A 30-day comparative test of three antistain solutions made in June, 1924, by the Virginia & Rainy Lake Co., of Virginia, Minn., is given below:

The conditions of the above test were unusually favorable to bluestain development, as the test material was bulk-piled and then covered with wet blankets. Of the three solutions tested, the sodium bicarbonate solution appeared to give the best results.

COST OF DIPPING

It has been estimated that approximately 50 pounds, or 6 gallons, of solution are needed to dip 1,000 board feet of pine lumber. About 16,000 board feet of lumber can therefore be treated with 100 gallons of solution. This will vary with many factors, such as species of wood, amount of heartwood, presence of "red heart" in the heartwood boards, chemical used, moisture content of the green boards, length of time the boards remain in the solution, etc.

The cost of soda dipping, as well as the method used in computing it, varies considerably within the same general logging region and from mill to mill. The cost per 1,000 board feet at 20 representative pine mills of the South (1925) was found to range from 5 to 35 cents. In a group of 5 mills cutting southern hardwoods the cost ranged from 9 to 50 cents, and at 3 pine mills in the Lake States region the cost ranged from 9 to 75 cents. The high cost of dipping at one mill was traceable to the extra charge for transportation and rehandling due to the fact that the dip tank was not built into the green chain but was placed as a separate unit some distance from the sorting table. The costs will naturally vary with changes in cost of materials and cost of labor. In 1929 the cost for dipping would be somewhat higher than the figures given above.

EQUIPMENT FOR DIPPING

For hand dipping small amounts of lumber, lath, and other products and for experimental purposes a simple wooden tank is found convenient and inexpensive. A small-sized tank of this type is shown in Figure 26. A return-steam coil for heating the solutions and a perforated pipe for use in steaming the lumber can easily be fitted to the bottom of the tank. Many of the tanks or vats at the larger mills are built into the green chains and are constructed mainly

42717°-29-5

FIGURE 28.-View of the principal parts of a metal dipping tank commonly used in the prevention of sap stain. The by-pass is clearly

shown and can be operated by foot power.

The drip-pan feature is missing in this installation

of wood. Specially constructed all-steel vats designed with heating coils and with a by-pass to allow certain grades of lumber to pass over the tank (figs. 27 and 28) are in use at many mills where soda dipping is practiced. These tanks are 10 feet wide and are made any length to suit conditions. Timbers as large as 6 by 8 inches can be handled over the chains, which have a speed of about 30 feet per minute. A mixing tank is used in connection with the dipping vat and is placed at some height above the dipping vat. The mixing tank holds about 250 gallons, and in it are mixed the solutions preparatory to filling the dipping vat by gravity flow. Care must be exercised not to use corrosive chemicals in metal tanks.

Thermometers to gauge the temperature of the solution, and hydrometers to test its strength at frequent intervals, are necessary additions to the equipment. The temperature of a soda solution

FIGURE 29.-A view of the type of steam cylinder used in the pressure steaming of certain hardwoods, such as sap gum. The cylinder holds one truck load of open-piled lumber of approximately 3,000 board feet

is an important item and should be checked regularly. Heat coils. with perforations or openings permitting the escape of live steam into the solution should not be used. Closed coils with sufficient steam to keep the solution between 140° and 160° F. are required. Heating the solution above 160° F. may cause a change in the chemicals and so weaken the dip solution.

The solution needs to be renewed as soon as it shows signs of weakening. Each board as it goes through the solution uses up a certain amount of the chemical in neutralizing the acid condition. of the sapwood. The passage through the solution of several thousand board feet of sapwood lumber soon reduces the alkalinity or strength of the chemical to a point where it is no longer effective as a stain preventive. It is doubtful whether the hydrometer can be depended on to do more than merely register the density of the

soda solutions regardless of their alkalinity. In order, therefore, to test at regular intervals the alkaline strength of the solution, some other method seems desirable. The following equipment was therefore devised for this purpose, and directions for determining the strength of soda solutions are given.

METHOD OF DETERMINING THE STRENGTH OF SODA DIP SOLUTION

APPARATUS AND CHEMICALS

1 Nefis automatic acidity tester.

2 twenty-five cubic centimeter cylinders graduated in one-fifth cubic centimeter.

1 or 2 ordinary white coffee cups.

2 glass stirring rods.

2 liters of one-tenth normal sulphuric acid.

3 ounces of methyl-orange indicator solution in a dropping bottle.

INSTRUCTIONS FOR SETTING UP THE APPARATUS

Be sure that the acid tester is clean and dry before putting it together. Any water in the bottle will change the strength of the acid and cause an error in the readings. When the bottle is dry, pour the normal sulphuric acid into the tester and set it up according to directions. Fill the burette or glass tube on the tester by squeezing the rubber bulb with one hand and closing the little hole in the glass tubing with the thumb of the other hand. Make certain that the burette is full even to the tip. This can be done by wasting a slight amount of the acid. The acid tester is so designed that when the little bulb on the top of the burette is partially filled and the pressure in the bottle is released by removing the thumb from the little tube, the liquid will siphon back, leaving the burette reading exactly zero. It is now ready for use.

INSTRUCTIONS FOR TESTING

Take a sample of the dip solution, fill the graduated cylinder to the 17-cubic centimeter mark and pour this amount into a coffee cup, without draining the cylinder. (Rinse the cylinder with water and hang it up to dry ready for the next test.) Add two or three drops of the methyl-orange indicator to the dip solution in the coffee cup. This will make the solution a yellow or orange color. Now add acid from the burette slowly until the solution in the coffee cup turns pink. This occurs when all the alkali has been neutralized. If this is done carefully, the pink will always appear at the same burette reading with the same sample. Read the burette graduation and divide the reading by two. This will give the percentage of bicarbonate present in the dip solution.

EFFECT OF SODA DIP UPON THE STRENGTH OF WOOD

Weiss and Barnum 44 in 1911 tested the effect of soda dip upon shortleaf and longleaf pine boards. The claim had been made that

44 Weiss, H. F., and Barnum, C. T., The Prevention of Sap Stain in Lumber: U. S. Dept. of Agri, F. S. Circ. 192: 1-19; 1911.

boards dipped in a sodium bicarbonate solution became brash and brittle. The tests made show that the soda-dipped wood is slightly stronger, stiffer, and tougher than untreated wood, but the difference is so small that the normal variations in the wood may account for it. The hardness tests indicated a slight increase in surface hardness of the soda-dipped shortleaf pine and no increase in case of the dipped longleaf pine. The hardness increase in shortleaf pine disappeared after the boards became thoroughly air-dry. As a result of the above tests the conclusion was reached that sodium bicarbonate treatment does not affect the strength properties of the wood.

STEAMING

PRESSURE STEAMING

In certain regions of the South lumber green from the saw is steamed under pressure previous to seasoning in the yard. The equipment used consists of heavy plate metal cylinders equipped with track, trucks, and a close-fitting door. (Fig. 29.) Certain types of these cylinders are known as Kraetzer Preparators, and the product is sometimes termed "Kraetzer-Cured " lumber. It is apparent that these cylinders represent a considerable outlay of capital. Along with the small capacity and large steam consumption this equipment is limited in its use to certain types of mills.45

The schedules for steaming sap gum in these cylinders vary considerably. For 4/4 sap gum the time of steaming ranges from 15 to 40 minutes and the gauge pressure from 20 to 30 pounds. For 6/4 stock, 20 to 30 minutes at approximately 30 pounds pressure is used. For 8/4 stock, 45 minutes at 30 pounds pressure is frequently used. Red gum, as well as red oak is treated by this process with the following schedules: 6/4-inch red gum, 40 minutes at 15 pounds pressure; 8/4-inch red gum, 50 minutes at 10 pounds pressure; 6/4-inch red oak, 30 minutes at 25 pounds pressure. In most cases pressure within the cylinders may drop considerably below the listed pressures due to expansion of pipe and cylinder and to condensation of steam.

The mills using this method of seasoning claim that it appreciably shortens the air-seasoning period, prevents stain, mold, and decay, eliminates certain mechanical defects common to air-dried stock, and changes the color of sap gum from white to pink. Care must be taken, however, to avoid severe surface checking. Rapid handling from the cylinder to the pile is essential in preventing sap stain.

In addition to the above advantages, it is of interest to note that high temperatures maintained for sufficient periods of time are effective in killing the fungi and insects present in the green boards. It has been reported that piling the lumber to allow sufficient slope for draining the condensed moisture is beneficial in removing the surface sap, and therefore the board surfaces are less favorable to fungous growth.

Steam boxes of various design are in general use in the cooperage and other industries where the stock is steamed at atmospheric pressure for several hours in order to soften the wood fibers prior to

45 Murial, C. J. Steaming Gum Lumber Ahead of Yard Drying: Hardwood Record 53:22-23; May 10, 1922.