United States

Department of

Agriculture

Forest Service

Pacific Northwest

Research Station

General Technical

Report

PNW-GTR-676

July 2006

A Review of DoubleDiffusion Wood

Preservation Suitable 

for Alaska

K. Josephine Pavia
The Forest Service of the U.S. Department of Agriculture is dedicated to the principle of

multiple use management of the Nation’s forest resources for sustained yields of wood,

water, forage, wildlife, and recreation. Through forestry research, cooperation with the 

States and private forest owners, and management of the National Forests and National

grasslands, it strives—as directed by Congress—to provide increasingly greater service 

to a growing Nation.

The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and

activities on the basis of race, color, national origin, age, disability, and where applicable sex,

marital status, familial status, parental status, religion, sexual orientation genetic information,

political beliefs, reprisal, or because all or part of an individual’s income is derived from any

public assistance program. (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 USDA’s TARGET Center at (202) 720-2600

(voice and TDD).

To file a complaint of discrimination, write USDA, Director, Office of Civil Rights, 1400

Independence Avenue, S.W. Washington, DC 20250-9410 or call (800) 795-3272 (voice) 

or (202) 720-6382 (TDD). USDA is an equal opportunity provider and employer.

Author

K. Josephine Pavia is a forest products technologist student trainee, Alaska Wood 

Utilization Research and Development Center, 204 Siginaka Way, Sitka, AK 99835.

This work was published in cooperation with Oregon State University. 

Pesticide Precautionary Statement

This publication reports research involving pesticides. It does not contain 

recommendations for their use, nor does it imply that the uses discussed here

have been registered. All uses of pesticides must be registered by appropriate

state or federal agencies, or both, before they can be recommended.

CAUTION: Pesticides can be injurious to humans, domestic animals, desirable

plants, and fish or other wildlife—if they are not handled or applied properly.

Use all pesticides selectively and carefully. Follow recommended practices for

the disposal of surplus pesticides and pesticide containers.
Abstract

Pavia, K. Josephine. 2006. A review of double-diffusion wood preservation suitable for Alaska. Gen. Tech. Rep. PNW-GTR-676. Portland, OR: U.S. Department

of Agriculture, Forest Service, Pacific Northwest Research Station. 23 p. 

Currently, all treated lumber used in Alaska is imported from the 48 contiguous 

states and Canada because there are no wood-treating facilities in Alaska. This

report explores conventional and alternative wood-treating methods and reviews

previous studies and laboratory tests on treated wood. In investigating wood treatment as a possible processing option for Alaska forest products manufacturers, the

double-diffusion method of using sodium fluoride followed by a copper sulfate

appeared to be the most advantageous approach. This method of treating wood was

identified because it can be used to treat freshly cut or green wood. This was an

important factor to consider, owing to the limited drying capacity in Alaska. Little

information was available as to the chemical retention after treating and its resistance to leaching.

Keywords: Wood preservation, double-diffusion, Alaska, nonpressured 

treatment, lumber, copper sulfate.

A Review of Double-Diffusion Wood Preservation Suitable for Alaska
Contents

1 Introduction

2 Literature Review

2 Wood Preservation

2 Treated Wood in Alaska

3 Conventional Wood Treating

6 Alternative Wood Treating

8 Double-Diffusion Wood Treating

18 Discussion

19 Acknowledgments

19 English Equivalents

19 Literature Cited

GENERAL TECHNICAL REPORT PNW-GTR-676
Introduction

For decades, the wood industry in Alaska focused on producing cants and chips 

for export to Asia (USDA FS 1999). Because exporting rules and the Asian market

have changed drastically, producers in Alaska are looking into other uses for their

wood. Currently, all treated lumber used in Alaska is imported from the 48 contiguous states and Canada because there are no wood-treating facilities in Alaska. Over

10 million board feet (mmbf) of treated lumber are imported every year (McDowell

Group 1998). 

There are many methods for treating wood. The conventional method for treating wood uses combinations of vacuum and pressure to force chemical into the cell

lumens (Zabel and Morrell 1992). Alternative wood-treating methods are nonpressure processes that include brushing, spraying, dipping, and many variations of

soaking (Hunt and Garratt 1967). Each method has its strengths and its weaknesses,

and has different equipment requirements and chemicals. Treating schedules have

been fully developed for some methods and chemical combinations, but some

processes have been less thoroughly examined. 

The double-diffusion method of treating wood was identified because it can be

used to treat freshly cut or green wood. Owing to the limited drying capacity in

Alaska, this was an important factor to consider. The double-diffusion method is

based on sequentially treating green wood in two aqueous chemical solutions that

react within the wood matrix to form a precipitate that is highly resistant to leaching and toxic to fungi. Sodium fluoride and copper sulfate are potential components

for this process because each chemical could be shipped in crystalline form to producers, and neither is labeled as a restricted-use pesticide. The literature advocates

the use of sodium fluoride (Baechler 1963) in the double-diffusion process; however, this use is not included on the sodium fluoride label currently registered with

the U.S. Environmental Protection Agency (EPA). The copper sulfate label shows

that EPA allows it to be used in wood treatment and requires a first solution of

sodium salt or sodium chromate (Griffin 1997). The label indicates that the wood is

to be sequentially soaked in each solution for up to 3 days, without regard to wood

species or retention. 

The lack of information on double-diffusion treatment by using sodium fluoride and copper sulfate led to investigating the effect on chemical retention of treatment times of up to 3 days in each solution. Because chemicals that diffuse into the

wood matrix could leach out during service, the extent of such leaching was also

investigated. During this work, the potential for selective chemical absorption from

The double-diffusion

method of treating

wood was identified

because it can be

used to treat freshly

cut or green wood. 

A Review of Double-Diffusion Wood Preservation Suitable for Alaska

1
the solution to the wood, the migration of chemicals after a 30-day diffusion period,

and the location of chemicals after leaching were also examined. Results of this

investigation are part of a collective effort to bring useable technical information 

to Alaska forest product manufacturers about a wood treating method that will 

successfully treat locally grown species.

Literature Review

Wood Preservation

Wood preservation has existed for millennia. Wooden ships needed protection from 

marine borers and decay fungi. Initially, shipbuilders used wood that had natural

durability against biotic attack. As the availability of those species lessened, shipbuilders looked for treatments that could preserve the wood or at least extend the

service life of a ship until it reached its destination. Today, wood preservation plays

an important role in our lives. Treated wood is used in foundations, decks, playgrounds, fences, utility poles, railroad ties, and a host of other applications (Zabel

and Morrell 1992). 

The amount of treatment depends on the level of protection needed. Decay risk,

length of service life, cost of treatment, and end-of-life disposal are all considered

when determining which treatment method and chemicals to use. There are shortand long-term levels of protection. Short-term protection, such as dipping in a

chemical, is used to minimize sapstain damage on fresh-cut lumber. Long-term

protection is used to extend the service life of wood used as an end product. Longterm protection is further divided into aboveground or ground-contact levels of

protection. Wood in contact with the ground requires more treatment because the

decay risk is higher. There are a variety of methods for delivering chemicals into

the wood (Zabel and Morrell 1992).

Treated Wood in Alaska

Market—

The McDowell Group (1998) estimated that the market for treated lumber in 

Alaska was 10 to 15 mmbf per year. Demand for treated dimensional lumber was

heaviest in southeast Alaska, which represented 25 to 30 percent of that estimate.

The Alaska Railroad (2003) also uses an additional 2 to 3 mmbf of treated railroad

ties every year. Since 1996, they have replaced 520,000 ties, and there are plans to

add track to the system. 

2

GENERAL TECHNICAL REPORT PNW-GTR-676
All the treated lumber used in Alaska is imported from the 48 contiguous states

and Canada because there are no wood-treating facilities in Alaska. For decades,

the wood industry had focused on producing cants and chips for export to Asia

(USDA FS 1999). Because exporting rules and the Asian market have changed

drastically, producers in Alaska are looking into other uses for their wood. One 

of those other uses could be treated wood products. 

Raw material—

Much of Alaska is publicly owned. Therefore, mills are dependent upon public 

lands, mainly the Tongass National Forest, for their timber supply. In fiscal year

2003, 115 mmbf were offered for sale.1 The amount available each year is subject

to change owing to legislative and political issues. The uncertainty of the supply

from year to year limits the amount of credit banks are willing to extend to mills.

Thus, mills trying to adapt to changing markets are hindered by the lack of capital

available for investing in manufacturing equipment. 

Alaska has four commercial softwood species: Sitka spruce (Picea sitchensis

(Bong.) Carr.), white spruce (Picea glauca (Moench) Voss), western hemlock

(Tsuga heterophylla (Raf.) Sarg.), and yellow-cedar (Chamaecyparis nootkatensis

(D. Don) Spach). Spruces and hemlocks are used to make dimension lumber. They

have very little natural resistance to decay, and therefore, would have to be treated

to withstand the moderate decay risk typical of southeast Alaska and the low decay

risk in the interior region (Hunt and Garratt 1967, Scheffer 1971). Yellow-cedar is

primarily used for decking and other exterior uses because it is naturally resistant

to decay. 

Sitka spruce and western hemlock are most abundant in southeast Alaska,

whereas white spruce is abundant in interior Alaska. Western hemlock is considered moderately difficult to treat, whereas Sitka and white spruce are considered

difficult to treat (USDA FS 1999). Therefore, it is possible that a preservation

process successfully used to treat Alaska-grown Sitka spruce would also be able 

to treat white spruce. 

Conventional Wood Treating 

The conventional method for treating wood uses combinations of vacuum and 

pressure to force chemicals into the cell lumens. This process produces a deep, 

A Review of Double-Diffusion Wood Preservation Suitable for Alaska

3

1 Brink, S. 2003. Personal correspondence. Former deputy regional forester, natural resources, 

U.S. Department of Agriculture, Forest Service, Alaska Region, 709 W 9th Street, P.O. Box 21628,

Juneau, AK 99802.
uniform penetration of chemicals in wood for applications requiring a long, reliable

service life in regions with high decay hazards (Zabel and Morrell 1992). 

Equipment—

Conventional treating requires treatment cylinders that are typically 2 to 3 m in 

diameter, and built to handle pressures around 1034 kPa. The length of the cylinder

is based on product requirements, but can extend up to 55 m long. These cylinders

are supported by pumps, chemical tanks, thermometers, gauges, controllers, piping,

valves, a boiler, and wood transporting systems (Hunt and Garratt 1967). Tracks 

on both the infeed and outfeed of the treating cylinder allow lumber on trams to be

rolled in and out of the cylinder. The outfeed area must capture any liquid coming

off the treated wood to avoid environmental contamination. In addition, this area is

usually covered to avoid rainwater contamination, as all water runoff must be captured and cleaned. The capital investment for this equipment can easily exceed 

$1 million (Reader 2000). 

Chemical combinations—

Conventional treating methods can use a host of waterborne or oilborne chemical 

combinations for treating. Many of these chemicals are listed as restricted-use pesticides by the EPA, meaning they can only be applied by certified pesticide applicators (Zabel and Morrell 1992). 

Presently, the waterborne chemicals that are commercially used include chromated copper arsenate (CCA), ammoniacal copper quaternary (ACQ), copper azole

(CA), and ammoniacal copper zinc arsenate (ACZA). Copper and arsenic are both

excellent fungicides, and arsenic also protects wood from insects and marine borers

(Zabel and Morrell 1992). Chromium bonds with the lignin inside the wood matrix

as well as forming a complex with the copper and arsenic, thereby limiting the

leaching of chemicals while the wood is in service (Hartford 1986). Some waterborne chemicals can be shipped dry and mixed onsite by using a local water supply, reducing transportation costs, whereas others are shipped as concentrates and

diluted onsite. Treating wood with waterborne chemicals leaves the wood surfaces

clean and paintable. 

Oilborne chemicals, like creosote, have existed the longest and have proven to

be very reliable preservatives. Creosote is a byproduct made during the manufacturing of coke that is used for steel production. Other major oilborne wood preservatives include pentachlorophenol, copper naphthenate, and copper-8-quinolinolate.

4

GENERAL TECHNICAL REPORT PNW-GTR-676
Most oilborne chemicals are transported to treating facilities as concentrates or, in

the case of pentachlorophenol, in solid blocks. 

Treatment processes—

Conventional pressure treating is divided into two processes, the full-cell and the

empty-cell. The full-cell, or Bethell process, is used when maximum retention is

paramount. A vacuum is first used to remove some air from the wood, and then

preservative is added while increasing pressure. The empty-cell process is used

when limited preservative retentions are needed. This process does not use a vacuum,

but pressure is introduced either before the preservative or immediately after the

preservative is added. Variations in the pressure applied have been further named 

as either the Rueping or Lowry empty-cell processes. Both full- and empty-cell

processes require dried wood, unless some form of conditioning can be performed

in the cylinder prior to treating (Hunt and Garratt 1967). 

Lumber or poles can be treated in less than 20 hours depending on specifications

and wood species. Because the wood is secured in the cylinder, the environment is

controllable. This allows treaters the option to adjust retention and penetration in

order to meet end-user specifications. 

Pretreatments—

Over the years, there have been many mechanical innovations used to aid preservative penetration including incising, radial drilling, through-boring, and kerfing.

Although they have been proven effective in many ground line applications for

posts and timber, they are destructive, and care must be taken to maintain required

mechanical strength properties. Incising also reduces the aesthetic quality of lumber if a smooth surface is desired.

Refractory issues—

Some commercially important refractory species, such as Douglas-fir (Pseudotsuga

menziesii (Mirb.) Franco) and spruces, have been excluded from certain end uses

because of the inability to attain the required preservative penetration, despite

attaining the recommended chemical retention (Baines and Saur 1985). Lebow and

Morrell (1993) had mixed results pressure treating Sitka spruce. None of the charges

that used CCA achieved the American Wood Preservers’ Association (AWPA) specifications for penetration despite incising, whereas 12 of 14 charges that used ACZA

met both penetration and retention specifications. Blew et al. (1967) found that

A Review of Double-Diffusion Wood Preservation Suitable for Alaska

5
pressure-treating wood grown in Alaska offered less protection than treating the

same species grown in Oregon. These results were based on retention differences 

in round and sawn wood in Sitka spruce and other species.

Others factors to consider in Alaska—

Even if enough chemical can be impregnated in Sitka spruce grown in Alaska by 

using conventional wood-treating processes, there are still other factors to consider.

The capital investment in treating cylinders of any size is unfeasible for most

Alaskans. The climate in Alaska usually forces mills to close during winter, thus

reducing the production time available to help repay any capital investment. The

low annual production for any one mill, often less than 1 mmbf per year, results in

a high capital cost per unit treated.2 Transporting chemicals, especially oilborne

chemicals, over the marine highway system in southeast Alaska can be very costly,

cutting into an already limited profit margin. Because many Alaska wood manufacturers use portable processing equipment, the treating system should also be capable of moving seasonally or as the harvest location changes. Treating fresh-cut

wood is required because there is a very limited amount of drying capacity available. Taking into account all of these factors, conventional treating does not appear

advantageous for Alaska. Therefore, alternative methods of treating wood ought to

be investigated. 

Alternative Wood Treating 

Alternative wood-treating methods include nonpressure processes such as brushing,

spraying, dipping, and many variations of soaking (Hunt and Garratt 1967). Many

alternative treating methods require much less equipment than conventional methods

and are typically limited to small-scale applications by homeowners and farmers

(Zabel and Morrell 1992). 

Brushing and spraying—

Typically, oilborne preservatives are used when treating wood by brushing or 

spraying, but waterborne preservatives can also be used. Penetration via these

processes is shallow, and therefore protection is limited. Abrasion or checking can

easily break the envelope of protection. Wood has to be dry and warm enough to

avoid congealing of the oilborne preservative on the wood surface (Hunt and

Garratt 1967). 

6

The treating system

should also be 

capable of moving

seasonally or as the

harvest location

changes. 

GENERAL TECHNICAL REPORT PNW-GTR-676

2 Kilborn, K. 2002. Personal communication. Former team leader, Alaska Wood Utilization 

Research and Development Center, 204 Siginaka Way, Sitka, AK 99835.
Dipping and soaking—

Hunt and Garratt (1967) differentiated numerous treatment processes that involved 

dipping or soaking wood. For example, dipping consists of momentarily immersing

wood in a bath of preservative, whereas steeping consists of submerging wood for

several days or even weeks in an open container. With steeping, dried wood is

treated with waterborne chemicals. Cold soaking is similar to steeping except that

wood is soaked in unheated oilborne chemicals for 2 days to 1 week. 

The thermal process involves immersing dried wood in successive baths of hot 

and cool preservative. The purpose of the hot and cool baths is to form a partial

vacuum, whereby atmospheric pressure would force the preservative into the wood.

Either oilborne or waterborne solutions can be used with this method if the temperature does not cause excessive chemical loss through evaporation. Depending on

the standard, the hot bath is around 102 °C and the cool bath around 38 °C. Several

variations of this method were patented. In one variation, the wood was heated in a

kiln instead of a hot bath and then submerged in the cool bath. The theory of creating a partial vacuum by using hot and cool baths was improved by actually creating

a vacuum in an airtight container by exhausting the air with a pump (Hunt and

Garratt 1967). 

Diffusion methods are similar to steeping in that there is bulk flow of solution

into the wood. Yet, the diffusion method has a second mechanism for moving preservative into the wood by using a diffusion period. Wood is wet-stacked for a 

period in order to facilitate diffusion. The theory of diffusion states that chemicals

will move from zones of higher concentration (treating solution) to those with

lower concentration (water in wood). Therefore, green wood and waterborne chemicals are used for diffusion treatments. This diffusion method typically involves

soaking wood in solutions, but theoretically can extend to the use of pastes and

wraps to deliver chemicals into the wood (Hunt and Garratt 1967). 

Single-diffusion applications that use boron have been commercially accepted

in New Zealand, Australia, and New Guinea for decades, and account for 28 percent of all wood treated in the region (Vinden et al. 1997). Chemicals placed in the

wood only by diffusion, however, are susceptible to leaching, because chemicals

that diffuse into the wood matrix can easily leach out during service. Products

treated by the diffusion method are used in low-hazard building timbers, or outof-ground contact (Vinden 1990).

The double-diffusion process was developed to overcome leaching issues 

associated with the single-diffusion processes. In this method, green wood is

sequentially soaked in two aqueous chemical solutions that react within the wood

A Review of Double-Diffusion Wood Preservation Suitable for Alaska

7
matrix to form a precipitate that is highly resistant to leaching and toxic to fungi

and termites (Baechler 1953). 

Other factors to consider in Alaska—

There are many factors to consider when comparing alternative treatment methods 

for use in Alaska. Dipping is only recommended as a method to deliver long-term

wood protection for wood that has been dried and is impractical to treat by more

effective methods. Limited drying capacity in Alaska makes dipping impractical.

The schedule for the steeping method recommends steeping 1 day for each 25 mm

of material thickness plus 1 more day for good measure, but penetration rarely

exceeds 6 mm. The poor penetration and the requirement for dried wood eliminate

steeping as a choice for Alaska. The thermal process can attain suitable penetration,

but the hot bath temperature may be unattainable or not maintainable in Alaska. In

addition, the hot and cool solutions need to be pumped into and out of the treatment tank, or the wood must be moved between two separate tanks. This requires

either pumps or equipment to move the wood back and forth between tanks, and

equipment to heat the solution. The vacuum method requires a sealed container 

and only works well with easily treated wood, again precluding refractory species.

Diffusion methods use green wood, open tanks, and are the suggested alternative

for treating refractory species (Hunt and Garratt 1967). Taking into account all 

of these factors, the alternative wood treating method of diffusion, particularly 

double-diffusion, appears most suitable for Alaska. 

Double-Diffusion Wood Treating

As previously stated, treating wood by diffusion typically refers to soaking wood, 

but theoretically can also extend to the use of pastes and wraps to deliver chemicals

into the wood. Two mechanisms help move preservative into the wood: bulk flow

and diffusion (Greaves 1990, Hunt and Garratt 1967). Bulk flow is considered the

initial mechanism of treatment by the diffusion method, and consists of liquid flowing into the wood owing to a pressure difference. The second mechanism is diffusion whereby the chemical absorbed in the bulk flow phase becomes more evenly

distributed as it moves from areas of high to lower concentration. This allows the

chemicals to penetrate deeper and more uniformly into the wood (Vinden 1990).

Baechler (1953) noticed a possible third mechanism: capillary pull. If the water

column inside the wood matrix was still continuous, evaporation from the top of

the post would draw solution upward. Capillary pull is a form of bulk flow that

mimics a tree’s natural water transport system; it is limited to extremely green

posts, treated upright in a barrel with post tops exposed. 

8

GENERAL TECHNICAL REPORT PNW-GTR-676
Theory of diffusion—

If the wood is at its highest possible moisture content and there is no interaction 

with the wood substance, the rate of diffusion of such chemicals should follow

Fick’s law. This law states that the rate of transfer per unit area of a section equals

the negative of the diffusion coefficient times the derivative of the concentration

with respect to the space coordinate measured normal to the section (i.e., the direction of diffusion). The rate of diffusion is greatest in the longitudinal direction and

lowest in the transverse directions (Vinden 1983). 

Mathematical models can help predict real world results, establishing relationships between variables, and optimizing treatment schedules. Models must take

into account the moisture content and density of the wood, the interactions between

the wood matrix and the preservative, temperature, preservative retention, time and

type of wood (heartwood, sapwood, earlywood, and latewood), as well as the concentration of the preservative. Vinden (1984) compared the calculated mathematical

models for steady-state and non-steady-state diffusion coefficients for copper ions

through saturated samples of Scotspine (Pinus sylvestris L.), Norway spruce (Picea

abies (L.) Karst.), and European white birch (Betula pendula Roth). His data indicated that the pathway for diffusion was limited to the area of the free water in 

the lumens, and that that diffusion ceased below the fiber saturation point. He also

found that the steady-state diffusion coefficient for spruce air-dried and resaturated

wood was significantly lower than the coefficient for spruce in the green condition.

This may be because the pathway for diffusion is slowed by pit aspiration (Flynn

1995) resulting from increased capillary tension caused by the removal of free

water in lumens during drying (Siau 1984), highlighting another raw material variable not previously mentioned. Therefore, the coefficients of diffusion differ for

green and previously dried wood. Vinden (1984) also found that during the initial

or non-steady state diffusion, the coefficient of diffusion will deviate from Fick’s

law, owing to the number of fixation sites (hydroxyl groups) within the wood

matrix. He found that all the fixation sites must be filled before diffusion proceeds.

Other researchers have also shown that copper ions fix to the wood matrix (Bland

1963, Cooper 1991, Jin and Archer 1991). 

Although explainable in mathematical terms, the numerous variables have a

significant impact on the retention and penetration of preservatives. Therefore, pilot

studies and chemical retention analyses are still needed. 

Equipment—

Treating by double-diffusion requires that the wood be soaked in two chemicals 

sequentially and then wet stacked for a period of time. Depending on the amount

A Review of Double-Diffusion Wood Preservation Suitable for Alaska

9
and size of the wood to be treated, the double-diffusion method can require fairly

simple equipment. Each chemical could be pumped into and out of one treatment

tank, or the wood could be moved between two separate tanks. This requires either

pumps or the ability to move the wood back and forth between tanks. The material

for the tanks can be stainless steel, wood with plastic lining, or other material

depending on the corrosivity of the chemicals employed. Tank size would depend

on the product being produced. Fence posts could be treated upright in a barrel,

whereas decking would have to be fully submerged. Depending on the amount of

wood treated per month or the volume of chemicals used per year, containment

equipment around the tanks may be necessary (EPA 1996). Depending on species

and moisture content, the buoyancy of wood may make hold-down hardware necessary. As with all wood-treating methods, equipment is needed to transport the wood

to and from the treating vessel. Personnel protective equipment, as mandated by the

Material Data Safety Sheets for each chemical and the Occupational Safety and

Health Administration, is also needed. 

Chemical combinations—

Ideally, the two chemicals used in the double-diffusion method will form a precipitate that is highly resistant to leaching and toxic to fungi and termites. To be toxic

and insoluble after forming a precipitate, salts of very strong acids are used with

weakly basic metals (Baechler 1953). Baechler (1953) initially reacted nickel, zinc,

or copper with chromate, fluoride, arsenate, borate, or phosphate. Advantages and

disadvantages for each chemical are given in table 1. 

10

Ideally, the two

chemicals used in

the double-diffusion

method will form a

precipitate that is

highly resistant to

leaching and toxic to

fungi and termites.

GENERAL TECHNICAL REPORT PNW-GTR-676

Table 1—Relative advantages and disadvantages of chemicals used in 

double-diffusion treatments

Chemical Advantages Disadvantages

Copper More toxic to fungi More corrosive to tank

More economical

Nickel Less corrosive to tank Less toxic to fungi

Less economical

Zinc Less corrosive to tank Less toxic to fungi

Reacting with:

Arsenic Restricted use Restricted use

Boron Did not form an insoluble 

precipitate with any metal

Chromium Restricted use Restricted use

Fluoride Consumers familiar (toothpaste) Did not form an insoluble 

precipitate with nickel or zinc

Phosphorous Helps fix copper inside wood Does not contribute to toxicity

Source: Baechler 1953.
Restricted use is listed as both an advantage and a disadvantage for two chemicals. These chemicals are highly toxic, making personnel training and extra containment equipment essential. The disadvantage would be the costs associated with

the added safety measures. The advantage would be the awareness of toxicity personnel would gain from training. Recent efforts to revive double-diffusion as an

effective but low-cost treatment option for rural areas have focused on sodium fluoride and copper sulfate (Hoffman 2002a, 2002b, 2002c; Kilborn et al. 2003, [and

see footnote 3]; Reader 2000; Wheat et al. 1996).

Chemical labels—

Treaters have to legally abide by the wording on the chemical labels. Chemical 

labels are proprietary to a given company and have either an EPA registration number or NSF-60 certification. Labels contain information on the uses for which the

chemical manufacturer is willing to take liability, based on past research. It is illegal to use pesticides for nonlabeled uses or to use them at levels above or below

label recommendations. 

The copper sulfate labels from Old Bridge Chemicals, Inc.4 (2000) and Chem

One, Inc. (2000) have the same wording for use in a wood treatment. Both labels

are for peeled, green posts treated “butt end down first in the copper sulfate solution for three days, then butt end down in sodium chromate solution for two days,

and finally turn the post upside down in the sodium chromate solution for one additional day.” 

The label for Blue Viking’s Copper Sulfate Instant (Griffin 1997) states that the

first solution is a solution of sodium salt or sodium chromate. Therefore, sodium

fluoride could be used with this product label. It states that green material is soaked

in the sodium solution for up to 3 days, and then soaked in the Blue Viking Copper

Sulfate Instant solution for up to 3 additional days. 

The only registered label found for sodium fluoride states: “For Pesticide

Formulation Use: Only in formulation into a fungicide for wood preservation”

(Osmose 2002). As the label stands, the term “formulation” precludes the use of

sodium fluoride in double-diffusion. This is because the wood is treated sequentially in the two chemical solutions, and the formulation of copper fluoride could

not occur until after the chemicals are inside the wood matrix. 

A Review of Double-Diffusion Wood Preservation Suitable for Alaska

11

3 Kilborn, K.; Crawford, D.M.; Lebow, S.T.; Lebow, P.K. 2003. Double-diffusion treatment of

Alaska inter-coastal wood species [Poster]. In: Forest Products Society annual meeting; 2003

June; Bellevue, WA.

4 The use of trade or firm names in this publication is for reader information and does not imply

endorsement by the U.S. Department of Agriculture of any product or service.
According to Curtis (2003, see footnote 5), Tyonek’s Wood Double-Diffusion

Treatment Plant in Kenai Borough, Alaska, had a sodium fluoride label that included atmospheric pressure immersion. Because the plant is no longer operating, the

whereabouts and status of that label are unknown. Besides the legal issue with the

use of sodium fluoride, there is not enough information on these labels for someone

to develop treating schedules based on wood species, retention, and penetration. 

Preservative threshold—

The minimum amount of preservative needed to prevent wood decay by selected 

fungi can be determined by using AWPA Standard E10-01 Soil-Block Method

(AWPA 2001b). The standard treats sapwood test blocks of a nondurable conifer

(e.g., southern pine [Pinus spp.]) or a medium-density angiosperm (e.g., sweetgum

[Liquidambar styraciflua L.]) with different concentrations of the chemical. A

minimum of three species each of brown rot and white rot fungi are required when

determining thresholds of new preservatives. Depending on the size of the test

blocks and fungi used, the incubation period extends from 8 to 24 weeks. The

threshold is then calculated by plotting weight lost after incubation against chemical retention to determine the point where fungal-induced weight loss ceases.

Duncan (1958) reported that the threshold for a given preservative changes with

wood species even within a genus, e.g., Pinus. Therefore, the wood species used in

the soil block test should match the wood species in question for the preservative

application. 

Baechler and Roth (1956) conducted decay tests by using Neolentinus lepideus

Fr., Gloeophyllum trabeum (Pers.) Murr., and Postia placenta (Fr.) M. Larsen and

Lombard fungi on 19 mm southern pine cubes treated with copper sulfate, zinc

chloride, sodium arsenate, sodium borate, sodium fluoride, or sodium dichromate

water-borne solutions. The only reference to treating schedules was: “the cubes

were treated to refusal with solutions of known concentration.” The threshold for

copper sulfate and sodium fluoride are given in table 2. Units were converted from

pounds per cubic foot (lb/ft3) to percentage on a weight per weight basis (percent

wt/wt) by using a specific gravity for southern pine of 0.51 (USDA FS 1999). 

These tests were not standardized nor was it stated how the thresholds were

determined. Furthermore, because cubes were treated with only one chemical for

12

GENERAL TECHNICAL REPORT PNW-GTR-676

5 Curtis, K. 2003. Personal communication. Director, Ketchikan Wood Technology Center, 

7559 N Tongass, P.O. Box 519, Ward Cove, AK 99928.
each decay test, no inferences can be made about the combined fungicidal effect 

of copper and fluoride. Copper is almost always used with another biocide, i.e.,

chromated copper arsenate, ammoniacal copper arsenate, copper naphthenate, and 

copper-8-quinolinolate. Cowling (1957) presented threshold values for several

preservatives inoculated with 18 wood-destroying fungi, including the 3 fungi listed

in table 2. The threshold reported for copper (as metal) in copper naphthenate was

0.50 percent wt/wt. This value may be a more accurate threshold assumption for

copper in copper fluoride than those listed in table 2. 

Panek (1963) immersed southern yellow pine (Pinus palustris P. Mill.) poles

for 15 minutes to 4 hours in 20 or 30 percent aqueous ammonium bifluoride. Pole

conditions after months of air-seasoning were compared to fluoride retentions. A

retention of 0.8 kg/m3 was ascertained as an above-ground fluoride threshold for

the outer 25 mm of southern yellow pine poles. The condition of the poles was

rated for one of six categories; no visible stain, light, medium, or heavy sapwood

stain, incipient decay, or decay. For wood with a specific gravity of 0.51, that

threshold could be expressed as 0.16 percent wt/wt (USDA FS 1999). Therefore,

0.16 percent wt/wt could be interpreted as an above-ground fluoride threshold

based on visual inspection of poles not in ground contact. 

Previous studies—

Other investigations into the treating of wood by using the double-diffusion method

were conducted by the U.S. Department of Agriculture, Forest Service. The first

double-diffusion study (Baechler 1953) resulted from increased interest in treating

fence posts for farm use. In 1941, 100 green southern pine posts were treated in

copper sulfate followed by sodium arsenate. After treatment, the posts were dried

and installed in a fence post plot in the Harrison Experimental Forest in Mississippi.

Eleven years later, only one failure occurred and only a few had decay. Five posts

have failed after 22 years (Blew and Kulp 1964), and a total of eight posts had

A Review of Double-Diffusion Wood Preservation Suitable for Alaska

13

Table 2—Threshold concentrations for copper sulfate and 

sodium fluoride with southern pine sapwood

Retention threshold range

Fungus Copper sulfate Sodium fluoride

Percent wt/wt

Neolentinus lepideus — – 0.59 0.26 – 0.41

Gloeophyllum trabeum 0.94 – 1.31 .49 – .59

Postia placenta .96 – 1.67 .49 – .57

— = lower threshold immeasurable.

Source: Baechler and Roth 1956.
failed after 29 years (Gjovik and Davidson 1975). The incomplete copies of these

reports did not indicate the service life of untreated southern pine posts in this plot.

Because 92 percent of the treated posts were sound after 32 years of service, it

would be safe to say that the double-diffusion method delivered satisfactory

amounts of chemical into the wood matrix. 

Laboratory tests were also part of Baechler’s (1953) initial study. Jack pine

(Pinus banksiana Lamb.) posts were treated with copper sulfate, followed by 

either disodium phosphate or sodium fluoride. Copper sulfate and sodium fluoride

absorptions by jack pine posts treated by the double-diffusion process are given 

in table 3. 

Several fence posts were treated by double-diffusion at the Matanuska

Experimental Station farm in Palmer, Alaska, in 1954. Species included Alaskagrown white spruce, paper birch (Betula papyrifera Marsh), balsam poplar

(Populas balsamifera L.), and quaking aspen (Populas tremuloides Michx.). Posts

were treated for 3 days in 8-percent copper sulfate solution and then treated for 3

days in 11-percent sodium chromate solution. After 32 years in service, 100 percent

of the aspen, balsam poplar, and white spruce posts were sound, whereas only 58

percent of the paper birch posts were sound. The controls for aspen, balsam poplar,

paper birch, and white spruce failed after 9, 4, 7, and 9 years, respectively (Mayer

et al. 1995). 

Baechler et al. (1959) treated several species of hardwood posts native to the

Southeast United States. The wood was completely submerged for treatment to

replicate larger scale commercial-type treating, permitting a more efficient utilization of hardwoods than the method of treating upright in a barrel. Treating was 

conducted at ambient temperature, with only one solution concentration for each

chemical used. Treatment times were 2, 1, 2, or 3 days for both tanks. The first

tank was zinc sulfate and arsenic acid. The second tank was sodium chromate. Five

14

Several fence posts

were treated by 

double-diffusion 

at the Matanuska

Experimental Station

farm in Palmer,

Alaska, in 1954. 

GENERAL TECHNICAL REPORT PNW-GTR-676

Table 3—Copper sulfate and sodium fluoride absorptions by jack pine posts treated by the 

double-diffusion process

Treating schedule

Copper sulfate Sodium fluoride Chemical absorption

ID Time Concentrate Time Concentrate Copper sulfate Sodium fluoride

Days Percent Days Percent - - - - - - - - Percent wt/wt - - - - - - - -

A 1 7.95 4 3.2 1.42 1.04

B 2 7.95 4 3.2 2.35 .93

C 2 7.95 7 3.2 2.16 1.27

Source: Baechler 1953.
posts from each treatment and species group were analyzed for chemical retention

and penetration. The remaining 25 posts were installed in a test plot at the

Whitehall Experimental Forest in Georgia. Chemical analyses showed that sapwood

was much more treatable than heartwood, and that “double-diffusion appears to

offer considerable promise.” After 29 years in service, only 3 of 25 pine posts had

failed. The overall service lives for white oak (Quercus garryana Dougl. ex Hook.),

red oak (Quercus falcate Michx.), and yellow-poplar (Liriodendron tulipifera L.)

for all of the treatment times combined were 16.3, 16.4, and 16.2 years, respectively (Vick and Baechler 1986). 

Twelve species of wood grown in Hawaii were treated by double-diffusion in

1960, as a demonstration of the process for local landowners, salesmen, and industry personnel. At the time, commercially treated posts were not readily available

and this method appeared feasible. Copper sulfate was used as the first solution,

followed by sodium chromate. Freshly peeled posts were treated butt down for 3

days in each solution, by using one barrel for each solution. Discs were cut from

the top, middle, and butt after a 2-week diffusion period, and analyzed for chemical

retention. Analyses showed that the chemical retention for most of the species were

within a satisfactory range, based on the desire to retain equal amounts of each

chemical. The demonstration showed promise for a commercial double-diffusion

treating operation using Hawaiian species (Smith and Baechler 1961).

Baechler (1963) explicitly told farmers “How to treat fence posts by double

diffusion.” This report recommended sodium fluoride and copper sulfate as the first

and second treatment solutions, respectively. 

The double-diffusion process was investigated in the late 1960s for its ability 

to treat Engelmann spruce (Picea engelmannii Parry ex Engelm.), lodgepole pine

(Pinus contorta Dougl. ex Loud.), and Rocky Mountain Douglas-fir (Pseudotsuga

menziesii (Mirb.) Franco var. glauca (Beissn.) Franco) posts. These species resist

conventional treatment. One hundred and twenty-six posts of each species were

treated with one of the four treatment combinations given in table 4. Posts were

fully submerged in solution for treatment. Sixty posts were analyzed for sapwood

thickness, and chemical retention and penetration. The remaining 225 treated posts

and 75 untreated posts were installed in a fence post plot at the Central Plains

Experimental Range in Colorado (Markstrom et al. 1970). 

Markstrom et al. (1970) found that Engelmann spruce and lodgepole pine could

be successfully treated based on the average penetration exceeding 19 mm. Full

sapwood penetration occurred in Rocky Mountain Douglas-fir, but the average

minimum penetration was less than 19 mm for all treatments. They also found that

A Review of Double-Diffusion Wood Preservation Suitable for Alaska

15
both Engelmann spruce and lodgepole pine could be treated to meet the 6.41 kg/m3

of CCA retention specified by AWPA Standard C5-00 fence posts (AWPA 2001a).

Thirty years later, all of the treated posts withstood a 22.7-kg load applied laterally at the top of the post. Untreated posts had service lives of 16, 17, and 9 years

for Engelmann spruce, lodgepole pine, and Douglas-fir, respectively. All untreated

posts failed at or near the ground line (Markstrom and Gjovik 1999). 

By 1985, double-diffusion studies were extended to treating railroad ties in 

an effort to demonstrate the use of nonpressure processes to treat native Alaska

species. Western hemlock ties and timbers, and Sitka spruce and yellow-cedar 

timbers were in the combinations shown in table 5.6

The first solution was heated for half of the charges. The goal of heating to 82

to 88 °C was unattainable; the actual temperature never exceeded 52 °C. Treating

with a heated solution is referred to as the modified double-diffusion process (see

footnote 6). Forty-eight hemlock ties went into the railroad track near Palmer,

Alaska, and are still in the track (see footnote 2). The remaining wood was to be

analyzed for chemical retention and penetration, but no reports of the results were

found.

To increase retention with the double-diffusion process, the use of ultrasonic

energy was investigated. Alaska-grown white spruce was treated with approximately 4 percent sodium fluoride while ultrasonic energy was applied. Wheat et al.

(1996) found that using ultrasonic energy during treatment increased chemical

retention. However, the second treatment in the double-diffusion process was not

16

GENERAL TECHNICAL REPORT PNW-GTR-676

6 Gjovik, L.R. 1985. Double-diffusion treatment study for southeast Alaska. Study 3-85-37. 

9 p. Unpublished progress report 1. On file with: USDA Forest Service, Forest Products

Laboratory, One Gifford Pinchot Drive, Madison, WI 53726.

Table 4—Treatment schedule used in fence post study

First solution Second solution

10 percent copper 13 percent sodium 

sulfate chromate / arsenic acid

ID Pretreatment Time Temperature Time Temperature

Days Fahrenheit Days Fahrenheit

A — 1 Ambient 1 Ambient

B — 3 Ambient 3 Ambient

C — 1/3 200° 1 Ambient

D Incising 1 Ambient 1 Ambient

— = no pretreatment occurred.

Source: Markstrom et al. 1970.
used. Therefore, it is not known if the additional chemical uptake would remain in

the wood matrix during submersion into the second treatment solution.

In 1995, the Wood in Transportation Program awarded Tyonek Native

Corporation a grant to develop a double-diffusion treating facility. The facility

would, in turn, use locally grown species of wood and provide long-term employment for local residents. Operations began in 1997, by treating wood for a bridge 

to be built near Fairbanks, Alaska (Russell and Kilborn 1997). However, operations

ceased shortly thereafter owing to lack of infrastructure. 

Treating demonstrations using ponderosa pine (Pinus ponderosa P.& C.

Lawson) posts and poles have also taken place in Colorado, Arizona, and Utah.

Posts were treated by using sodium fluoride and then copper sulfate. Because these

demonstrations were to inform the public of a low-cost wood preservation treatment for their refractory species, only one charge of wood was treated at each site.

Chemical penetration and retention were not assessed (Reader 2000).

The most recent double-diffusion field project took place near Copper Center,

Alaska. To access proposed agricultural land, the Trans Alaska Pipeline had to be

crossed by using a bridge. The State of Alaska and Alyeska Pipeline Service opted

to use Sitka spruce for the bridge abutments, because wood abutments would not

transfer heat to the soil and disturb the permafrost supporting the pipeline. Timbers

were treated with heated 4-percent sodium fluoride and then 10-percent copper sulfate (Hoffman 2002a). Samples were treated along with timbers for the purpose of

analyzing chemical retention and penetration, but no results have been made available at this time. 

A Review of Double-Diffusion Wood Preservation Suitable for Alaska

17

Table 5—Treatment schedule used in the Alaska demonstration project

Chemical Concentrate Temperature

Percent

First solution:

Sodium fluoride 4 Ambient

Sodium fluoride 4 Hot

Copper sulfate 8 Ambient

Copper sulfate 8 Hot

Second solution:

Copper sulfate 8 Ambient

Copper sulfate 8 Ambient

Sodium chromate/arsenic acid 11 Ambient

Sodium chromate/arsenic acid 11 Ambient

Source: Gjovik 1985 (see footnote 6).
In the most recent laboratory study on double-diffusion, railroad ties from

western hemlock, Sitka spruce, and white spruce grown in Alaska were treated with

4-percent sodium fluoride followed by 8-percent copper sulfate. Ties were fully

immersed for 20, 10, 5, or 2.5 days in each solution. After a 2-week diffusion period,

copper content was analyzed. Fluoride content was estimated from copper content

based on a previous study indicating that fluoride was found in excess of copper.

Because copper sulfate labels limit treatment time to 3 days, only the chemical

retentions for the 2.5-day treatment are given in table 6. The authors did not discuss

the implications of the chemical analyses (see footnote 3). Furthermore, it is unknown

if decay tests will be performed on these treated ties in order to determine the actual

copper and fluoride thresholds needed for service in Alaska.

Discussion

This review found double-diffusion as the most advantageous wood-treating method

that could potentially treat locally grown species in Alaska. Although there appear

to be many studies treating wood by double-diffusion, several gaps still exist in the

literature. 

• Although copper sulfate manufacturers include wood treatment on their 

label, it is not clear why a maximum treatment time of 3 days in each 

solution was selected. It is unclear if a 3-day treatment can deliver enough

chemical under all conditions of initial moisture content, density, temperature of the wood or solution, and type of wood (species, heartwood, 

sapwood, earlywood, and latewood). Furthermore, there is no established

threshold for copper and fluoride for double-diffusion. 

• The only report found on the leach resistance of chemical precipitates 

formed in double-diffusion treatments did not include copper fluoride. The

18

Several gaps still

exist in the literature. 

GENERAL TECHNICAL REPORT PNW-GTR-676

Table 6—Chemical retentions for Alaska-grown railroad ties

Copper Fluoride (estimated)

Species MC 0-13 mm 13-25 mm 0-13 mm 13-25 mm

Percent - - - Percent wt/wt - - - - - - Percent wt/wt - - -

Sitka spruce 28 0.22 0.06 0.13 0.04

Sitka spruce 34 .26 .08 .15 .05

Western hemlock 26 .28 .06 .17 .04

Western hemlock 31 .48 .09 .28 .05

White spruce 39 .37 .08 .22 .05

White spruce 32 .44 .16 .26 .09

MC = moisture content.

Source: Kilborn et al. 2003 (see footnote 3).
report only included copper arsenate, copper chromate, nickel arsenate, 

nickel chromate, and magnesium ammonium arsenate (Baechler 1941). 

It has never been established that copper sulfate forms an insoluble 

precipitate with sodium fluoride. 

• Optimum treatment schedules to deliver adequate copper and fluoride 

retentions into Sitka spruce grown in Alaska, and the optimal length of 

time for the treated wood to be wet-stacked to facilitate the diffusion 

remain unknown. 

• Previous studies were limited to posts, poles, and railroad ties, all of which 

are likely to have more easily treated sapwood. The treatment times for 

dimensional lumber that do no contain any sapwood are unknown.

Acknowledgments

The following are acknowledged for their helpful comments and contributions to 

this project: Michael R. Milota and Jeffrey J. Morrell, Department of Wood Science

and Engineering, Oregon State University.

English Equivalents

When you know: Multiply by: To find:

Meters (m) 3.28 Feet

Millimeters (mm) .03937 Inch

Kilograms (kg) 2.205 Pounds

Kilograms per cubic meter (kg/m3) .06243 Pounds per cubic feet

Degrees Celsius (C) 1.8 C + 32 Degrees Fahrenheit

Kilopascals (kPa) .14504 Pounds per square inch

Cubic meters 2,360 Million board feet (mmbf)

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23
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