Distribution of 

Borates Around Point 

Source Injections in 

Wood Members 

Exposed Outside 

Rodney C. De Groot 

Colin C. Felton 

Douglas M. Crawford 

United States

Department of 

Agriculture 

Forest Service 

Forest 

Products 

Laboratory 

Research 

Note 

FPL−RN−0275

Pile 

Pile cross 

section 

Deck 

Cap 

Stringer 
Abstract 

In bridge timbers, wood decay is usually found where water has 

accessed the end-grain surfaces. In preservative-treated members, 

end-grain surfaces are most likely to be those resulting from on-site 

framing cuts or borings. Because these at-risk surfaces are easy to 

see, it seems feasible to establish a program where diffusible preservatives are repetitively inserted into those critical areas spatially 

distributed in a grid and on a schedule that will ensure protection, 

thereby extending the life of the entire structure. The objective of 

this study was to determine the vertical and lateral distribution and 

the post-treatment behavior of injected and inserted borate 

preservatives in wood exposed to natural wetting in field exposure. 

During this 1- and 2-year exposure, rain wetting elevated the moisture content of the wood enough to support growth of decay fungi 

in wood not protected by borates. Point source treatments consisted 

of either borate solutions or fused borate rods that were injected or 

inserted, respectively, into predrilled holes. The longitudinal 

movement of borates applied as either glycol or aqueous solutions 

was generally greater than that occurring with treatment of borate 

rods only. Lateral distribution of borates was similar among treatments. In Southern Pine, differences in both vertical and longitudinal movement of borate from the insertion holes were associated 

with the type of closure used. Results indicate that borates can be 

included in a maintenance program consisting of time-sequenced 

treatment of critical regions of wood bridges that are at risk for 

internal decay. Grids for placement of point sources of diffusible 

borates in engineered wood structures could be developed on a 

wood-species-specific basis. Such treatments would complement 

the exterior shell of protection provided by the original pressure 

treatment and enhance long-term durability. 

Keywords: borates, preservative, diffusible preservative, post 

treatment, remedial treatment 

March 2000 

De Groot, Rodney C.; Felton, Colin C.; Crawford, Douglas M. 2000. 

Distribution of borates around point source injections in wood members 

exposed outside. Res. Note FPL–RN–0275. Madison, WI: U.S. Department 

of Agriculture, Forest Service, Forest Products Laboratory. 5 p. 

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Distribution of Borates Around 

Point Source Injections in Wood 

Members Exposed Outside

Rodney C. De Groot, Research Plant Pathologist (retired) 

Forest Products Laboratory, Madison, Wisconsin 

Colin C. Felton, Ph.D., Chemical Engineer, Manager 

Global Resource Technologies, LLC, Madison, Wisconsin

Douglas M. Crawford, Forest Products Technologist

Forest Products Laboratory, Madison, Wisconsin 

Introduction 

In bridge timbers, critical wetting that leads to wood decay 

is, in the majority of cases, found at end-grain surfaces 

(Eslyn and Clark 1979). In preservative-treated members, 

end-grain surfaces are likely to be those resulting from onsite framing cuts or borings. Wood at joints is also vulnerable 

because water may be held or trapped in the joint interface 

or continuously supplied by capillary movement to the end 

grain of associated fastener holes as it is absorbed into 

the wood. 

The locations where structural members in wood bridges are 

at risk for decay are known. Therefore, it seems technically 

feasible to establish a maintenance program where diffusible 

preservatives are repetitively inserted into those critical areas 

in a distribution grid and on a schedule that will ensure protection, thereby extending the service life of the entire structure. The insertion of solid, fused borate rods into affected 

members is a method that could be used for this purpose 

(Dietz and Schmidt 1988a, Dirol 1988, McCarthy and others. 

1993, Ruddick and Kundzewicz 1992). Dickinson (1990) 

regards the development of fused borate rods for in situ 

treatments as a major advance in the use of borates. However, Dickinson (1990) and Schmidt (1990) emphasize the 

need to understand the moisture characteristics of wood in 

service, which are critical to the development of effective 

treating practices with diffusible preservatives. Difficulties in 

protecting millwork (joinery) and decking above ground are 

summarized by Schmidt (1990), who emphasizes the need 

for more field trials in the United States to show the specific 

benefits of using fused borate rods. 

Borate solutions can also be injected into wood members. A 

conceptual advantage of injected borate solutions compared 

with insertion of fused borate rods is that immediate distribution of boron throughout a critical area can be accomplished 

at the time of treatment. Edlund and others (1983) observed 

faster diffusion of boron from a borate and glycol solution 

than that from fused borate rods in window joinery. Our 

observations (De Groot and Felton 1998) of generally limited 

movement of injected solutions in dry wood within weeks 

after injection led us to question whether benefits from 

borate injections were substantially greater than benefits 

derived from solid rods. The limited lateral movement of 

borates from the longitudinal flow of injected chemical could 

limit the immediate effectiveness of these treatments. 

Ultimate protection of the treated member would, as with 

solid fused borate rods, depend upon diffusion of the borates 

throughout the wood. 

Diffusion of borates from point source applications usually is 

greater along the wood grain than across the grain. Longitudinal diffusion of borates to distances greater than 120 mm 

from the injection point (depot) occurred in sapwood portions of pine (Pinus sylvestris) joinery treated with fused 

borate rods and with borate and glycol solutions (Edlund 

and others 1983). There was faster diffusion of borates in 

the fiber direction than across the grain. It was recommended 

that, in practice, a spatial (longitudinal) distance of 20 cm 

between depots not be exceeded. In laboratory studies, 

borates diffused through glue lines and, after time in the 

field, diffused into the heartwood. Longitudinal diffusion of 

borates adequate to protect eucalyptus (Eucalyptus obliqua)

in an accelerated field simulator was only 20 to 50 mm from 

the fused rods (Greaves and others 1982). Only minimal 

lateral diffusion of borate into Douglas-fir heartwood resulted from application of sodium borate solutions to one 

face of Douglas-fir heartwood (Grace and Yamamoto 1994). 

Highley and Ferge (1995) monitored the movement of borate 

from fused boron rods that were placed in holes drilled into 

the upper surface of 15.2- by 15.2-cm timbers that were 

exposed for 2 years under field conditions near Madison, 

Wisconsin. Both transverse and longitudinal movements of 

boron from rods were greatest in Southern Pine, which was 

mostly sapwood and had the highest moisture content, and 

were least in Douglas-fir in which the moisture content in the 

field did not exceed 21%. On the basis of those observations, 

Highley and Ferge suggested that the spacing of borate rods 

shown in Table 1 was adequate for protection. 
2 

In field settings, insertion of borates into bridge members and 

other engineered structures may have to be from the underside or the lateral surface of members. Little attention has 

been given to vertical redistribution of borates, but Dietz and 

Schmidt (1988b) did observe in deck members that the 

plume of diffused borate was longer on the bottom than on 

the topside of treated members. 

In this study, we conducted experiments to determine the 

post-treatment behavior and the vertical and lateral distribution of injected and inserted borate preservatives in wood 

exposed to natural wetting in field exposure. Species of wood 

used in this study are either currently being used or being 

considered for use in transportation structures within the 

United States. 

Materials and Methods 

This investigation was conducted with wood members that 

were not treated with preservatives or were experimentally 

treated with creosote in a treatment cycle that was intended 

to give only an inadequate treatment. The distribution of 

preservative about the injection point (depot) was observed 

in wood members that were exposed for either 1 or 2 years in 

the field. Usually, 16 to 20 replicate units of each wood 

species were used, but as a result of constraints in availability 

of some materials, sometimes fewer replicates were used 

(Table 2). Only an overview of the sample preparation is 

given here. Specific details of sample preparation and 

treatment procedures were described by De Groot and 

Felton (1998). 

In this study, we used solid members of eastern cottonwood 

(Populus deltoides Bartr. Ex Marsh.) and Southern Pine 

(probably longleaf pine, Pinus palustris Mill.) and laminated 

beams of red maple (Acer rubrum L.) or Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). Southern Pine and cottonwood members (Table 2) typified the cross-sectional 

dimensions of individual members of a stress-laminated 

bridge deck. The laminated beams of red maple and Douglasfir, although narrower than would probably be used in a 

bridge, were composed of 5-cm-thick laminates, which are 

typical of that type of construction. 

All untreated members were from the same sources as the 

members used to monitor preservative distribution soon after 

treatment (De Groot and Felton 1998). To the extent possible, members in this study were end-matched with members 

used in the prior study. All materials were cut into lengths of 

30 cm prior to borate treatments. Lateral surfaces of laminated beams (Douglas-fir and red maple) were coated with 

a commercial water repellent prior to cutting into 30-cm 

lengths. 

Members to be treated with creosote were cut into 60-cm 

lengths and treated with creosote meeting AWPA standard 

P1/P13 (AWPA 1998a). The treating schedule was intended 

to produce only a shell treatment, but the laminates of red 

maple and Southern Pine and some Douglas-fir laminates 

received almost thorough penetration. Following treatment, 

the members were longitudinally bisected into two 

30-cm lengths. 

All borate treatments (Table 3) were inserted at midlength 

into 9.5-mm-diameter holes that were drilled through the 

narrow face of each 30-cm-long unit. Holes were positioned 

equidistant from both sides of the units. With the laminated 

units, the insertion hole penetrated through several laminates. 

Solid borate rods were inserted into the holes, and then the 

holes were sealed with either a hardwood plug or an injection 

fitting. Solutions were injected through a nozzle pressed 

against commercially available fittings that were driven into 

the 9.5-mm-diameter holes in the wood units. When both a 

rod and solution were used, the rod was inserted, the hole 

sealed with a fitting, and the solution injected to refusal. 

All treatments were completed at the USDA Forest Service, 

Forest Products Laboratory (FPL), in Madison, Wisconsin. 

Within 1 month of treatment, all units were shipped to the 

Harrison Experimental Forest in southern Mississippi and 

exposed above ground in January 1996. Ten replicates were 

juxtaposed with the longitudinal axis of each replicate in an 

east–west direction (Fig. 1). Insertion holes were positioned 

to the bottom of the replicate (toward the ground). Replicates 

were supported by a concrete base, approximately 0.3 m 

above ground. Replicates of each treatment were held tightly 

together with metal banding. Units were removed from the 

field in January of either 1997 or 1998. After exposure, units 

were shipped back to FPL where they were dried to a 

constant weight in a heated room (estimated 6% relative 

humidity). 

Lateral and vertical movement of borate from the injection 

hole was determined visually on the cross-sectional surface 

of a cut made through the insertion port. Longitudinal 

movement was visually determined on surfaces of cross 

sections that were cut sequentially at distances of 2.5 cm 

from the injection point (Fig. 2). Solutions of turmeric followed by a hydrogen chloride–salicylic acid solution were 

Table 1—Spacing of borate rods for adequate protection 

 Borate rod spacing (mm) 

Species Across the grain Along the grain 

Southern Pine 51 305 

Red oak 25 152 

White oak 25 76 

Douglas-fir Recommendations 

could not be 

developed 

Recommendations 

could not be 

developed 
3 

Table 2—Mean indices of diffusion for borates from insertion point after 1 or 2 years of field exposurea

Type of borate 

 treatment 

Number of 

replicates 

per 

treatment 

Size of 

member, 

width, depth 

(cm) 

Maximum 

longitudinal 

diffusion 

(parallel to 

grain) (cm) 

Maximum 

diffusion above 

depot (insertion 

hole) (cm) 

At point of 

insertion, depth 

below surface 

not protected 

by diffusion 

(cm) 

Maximum 

width of 

diffusion 

plume (cm) 

1-year exposure 

Cottonwood without creosote treatment 

None 20 5, 14 

Borate/glycol 20 5, 14 11.88 (0.523) 1.41 (0.661) 0.49 (0.919) 4.29 (1.16) 

Borate/H2O 20 5, 14 11.16 (1.37) 1.99 (0.77) 0.325 (0.55) 4.40 (0.91) 

Fused borate rod 

(wood plug) 

17 5, 14 5.22 (1.88) 0.50 (1.75) 0.312 (0.86) 4.36 (1.28) 

Borate/glycol/rod 16 5, 14 10.31 (1.80) 2.88 (0.73) 0.235 (0.52) 4.90 (0.34) 

Creosote-treated cottonwood 

None 14 5, 14 

Borate/glycol 16 5, 14 9.9 (2.22) 1.45 (0.50) 1.08 (0.61) 3.19 (0.30) 

Fused borate rod 

(wood plug) 

16 5, 14 9.45 (3.00) 0.84 (0.52) 0.83 (0.85) 3.23 (0.41) 

Southern Pine without creosote treatment 

None 15 4, 18 

Borate/glycol 16 4, 18 4.95 (2.16) 2.28 (0.47) 0.34 (0.52) 3.58 (0.10) 

Borate/H2O 16 4, 18 3.75 (2.68) 1.42 (1.40) 0.14 (0.38) 3.38 (0.38) 

Fused borate rod 

(wood plug) 

16 4, 18 3.30 (1.87) 1.94 (0.58) 0.03 (0.12) 3.56 (0.20) 

Fused borate rod 

(wood plug) 

16 4, 18 6.45 (3.26) 1.28 (1.14) 0.09 (0.26) 3.43 (0.27) 

Borate/glycol/rod 16 4, 18 5.40 (2.88) 2.41 (0.45) 0.0 (0.00) 3.55 (0.20) 

2-year exposure 

Red maple without creosote treatment 

None 16 7, 18 

Borate/glycol 16 7, 18 7.50 (1.16) 0.84 (0.72) 0.06 (0.24) 3.94 (0.58) 

Fused borate rod 

(wood plug) 

16 7, 18 4.50 (2.38) 0.76 (1.63) 0.0 (0.0) 3.89 (0.42) 

Borate/H2O 15 7, 18 9.28 (0.82) 1.25 (0.43) 0.15 (0.38) 3.37 (0.43) 

Creosote-treated red maple 

None 18 7, 18 

Borate/glycol 20 7, 18 6.84 (2.77) 2.04 (3.46) 1.15 (0.90) 2.18 (0.72) 

Borate/H2O 20 7, 18 9.96 (2.77) 1.75 (2.67) 0.57 (0.73) 1.45 (0.44) 

Douglas-fir without creosote treatment 

None 16 7, 15 

Borate/glycol 16 7, 15 5.40 (4.11) 1.35 (0.33) 0.00 (0.00) 3.47 (0.41) 

Fused borate rod 

(wood plug) 

16 7, 15 0.00 (0.00) 1.73 (0.45) 0.86 (0.71) 3.66 (0.29) 

Creosote-treated Douglas-fir 

None 14 7, 15 

Borate/glycol 15 7, 15 4.32 (2.00) 0.91 (0.36) 0.00 (0.00) 3.75 (1.22) 

a

Borate–H2O = 15% Na2B8O13 ⋅ 4 H2O (saturated); borate–glycol = 15% Na2B8O13 ⋅ 4 H2O; 26% ethylene glycol, 

 borate rod = fused disodium octaborate (borate [B2O3] equivalent of 82.0%). Each rod contains 2.14 g Na2B8O13. 

 Data are means; standard deviations are in parentheses. 

Table 3—Specifications of preservative treatments 

Solution Composition Specific gravity at 20o

C 

Borate–H2O 15% Na2B8O13 ⋅ 4 H2O (saturated) 1.00 

Borate–glycol 15% Na2B8O13 ⋅ 4 H2O, 26% in ethylene glycol (saturated) 1.16 

Fused borate rods 2.5 cm long by 0.8 cm diameter, 2.14 g 15% Na2B8O13 ⋅ 4 H2O each — 
4 

used to indicate the presence of boron at a threshold of 

≥0.25 BAE (AWPA 1998b). With this methodology, the 

distance that the borate diffused and the maximum width of 

diffusion at the insertion point were recorded above the 

insertion hole. The distance was recorded from the surface, at 

the point of insertion, to the depth below the surface that was 

not protected by borate. The maximum longitudinal distance 

that borate diffused was also recorded. Means and standard 

deviations of these measurements are reported in Table 2.

Results and Discussion 

Comparisons among treatments were limited to within comparable woods for the same exposures. Visual measurements 

of curcumin on sawn surfaces are not precise, but they did 

demonstrate differences among treatments in patterns of 

distribution. Not one of the distribution patterns that we 

observed was influenced by pre-existing decay (Edlund 

1982), because all wood materials were sound at the time of 

treatment. In untreated wood, advanced decay was observed 

at time of removal from the field in the portions unprotected 

by borates. In members exposed for 2 years, the boundary 

between positive curcumin reaction and visible decay was 

quite distinct. Therefore, the observed patterns were accepted 

as having practical value in defining a treatment grid for 

long-term maintenance of wood bridges and similar structures. No decay was visible in wood members treated with 

creosote. 

The longitudinal (with the wood grain) movement of borates 

applied as either glycol or aqueous solution was generally 

greater than that occurring with treatment using borate rods 

only. An exception occurred in the Southern Pine that was 

exposed for 1 year. Maximum longitudinal penetration occurred in Southern Pine members in which the insertion holes 

were sealed with a solid wood plug after a fused borate rod 

was inserted. Edlund and others (1983) observed a more 

rapid diffusion of borate from a borate–glycol solution than 

from fused borate rods in window joinery. However, that 

difference was reduced over time. In this study, the difference in longitudinal diffusion had practical significance. 

Vertical movement was characterized by a large relative 

variation about the respective mean of each treatment. With 

aqueous and glycol solutions, vertical movement of borate 

above the insertion ports in cottonwood and red maple was 

greater than that observed with solid rods. In Southern Pine, 

the vertical distribution of borates from treatment solutions 

was comparable with that from the solid rods in holes sealed 

with an injection fitting. Vertical distribution of borate from 

solid rods in holes sealed with a wood plug was less than 

other treatments in Southern Pine. Direct comparisons of 

solid rods and solutions in both vertical and longitudinal 

movement from the insertion depot, therefore, may be valid 

only if the same closures were used with all treatments. In 

Southern Pine, remnants of the solid fused rods were still in 

the insertion holes (depots) even after 2 years of exposure. In 

depots that had been sealed with a wood plug, fused rods 

were positioned near the base (plugged end) of the hole. The 

rods appeared to be resting on top of the wood plugs. In 

depots that had been sealed with a commercial injection 

fitting, the fused borate rods were at the upper end of the 

hole. During field exposure, the injection fittings moved into 

the insertion holes but the wood plugs remained at the surface. The effect of this was that the injection fittings forced 

the solid borate rod towards the top end of the insertion 

Figure 1—Units of treatment held together with metal 

banding and exposed in southern Mississippi in east–

west direction longitudinally. Insertion holes were on the 

side toward the ground. 

Figure 2—Sequential cross sections of cottonwood 

members treated with fused borate rods and exposed 

in the field for 1 year. Injection ports are shown in 

samples marked A on the left. 
5 

holes. The various positions of the solid rods within the 

Southern Pine members probably contributed to the differences in both vertical and longitudinal movement of borate 

that are associated with the type of closure that was used. In 

Douglas-fir that was not treated with creosote, vertical diffusion was somewhat greater with the borate rods than with the 

borate–glycol solutions. 

With all treatments, the amount of wood near the opening of 

the entry port not yielding a positive response to curcumin 

(due to presence of enclosure in entry port) was usually less 

than 1 cm. The largest width not showing presence of borate 

occurred with the creosote-treated red maple. Lateral distribution of borates, as measured by maximum width of diffusion plume, was similar among treatments. 

Diffusion of borate from injection ports also occurred in 

creosote-treated wood. When comparisons could be made, 

the amount of borate movement in creosote-treated wood was 

slightly less than that which occurred in untreated wood. 

Evidence of borate movement in creosote-treated wood 

indicates that these treatments would be functional in large 

creosote-treated timbers. Borate movement also suggests that 

insertion of borate solutions or rods into increment borer 

holes and then plugging the holes with a wood plug could be 

an acceptable means of protecting those holes after removing 

an increment boring. 

Conclusions 

Borates can be included in a maintenance program that consists of time-sequenced treatment of critical regions of wood 

bridges that are at risk for internal decay. Grids for placement of point sources of diffusible borates in wood members 

in bridges or in other aboveground engineered wood structures could be developed on a wood species-specific basis. 

Such treatments would complement the exterior shell of 

protection provided by the original pressure treatment and 

would enhance long-term durability. 

Acknowledgment 

This study was jointly sponsored by the USDA Forest 

Service, Forest Products Laboratory, and the Federal 

Highway Administration under the ISTEA Timber Bridge 

Research Program. 

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