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the National Dry Kiln Co. (in The Spokesman, house organ of that company, for January, 1927), suggests another method of determining the length of steaming, the first step of which is described by him as follows:

Close the kiln when empty and heat it with steam spray at medium volume and with no heating coils turned on. The temperature (dry bulb) of the room will gradually mount until it reaches its practical maximum. Note what this is and use it as a guide in future operations. When there is lumber in the kiln the temperature may go 5° to 20° higher.

The method depends upon the fact that as long as the lumber is absorbing heat the kiln temperature resulting from the spray will not reach its maximum but will rapidly rise to it when the lumber becomes heated through and stops absorbing heat, which is an indication that the steaming has been carried far enough. Mr. Krick stated that this method had been developed and was followed by the kiln operators at the plant of the Hayes-Ionia Co., Grand Rapids, Mich. It was found that where the empty kiln reached a maximum of 140° the loaded kiln at the end of the steaming process would rise quite rapidly to 152°.

Both preliminary and intermediate steamings differ from final steaming, in that they are parts of the kiln-drying process, while final steaming is a tempering or conditioning process.

Steaming treatments do not contribute moisture to the surface of the wood as rapidly as might be expected. Steaming at 100 per cent humidity and 141° F. corresponds with a moisture content of about 26 per cent in the wood, and it will not take up moisture to a greater degree except while the wood is at a lower temperature than the air. Steaming at higher temperatures lowers the moistening effect on the wood as, of course, does steaming at less than saturated humidity.

This question of the equilibrium moisture content of the wood at the steaming temperature and humidity has, however, only an incidental bearing on the subject, because the main purpose of steaming is to heat the lumber, and doing this will require the condensation of a certain volume of steam, which condensation occurs on the surface of the wood regardless of whether the wood is or is not thirsty for moisture. But the amount of condensation represented by this heat transfer is much smaller than is generally supposed.

Intermediate steaming treatments have been observed to add as much as 3 to 5 per cent moisture to the wood, but the wood must be pretty dry and the steaming rather thorough in order to produce so large an effect.

Initial kiln steaming seems to be a comparatively recent development. In Department of Agriculture Bulletin 509, published in 1917, and discussing kiln-drying theory as applied to the Tiemann water-spray kiln, the author, H. D. Tiemann, says:

Preliminary steaming may be used in connection with this kiln, but experiments indicate that ordinarily it is not desirable, since the high humidity which can be secured gives as good results.

In his Kiln-Drying of Lumber, first published in 1917 and revised in 1920, this author has not anywhere discussed preliminary kiln steaming, though he does discuss pressure steaming in cylinders as preliminary to either kiln-drying or air-drying.

In The Kiln-Drying Handbook, by Rolf Thelen (Department of Agriculture Bulletin 1136), published in 1923, there is a recommendation of preliminary steaming for air-dried lumber of one and one-half to two hours per inch of thickness. A preliminary steaming, as indicated on page 33, has been mentioned for the relief of air-drying stresses in partly dried stock. This treatment is also recommended for green stock, not to relieve stresses but to warm the stock thoroughly before the drying operation begins. It is not necessary to steam green stock so long as partly seasoned stock, one hour per inch of thickness being sufficient. The temperature may be from 10° to 15° above the starting point of the schedule.

In Practical Kiln-Drying, by E. U. Kettle, published in 1923, a table on page 68 gives four schedules for inch hardwoods at moisture contents of 15 to 40 per cent, which call for one to three days at 100 per cent humidity, and the text on page 69 shows that these are obtained by use of the steam sprays.

In Dry-Kiln Practice, 1925, by Prof. H. L. Henderson (Bulletin 16, New York State College of Forestry), preliminary steamings of three hours per inch of thickness for hardwoods and two hours for softwoods are recommended where the air circulation is adequate.

In The Kiln-Drying of Lumber, 1926, by Koehler and Thelen, page 179 recommends steaming green lumber one hour per inch of thickness at 100 per cent humidity, with the following recommendation for air-dried lumber:

If air-dry lumber contains less than 18 per cent moisture, it should not be steamed at 100 per cent humidity, but at 65 to 85 per cent, for 10 to 30 hours, the time depending upon the moisture content and the thickness of the stock. The temperature during this steaming period should be between 160° and 180° F. For moisture contents much lower than 18 per cent, the humidity should be between 60 and 70 per cent. If high humidities are used on dry stock, not only are temporary internal stresses set up but moisture is added which later must be evaporated.

In Department of Agriculture Circular 421, by L. V. Teesdale, 1926, the kiln-drying schedules given for gum up to 12 inches thick contain instructions to steam green stock four hours at 180° before kiln-drying is started. Besides heating the lumber rapidly, the purpose of the steaming treatment in the case of red gum is to darken the sapwood and to assure sterilization of the stock.

In The Kiln-Drying of Long Leaf Pine, by L. V. Teesdale (Southern Lumbermen, December 17, 1927, p. 169), various schedules used in the tests made for the Forest Products Laboratory at the plant of the Kaul Lumber Co., Tuscaloosa, Ala., are published. For a period of five hours after the steam is turned on there is maintained à temperature at the dry bulb of 175° or 180° F. and wet bulb 5° lower, a humidity of 89 per cent, which was nearly equivalent to a steaming treatment.

In the course of lectures prepared as a kiln-drying course for the British Columbia Forest Products Laboratory, 1926, by H. L. Jenkins, and applying to Douglas fir and other west coast softwoods, lecture 5 recommends an initial steaming of three hours per inch of thickness for green lumber, and one or two hours additional for partly air-dried lumber.

The intermediate steamings recommended by the various authorities range from one-half to four hours. For relief of casehardening

the manual by Henderson recommends instead a high humidity treatment at 70 to 85 per cent for 6 to 24 hours; and The Kiln-Drying of Lumber, by Koehler and Thelen, recommends steaming for one-half to three hours at temperatures of 165° to 185°, using 100 per cent humidity if the stock has 17 per cent moisture in the center, 75 to 85 per cent humidity for stock ranging from 15 to 17 per cent, and 60 to 70 per cent humidity for stock under 15 per cent moisture in the center.

A latter authority than any of those quoted, is the revised edition of Department of Agriculture Circular 1136, entitled "The Kiln Drying Handbook," by Rolf Thelen, in charge of the section of timber physics at the Forest Products Laboratory. It gives the latest conclusions, differing somewhat from the older texts. In this publication will be found a discussion of preliminary steaming of

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FIGURE 58. The usual power transfer consists of a winch mounted on the transfer car and capable of pulling the transfer, or pulling cars on or off, by a cable led from the winch through suitable fair-leaders. The form illustrated here consists of a tractor coupled to the transfer car and also carrying a power winch. It may be seen also in Figure 54. Photograph courtesy of Forest Products Laboratory

green lumber; and such steaming of air-dried stock is recommended to be conducted at a humidity corresponding to an equilibrium moisture content 2 or 3 per cent higher than the seasoned condition of the surface. The further statement is made that steaming treatments for wood should not approach 100 per cent humidity unless the wood contains 18 per cent or more of moisture. In the discussion of casehardening and relief by steaming, compression set is brought in as an important factor. It is stated that "steaming is often unessential, and it is far better not to steam air-dried lumber than to give the treatment improperly."

Although kiln trucks are usually provided with roller-bearing wheels, kiln loads are rather heavy to push about by hand. Where the movement is all in one direction, as through a progressive kiln, some incline can be given the track and a track stop used to limit the movement. Mechanical power is useful and is usually installed

in connection with the transfer. In Figure 54 is shown a motor truck coupled to the transfer car on which the load is standing, capable of moving the transfer and of moving loads on and off it by the use of a cable led from the motor winding drum. Figure 58 shows a somewhat better view of this arrangement.

KILN-DRYING OF SAWMILL PRODUCT OTHER THAN LUMBER

About the only forms of sawmill product beside lumber to be seasoned in dry kilns are shingles and small dimension, which are produced only by a limited number of mills in connection with lumber, and which are often manufactured at plants which have no longlumber product.

The kiln-drying of small dimension can best be accomplished in compartment kilns with a very mild operating schedule, though material which runs fairly even in drying quality can be handled through a good progressive kiln equipped for maintenance of a relatively high humidity. Kiln-drying of these small sizes can not be hurried as the drying must be very slow and even in order to avoid damage by checking. In larger-dimension sizes in certain woods the process becomes very slow indeed; oak wagon bolsters sometimes require as much as six months in the kiln under the extremely slow schedule required for such stock. Good air circulation, moderate temperatures, and high humidities are the usual requirements; softwood dimension can often be handled by a schedule more severe than most hardwoods would tolerate. Small dimension is kilnpiled on the usual foundation bunks, with the addition perhaps of a plank foundation; but short and irregular forms, as, for instance, rough-shaped last blocks, may be piled in slatted bins or boxes on the bunks. Wherever possible the regular tier piling is used, so that each piece will be held true and straight by the weight of the pile

above.

THE KILN-DRYING OF SHINGLES

Shingles are sorted and bundled green from the saw, and the bundles are then stacked upon kiln bunks, usually with center openings in the load to promote air circulation, and kiln-dried, mostly in progressive kilns. The bunks are piled to usual width of about 6 feet and of the same height, and to whatever length the foundation planks will carry. A kiln load will contain 170 to 300 bundles. The moisture dries out of these bundles rather slowly because not much air circulation takes place inside the bundles. The dry kilns used for shingles rarely have steam-spray apparatus, and whatever humidity is carried is derived from the moisture in the shingles. Shingles dry with practically no development of the usual kiln defects such as warp and checking, but if dried too severely or at too high temperatures, the texture of the wood is injured and the shingles become brash and brittle.

Another kiln-drying defect peculiar to shingles is the development of collapse in certain of the shingles in a bundle. These are always from very wet or "sinker" stock, which under the high kiln heat becomes so soft that the wood cells flatten together. Where wood subject to this defect is being sawed collapse can be avoided by using

29055°-29-8

temperatures below 140° F. until the shingles are dried down to fiber saturation point or about 25 per cent moisture content, after which the heat may be somewhat increased; but it is rather difficult so to control a progressive kiln as to get this result. An easier way is to segregate the shingles from sinker bolts (they can easily be recog nized at the time of sawing them) and give them some air-drying in loose form before mingling them with the other shingles.

MAKING MOISTURE-CONTENT TESTS

The standard method of making moisture-test determinations requires an accurate scale with a capacity of 2 to 4 ounces and a suitable means of drying the sample to "oven dry" condition, by which is understood drying them at a temperature approximating the boiling point of water (212° F.) until repeated weighings show no further loss of weight. The Forest Products Laboratory is authority for the statement that such drying will reduce the moisture content in the sample to about 0.1 per cent of the dry weight of the wood, and this small remainder of moisture is treated as negligible; the weight in that condition is taken as absolutely dry weight.

Any precision scale of suitable capacity may be used, though one with weights attached is usually preferred because of the convenient decimal arrangement of metric units; but a scale graduated in decimal fractions of any ordinary avoirdupois unit, such as the oùnce, is equally convenient. To produce a moisture content reading accurate to one-tenth of 1 per cent, the scale must be accurate to 1 part in 1,000. If, for instance, it has a capacity of 100 grams, it must be able to indicate differences of one-tenth gram. Such scales range in price from $20 to $50 or more. A number of special scales for the purpose are on the market.

For drying specimens small electric heaters with racks for the specimens above the heater unit are available at prices of $15 or under; but the most useful apparatus is a small electric oven of brass-bound insulating board, with an accurate thermometer and a thermostatic switch which can be set to regulate the heat in the oven and maintain it within a degree or so of the desired temperature. Such an oven can be purchased for about $35.

In making the test several representative boards are selected. As boards are usually driest near the ends, about 2 feet are sawed off the end of each test board, and then the test section is sawed across the board, one-half to 1 inch in length. (Fig. 59.) The shorter the section the more quickly it can be dried in the electric oven, but the more sensitive is it to possible change in moisture content after being cut and before being weighed; and if there is any time lag here, the 1-inch section will give more accurate results.

The section is them promptly weighed and the weight recorded on it. It is then dried in the oven until it ceases to lose weight and is again weighed to find the dry weight. If it has lost any mass between the two weighings, the difference will be increased; so before the first weighing any loose splinters or sawdust are carefully removed. The moisture content the specimen had at the first weighing and lost in the drying may now be computed by either of the following

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