TIRATINe WOOL) IN PIENTACILP0IPHEIN0L
SOLUTIONS IBY T11 COLU-SOAIINC
RIevised March 1948
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No. I 144
UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST PRODUCTS LABORATORY
In Cooperation with the University of Wisconsin
TREATING WOOD IN PENTACHLOROPHENOL SOLUTIONS
BY THE COLD-SOAKING METHOD
J. OSCAR BLEW, Technologist
Cold soaking is a limited-purpose treatment for the preservation of
wood. In treating by the cold-soaking method, the wood is simply submerged
in the preservative solution and allowed to soak for a few hours or days,
Wood that is easy to treat, such as sapwood of pine or round pine posts con-
taining a substantial sapwood volume, may absorb enough preservative and be
sufficiently well penetrated during the treatment to provide a high degree
of protection from decay and termites. In woods more difficult to treat,
including the heartwood of most species, the results may be much less sat-
isfactory. Treatment by cold soal:ing, because it permits greater absorptions
and deeper penetrations of preservative, will provide more satisfactory re-
sults than are obtainable by short dipping treatments.
Cold soaking can sometimes be used to advantage when it is impracti-
cal to use the more effective impregnation treatments. It has the advantage
of being relatively simple and moderate in cost. It can be used with a
variety of preservatives to meet different requirements as to color, clean-
liness, paintability, freedom from odor, or other properties. Seldom, how-
ever, will treatment by cold-soaking methods give as much protection from
decay or insects as treatment by pressure or by hot-and-cold bath methods.
Despite their greater first cost, these methods will often be cheaper in the
long run because of the longer life they provide.
Any preservative dissolved in water or in thin oils may be applied
by cold soaking. The use of water solutions by cold soaking is often called
the steeping process and is described in another publication.1 There are a
number of preservatives, consisting of toxic chemicals dissolved in kero-
sene, domestic fuel oil, or in si.ailar oils, that can be applied by cold
soaking. Pentachlorophenol is one such chemical, more commonly used than
the others, with which the Forest Products Laboratory has experimented con-
siderably. This report deals primarily with the use of pentachlorophenol
-A partial list of publications describing various preservatives and non-
pressure processes will be found at the end of this report. They are
available from the Forest Products Laboratory on request.
Rept. No. R1445 (Revised)
solutions, but most of the information it contains will apply as well to
other preservatives of similar type. Coal-tar creosote, diluted with not
more than an equal volume of low-viscosity fuel oil,and light oils contain-
ing copper naiphthenate have been applied to fence posts by the cold-soaking
method with treating results similar to those obtained with pentachlorophenol
Solutions of pentachlorophenol in light petroleum oil solvents have
been widely used in recent years for the treatment of window sash and frames,
also for treating other products where a clean, odorless, and nonswelling
treatment is required, particularly when the wood'must be painted after treat-
ment. The treating solutions are usually applied by short dipping applications
since the produ-icts are generally not exposed to conditions favoring rapid
decay or insect attack and thus do not require a more thorough treatment. The
high toxicity, the resistance to leaching, and results of limited service
tests indicate that pentachlcrophenol solutions with a more thorough applica-
tion may also be suitable for the treatment of lumber, fence posts, or other
wood products to be used in contact with the ground.
Pentachlorophenol is available in two forms -- dry flake, and concen-
trated solution. When purchased dry it is in the form of dark gray flakes
or crystals having a slight odor. When handled in this form, the chemical
has a tendency to dust into the air. The dust is irritating to the mucous
membranes of the eyes, nose and throat, thus the use of protective goggles
and dust masks is recommended for workers mixing the solutions. Heating
equipment is generally required in order to prepare solutions from the dry
It is generally more convenient for the user without special mixing
equipment to purchase a concentrated solution of pentachlorophenol and dilute
it with the desired solvent. Concentrates are available requiring dilution
with 2 to 12 or more parts of solvent (by volume) to prepare a 5 percent
treating solution. Concentrates are either dark or light colored depending
upon the use requirements of the material to be treated and the percentage
of pentachlorophenol in the concentrate. Light-colored concentrates usually
contain less preservative than those of a darker color.
For treatments requiring a clean, paintable surface, light-colored con-
centrates and solvents, such as naphtha, mineral spirits, or kerosene, should
be used. Where such properties in the treated wood are not required, as in
fence posts or rough lumber, the darker-colored concentrates and lower-priced
petroleum oil solvents and even used crar.kcase oil, if reasonably clean, can
be used. The use of dark-colored solutions makes it relatively easy to
determine the dppth Df penetration in the wcc-d after treatment. In making
the selection, however, other requirements such as drying rate, fire hazard
and penetrating properties should be considered. There is some evidence to
Indicate that p(rntachlorplhcnol solutions prepared from the light oil solvents
may provide less protection than those prepared from the heavier type of oils.
Rept. No. BlI45
In mixing the treating solutions, it is important to have the proper
proportions of toxic chemical and solvent. The weighing of all ingredients
is recommended, but in the absence of accurate weighing equipment, an accu-
rate liquid measure can be used providing the weight per gallon of both
preservative concentrate and solvent and the percentage of chemical in the
concentrate are known. If complete instructions for preparing the solution
cannQt be obtained, the proper quantities of concentrate and solvent can be
arrived at by using either of the following formulas:
1. For obtaining the required number of pounds of solvent for each
pound of concentrate.
Pounds of solvent to be added to each pound of concentrate = a b
Where a = toxic concentration of concentrate expressed decimally.
b = toxic concentration of desired treating solution expressed
If the concentrate contailq 24 percent pentachlorophenol and a
5 percent treating solution is desired, the weight of solvent
required per pound of concentrate would be 0.24 0.05 = 3.8 pounds.
2. For obtaining the required number of gallons of solvent for each
gallon of concentrate.
Gallons of solvent required per gallon of concentrate = (a ) (-?)
Where a = toxic concentration of concentrate expressed decimally.
b = toxic concentration of desired solution expressed
c = weight per gallon of concentrate.
d = weight per gallon of solvent.
A gallon of concentrate containiLg 16 percent pentachlorophenol
weighing 8.15 pounds is to be mi;ed with fuel oil weighing 7
pounds per gallon to obtain a 5 percent treating solution. The
number of gallons of solvent required per gallon of concentrate
would be 0.16 o0.05 x 815 = 2.56 gallons.
Rept. No. R1445
To obtain satisfactory results by cold-soaking treatment, the material
must be well seasoned and free from outer and inner bark. A seasoning period
of 2 to 6 months or longer, during the warmer, drier seasons of the year, is
generally sufficient if the wood is open-piled at least a foot above the
ground and is well ventilated. Material seasoned for a year or longer and
even that seasoned for only several months under unfavorable conditions will
probably absorb greater or even excessive quantities of preservative, but in
wood that is thus handled seasoning is apt to be accompanied by decay. Such
seasoning is therefore not encouraged.
The material should be cut to final dimensions and all boring or framing
should be done before treatment. If cutting or boring is required after
treatment, the cut surfaces should be treated with a liberal quantity of the
preservative applied by flooding or generous brushing.
The equipment required for treatment by the cold-soaking method con-
sists mainly of a leak-proof tank of sufficient size and strength to accom-
modate the completely submerged material for the full time of the treatment.
Some arrangement should be provided for keeping the material from floating
above the level of the solution. Suitable weights, or cross bracing at-
tached to the sides of the tank generally accomplish this purpose,
In making the treatment, the wood is simply submerged in the preserva-
tive solution and allowed to soak for a predetermined time, or until the
desired quantity of preservative has been absorbed and good penetrations are
obtained. For round fence posts of easily treated woods, a soaking period
of 48 hours produces good penetrations. For various species treated by the
Forest Products Laboratory, preservative absorptions during such periods
have averaged approximately 90 percent in softwoods and 70 percent in hard-
woods of those observed after continued soaking as long as one week. Soaking
periods longer than one week do not appear to be justified except in special
Higher absorptions and some improvement in preservative penetration
can be expected in cold soaking if the depth of immersion is increased.
Limited soaking tests on southern yellow pine sapwood and Douglas-fir heart-
wood lumber cf short lengths have shown absorption increases from 2' to 100
percent or more due to pressure increases at tank depths of 7 to 10 feet.
Where suitable tank equipment can be provided increasing the depth of immer-
sion can be used as a means for obtaining greater absorptions in woods that
are difficult to treat, or for reducing the soaking time for easily treated
woods such as pine. The use of a vertical tank (fig. 1) can likewise be
expected to provide improved treatment in the butts of fence posts providing
the posts are placed in the tank and soaked with the butt ends down. This
method of treatment,however, cannot be expected to materially improve pene-
trations in woods, such as aspen (popple) or basswood, that normally absorb
large quantities of preservative but show erratic penetrations.
Rept. No. R1445
The tank shown in the illustration was made by welding two hot-water
storage tanks. Oil drums welded together would be more appropriate if a
considerable number of posts are to be treated. The size, shape, and char-
acter of treating tank should be determined by the amount and character of
wood to be treated and by the character of acceptable containers available.
Th@ following rough classification, based upon a limited number of
species of round fence posts treated by the Forest Products Laboratory,
should be helpful in the selection of woods that are best adapted to the
cold-soaking treatment. The results indicate that the woods of group 1 are
to be preferred.
Group 1. Absorptions fair to good, and penetrations in sapwood reasonably
good, after a soaking period of 48 hours.
Round softwood posts
Round hardwood posts
Oak, black (high sapwood
Oak, red (high sapwood
Oak, southern red
Group 2. Absorptions fair to good, and penetrations in sapwood fair, after
a soaking period of 48 hours or longer.
Bound hardwood posts
Group 3. Absorptions good, but transverse sapwood penetrations generally
poor, after a soaking period of 48 hours or longer. (Good end penetration
is usually obtained, thus material not over 2 to 3 feet in length can be
expected to show satisfactory treatment.)
Round hardwood posts
Birch, white or
Tupelo, swamp (blackgum)
Rept. No. E1445
Group 4. Absorptions and penetrations poor to fair after a soaking period
of 48 hours or longer.
Round softwood posts Round hardwood posts
Douglas-fir (mountain Ash, green
Larch, European Catalpa
Spruce, Norway Hackberry
*Penetrations in some cases fair to good but not uniformly so.
Cost and Effectiveness of Treatment
The cost of treatment by cold soaking can be expected to vary to a
considerable extent according to the cost of the treating solution used, the
size of the material, the number of pieces being treated, the quantity of
solution absorbed, labor costs, and equipment costs.
In the treatment of fence posts by the Forest Products Laboratory with
domestic No. 2 fuel oil containing 5 percent pentachlorophenol, preservative
costs (1948 figures) alone varied from 6-1/2 cents to 62 cents per cubic foot
of wood treated. The preservative cost will vary with the price per gallon
of the preservative as well as with the amount absorbed per unit volume of
wood. Costs for labor, equipment, and the surplus preservative that is
required to keep the wood covered during treatment, but which remains after
treatment, should be added to the cost of the preservative in arriving at the
total cost of treatment. Round posts of some woods in Group 4 absorbed less
than 1/4 gallon per cubic foot of wood during a 48-hour soaking period while
those of some woods in Groups 1 and 3 absorbed as much as 2 gallons per cubic
foot in the same period. A post 7 feet long with an average diameter of 5
inches has a volume 3f approximately 1 cubic foot. A gallon of 1 to 10 con-
centrate, costing $2.15, diluted with 10 gallons of domestic No. 2 fuel oil
(12 cents per gallon) costing $1.20 would produce a treating solution at a
cost of 50.5 cents per gallon.
The effectiveness of the cold-soaking treatment of wood with penta-
Chlorophenol solutions has not yet been fully established since this treat-
ment h&s not been applied for a long enough period. Conclusive service infor-
nmation is not available at the present time, except on posts known to be
inadequately treated (see last paragraph). Experiments, however, on southern
yellow pine fence posts treated by pressure impregnation with 6.4 and 6.7
pounds per cubic foot of 5 percent and 5 percent pentachlorophenol, respec-
tively, in spent crankcase oil indicate that good results may be expected
from material treated with these preservatives if sufficient absorption and
good penetrations are obtained. The posts have been in test in Mississippi
for approximately 11 years. In that time all cf 99 posts treated with the
Bept. No. Rl45
5 percent solution and all but one of the 99 posts treated with the 3 per-
cent solution were in good or serviceable condition,. These results with
the 5 percent solution are similar to those obtained with posts in the same
test treated with 6 pounds of coal-tar creosote per cubic foot. All of the
untreated posts installed at the same time have been destroyed because of
decay and termite attack. The average life of the untreated posts was 3.3
Since retentions as high as 5 or 6 pounds per cubic foot and penetra-
tions comparable to those obtained by pressure treatment are generally not
obtainable through cold soaking, the effectiveness of this method of treat-
ment may not be so high as that reported for the pressure-treated posts.
The Forest Products Laboratory during 1942 and 1943 installed near
Madison, Wis., experimental posts of several species treated with pentachloro-
phenol solutions by cold soaking. Later installations have included treat-
ments by cold soaking with fuel oil containing copper naphthenate and with
solutions containing 50 percent coal-tar creosote and 50 percent fuel oil.
Most of these experimental posts were in good or serviceable condition when
last inspected during the fall of 1947. However, there have been a high per-
centage of premature failures after 4 years of service in aspen and white
birch posts due to inadequate preservative penetrations in the ground line
zone of these posts. These and similar early failures reported on tests in
Minnesota on treated aspen posts indicate that all woods cannot be treated
satisfactorily by the cold-soaking method. The user of the method should
therefore select only those woods which experience has shown to take treat-
ment satisfactorily. The cold-soaking treatment of posts of woods in Group 3
(see page 5) cannot generally be recommended.
Rept. No. R1445
List of Forest Products Laboratory Publications
on Nonpressure Treatments
Preservative treatment of farm timbers, U. S. Dept. Agr. Farmers'
Bull. 744. Revised 1928.
B154 Methods of applying wood preservatives, (revised 1944).
B1695 Preservatives for wood poles (1946).
R621 Preservative of tin-er by the steeping process, (revised 1945).
RP.761 Preser,,vive treatment and staining of shingles, (revised 1947).
B919 Preae:-'..-ve treatment of window sash and other millwork,
(re "i-id 1)'5).
P1190 Testing wood preservatives, (1938).
R149 Wood preservatives, (revised 1944).
R1468 Selecting a suitable method for treating fence posts (1946).
The Osmose process (1944)
Chrocmated zinc chloride (1944)
List of coal-tar creosote producers and dealers in the
United States (1941)
Zinc chloride producers and dealers in the United States (1947)
Some companies that design or build treating plants (1948)
Partial list of suppliers of pentachlorophencl wood preservatives
for the treatment of fence posts, poles, and structural lumber
and timbers (1945)
Rept. No. B1445 -8-
HINGED ARM FOR
Figure 1.-An arrangement for treating posts with pentachlorphenol solutions on a
small scale. The main part of the system Is the upright tank so that pressure
caused by the height of the liquid causes deeper penetration Into the butts of
the posts. Piping shown for a circulation system is helpful but it Is not
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