Influence of the Wash From Bronze

on the Weathering of Marble

United States Department of Commerce

National Bureau of Standards

Building Materials and Structures Report 137
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UNITED STATES DEPARTMENT OF COMMERCE • Sinclair Weeks, Secretary NATIONAL BUREAU OF STANDARDS • A, V, Asi\n, Director

Influence of the Wash From Bronze on the

Weathering of Marble

Daniel W. Kessler and Ross E. Anderson

Building Materials and Structures Report 137

Issued 1953 JUN 2 3 1953

For sale by the Superintendent of Documents, U. S. Government Printing Office Washington 25, D. C.

Price IS cents
Contents

Page

1. Introduction 1 2. Outline of program 1 3. Description of materials 3

4. Tests on fresh marble 3

5. Description of exposure tests 3

6. Results of exposure tests 5

7. Observations on various marbles exposed to bronze 7

8. Discussion of results 8

8.1. Exposure tests 8

a. Untreated specimens 8

b. Paraffin-treated specimens 8

c. Warping of marble 8

8.2. Observations on structures 8

8.3. Effect of infiltrating matter 10

9. Summary 10

10. References 11

II
Influence of the Wash From Bronze on the

Weathering of Marble

Daniel W. Kessler and Ross E. Anderson

This investigation was made to determine whether rain water flowing over bronze work

and then over marble in monuments or other structures causes deterioration of the marble. Experimental work included exposure tests, extending over a period of I'-P/i years, in which

many specimens of a white calcite marble were exposed to the wash from three bronzes of typical compositions. Tests for flexural strength, abrasive resistance, absorption, and bulk

density were made on some of the specimens after exposure periods of 5, 10, and l'3}i years

to determine the changes in test values. Several buildings and monuments were inspected

where various marbles had been exposed to the wash from bronze for periods ranging from

20 to 98 years.

It was found that some marbles are not appreciably injured by such conditions, whereas

others showed advanced stages of disintegration within a period of 30 years. Some marbles

develop both green and brown discolorations, and disintegration usually occurs in the brown

areas. The green staining matter does not appear to have any appreciable affect on the durabiUty of marble. Treatment of the marble with molten paraffin to seal the pores does not prevent discoloration except for a period of 2 or 3 years. Tests indicated that this treat- ment materially reduces the flexural strength and abrasive resistance of marble.

1. Introduction

The performance of marble in certain monuments, where it "w as used to support bronze castings, gave evidence that the wash from bronze is injurious to marble. Both green and brown dis- colorations appeared and deterioration of the

marble occurred in the broAva areas. Because

there was no definite proof that the brown stains were caused by bronze, the deterioration could not

be properly ascribed to this metal. Dark stains can be caused by other materials, either in the

masonry or nearby.

Such structures are commonly erected without

providing a means of diverting the wash from the

bronze away from the marble, and stains often occur within a short time. Some valuable monuments, illustrated in figures 1 and 2, have shown

appreciable deterioration within 20 years. Frequently, different blocks of marble from the same

source show different degrees of resistance, as

illustrated in figxu-e 3. Although there are various manifestations of weathering in marble and some of these appear more prominently on certain marbles than others,

it will suffice for present purposes to consider only

the more serious phase that gradually destroys

the bond between grains. Often the first outward

manifestation of this condition is a crack formation. The final stage, commonly called "sugaring",

results when the bond between grains is so reduced

that criunbling occurs. During the com'se of weathering the marble loses strength and increases

in porosity and over-all dimensions. Some of the

marbles are much more resistant to this kind of

weathering than others, and the more resistant

varieties may not show visible effects for many

years, even though exposed to the most severe

conditions.

About 15 years ago plans were made for a study

at the Bureau to determine whether the brown

stains on marble are caused by bronze and whether

there is an accelerated weathering action in the

discolored parts of the marble. It was hoped that

such an investigation would add to the knowledge

of weathering phenomena in masonry as well as to serve an economic purpose in pointing out the need

of improved designs for such structm-es.

2. Outline of Program

It was decided to experiment with tkree t^-pical

bronze compositions on specimens of one representative marble. In order to accelerate the action,

the bronzes were to be cut into fine shavings and

exposed to the weather above the marble so that

rain water would trickle down through the shavings and over the faces of the marble specimens.

A sufficient number of specimens were to be used

so that some coidd be removed and tested at the

end of 5, 10, and 15 years of exposure. It was hoped

that changes in test values for flexm-al strength,

abrasive resistance, absorption, and bulk density

during weathering would give satisfactory indications of the rates of deterioration as compared with

control specimens exposed to the same weathering

conditions but with no bronze in contact.

260067—53 1
•3

FiGUKE i. (irtiitt MoHii incut, Washington, D. C, from the .soutli ircat showtny all but one of the bronze groups and the supporting marble.

h

Figure 3. Block of badly cracked marble under the nortJ statuary group of the Grant Monument showing the wors. condition in this structure. 2
3. Description of Materials

The bronze compositions selected for the tests were (A) 90 parts copper, 8 parts tin, 2 parts lead;

(B) 88 parts copper, 10 parts tin, 2 parts zinc;

(C) 80 parts copper, 3 parts tin, 16 parts zinc, 1 part lead. These were cast into cylindrical ingots

by the foundry of the Bureau's Metallurgical

Division and turned in a lathe to secure a supply

of shavings for the tests. The Vermont ^Marble Co. supplied about 100

marble slabs, 24 by 4.25 by 1.5 in. m size. This

firm treated twelve of the slabs by soaking them in molten paraffin to determine if such treatment

would prevent discolorations. The marble was a medium-grained, white calcite from a quarry in Colorado. The same marble has been used in several prominent structm-es, such as the Lincoln

Memorial in Washington, D. C. and the Tomb of

the Unlmown Soldier in Arlington, Yd.

i

! 4. Tests on the Fresh Marble

i It was necessary to determine reliable average

test values of the marble in its origmal condition

I for comparison with those obtained after various

" periods of weathering. As the marble was formd

I to be rather variable from one specimen to an-

' other, many tests were made and averaged for

j each property.

I In the flexural-strength determinations, the

j slabs were first supported near the ends and loaded

i in the middle. In order to conserve material the

' two end pieces resulting from the tests on long

j spans were then tested on shorter spans, the span-

! depth ratio in the first test being about 14 and in the second about 6.7, corresponding, respectively,

to spans of 21 and 10 in. The modulus-of-rupture

values obtained on the 6.7 span-depth ratio aver- aged about 13 percent higher than those for the

14 ratio, and it became evident that a con-ection

must be made to reduce all values to a common

basis. Additional tests were then made on

shorter spans in order to derive a cmwe showing

the relation between modulus of rupture and

span. This cmve was then used to reduce all values to those equivalent for a span of 21 in. The curve was also used to correct modulus-ofrupture values obtained on weathered specimens.

Abrasive-resistance tests were made on a testing

machine [1] ^ that determines the abrasion loss under controlled conditions of pressure, speed, and

time with a standard abrasive grain used in the

dry state. It was known that the rate of abrasion was influenced to some extent by the humidity of

the air during the test.^ Because it was not

feasible to control the humidity of the laborator}^,

• Figures in brackets indicate the literature references at the end of this paper.

2 High humidity causes the specimen to abrade faster, and low humidity

has an opposite effect. Tests made to determine the extent of this effect indicated that a resistance value determined at 12 percent relative humidity would be about 17 percent higher than the value at 30 percent humidity, and if the same specimens were retested at 70 percent humidity, the value would be about 20 percent below that at 30 percent humidity.

or to make all tests at one humidity, a considerable number of tests were made on one uniform

marble at various hmnidilics to derive corrections

for the results on the iiuu l)le used in the exposures.

All values were then corrected to 30-porcent relative humidity. Sixty-two tests were made on the

fresh marble, and the average of all was used for the basis of comparison in later tests. Absorption and hulk density values were determined on the specimens used for abrasion

tests. Standard methods ^ were followed, except

that the specimens were somewhat smaller than

the methods recommend.

Table 1 gives the data on properties of the fresh

marble. The histograms in figure 4 show the

distributions of the test values. Skewness is in

evidence, particularly for the results on abrasive

resistance. This peculiar distribution of test values and the wide scatter are believed to be

mainly responsible for irregularities in some of the curves in figures 8 to 12.

Table 1. Summary of test data on fresh marble

Property

Modulus of ruptm'e. Abrasive resistance ' Absorption Bulk density 2

Number

of tests Average

1,436 psi—

8.8 (/-/„)-.. 0.093%

2.7048 g/ml

Standard

deviation

209 psi

1.41 0.017%

0.0009 g/ml.

Coedicient

of varia- tion (%)

14.6

16.0

18.2

0.033

' The apparatus and test procedure used to determine the abrasive-resist" ance values are described in BS J. Research 11, (1933) RP 612, Wear resistance

of natural stone flooring. The symbol Ha is used to designate the resistance

values.

2 Bulk density is obtained by dividing the dry weight of the specimen in grams by the over-all volume in milliliters, including the volume of the pores.

5. Description of Exposure Tests

Figure 5 shows some of the specimens exposed on the limestone coping of a building at the

National Bm-eau of Standards. All specimens were

freely exposed, with no protection from the

weather or sunlight. Each marble slab was

supported on strips of marble attached to the

limestone with plastic calkmg material. The

marble slabs were attached to the strips in the same way. The bronze shavings were placed in 22-gage phosphor bronze boxes at the center of

each marble slab and held in position by strips of

the metal passing below the marble. Each box

contained 1.8 kg of bronze shavings, and as the

boxes were open at top and bottom, rain water

could trickle domi tlu'ough the shavings and over

the faces of the marble slabs. There were 15 imtreated specimens for each

of the 3 bronzes, and 15 slabs of untreated

marble were exposed in the same way mthout

bronze m contact for control tests. Six of the

12 specimens treated with paraffin were exposed

without bronze, and the other 6 were exposed with

bronze on top. There were only 2 specimens of

each bronze on the treated marble.

i ASTM Designation C97-47

3
Modulus of Rupture, psi

Spread

20 -

10 _

0.06 0.08 0.10 0.12

Absorption, %

0.14 2.703 2.704 2.7D5 2.706

Bulk Density, gm/ml

Figure 4. Histograms of the test data on fresh marble used in the exposure tests. This shows a remarkably high variation in the marble from one specimen to another.

Figure 5. Some of the specimens in the exposure test and

the method used for exposing the marble to the wash from

bronze shavings.

Figure 6. Various marble specimens after 10 years of exposure to bronze shavings. The dark areas were brown, and Immediately under the bronze, green

discolorations occurred.

A \^

4
j 6. Results of Exposure Tests

The specimens were examined periodically during

I the first 2 years for staining effects. Faint-green

stains on the marble occurred within a year, and

brown stains were noted after 18 months. The

i brown discolorations occurred mainly where thei'e was little shading from the smi ; the green appeared on the lower faces of the slabs, under the bronze

shavings, and under the bronze strips used to secure the boxes to the marble. The darkest part

of the brown stains extended about 2 in. from the

boxes, as shown in figm-e 6, but in most cases the

marble was appreciably darkened on the top face

; to the ends. The same kinds of stains occurred

' on the paraffin-treated marble as on the untreated,

but the treatment appeared to retard the staining

somewhat. At the end of 5 years there was no

appreciable difference in the amount of discolora-

' tion on the treated and untreated marble.

Specimens removed for tests at the end of 10 years were found to be warped appreciably.

, Most of them had sagged imder the weights, but

two had warped in the opposite direction. The

average sag for the specimens exposed to bronzes

i A, B, and C were, respective!}^, 0.009, 0.014, and

0.017 in. The control specimens sagged more,

and the average was 0.033 in. One of the treated

specimens without bronze sagged 0.25 in., and

I the other warped up at the center 0.024 in. Of

' the three treated specimens with bronze, one

j showed no change, one sagged 0.041 in., and the

third warped upward at the middle 0.020 in.

I The changes in properties for the different

j exposure periods are shown in figures 7 to 15. At the end of the 5-year period, the marble

I slabs were broken only in the middle for flexural

i strength. At the ends of the other test periods,

! the strength was determined by loading the speci- mens first in the middle, then breaking the two

i end pieces by loading them in the middle. The

slabs subjected to bronze wash were discolored

j green at the center, both on top and bottom, so

ithe tests on the long specimens gave the modulus

jof rupture in a green area, while the breaks at the

quarter points were in regions where the specimens were somewhat brovvn. The curves in figures 7

i and 8 indicate that the loss of strength of the marble was least in the gi'een areas, greater in the

brown areas, and greatest in the control specimens.

Figures 9 and 10 show the changes in abrasive

resistance for different exposures. At the end of

the 5-year period, specimens were cut only from

the brown areas, but for other periods they were

cut from both brown and green areas. The

curves are rather variable in trend, but there is some indication that the marble was hardened in the brown areas but not definitely changed in the

green. The apparent increase in hardness for five groups of specimens, including the controls at the

final tests, can probably be attributed to the variability of the marble rather than an actual increase.

Figures 11 and 12 show that definite increases

in absorption occiu'i-ed in brown ai'eas for all speci- mens. In the green areas caused by bronze A

the absorption values showcid no appreciable

change, but in those for bronzes B and C the

values were similar to those for the brown areas. All of the increases for the discolored areas were

small in comparison with those for the control

specimens.

Because the bulk density of the marble was much less variable than other properties, it was

expected that the changes in this property would

give more reliable indications of weathering than

strength, hardness, or absorption. The curves in figure 13 are somewhat more regular in trend than

those in figures 9 to 12, inclusive. Decreases in

bulk density were more pronounced in the brown

areas than in the gTeen, but changes were much

greater in the control specimens.

Some tests were made to determine whether the

copper compounds, deposited in the pores of the

surface layers, caused an appreciable increase in density of the stained portions. The results indicated that the interior of the marble, not reached

by the stains, was appreciably higher in density

than the stained portions. Similar indications were obtained for the control specimens when the

sm-face layers and interior portions were tested

separately.

0 5 10 IJ^ TIME,YEARS Figure 7. Results of flexure tests after three exposure

periods. The first group of curves show the changes in strength at the center of the specimens (which was mainly in an area of green discoloration), whereas

the lower group shows the strength \vhere the marble was somewhat brown.

A, Change at midsection (green areas'!; B, change at quarter-section (brown

areas); O. marble with bronze A; X, marble with bronze B; , marble

with bronze C.

TIME, YEARS Figure 8. Average changes in flexural strength in the brown and green areas, along with changes in this property

that occurred in the control specimens.

(Si, In green areas; , in brown areas; A, in control specimens.

5
TIME, YEARS Figure 9. Results of abrasive-hardness tests on marble

after three exposure periods. The variability in trends of these curves is believed to be largly due to variability in the marble from one specimen to another. A, Changes in brown areas; B, changes in gi'een areas; O. marble with bronze A; X,

marble with bronze B; marble with bronze C.

TIME.YEARS Figure 10. Average changes in abrasive hardness in brown

and green areas, along with changes in this property thai occurred in the control specimens.

®, In green areas; , in brown areas; A, in control specimens.

Figure 11. Changes in absorption of the marble in brown

and green areas for two exposure periods.

A, absorption in green areas; B, absorption in brown areas; O. marble with

bronze A; X, marble with bronze B; , marble with bronze C.

Figure 12. Average changes in absorption for brown and

green areas, along with the changes that occurred in this property in the control specimens.

®, In green areas; , in brovm areas, A, in control specimens.

TIME IN YEARS Figure 13. Changes in hulk density of the marble for two' exposure periods.

A, Change in green areas; B, change in brown areas; O. marble with bronze A; X, marble with bronze B; A, marble with bronze C.

" - 0.20

Figure 14. Average changes in bulk density for the brown

and green areas along with the changes that occurred in

this property in the control specimens.

<S>, In green areas; in brown areas; A, in control specimens.

ijJFiGURE 15. Changes in strength, abrasive hardness, and

absorption of the treated specimens. The treatment appeared to accelerate the weathering action rather than

retard it. A, Not e.\posed to bronze; B, exposed to bronze; O. absorption; 3, abrasive

resistance ;©, flexural.

6
Changes in flexural strength, abrasive resist- ance, and absorption for specimens treated with

paraffin are shown in figure 15. Bulk-density

changes were determined, but they were small in comparison with those of the other properties and

are not shown in the graphs. It was assumed

that the only effect of the treatment on the

properties of the marble would be to reduce the

absorption. For this reason, and the fact that

only a few specimens were available, no tests were made before exposure. However, the low flexural- strength and abrasive-resistance values obtained

at the end of the first exposure period indicate that

there was a pronounced effect of the treatment on

these properties as soon as the treatment was

applied. As there \yere no data for the original test values, the curves i'ii figure 15 are sho^\^^ in dashed

lines, for the first 5 years, starting from the zero

point.

7. Observations on Various Marbles

Exposed to Bronze

A few inspections of various marbles in monuments and buildings were made to determine if all marbles are affected alil^e when exposed to the

wash from bronze. All of these installations were

in Washington, D. C, except the McKinley Alonument. No records were available to definitely

identify the marbles in the monument bases as

to source or common names. In the following notes effort is made to identify these marbles

from their appearance, but there is some doubt

as to the correct designations of those in the Lafayette and Rochambeau monuments. The

compositions of the bronzes used in the various

castings Avere not knoA^Ti. (Numerals in parenthesis

following the names of the structures indicate the

years of erection.)

Georgia Marhle. The Corcoran Art Gallery

(1906) has two bronze lions, either side of the

entrance, mounted on pedestals of coarsely cvjstalline, calcite marble from Georgia. Green stains on the marble are abundant, but no brown discolorations or pronounced weathering was found. A

few cracks in a corner block of the south pedestal

appear to be due to a defect in the marble.

Two large bronze lanterns are mounted on

marble pedestals in front of the Pan American

Building (1908). The pedestals are Georgia

marble, in part, but they show no appreciable

deterioration. Georgia marble steps abutting on

the pedestals show small areas of broA\Ti discoloration and a minor amount of cracking.

Alum Marhle. The coarsely crystalline calcite marble, formerly quarried at Texas, Md., was

used in the bases of two equestrian monuments.

These are the Jackson AIonument(1853) in Lafayette Square and the Washington Equestrian

Monument (1860) in Washington Circle. Neither

of these shows prominent staining of any kind,

and apparently the marble is in good condition.

Some cracks in one corner of the latter appear to

be due to a faulty block i-aflicr lliau a wcatlicring

effect. Carrara Marble. I'hc Lafavctt(! Monument

(1891), in Lafayette Square, is Ix'licved to be

Carrara marl)le. It has a largo amount of bronze

mounted on a central pe<U>stal of marble. Jioth

green and ])rown stains on the marble arc; abundant. Near the base, where the brown stains arc most pronounced, there are several large cracks

in the marble.

Beaver Dam Marhle. The Rochambeau Monument, (1902) in Lafayette Square, has several

bronze figures mounted on a white marble, which

appears to be the Beaver Dam dolomite. Discolorations on the marble are slight, and there seems

to be no deterioration.

Vermont White Marhle: The Grant Monument

(1922) showTi in figure 1 consists of seven mai-ble

bases supportmg large bronze figures. Several

blocks have prominent browii stains, accompanied

by crackmg of the marble. The worst condition

is shown m figure 3, but many blocks appear to be

in good condition. The comer block mider the

south group is projecting considerably over the

edge of the block below. Tbis is believed to indi- cate that the injm-ed block has midergone a swelling action.

This same marble was used in the American

Red Cross Buildmg on Seventeenth Street. Two

bronze lampposts are attached to the marble

cheek walls at the maua entrance. One block in

the north cheek wall is showdng a condition of

incipient cracking similar to the more advanced

stage showaa in figm-e 3. None of the other blocks near these lamp posts show a similar condition.

The McKinley Monument, in Buffalo, N. Y.

(1907), is surx'oimded bv several marble posts, and

near the top of each post is a bronze collar supporting a bronze cham. Figm-e 2 iUustrates how

seriously the marble is cracked. Above the collars the marble appears to be in good condition.

When this structure Avas examined about 20 years

ago, the browTi stains were confined to a few mches

below the collars, and incipient cracldng was visi- ble m these discolored areas. This seems to prove

that the deterioration was caused b}" the bronze

rather than some substance coming up from below.

Tennessee Marhle: The onh' case examined

where this marble is exposed to the wash from

bronze is at the National Gallery of. Ai-t. Small

lampposts are mounted on the marble at the east and west ends of the building. Only green stains were fomid, and there appeared to be no deterioration of the marble.

Colorado Yule Alarble: In the Lincoln Memorial (1916) this marble forms the exterior. The

only part of the marble in contact with bronze on

the exterior is on the coping. Bronze cramps were

used to anchor the copmg blocks together, and

they \vere sunk into the marble about ]i in. below

the sm-face. The recess above the bronze was

filled with mortar. When this was examined, some of the mortar had weathered away, and m

7
other cases the mortar had shrunk away from the

marble enough to expose the bronze to rain water.

All of the joints were examined, and only three were showing the characteristic staining and

cracking of the marble. More cases were fomid

where the cramps had broken off the end of blocks

because of temperature strains.

South Dover Marble: This dolomite marble

forms the exterior of the National Academy of

Science Bunding. The building has bronze wmdow frames, a copper roof, and considerable orna- mental copper work on the coping. This case is of value in comparmg the effects of bronze and

copper. Both green and brown stains occur in

abundance even on the marble steps at the front,

which are about 16 ft from the building. Crack

formations often occm- hi the brown discolored

areas, and some of the marble is sugaring. It is evident that copper is the element in the bronze

that causes discolorations, and the stains from

this metal may be either green or brown. Sugaring of the mai'ble was foimd where the marble was

exposed only to the wash from copper and also

where both metals may have had an effect. This

case also shows that the dolomites may be as much affected by such conditions as calcite

marbles.

Most of the structures described above were

exposed to the sun for at least half of the time.

In the Lafayette and parts of the Grant monuments the marble was shaded dming a large portion of the time.

No definite relations could be found m these

observations between the locations of brown

stains and the exposm-e to simlight such as indicated by the exposure tests. Brown discolorations were found where the marble was shaded

most of the time, and both green and brown stains

appeared where there was very little shading.

8. Discussion of Results

The data obtained in the exposure tests on one

particular marble show that this marble was

stained both green and brown by the wash from

bronze. It weathered appreciably faster in the

brown areas than in the green, and faster still in

the control specimens. The weatherhig in all cases was so mild that crack formations did not

occur. The foUoAving reasons probably account

for this behavior: (1) This marble was one of the more resistant varieties, and (2) the exposure

period was relativeh^ short.

Information obtained from observations on

structures indicates plainly that some marbles are not seriously affected by the wash from bronze

for many years, whereas others show crack

formations under such conditions in less than 20

years. In the following discussion an effort is made to interpret the resiilts with reference to data available on weathering processes.

8.1. Exposure Tests

Each set of curves (figs. 4, and 7 to 15) is based on differences in test values between the fresh

marble and weathered marble. The average

values of strength, abrasive resistance, absorption,

and bulk density on the fresh marble were determined from numerous tests, and are believed to be reliable ; but because the amount of marble was

limited, it was possible to determine the cor- responding values after each period of weathering on only 5 to 6 specimens. Chance selection

of a few specimens from a material that varies

considerably does not give as reliable aver- age values for the properties as a larger

number would. This probably accounts for the

fact that some of the curves are irregular in trend,

especially those for abrasive resistance.

a. Untreated Specimens

The most prominent feature of the exposure

tests was that the control specimens weathered more rapidly than those exposed to the wash

from bronze. In view of the unmistakable fact

that some marbles deteriorate rapidly under

bronze but are durable under normal conditions,

it is important to find some explanation for the

variable performance. A possible explanation is that copper compounds forming in the pores may

act, to some extent, as an added binding material

during the early period of exposm-e. After a

larger amount has accumulated in the pores, it might act as a disrupting force. It is well ImowTi

that iron compoimds form a binding material in various types of rocks. However, rock strata are commonlj^ formed under high pressures from all dhections, and under such conditions the infiltra- tion of extraneous matter cordd not disrupt the

stone. Conditions ui a building or moniunent

are quite different because the blocks are not

restrained on the exposed face. Tests on numerous samples of marble after

weathering under normal conditions have shown

that prominent increases in absorption, and significant decreases in bulk density occur. Decrease

in strength are appreciable, but little information

is available on changes in abrasive resistance

during weathering. Stone workers believe that

exposure to the weather increases the hardness of stone. In the abrasion test, it seems logical to believe that the resistance of specimens would not

be greatly changed unless the individual crystals

undergo deterioration or unless the bond between

grains becomes so weak that abrasion would result

in crumbling the marble. Examination of sugared

marble after long periods of exposure indicates

that the individual grains do not suffer appreciable

deterioration. For this reason it is believed that

absorption, strength, and bulk-density determinations are of more value for indicating the amount

of weathering than abrasive resistance. In the

tests on specimens after weathering under bronze,

the results for these three properties are reasonably

concordant.

8
b. Paraffin-Treated Specimens

I Tests on paraffin-treated specimens were not

' planned in the original program, but some speci-

! mens were included at the request of the marble

producer who supplied the specimens. The limited

j number of pieces precluded a more thorough study

I of the value of the treatment. It was known

' that this treatment was quite effective in sealing

the pores of sandstones and limestones of the more

porous varieties, and that the treatment deteriorated slightly during an exposure period of 10

I years [2]. No previous data were available to indicate that the treatment reduces the strength

j and abrasive resistance of stone.

|| The effect of the treatment on strength was

thought to be due to the wax acting as a lubricant

I between crystals. In order to check this assumtion, some specimens were soaked in a light lubrieating oil until it had penetrated entirely through

I the marble. Flexure tests were then made on the

J specimens, and the resulting modulus of rupture

^ was 30 percent lower than the average for fresh

I untreated marble. As the fractiu-e in this test is mainly between crystal surfaces, it is concluded

that both the wax and oil caused a marked reducI tion in the bond between crystals.

When specimens were cut from the treated slabs,

I they had a wax-penetrated layer at top and boti tom, with a layer between that was not penetrated,

j When these specimens were immersed in water

the portions between penetrated layers could abI sorb as readily as untreated marble. For this reason the absorption results are useful only for

indicating the rate of deterioration of the

i| treatment.

1 The absorption of the control specimens in the

: treated group increased considerably faster bej tween 5 and 10 years than the controls in the

untreated group. The apparent decrease in abj| sorption rate between 10 and 13K years might be

'{ explained by the fact that the depth of penetra-

'i tion in different parts of the marble was variable,

j It could be seen that the depth of penetration was

!! variable when the slabs were broken in flexure and

{ that the depth varied from H to % in. The number

of specimens available was too limited to get more

reliable average values.

The curves indicate that there was a slight in- crease in abrasive resistance of the marble after 5 years for the control specimens and also for those

exposed to bronze. The flexural strength decreased slightly in both groups between 5 and 10

years, but in the control group there was a slight

increase after 10 years. In general, the tests indi- cate that the control specimens deteriorated more

j rapidly than those exposed to bronze. Apparently

there was some improvement in qualities of the

marble as the wax weathered away.

Considerable confidence has been manifested in

the paraffin treatment in the past and it has been

used as a preservative on various materials, such

as the granite in Cleopatra's Needle and on sandstone b\iildings in New York City [3]. The exposure tests as well as others [2] have; indicated that

the life of the treatment is short compared to the

normal life of most stones. Probably the effects on the strength and abrasive resistance of stone

would not be a serious consideration.

c. Warping of Marble

Marble slabs have been observed to warp under

various conditions of exposure. The direction of

warpage seems to be determined by gravity in some cases, but not all. The phenomenon is probably an intergranular effect and evidently has some

connection with temperature changes. It has

been reported that stoneworkers sometimes bend

marble slabs under hot water to make them con- form to a desired curve. Adams [4] has proved

that marble can be made to assimie new shapes by

subjecting it to high pressures in closed heated

containers.

The comparatively large amount of warping

that occurred in the control specimens that have

been treated with molten paraffin seems to indicate

that the wax facilitated slippage between crystals.

The fact that marble sometimes warps upward

against the force of gravity has never been satis- factorily explained.

8.2. Observations on Structures

Inspection of various marbles exposed to the

wash from bronze indicates prominent differences

in their resistance to deterioration. Three of the

eight marbles that were examined had developed

brown discolorations and were showing crack for- mations. Another marble was little affected \vith

either green or brown stains, and no significant

amount of deterioration could be found on the

marble of two monuments in which it was used,

although both of these were nearly 100 years old. One dolomite marble in a monument base for 50

years was in good condition whereas another marble of the same type in a building 35 years old was

showing crack formations and occasional sugaring

in the brown areas. Calcite marbles, which are considered to have satisfactory durability under

ordinary conditions of use, were found in two

monuments where they were exposed to the wash

from bronze. In both cases there were many

blocks showing advanced stages of weathering for

portions affected by dark stains. These marbles

seemed to be more readily affected by such conditions than any examined. One of these marbles

in a building 20 years old was exposed to the action

of bronze under lampposts. One block showed

plainly the beginning of the action that had

reached an advanced stage in the two monuments

in less than 40 years. The inspections indicate that some of the marbles readily develop green discolorations due to the

wash from bronze, but in these there appears to be

no serious weathering action. Iq larger structures, where several blocks of marble from the same source were used, it was noted that different

9
blocks have various degrees of resistance to deterioration. The proximity of marble to bronze seems to be unimportant, and frequently blocks

in direct contact with the metal were in good condition, whereas others several feet away were

badly cracked. Mortar is readily discolored a uniform shade of green, and in some instances it presented a prominent contrast with the adjoining marble. No case was found where the mortar was stained brown, and the green staining matter

appeared to have no detrimental effect.

8.3. Effect of Infiltrating Matter

It has long been known that marble undergoes some change in structure when it is subjected to cycles of heating and cooling. Souder and Hidnert [5], in determining the thermal expansion of various marbles, found that when a specimen is measured at room temperature then heated to 100° C, the rate of expansion increases as the temperature increases. Also, when the temperatm-e

is lowered to the starting point, the specimen does

not return to its original length but retains an

increment of growth. Each repetition of the temperature cycle resulted in an additional increase in length. Such effects are probably due to the peculiar expansion characteristics of calcite crystals.

The thermal coefficient has been determined as 22X10""® per deg C along the principal axis and

—4X10""" per deg C normal to that axis [6]. The

simultaneous expansion in one direction and contraction in another for each change in temperature evidently causes stresses within a closely

packed mass of crystals like marble and a complicated condition of stresses and movements among

the crystals, which results in slippage along contiguous surfaces. The fact that marble takes on an increment of growth during succeeding temperature cycles indicates that readjustment of the

movements between crystal sm-faces is not completed during the first cycle.

Different marbles show appreciable differences

in rates of growth for repeated cycles of heating

and cooling, but generally the rates decrease with

the number of cycles. It has been found that if the marble is heated while standing in a small

quantity of a dilute salt solution, such as gypsum,

the grow th of the specimen is greatly increased.

Hepeated cycles under such conditions result in an almost constant rate of growth of the specimen.

If lime water is used instead of the gypsum solution, most marbles show a similar effect, but the

rates of growth are somewhat lower than when

gypsum is used. In this test there is a pronounced

difference in rates of growth for dift'erent marbles,

and a fair degree of coi'relation has been found

l^etween these rates and the durability of different

marbles [7]. The reason for this behavior is believed to be

that, during each heating, some of the solid matter

in solution is carried into the pores while the marble

is under stress. This solid matter acts as an obstacle to readjustment dming the cooling and thus causes a more rapid growth.

It seems likely that any solid matter carried into

the pores of marble would have a similar effect. One case has been noted where a block of marble was stained green from vegetation. The block

crumbled at the edges during an exposure of about

15 years. The same kind of marble is known to be durable under normal conditions of exposure.

Studies on old marble structures in Athens by

Sofianopoulos [8] indicate that most of the deterioration is in discolored areas where the marble is somewhat sheltered from the cleaning effects of

rains. He states that these areas are covered with

a layer of fine dust containing clay, silica, and

soot, which holds moisture and atmospheric gases

by absorption. This authority found evidence

that the byproducts of chemical reactions gave

rise to discoloration and mechanical forces that

exert a destructive action.

Such performance seems to offer an explanation

of the effect of bronze stains on marble. It does

not explain why brown stains produce a more

severe condition than green stains. Possibly the

specific gravity of the solid matter may be a codsideration. The specific gravity of cupric oxide,

which seems to be the main constituent of the

browTL staining matter, is approximately 6, and

that of malachite (basic cupric carbonate) is 4. A

somewhat analogous effect was found in experiments with gypsum and calcium hydroxide; the

specific gravity of the first is 2.3, and that of the

other is 2.0. However, the difference in solubility

of these calcium compounds evidently causes a

larger amount of the gypsum to be carried into

the pores.

9. Summary

The peculiar performance of different marbles

when exposed to various conditions of use tends

to emphasize the need for a more intensive study

of the destructive forces involved in weathering

phenomena. The writers have set forth their

interpretation of the results obtained in experimental studies, with due regard to the unmistak-

| able evidences shown by the performance of dif- i ferent marbles in structures. It is possible that ; the anomalous experimental results are due to some factor, such as the mortar between blocks

in masonry, which plays an unexpected part in the weathering process and was not reproduced in the exposure tests. The findings should emphasize the desirability of diverting the wash from

bronze away from the marble wherever possible.

Inasmuch as the resistance of different marbles

to such conditions cannot be feasibly determined

in the laboratory, the selection of material should

be based on past performance.

The following conclusions are believed to be

justified:

1. Rain water flowing over bronze work and

then over marble affects different marbles in dif- ferent waj'-s. Marbles from some sources are discolored green, and those from other sources develop both green and brown stains.

10
2. Marbles that develop only green stains,

appear to withstand weather action as well

under bronze as under normal exposures, whereas some of those that develop brown stains deteri- orate more rapidly in the brown areas.

3. Copper is the main element in bronze that

causes stains and deterioration of the marble.

The green discolorations probably result from the

formation of basic cupric carbonate in the pores

of marble, whereas cupric oxide forms the main

portion of the brown stains. Marble exposed

to the splash below copper roofs develops the same lands of discolorations that occur under

bronze.

4. Dolomite marbles are affected in the same way as calcite marbles, and different dolomites

show different degrees of resistance to the action

of the wash from bronze.

5. Treatment of the marble with molten paraffin

to seal the pores does not prevent discolorations

due to bronze wash, except for 2 or 3 years. This treatment reduces the flexural strength and

abrasive resistance of marble.

6. Wherever bronze or copper is mounted on

marble, an effective means should be provided

for diverting tlu; wash from tlie metal away

from the marble.

10. References

[1] Daniel W. Ko.ssler, Wcuir rcsislancc^ of natural .stone flooring, BS .1. Rcscarc-h 12, 353 (1933) RPGGl.

[2] Daniel W. Kessier, J^.xtcrior waterproofing materials

for masonry, NBS J. Research 14, 317 (1935)

RP771.

[3] G. P. Merrill, Stones for building and decoration,

p494 (.John Wiley & Sons, New York, N. Y., 1908).

[4] Frank Dawson Adams, An experimental investigatio/i into the flow of marble, Trans. Roval Soc. (London)

[A] 195, 363-401 (1901).

[o] Wilmer H. Souder and Peter Hidnert, Thermal ex- pansion of insulating materials, BS Sci. Pap. 15, 387 (1918) S352.

[6] Walter H. .Johnson and WlUard H. Parsons, Thermal

expansion of concrete aggregate materials, NBS

J. Research 32, 101 (1944) RPl.578.

[7J Daniel W. Kessier, A new durability test for marble,

Am. Soc. for Testing Materials, Bui. No. 159,

p. 45 (July 1949).

[8] A. J. Sofianopoulos, Conservation of ancient marble

monuments, J. Chem. Educ. 28, 79 (1951).

Washington, January 26, 1953.

11
I4i


BUILDING MATERIALS AND STRUCTURES REPORTS

[Continued from cover page ii] BMS32 Structural Properties of Two Brick-Concrete-Block Wall Constructions and a Concrete-Block Wall Construction Sponsored by the National Concrete Masonry

Association 15j5 BMS33 Plastic Calking Materials * BMS34 Performance Test of Floor Coverings for Use in Low-Cost Housing: Part 1 15^ BMS35 Stability of Sheathing Papers as Determined by Accelerated Aging * BMS36 Structural Properties of Wood-Frame Wall, Partition, Floor, and Roof Constructions

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BUILDING MATERIALS AND STRUCTURES REPORTS

[Continued from cover page iii] BMS82 Water Permeability of Walls Built of Masonry Units 25^ BMS83 Strength of Sleeve Joints in Copper Tubing Made With Various Lead-Base Solders 15^ BMS84 Survey of Roofing Materials in the South Central States 15^ BMS85 Dimensional Changes of Floor Coverings With Changes in Eelative Humidity and

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BMSllO Paints for Exterior Masonry Walls 20^ BMSlll Performance of a Coal-Fired Boiler Converted to Oil 15^ BMS112 Properties of Some Lightweight-Aggregate Concretes With and Without an Air- entraining Admixture lOfS BMS113 Fire Resistance of Structural Clay Tile Partitions 15^ BMS114 Temperature in a Test Bungalow With Some Radiant and Jacketed Space Heaters 25^ BMS115 A Study of a Baseboard Convector Heating System in a Test Bungalow 20^ BMS116 Preparation and Revision of Building Codes 15^ BMS117 Fire Resistance of Walls of Lightweight Aggregate Concrete Masonry Units 20^

BMS118 Stack Venting of Plumbing Fixtures 15^ BMS119 Wet Venting of Plumbing Fixtures 20^ BMS120 Fire Resistance of Walls of Gravel-Aggregate Concrete Masonry Units 15^ BMS121 Investigation of Failures of White-Coat Plasters 25^

BMS122 Physical Properties of Some Samples of Asbestos-Cement Siding 15^ BMS123 Fire Tests of Wood-Framed Walls and Partitions With Asbestos-Cement Facings 15^ BMS124 Fire Tests of Steel Columns Protected With Siliceous Aggregate Concrete 15^ BMS125 Stone Exposure Test Wall 30^ BMS126 The Self-Siphonage of Fixture Traps 20^S BMS127 Effect of Aging on the Soundness of Regularly Hydrated Dolomitic Lime Putties 15^ BMS128 Atmospheric Exposure Tests of Nailed Sheet Metal Building Materials 20^ BMS129 Fire Endurance of Shutters for Moving-Stairway Openings 10^ BMS130 Methods and Equipment for Testing Printed-Enamel Felt-Base Floor Covering 15^ BMS131 Fire Tests of Gunite Slabs and Partitions 15^ BMS132 Capacities of Plumbing Stacks in Buildings 20^ BMS133 Live Loads on Floors in Buildings 20^ BMS134 Fire Resistance of Concrete Floors 15^ BMS135 Fire Tests of Steel Columns Encased With Gypsum Lath and Plaster 15^ BMS136 Properties of Cavity Walls 15^ BMS137 Influence of the Wash From Bronze on the Weathering of Marble 15^

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