COLOR TESTS FOR DIFFERENTIATING

HEARTWOOD AND SAPWOOD IN

CERTAIN SOFTWOOD TREE SPECIES

May 1962

No. 2246

FOREST PRODUCTS LABORATORY

MADISON 5, WISCONSIN

UNITED STATES DEPARTMENT OF AGRICULTURE

F O R E ST S E R V I C E

In Cooperation with the University of Wisconsin
COLOR TESTS FOR DIFFERENTIATING HEARTWOOD AND

SAPWOOD IN CERTAIN SOFTWOOD TREE SPECIES

By

NORMAN P. KUTSCHA, Forest Products Technologist

and

IRVING B. SACHS, Forest Products Technologist

Forest Products Laboratory,1 Forest Service

U.S; Department of Agriculture

Summary

In certain softwood tree species, very little color difference exists between

the sapwood and heartwood. Being able to differentiate the sapwoodheartwood boundary in such species is of definite value to the research

forester and particularly the wood preservation industry for determining

depth of penetration in the sapwood. This report brings together the

published information on sapwood-heartwood indicators for certain softwood

species and also describes tests made to determine the effectiveness of

21 chemicals and dyes on 23 different species.

Introduction

In many woods, heartwood can be readily distinguished from sapwood by

heartwood's darker color. In such woods, however, as hemlock, most

spruces, the true firs, aspen, and cottonwood, there is very little color

difference between the heartwood and the sapwood (13) 2 Trees with this

characteristic are sometimes referred to as "sapwood trees" (3).

1Maintained at Madison, Wis., in cooperation with the University of

Wisconsin.

2Underlined numbers in parentheses refer to the Literature Cited at the end

of this report.

Report No. 2246
There are many situations in which it is necessary to distinguish the

heartwood-sapwood boundary in sapwood trees. For example, when using

these woods under conditions that are favorable to decay or insect attack, it

is often valuable to determine the proportion of heartwood present since

heartwood is frequently less susceptible to decay than sapwood. This greater

resistance is due to the presence of infiltration products, which are more or

less toxic to fungi and obnoxious to insects. In the penetration of preservatives, which is mostly confined to the sapwood, suitable means for determining the sapwood boundaries are necessary in establishing how much of the

sapwood is treated and whether or not the treated portion is thick enough to

give adequate protection. Undoubtedly, knowledge of whether heartwood or

sapwood tissue is being dealt with is of value in the use of glues and paints

since both types of tissue are different in chemical makeup and therefore

produce different bonding properties. This chemical difference would also

affect the preparation of pulp from these two types of tissues. Determining

percentage of sapwood to heartwood is also useful as an indicator of site or

exposure conditions. Width of sapwood may also be useful as an indicator

of the crown class of the growing tree.

The purpose of this report is to bring together published information on

sapwood-heartwood indicators, carry out necessary tests to validate this

information, and to rate these indicators as to effectiveness. This

information was compiled for particular use in the density survey of western

woods that is being carried out by the U.S. Forest Products Laboratory.

Part of this survey requires measuring the width of the sapwood on a large

number of increment cores. The measurements that are taken from the

cores are being used to correlate width of sapwood with various growth and

environmental factors. The measurements will also provide the information

that has been requested by the wood preservation industry on sapwood width

for particular species and diameter classes.

The criteria for an ideal sapwood-heartwood indicator (2) are: Is easy to

apply, either in the laboratory or in the field; gives sharp color differentiation

with clear-cut contrast; and gives uniform results even if the specimen is

decayed, weathered, or impregnated with a preservative.

The five types of tests that fulfill these criteria most closely and that were

used in this study on increment cores from 23 different tree species are:

(a) Starch tests--sapwood may contain more starch than heartwood.

(b) Sugar tests--sapwood may contain more sugar than heartwood.

(c) Organic stains--heartwood and sapwood may be different in chemical

composition; for example, when benzidine is applied, a red color is produced

if pinosylvin phenols are present in the heartwood (7)

Report No. 2246 -2-
(d) Inorganic stains --when ferric chloride is applied to the sapwood, a green

color will result if catechols are present, or a dark gray-blue will result if

pyrogallos are present (10)

(e) Hydrogen-ion activity--when bromcresol green, bromphenol blue, or

methyl orange are applied to species such as Douglas-fir, contrasting

colors are produced between the heartwood and sapwood because of a

difference in pH (5)

A number of other types of tests and techniques based on various chemical

and physical properties can be used to differentiate sapwood from heartwood

when no visible color difference exists. These include:

(a) Ultraviolet light and fluorescent pH indicators--the difference in acidity

might be made visible by using a fluorescent pH indicator that fluoresces

different colors depending on the pH of the specimen.

(b) Microscopic examination-- living cells may be present in the sapwood

and lacking in the heartwood.

(c) Differential electrical conductivity--a higher moisture content in the

sapwood may allow a greater flow of current than the drier heartwood.

(d) Differential heat transfer--a higher moisture content in the sapwood

may allow a more rapid flow of heat than the drier heartwood.

(e) Differential absorption rate of water, dyes, or carbon tetrachloride--

heartwood may be less permeable than sapwood tannins, gums,

resins, or salts infiltrate the cell walls or accumulate in the cell cavities

during heartwood formation.

(f) Differential diffusion rate of soap solution --when soap solution is forced

through the specimen, soap foam forms on opposite side of the more

permeable area showing a difference in permeability (12)

(g) Differential diffraction by soft X-rays --heartwood may contain certain

chemical constituents not found in the sapwood, such as gums, resins, and

salts of organic acid, which therefore could have a different effect on the

passage of X-rays.

(h) Difference in photographic image appearance of heartwood and sapwood

when specimen is placed in contact with film (Russell Effect). This is a

chemical phenomenon based on the presence or absence of unknown reducing

volatile materials in the specimen (9)

Report No. 2246 -3-
Most all of these tests were tried on a preliminary basis on specimens with

known sapwood-heartwood boundaries before it was decided to utilize only

the first five types as noted above.

A partial list of United States suppliers of dyes and stains is listed in

Appendix I.

Materials Tested

Between 10 and 15 increment cores of 23 different species were obtained

through the cooperation of three U.S. Forest Service Experiment Stations.

These cores were all untreated and arrived at the Laboratory at approximately

12 percent equilibrium moisture content (E. M. C.). Cores were obtained from

the Pacific Northwest Forest and Range Experiment Station, Intermountain

Forest and Range Experiment Station, and Pacific Southwest Forest and

Range Experiment Station,

Methods of Investigation

The cores were placed in trays made of single-faced, corrugated, white

paper for easy handling and to provide a white background for the comparison

of the color reactions. The trays were 15 inches long and 30 grooves wide,

which provided a space of 1 groove between each of the 15 cores. A 1- by

1/4-inch strip of wood was glued across the top and bottom of each tray

onto the corrugated side to give added support to the trays and provide a

means of numbering the cores.

Each tray contained cores from only one species, and the cores were spotglued into the grooves with rubber cement. All trays were labeled as to

species, and the cores in each tray were numbered. The trays were placed in

a room in which the temperature was 80° F. and the relative humidity was

65 percent to condition the specimens to 12 percent E.M.C. The cores that

are being tested in the density survey are being received in an air-dry

condition (approximately 12 percent E. M. C.). Thus, it was desirable to

have all the cores at the same moisture content since this tended to

eliminate any variation in reactions caused by different amounts of moisture

in different cores of the same species that were collected from different

trees on different sites.

Report No. 2246 -4-
A 4- by 6-inch card was made out for each core. On this card was recorded

the species, tray number, core number, chemical and date on which it was

applied, and color of the sapwood and heartwood immediately after application,

after 1 hour, and after 24 hours.

Each of 21 solutions was prepared and placed in a glass-stoppered, 60-

millimeter eyedropper bottle. The solutions were applied in a series of

continuous drops to the cores.

Solution 1

Alizarine - Iodine3

69.4 milliliters methanol

0.3 gram iodine (crystals)

0.1 gram Alizarine Red S Indicator

0.5 milliliter sulfuric acid (concentrated)

29.7 milliliters water

Mix iodine with methanol, add acid to this mixture; mix alizarine with water,

add iodine -methanol-acid mixture to alizarine-water mixture; resultant

solution is stable.

Solution 2

Alizarine Red S 0.75 percent (14)

99.25 milliliters water

0.75 gram Alizarine Red S Indicator

Solution 3

Ammonium Bichromate 6 percent (5)

6 grams ammonium bichromate

94 milliliters water

Solution 4

Benedict’s Solution (5)

10.00 grams sodium carbonate

17.38 grams sodium citrate

1.73 grams copper sulfate - CuSO4. 5H2O

Enough water to make 100 milliliters

3Described in a personal communication to R. M. Lindgren, U.S. Forest

Products Laboratory, from L. J. Wildes, T&TI, Atlantic Coast Line

Railroad Company, Jacksonville, Fla.

Report No. 2246 -5-
With aid of heat dissolve 17.38 grams of sodium citrate and 10 grams of

sodium carbonate in 80 milliliters of water. Filter, if necessary, and

dilute to 85 milliliters. Dissolve 1.73 grams of copper sulfate in

10 milliliters of water. Pour this solution, with constant stirring, into the

carbonate-citrate solution, and make up to 100 milliliters total volume.

Solution 5

Benzidine (14)

Solution A

1 gram Benzidine

5 milliliters of 25 percent hydrochloric acid

(5 milliliters of concentrated acid plus 2.45 milliliters of water

makes 25 percent hydrochloric acid)

194 milliliters water

Solution B

20 grams sodium nitrite

180 grams water

Solutions A and B must be stored in separate bottles; A gradually deteriorates

and forms a precipitate. When ready to make test, mix solutions together

in equal amounts. Mixture is stable for only 2 or 3 hours, after which

reaction to it becomes slower.

Solution 6

Bromcresol Green 0.0413 percent (8)

0.0413 gram Bromcresol Green

100 milliliters water

Solution 7

Bromphenol Blue 0.0413 percent (5)

0.0413 gram Bromphenol Blue

100 milliliters water

Solution 8

Fehling's Solution (5)

3.5 grams copper sulphate - CuSO4. 5H2O

17.3 grams potassium sodium tartrate

6.0 grams sodium hydroxide

100.0 milliliters water

Report No. 2246 -6-
Solution 9

Ferric Chloride 10 percent (2)

10 grams ferric chloride

90 milliliters water

Solution 10

Ferric Nitrate 0.1 N (5)

1.346 grams ferric nitrate--Fe (NO3 ) 3 9H2O

100 milliliters water

Solution 11

Ferrous Ammonium Sulfate 0.1 N (5)

1.96 grams ferrous ammonium sulfate --Fe (NH4SO4 ) 2 .6H2O

0.10 milliliters sulfuric acid

100 milliliters water

Solution 12

Ferrous Sulfate 0.1 N (5)

1.39 grams ferrous sulfate--FeSO4 .7H2O

0.10 milliliter sulfuric acid

100 milliliters water

Solution 13

Harlco Indicator --Hartman-Liddon Company (Appendix I)

Universal Wide Range pH Indicator

Solution 14

Hydrochloric Acid - Methanol (6)

2.5 milliliters hydrochloric acid

100 milliliters methyl alcohol

Solution 15

Iodine 2-1/2 percent (4)

1.25 grams iodine

48.75 milliliters ethyl alcohol

Report No. 2246 -7-
Solution 16

Methyl Orange 0.1 percent (5)

99.9 milliliters water

0.1 gram methyl orange

Solution 17

Perchloric Acid 40 percent (5)

100 milliliters of 70 percent perchloric acid

75 milliliters water

Very caustic, degrades wood

Solution 18

Per-Osmic Acid 1 percent (15)

1 gram Per-Osmic Acid

99 milliliters water

Very caustic, degrades wood

Solution 19

Phenol-HCl -Ethanol + U.V. (1)

10 milliliters phenol

5 milliliters hydrochloric acid

45 milliliters 95-percent ethanol

Melt phenol by heating in a water bath. Add mixture of acid and ethanol to

phenol. After application of chemicals to wood, the wood must be exposed to

direct sunlight or to rays of an ultraviolet lamp for 2 to 4 minutes to bring

about reaction.

Solution 20

Potassium Iodide - Iodine (5)

0.5 gram potassium iodide

24.5 milliliters water

0.5 gram iodine

24.5 milliliters ethanol

Dissolve potassium iodide in water, dissolve iodine in ethanol, mix both

solutions together.

Report No. 2246 -8-
Solution 21

Triplex Soil Indicator (11)

Triplex Indicator Solution for Heelige-Truog Soil Testers - Hellige

Inc. (Appendix I)

No, 697-27, Reagent N

Results and Conclusions

The various chemicals tested were rated for each particular species as

follows: A, effective; B, fairly good; and C, not useful for differentiating

sapwood from heartwood (table 1). N indicates that according to the

literature the chemical is not useful and therefore no test was made. The

colors given in table 2 are only approximate since they refer to all the

species tested. For some tests, the colors in the sapwood and heartwood

were actually reversed from those given in the tables but in every one of

these cases the sapwood-heartwood boundary was still delineated (5)

In utilizing the results given in table 1, one should keep in mind that in many

cases only one test was made with a particular chemical on a single core

of a species. Also, very few of the tests, even those rated as A are

infallible, and it is recommended that more than one be used on different

areas of the same specimen to determine most positively the sapwoodheartwood boundary.

Very different results might be obtained if the cores that were tested had

not been untreated and at 12 percent E.M.C. Some of the factors which can

affect the results of similar tests on specimens that do not meet these

conditions include (2)

(a) Presence of a preservative.

(b) Immersion in seawater--may result in the deposit of minute quantities

of salts, which could give a false reaction.

(c) Presence of extractives in the bark--extractives may diffuse into the

wood and interfere with the reaction given by the sapwood.

(d) Green wood from the living tree--may react differently than seasoned

wood because of differences in moisture content or chemical changes caused

by seasoning.

Report No. 2246 -9-
(e) Freshly exposed surface--may be more effective than an aged or

weathered surface.

(f) Abnormal environmental conditions --soil conditions, for example, may

affect pH conditions in the growing tree.

(g) Time of felling --presence of sugars and starches in the sapwood is

dependent on whether the tree is felled during the growing period or dormant

period.

(h) Wet wood--may give a weaker reaction due to a dilution effect.

(i) Presence of wound heartwood, false heartwood, or included sapwood may

confuse the test results.

(j) Presence of decay --chemical changes brought about by decay may affect

the results.

Report No. 2246 -10-
Literature Cited

Report No. 2246 -11-
Report No. 2246 -12-
Appendix I

Partial List of United States Suppliers of Dyes and Stains1

Allied Chemical Corporation, National Aniline Division, 40 Rector Street,

New York 6, N.Y.

Chroma (formerly G. Grübler & Company), Roboz Surgical Instrument

Company, 2500 Wisconsin Avenue, Northwest, Washington 7, D.C.

Difco Laboratories, Inc., 920 Henry Street, Detroit 1, Mich.

Eastman Kodak Company, Chemical Sales Division, Rochester, N. Y.

Hartman-Leddon Company, Philadelphia 43, Pa.

Hellige, Inc., 877 Stewart Avenue, Garden City, N.Y.

Matheson Coleman & Bell, Division of the Matheson Company, Norwood, O.

Merck & Company, Inc., Rahway, N. J.

1 list was prepared for informational purposes and the inclusion of

names in it implies no endorsement as to quality and prices. The list is

known to be incomplete and, upon request, other names will be added at

the time of the next revision.

Report No. 2246 -13- 2. -18
Table 1 .--Rating

1 of indicator solutions used for heartwood-sapwood differentiation in some s
Table l.--Rating

1 of indicator solutions used for heartwood-sapwood differentiation in some sof
Table 2 .--Color reactions of sapwood and heartwood of some softwoods

in response to various chemical indicators