United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Research

Note

FPL–RN–0269

Effect of Prestain on the

Treatability of Western

Hemlock With Chromated

Copper Arsenate

Stan T. Lebow

Steven A. Halverson
Abstract

To enhance the appearance of the treated product, a stain is

often sprayed onto western hemlock lumber (Tsuga heterophylla (Raf.) Sarg.) prior to treatment with chromated copper

arsenate (CCA-C). There is concern that this stain might

interfere with the penetration and retention of CCA-C during

the subsequent pressure treatment. This study compared the

penetration and retention of CCA-C in prestained and nonstained end-matched specimens of western hemlock lumber.

Results indicate that the weight gain retention of CCA-C

solution was slightly but significantly impaired by the

prestain treatment. Assayed concentration levels of copper,

chromium, and arsenic detected in the treated wood were

also slightly less in the prestained samples than in the nonstained samples, especially for the inner assay zone. No

significant difference in average penetration was observed

between the prestained and nonstained specimens, although

two of the prestained specimens did have much less penetration than did their nonstained counterparts. The differences

in treatability noted in this study were too small to be of

concern in commercial practice.

Keywords: prestain, western hemlock, chromated copper

arsenate, treatability

May 1999

Lebow, Stan T.; Halverson, Steven A. 1999. Effect of prestain on the

treatability of western hemlock with chromated copper arsenate.

Res. Note FPL–RN–0269. Madison, WI: U.S. Department of Agriculture,

Forest Service, Forest Products Laboratory. 5 p.

A limited number of free copies of this publication are available to the

public from the Forest Products Laboratory, One Gifford Pinchot Drive,

Madison, WI 53705–2398. Laboratory publications are sent to hundreds

of libraries in the United States and elsewhere.

The Forest Products Laboratory is maintained in cooperation with the

University of Wisconsin.

The United States Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national

origin, gender, religion, age, disability, political beliefs, sexual orientation,

or marital or familial status. (Not all prohibited bases apply to all programs.) Persons with disabilities who require alternative means for communication of program information (braille, large print, audiotape, etc.) should

contact the USDA’s TARGET Center at (202) 720–2600 (voice and TDD).

To file a complaint of discrimination, write USDA, Director, Office of Civil

Rights, Room 326-W, Whitten Building, 14th and Independence Avenue,

SW, Washington, DC 20250–9410, or call (202) 720–5964 (voice and

TDD). USDA is an equal employment opportunity employer.
Effect of Prestain on the Treatability

of Western Hemlock With

Chromated Copper Arsenate

Stan T. Lebow, Forest Products Technologist

Steven A. Halverson, Physical Science Technician

Forest Products Laboratory, Madison, Wisconsin

Introduction

The use of wood treated with chromated copper arsenate

(CCA-C) has increased rapidly during the past two decades,

primarily because of the expanded use in residential applications such as in decks (AWPI 1996). However, some consumers dislike the greenish color of the treated wood.

Accordingly, CCA-C producers and wood treaters developed

methods to color the wood to enhance its appearance. One

method is to spray the wood with a “prestain” prior to treatment. This practice is sometimes used during production of

treated Southern Pine but has become particularly common

on the West Coast, where much of the CCA-C treatment is

performed on western hemlock (Tsuga heterophylla (Raf.)

Sarg.).1

One concern with the practice of prestaining is that it may

form a coating that becomes a barrier to the subsequent

penetration of CCA-C during preservative treatment. This is

more of a concern with western hemlock than with Southern

Pine, because western hemlock is only moderately treatable

and exhibits wide variability in treatability (Cooper and Ross

1977; Kumar and Morrell 1989; Lebow and others 1996;

Lebow and Morrell 1989). There is also concern that the

complexation of CCA-C with the wood may be altered by the

surface coating, thus increasing the leachability of the treated

wood.

This paper describes a study in which the treatability of

western hemlock is compared with end-matched decking

specimens, prestained or nonstained. In a related paper,

Lebow and Evans [in press] discuss the effect of

prestaining on the rate of leaching of CCA-C components.

 

1

Western hemlock is produced as part of the Hem-Fir species

mix, of which it is the largest component.

Materials and Methods

Ten No. 1 Grade western hemlock boards, standard 38 by

140 mm (nominal 2 by 6 in.) and 2.4 m (8 ft) in length, were

obtained from a mill in western Washington. No attempt was

made to select for sapwood or a specific grain orientation.

Each board was conditioned to a uniform moisture content in

a room maintained at 23oC (74oF), then cut into five specimens. Two specimens were 610 mm (24 in.) in length, and

three specimens were 406 mm (16 in.) in length. Both ends

of each section were end-sealed with two coats of a neoprene

rubber sealant. The longer specimens were used in an aboveground leaching study (Lebow and Evans [in press]).

The prestain applied was a commercial water-soluble acrylic

polymer with an iron-oxide-based rust color. Eighteen hours

prior to preservative treatment, the prestain was brushed on

to one 406-mm- (16-in.-) long specimen that had been cut

from each board.

The one 406-mm- (16-in.-) long specimen cut from each

board that had not been prestained was weighed, pressuretreated with water using a full-cell treatment process, then reweighed. The vacuum was maintained at –85 kPa (25 inHg)

for 30 min; the pressure was maintained at 1.03 MPa

(150 lb/in2

) for 2 hours. These specimens were then discarded. The uptake of water during this preliminary treatment

was used to calculate the solution concentration needed to

achieve a CCA-C retention of 6.4 to 9.6 kg/m3

 (0.4 to

0.6 lb/ft3

). An oxide-based CCA-C solution (active ratio of

17.4% CuO, 46.9% CrO3, 35.7% As2O5) was prepared

accordingly. The remaining specimens were weighed and

treated in a single charge using the same treatment conditions

that were used for the water treatment. Following treatment,

the specimens were weighed again to determine the amount

of solution uptake. Each specimen was then placed into a

plastic bag and stored for 10 days at 23oC (74oF). The next

step was to remove each specimen from the bag and sticker

to air dry in a room maintained at 23oC (74oF) and 65%

relative humidity.
2

Analysis of Retention

Preservative retention was determined in the outer (bark side)

wide face of each piece. Assay samples were removed from

the boards by drilling holes to depths of 5 mm (0.2 in.) and

15 mm (0.6 in.) from the sample surface using a 6-mm-

(0.25-in.-) diameter bit. A minimum of 20 holes was drilled

along the length of each specimen, and the shavings were

collected for analysis. The shavings were analyzed as specified in A11-83 (AWPA 1998a). In short, the wood was dried,

ground, then digested in a mixture of hydrogen peroxide and

nitric acid. The resulting solution was analyzed for preservative components by atomic absorption spectroscopy.

Measurement of Penetration

One end of each specimen was cut off to allow observation

of penetration in the cross section. The cut surface was

sprayed with Chrome Azurol S reagent to increase the visibility of copper distribution in the wood (AWPA 1998b).

The maximum and minimum penetration values were then

measured on the wide and narrow faces of each specimen,

and the percentage of the cross section that was treated was

visually estimated.

The mean uptake retention of CCA-C solution and the mean

assay retention levels of copper, chromium, and arsenic were

compared for the prestained and nonstained specimens using

a paired t-test. The minimum penetration levels in the two

types of samples were also compared using a paired t-test.

Results and Discussion

Uptake

Application of prestain to specimens before treatment

slightly, but statistically significant, reduced uptake of the

treating solution (Fig. 1). The average uptake of the nonstained specimens was 6.9 kg/m3 (0.43 lb/ft3

), and that of the

prestained specimens was 6.0 kg/m3 (0.38 lb/ft3

). The effect

appeared to be the slightest in specimens with the greatest

uptake, and maximized in specimens with the poorest uptake.

In boards five and eight, which had the greatest uptake, the

prestained boards retained more solution than did the nonstained boards. This contrasted with specimens cut from

board four, in which the prestain retention was only half that

of the matching nonstained specimen.

Retention

A slight reduction in assayed concentration levels of copper,

chromium, and arsenic was also detected in specimens that

had been prestained before treatment (Table 1, Fig. 2). The

average total CCA-C retention in the 0- to 15-mm

(0- to 0.6-in.) assay zone was 9.3 kg/m3

 (0.58 lb/ft3) for the

nonstained specimens and 8.3 kg/m3

 (0.52 lb/ft3) for the

prestained specimens. Although the differences appear slight,

they were significant at the 90% or 95% confidence level in

several cases (Table 1). The relative difference was greatest

in the inner 5- to 15-mm (0.2- to 0.6-in.) assay zone.

0.6

0.5

0.4

0.3

0.2

0.1

0

CCA-C retention (lb/ft3)

CCA-C retention (kg/m3)

10

8

6

4

2

0

Board number

 1 2 3 4 5 6 7 8 9 10 Avg*

Nonstained

Prestained

Figure 1—Uptake retention of CCA-C solution in nonstained and prestained specimens. Average uptake

retention levels are significantly different at the 95% confidence level, as determined by a paired t-test.
3

Prestaining had a more strongly negative effect on retention

in specimens cut from board four, where retention levels in

the inner 5- to 15-mm (0.2- to 0.6-in.) assay zone were less

than half those for the matching nonstained specimen.

Prestaining also caused large retention reductions in the

specimen cut from board six.

As noted for the uptake retention levels, there were two

exceptions to the trend of higher loadings in the nonstained

samples. In specimens cut from boards five and eight, copper

and chromium retention levels in the inner 5- to 15-mm

(0.2- to 0.6-in.) assay zone were greater for the prestained

samples (Fig. 2a,b). Interestingly, copper and chromium

retention levels in the outer 0- to 5-mm (0- to 0.2-in.) assay

zone of these specimens were greater in the nonstained samples. Prestaining produced mixed effects on arsenic retention

in specimens cut from these two boards (Fig. 2c).

As shown in Table 2, the prestaining did not appear to affect

the proportions of chromium, copper, and arsenic that penetrated into the wood. Both the nonstained and prestained

specimens contained more chromium and less arsenic than

did the treating solution. Both types of specimens also contained a greater proportion of copper in the inner 5- to

15-mm (0.2- to 0.6-in.) assay zone than they did in the outer

0- to 5-mm (0- to 0.2-in.) assay zone.

Penetration

In general, penetration was good in both nonstained and

prestained specimens (Table 3). Several specimens in each

group were 100% penetrated. However, the pattern of penetration was erratic in some specimens, with large differences

in minimum and maximum penetration on the same face.

The presence of pith in specimens cut from four of the

boards appeared to have little effect on penetration.

The average minimum and maximum penetration and percentage of cross section penetrated appeared to be slightly

greater for the nonstained group. However, a paired

t-test comparison of average minimum penetrations on nonstained and prestained specimens revealed no significant

difference for either the wide or narrow face. An examination

of individual specimens revealed that penetration was not

consistently greater in either the nonstained or prestained

groups. As noted for the retention analysis, prestaining did

appear to have a strong negative effect on specimens cut

from boards four and six. Only 40% of the cross section on

the prestained specimen cut from board four was penetrated,

and 100% of the cross section was penetrated in the endmatched, nonstained specimen.

Conclusions

The use of a prestain prior to CCA-C treatment slightly, but

statistically significant, lessened the retention of CCA-C

components in the wood. However, the effect was generally

not large enough to cause concern about the use of prestaining in commercial charges. No significant difference in

penetration was detected for the prestained and nonstained

specimens. Prestaining did appear to have a strong negative

effect on the treatment of specimens cut from one or two of

the more refractory boards. This increases the possibility that

prestaining might have more of an effect when used with

more refractory wood species or with the use of milder

treating schedules.

Table 1—Average assay retention levels of copper, chromium, and arsenic in nonstained and

prestained specimens

Average retention in each assay zone (kg/m3

 (lb/ft3))

CCA- 0 to 15 mm (0 to 0.6 in.) 0 to 5 mm (0 to 0.2 in.) 5 to 15 mm (0.2 to 0.6 in.)

constituent Nonstained Prestained Nonstained Prestained Nonstained Prestained

CuO 1.6 (0.10) 1.4 (0.09)b 2.4 (0.15) 2.2 (0.14)a 1.3 (0.08) 1.0 (0.06)b

CrO3 4.8 (0.30) 4.3 (0.27)b 8.3 (0.52) 7.7 (0.48) 3.0 (0.19) 2.6 (0.16)a

As2O5 2.9 (0.18) 2.6 (0.16)a 5.0 (0.31) 4.6 (0.29) 1.8 (0.11) 1.6 (0.10)b

Total 9.3 (0.58) 8.3 (0.52)b 15.7 (0.98) 14.4 (0.91) 6.1 (0.38) 5.1 (0.32)b

a

Significantly different from nonstained specimens at the 90% confidence level, as determined by paired t-test. b

Significantly different from nonstained specimens at the 95% confidence level, as determined by paired t-test.
4

0.25

0.20

0.15

0.10

0.05

0

Copper oxide (lb/ft3)

Copper oxide (kg/m3)

0.4

3.2

2.4

1.6

0.8

0

Replicate number

 1 2 3 4 5 6 7 8 9 10

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

Chromium trioxide (lb/ft3)

0.5

0.4

0.3

0.2

0.1

0

Arsenic pentoxide (lb/ft3)

Chromium trioxide (kg/m3)

11.2

9.6

8.0

6.4

4.8

3.2

1.6

0

Arsenic pentoxide (kg/m3)

8.0

6.4

4.8

3.2

1.6

0

(a)

(b)

(c)

Nonstained, 0-5 mm

Nonstained, 5-15 mm

Stained, 0-5 mm

Stained, 5-15 mm

Figure 2—Assayed concentrations of (a) copper oxide, (b) chromium trioxide , and (c) arsenic pent

oxide retention in the outer (0- to 5-mm) and inner (5- to 15-mm) assay zones for end-matched,

nonstained and prestained specimens.
5

References

AWPA. 1998a. Book of standards. A11–93, standard

method for analysis of treated wood and treating solutions by

atomic absorption spectroscopy. Granbury, TX: American

Wood-Preservers’ Association.

AWPA. 1998b. Book of standards. A3–97, standard methods

for determining penetration of preservatives and fire retardants. Section 2. Method for determining penetration of

copper-containing preservatives. Granbury, TX: American

Wood-Preservers’ Association.

AWPI. 1996. The 1996 wood preservation industry

production statistical report. Fairfax, VA: American Wood

Preservers’ Institute,

Cooper, P.A.; Ross, N.A. 1977. Treatability of western

hemlock lumber from Coastal and Interior British Columbia

with waterborne preservatives. Forest Products Journal.

27(12): 36–39.

Kumar, S.; Morrell, J.J. 1989. Penetration and absorption

of different CCA compositions in six western conifers.

Forest Products Journal. 39(10): 19–24.

Lebow, S.T.; Evans, J.W. [In press]. The effect of a

prestain on the rate of copper, chromium, and arsenate release from western hemlock decking.

Lebow, S.T.; Morrell, J.J. 1989. Penetration of boron in

Douglas-fir and western hemlock lumber. Forest Products

Journal. 39(1): 67–70.

Lebow, S.T.; Morrell, J.J.; Milota, M.R. 1996. Western

wood species treated with chromated copper arsenate: effect

of moisture content. Forest Products Journal. 46(2): 67–70.

Table 2—Average ratio (percentage of 100) of copper, chromium, and arsenic retention

in nonstained and prestained specimens

Average ratio in each assay zone

0 to -15 mm

(0 to 0.6 in.)

0 to 5 mm

(0 to 0.2 in.)

5 to 15 mm

(0.2 to 0.6 in.)

CCAconstituentNonstainedPrestainedNonstainedPrestainedNonstainedPrestained

Copper 17.3 16.9 15.2 15.1 20.1 19.4

Chromium 51.7 51.3 53.4 52.8 49.6 49.3

Arsenic 30.9 31.8 31.4 32.1 30.3 31.3

Table 3—Penetration of CCA-C in nonstained and prestained specimens

Penetration on wide faces (mm (in.)) Penetration on narrow faces (mm (in.))

Minimum Maximum Minimum Maximum

Cross section

penetrated (%)

SpecimenNonstainedPrestainedNonstainedPrestainedNon

stained

PrestainedNon

stained

PrestainedNonstainedPrestained

1 6 (0.24) 6 (0.24) 19 (0.75) 19 (0.75) 32 (1.26) 32 (1.26) 70 (2.76) 70 (2.76) 95 90

2 5 (0.20) 9 (0.35) 19 (0.75) 19 (0.75) 6 (0.24) 7 (0.28) 37 (1.46) 31 (1.22) 70 70

3 12 (0.47) 19 (0.75) 19 (0.75) 19 (0.75) 49 (1.93) 70 (2.76) 61 (2.40) 70 (2.76) 95 100

4 19 (0.75) 1 (0.04) 19 (0.75) 18 (0.71) 70 (2.76) 1 (0.04) 70 (2.76) 19 (0.75) 100 40

5 5 (0.20) 19 (0.75) 19 (0.75) 19 (0.75) 9 (0.35) 70 (2.76) 40 (1.57) 70 (2.76) 70 100

6 5 (0.20) 1 (0.04) 19 (0.75) 19 (0.75) 7 (0.28) 8 (0.31) 16 (0.63) 40 (1.57) 70 45

7 4 (0.16) 5 (0.20) 19 (0.75) 19 (0.75) 16 (0.63) 3 (0.12) 35 (1.38) 35 (1.38) 80 70

8 19 (0.75) 19 (0.75) 19 (0.75) 19 (0.75) 70 (2.76) 70 (2.76) 70 (2.76) 70 (2.76) 100 100

9 19 (0.75) 7 (0.28) 19 (0.75) 19 (0.75) 70 (2.76) 35 (1.38) 70 (2.76) 70 (2.76) 100 90

10 3 (0.12) 2 (0.08) 19 (0.75) 19 (0.75) 14 (0.55) 3 (0.12) 35 (1.38) 22 (0.87) 70 85

Average10 (0.38) 9 (0.35) 19 (0.75) 19 (0.74) 34 (1.35) 30 (1.18) 50 (1.98) 50 (1.96) 85 79