mineral refractive indices searched

until the compound is identified. This method is particularly useful when the material of

interest is present in minute amounts or when X-ray or SEM equipment is not available. There

are many excellent texts on the use of the polarizing microscope and this technique is discussed in detail in reference [29]. Sets of oils for refractive index determinations are

commercially available. Refractive indices of the more common minerals are given in references

[30,31]. Additional information on the optical properties of minerals is given in the series

of Deer et al. [32]

.

3.7.3 Scanning Electron Microscope

The scanning electron microscope (SEM) has a magnification range of 20 to about 150,000

and a depth of field some 300 times greater than that of the optical microscope. With this

instrument, electrons strike a sample and secondary electrons are emitted from the sample

surface to form what appears to be three dimensional images. Cost of SEM's are in the same

range as X-ray diffraction equipment. Because of the valuable information which can be

obtained, this instrument is now found in many university and research laboratories. It may

be used to study polished or fractured surfaces, individual crystals or powders. The SEM

40

\
Figure 13. Thin section of adobe sample from Fort Bowie.

The specimen was viewed using polarized

transmitted light (magnification of 50X)

.

41
is used for examining the microfabric of adobes and soils, observation of clay particles

which are too small to be observed with optical microscopes, and the arrangement and

porosity of the silt and clay particles. If equipped with an energy dispersive X-ray analysis

attachment, SEM may also be used for chemical or elemental analysis of adobes and soils.

Figures 14-18 show examples of some applications of this technique to adobe studies.

Figure 14 is a scanning electron micrograph of a polished section of adobe from the church at

Tumacacori. The micrograph was taken at very low magnification (30X) to show the general

fabric of the coarse and fine aggregate and the clay-silt matrix. Figure 15 shows an elemental

map of a fine particle in the adobe from Tumacacori. Because the particle was suspected to

be gypsum the area was mapped for calcium and sulfur, and the two maps were superimposed.

Gypsum, which has filled a pore, is shown by the white areas in the photograph (magnification

500X) . The energy distribution of this light region, obtained by energy dispersive X-ray

analysis (EDXA) , is shown in figure 16. The prominent sulfur peak on the left and the calcium

peak on the right suggests the presence of gypsum (CaSO^ •2H2O)

.

During size fractionation of the adobe specimens, the soluble salt fractions can be

separated for elemental and other analysis. Figure 17 shows the energy distribution of a

bulk salt sample from Tumacacori, and indicates the elements potassium, magnesium, sulfur,

chlorine and calcium. This material is extremely hygroscopic. A small amount of the powder

previously dried at 110 °C was placed on a microscope slide and exposed to the laboratory

atmosphere, with a relative humidity of about 60 percent, for approximately one-half hour.

The resulting material was viewed through a polarizing microscope in transmitted light at a

magnification of 65X. The large volume of water absorbed by and eveloping the grains is

clearly evident (figure 18)

.

,

The elemental analyses that have been discussed in this section are qualitative determinations. Computers can be interfaced with SEM's [33] so that quantitative elemental

analyses can be made. Quantitative determinations of individual soluble salts in adobe

could provide valuable information in the investigation of the factors causing the deterioration of adobe.

42
Figure 14. Scanning electron micrograph of adobe sample

from Tumacacori (magnification of 30X)

.

43
Figure 15. Energy dispersive X-ray element map of adobe sample

from Tumacacori (magnification of 500X) . The

white regions indicate the presence of calcium and

sulfur.

44
I

Figure 16. Energy dispersive X-ray spectrom of white material

shown in figure 15. The two major peaks indicate

the presence of a material containing calcium

and sulfur, probably gypsum (CaSo -2 HO).

45
Figure 17. Energy dispersive X-ray spectrum of salts

extracted from adobe sample from Tumacacori.

The major peaks indicate the presence of the

elements potassium, magnesium, sulfur, chlorine,

and calcium.

46
Figure 18. Microscopic view of hygroscopic salts extracted

from adobe sample from Tumacacori. The salts were viewed with polarized transmitted light

(magnification of 65X)

.

47
4. SUMMARY AND RECOMMENDATIONS

Methods are described in this report for the characterization of those physical proper- '

I ties and mineralogical features of adobes which appear to have the most significant effects

on adobe durability. These features include particle size distribution, mineral composition j

of particle size fractions, presence of soluble salts, the microfabric of the silt-clay if

matrix, and the rheological response of adobes to moisture. In addition, methods are given '

for assessing the compatibility of repair materials and preservation techniques with the ] original adobe of historic structures. I

1

:i It is recommended that the analysis of an adobe should include the following determinations; color, pH, amount of soluble salts, particle size distribution, liquid and plastic

limits, and the X-ray diffraction "fingerprint." In addition, viewing an adobe specimen

through a binocular microscope can provide interesting information on the gross fabric of

the adobe. This complete analysis can be carried out within a week using equipment available

to most testing laboratories.

The definitive analysis of the mineral composition and the proper analysis of the fine

microfabric features of an adobe are more difficult than the preceeding determinations, and

require more sophisticated techniques. However, at times such analyses are justified. For

example, if there is any indication that the soil selected for making repairs to an adobe

structure contains a swelling clay, e.g., montmorillonite type, then a definitive identification

should be performed.

The characterization of the physical properties and mineralogical features of adobe should

become a part of the standard methodology developed for the preservation of adobe structures.

The methods described in this report can form the basis for this contribution to the standard

methodology.

48
5 . REFERENCES

[1] Clifton, J. R. , Preservation of Historic Adobe Structures - A Status Report, National

Bureau of Standards Technical Note 934 (1977).

[2] Soil Survey Manual, Handbook 18, United Stated Department of Agriculture

(Washington, D.C.).

[3] Willard, H. H. , Merritt, L. L. and Dean, J. A., Instrumental Method of Analysis,

4th Edition (D. Van Nostrand Co., Princeton, New Jersey, 1965).

[4] Mitchell, J. K. , Fundamentals of Soil Behavior (John Wiley & Sons, Inc.,

New York, 1976).

[5] Jackson, M. L. , In Chemistry of the Soil, 2nd Edition, edited by F. E. Bear,

(Reinhold Publishing Corporation, New York, 1964)

.

[6] Eyre, T. A., The Physical Properties of Adobe Used as a Building Material, The

University of New Mexico Bulletin, Engineering Series, Vol. 1 (No. 3 (Albuquerque,

New Mexico, 1935).

[7] Schwalen, H. C, Effect of Soil Texture Upon the Physical Characteristics of Adobe

Brick, Agricultural Experimental Station Technical Bulletin No. 58 (University of

Arizona, Tucson, Arizona, 1935).

[8] Webb, T. L. , Cilliers, T. F. , and Stutterheim, N., The Properties of Compacted Soil

and Soil-Cement Mixtures for Use in Building, National Building Research Institute

Series DR 2 (South African Council for Scientific and Industrial Research, 1950).

[9] Harrington, E. L. , Adobe as a Construction Material in Texas, Texas Engineering

Experiment Station, Bulletin No. 90 (1945).

[10] Standard Method of Test for Amount of Material in Soil Finer than the No. 200 (75 m)

Sieve, American Society for Testing and Materials Designation D1140-56.

[11] Standard Method for Particle-Size Analysis of Soils, American Society for Testing

and Materials Designation D1122-63.

[12] Tanner and Jackson, M. L. , Soil Science Society Proceedings No. 12 (Nov. 1947).

[13] Standard Specifications for Wire-Cloth Sieves for Testing Purposes, American Society

for Testing and Materials Designation Ell-61.

49.
[14] Yong, R. N. and Warrenton, B. P., Soil Properties and Behavior (Elsevier Scientific

Publishing Company, Inc., New York, 1975).

[15] Tuma, J. J., and Abdel-Hady, M. , Engineering Soil Mechanics, Prentice-Hall Inc.

(Englewood Cliffs, New Jersey, 1973).

[16] Standard Method of Test for Liquid Limits of Soil, ASTM Designation D423-66.

[17] Standard Method of Test for Plastic Limit and Plasticity Index of Soils, ASTM

Designation D424-59.

[18] Dawson, R. F. , Investigation of the Liquid Limit Test on Soils, Papers on Soils 1959

meeting, ASTM Special Technical Publication No. 254, pp. 190-195.

[19] Abdun-Nur, E. A., Plastic-Limit-Comparison of Cube and Standard Thread Test Methods,

pp. 212-215, reference 28.

[20] Seed, H. B. , Woodward, R. J. , and Lundgren, R. , Prediction of Swelling Potential for

Compacted Clays, Journal of the Soil Mechanics and Foundations Division, ASCE, Vol. 8i

(no. SMS), pp. 53-87 (1962).

[21] Klug, H. P. and Alexander, L. E. , X-ray Diffraction Procedures for Polycrystalline

and Amorphous Materials, 2nd Edition (John Wiley & Sons, Inc., New York, 1974).

[22] Fang, J. H. and Bloss, F. D. , X-ray Diffraction Tables (Southern Illinois

University Press, 1966).

[23] Gieseking, J. E. , Soil Components, Vol. 2, Inorganic Components (Springer-Verlag,

New York, 1975).

[24] Brindley, G. W. , in the X-ray Identification and Crystal Structures of Clay Minerals

edited by G. Brown (Mineralogical Society London, 1961).

[25] Grim, R. E. , Clay Mineralogy, 2nd Edition (McGraw-Hill Book Co., New York, 1968).

[26] Carroll, D. , Clay Minerals: A Guide to Their Identification, Special Paper 126

(The Geological Society of America, Boulder, CO, 1974).

[27] Kinter, E. B. and Diamond, S., A New Method for Preparation and Treatment of Oriented

Aggregate Specimens of Soil Clays for X-ray Diffraction Analysis, Soil Science 81 ,

2, 111-120 (1956).

50
I [28] Moreland, G. E. , Preparation of polished thin sections, Am. Mineralogist, _53^, 2070-

2079 (1968).

[29] Hartshorne, N. H. , and Stuart, A., Crystals and the Polarising Microscope, 4th Edition,

I (Edward Arnold & Qo. , London, 1970).

j [30] Heinrich, E. Wm. , Microscopic Identification of Minerals (McGraw-Hill Book Co.

,

I

j New York, 1965).

[31] Winchell, A. N. and Winchell, H. , The Microscopical Characters of Artificial Inorganic

1 Solid Substances: Optical Properties of Artificial Minerals (Academic Press,

New York, 1964).

I

I [32] Deer, W. A., Howie, R. A., and Zussman, J., Rock-Forming Minerals, Volumes 1-5

(John Wiley and Sons, Inc., New York, 1963).

[33] Yakowitz, H. , Myklebust, R. , and Heinrich, K. , The EDAX Editor 3, (1974).

51
APPENDIX - DESCRIPTION OF SOURCES OF ADOBE SAMPLES

Escalante An Indian farming conmunity inhabited from about

900 to 1450 A.D. It was located along the Gila River,

in Arizona.

Fort Bowie - An Army fort constructed on the Apache Pass in Arizona.

The adobe test samples were taken from the ruins of the

2nd fort, constructed in 1868.

Tumacacori National

Monument

A Jesuit Missionary settlement, of which the mission church

is the most important building. The church was constructed between 1802 and 1822. Tumacacori National

Monument is located about 40 miles south of Tucson,

Arizona.

52
NBS-n4A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN 977

2. Gov't Accessic

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Methods for Characterizing Adobe Building Materials

5. Publication Date June 1978

6. Performing Organization Code

7. AUTHOR(S)

James R. Clifton, Paul W. Brown, and Carl R. Robbins

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

U.S. Department of the Interior

National Park Service

Washington, D.C. 20240

13. Type of Report & Period Covered Final Report

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most si^ificant information. If document includes a significant

bibliography or literature survey, mention it here.)

Methods are described for the characterization of those physical

properties and mineralogical features of adobe which appear to have the

most significant affect on the durabilities of adobes. These methods

include determinations of color, pH, soluble salts, particle size distribution, liquid and plastic limits, and the X-ray "fingerprint" of adobe.

In addition, methods are given for the identification of the mineralogy

of adobe soils and for the examination of the microfabric of adobe.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of tfie first key word unless a proper name; separated by semicolons) Adobe building materials; adobe soil; color determination;

microfabric analysis; mineralogical analysis; particle size distribution;

pH; plastic and liquid limits; soluble salts.

18. AVAILABILITY [x] Unlimited

I I For Official Distribution. Do Not Release to NTIS

^ I Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. C13

I I Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

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(THIS REPORT)

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59

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published by the Department of Commerce in Part 10, Title 15, of the Code of Federal Regulations. The purpose

of the standards is to establish nationally recognized require- ments for products, and to provide all concerned interests with a basis for common understanding of the characteristics of the products. NBS administers this program as a supple- ment to the activities of the private sector standardizing

organizations.

Consumer Information Series—Practical information, based on NBS research and experience, covering areas of interest to the consumer. Easily understandable language and

illustrations provide useful background knowledge for shopping in today's technological marketplace.

Order above NBS publications from: Superintendent of Documents, Government Printing Office, Washington, D.C.

20402. Order following NBS publications—NBSIR's and FIPS from

the National Technical Information Services, Springfield, Va. 22161.

Federal Information Processing Standards Publications (FIPS PUB)—Publications in this series collectively constitute the Federal Information Processing Standards Register.

Register serves as the official source of information in the Federal Government regarding standards issued by NBS

pursuant to the Federal Property and Administrative Services Act of 1949 as amended, Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 11717

(38 FR 12315, dated May 11, 1973) and Part 6 of TiUe 15 CFR (Code of Federal Regulations).

NBS Interagency Reports (NBSIR)—A special series of

interim or final reports on work performed by NBS for outside sponsors (both government and non-government).

In general, initial distribution is handled by the sponsor;

public distribution is by the National Technical Information

Services (Springfield, Va. 22161) in paper copy or microfiche form.

BIBLIOGRAPHIC SUBSCRIPTION SERVICES

Hie following current-awareness and literature-survey bibli- ographies are issued periodically by the Bureau:

Cryogenic Data Center Current Awareness Service. A litera- ture survey issued biweekly. Annual subscription: Domestic, $25.00; Foreign, $30.00.

Liquified Natural Gas. A literature survey issued quarterly.

Annual subscription: $20.00.

Superconducting Devices and Materials. A literature survey

issued quarterly. Annual subscription: $30.00. Send subscription orders and remittances for the preceding bibliographic

services to National Bureau of Standards, Cryogenic Data

Center (275.02) Boulder, Colorado 80302.
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington, D.C. 20234

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