the average air in contact with the lumber generally. If the air inside the kiln is generally warmer than where the reading is taken, it will be apt to be also generally drier, as heating air reduces its relative moisture percentage; and if the readings are affected by any such variation, they will result in applying more heat and less humidity to the lumber in the kiln than is intended by the schedule.

FINAL STEAMING OF KILN-DRIED STOCK

Lumber coming out of a dry kiln is not in good condition to send at once through planing machines. Much of it is in a condition of superdryness on the surface, and there may be unequalized stresses in it which produce an effect of brittleness; if a slightly cupped board is flattened out under the planer rolls, the mechanical stress of bending adds to the internal stress and puts it beyond the fiber rupture point, and the board splits. Any board dried below 5 per cent moisture content may have this inherent brittleness even apart from internal stresses, and boards do split under the planer rolls which have an average moisture content considerably above that point. A good deal of brittleness and harsh working of the surface under the planer knives will be found in lumber fresh from the kiln even though the average moisture content is above 10 per cent, and largely because the surface dryness is considerably below the average of the piece throughout.

If such lumber is rough piled in a shed and allowed to lie for a week or 10 days before being sent to the planer, the surface reabsorbs some moisture by diffusion from the interior, and to a small degree by exposure to the air, even if close piled; and the internal stresses also disappear to a considerable degree, so that after such tempering or annealing storage the lumber will develop materially less planer degrade in the form of end checks and splits, loosened knots, etc. Such storage produces an added cost, which in some tests has exceeded the value of the benefits produced; and in many cases the facilities do not exist for applying it to the bulk of the kiln-dried product.

The questionnaire replies show that storage of lumber after kilndrying and before dressing or working is given for 5 days or under at 16 mills; for 15 to 60 days at 2 mills; for 30 days at 1 mill, and for 60 days or more at 5 mills. A final steaming treatment is reported by 5 mills, while 7 others use it on a portion of the product. With compartment kilns practically the same results can be produced with little cost by a final steaming of the lumber just before taking it from the kiln. As compartment kilns are generally equipped with steam sprays, this merely means a steaming of the entire kiln before releasing the charge. In a progressive kiln it would be necessary to limit such a steaming to the dry end; and though this could be done by providing steam sprays at that point and cutting of the space from the rest of the kiln by use of a drop curtain, progressive kilns are not usually equipped with these accessories. It is, however, quite common to provide steam sprays in the green end of progressive kilns for an initial spraying of the green stock when placed in the kiln, and sometimes a drop curtain is provided for shutting off this end of the kiln during such steaming,

which serves to heat the lumber thoroughly and start it to drying in good shape.

The final steaming is usually made at a humidity of 85 per cent or less, and with a somewhat advanced temperature. Koehler and Thelen (The Kiln-drying of Lumber, p. 180) recommend temperatures of 165° to 185° F. and a humidity of 100 per cent for one-half to three hours if the center of the stock contains over 17 per cent moisture; unless the stock is checked, when a humidity of 85 to 95 per cent is recommended, used for 10 to 20 hours. If the stock contains 15 to 17 per cent moisture, a humidity of 75 to 85 per cent should be used for 20 to 30 hours; or if the stock is below 15 per cent moisture, humidities of 60 to 70 per cent should be used for a like period. The purpose of the steaming is to heat the stock and render it more plastic and capable of adjustment of casehardening stresses, and to promote diffusion of moisture in the wood, thus equalizing its moisture content and also imparting somewhat additional moisture to the surface. It is not desirable to add too much surface moisture, as this steaming is the final process and is not followed by further kiln-drying. The process must also be modified somewhat according to the final average moisture content desired in the product. The eventual moisture content desired governs the degree of kiln-drying imparted to the stock before the final steaming process. The final steaming treatment is more important and more necessary with some species of wood than with others, and it is most commonly used in factory drying of hardwood, especially where they are to go into the cutting room directly from the kilns. In many cases it would be profitable to use this treatment on kiln-dried softwoods before sending them to the planer. It should be given careful experimental tests by individual manufacturers in all cases where there are planer degrades that might be reduced by using it, if the character of kilns are such as to be adaptable to it.

H. L. Henderson (Dry-Kiln Practice, p. 65) recommends use of a high humidity, 70 to 85 per cent, for 24 hours in place of a final steaming, for balancing up differences of moisture content in the product.

Only the more recent available kiln-drying literature stresses the importance of final steaming in connection with kiln-drying; though earlier issues mention it as occasionally desirable for relief of casehardening where it is found to exist. In The Kiln-Drying of Longleaf Pine, a report of Forest Products Laboratory tests in cooperation with the Southern Pine Association and the Kaul Lumber Co. at the latter's plant at Kaulton, Ala., by Laurence V. Teesdale (Southern Lumberman, December 17, 1927, pp. 161 to 170), five schedules used in the tests are printed. The final drying stages of these schedules ranged from 200° to 210° F., and humidities ranged from 39 to 51 per cent; and each schedule ended with a final steaming for five hours at 190° dry bulb and 182° wet bulb, or a relative humidity of 83 per cent; the schedules being for stock ranging from 1 to 2 inches in thickness. The following quotation from the report is interesting as showing why final steaming treatments sometimes fail of the intended result:

Schedules 1, 2, and 3 differ from the original schedules (found already in use by the kiln operator) only in the method of final conditioning or steaming just before the stock is unloaded. The original schedule provided for a con

ditioning treatment of three hours and to be obtained by retaining the lowest wet-bulb temperature used and lowering the dry-bulb temperature to 5° above the wet bulb. For example, the last three hours of schedule 2 would be: Dry bulb 175°; wet bulb, 170°; humidity 89 per cent. Actually this condition was not obtained. The operator maintained a constant wet-bulb temperature and set the thermostatic controller for a temperature 5° above the wet bulb. This, of course, cut off the steam supply to the heating coils, but the lumber and the kiln walls contained so much heat that the dry-bulb temperature did not actually drop to the line indicated by the drying schedule within the three hours. The temperature would drop to about 190° in one hour and to 185° in three hours. Further, the increase in humidity caused by the drop in temperature did not affect the stock in the manner intended, which was to add a little moisture to the surface to relieve casehardening.

Theoretically, and to get quickest results, it is proper to raise both wetbulb and dry-bulb_temperature above the highest temperatures used in the drying schedule. For example, if the final temperatures specified were dry bulb 200° and wet bulb 170°, the steaming schedule would be, say, dry bulb 210° and wet bulb 200°. However, it would be impractical to obtain a wetbulb temperature of 200° in the ordinary type of kiln. A practical compromise was obtained which seemed very satisfactory. As the temperature dropped quickly to 190°, the dry-bulb controller was set for that temperature and the wet-bulb controller set up to 185°. The sprays now operated, and the humidity actually increased from the moment of making the change at the controller. Even though the wet-bulb temperature seldom exceeded 182°, the increased humidity was beneficial to the extent that stock from runs made by this compromised schedule had less degrade than that from stock dried by the original. schedule where it was not steamed.

The last statement refers to the fact that, while steaming for three hours was intended by the original schedule, it was not actually accomplished for the reasons above explained.

Depreciation in value per M feet of stock dried during 1926 in accordance with the company's original schedule and run direct from the kiln to the planer

Depreciation in value per M feet of stock dried during 1926 in accordance with the Forest Products Laboratory's schedule and run direct from the kiln to the planer

The kiln-drying schedule used was practically the same for both these tables, aside from the final steaming, and produced a very small degrade as indicated in the first table, correspondingly reduc ing the saving in degrade shown by the second table as a result of the

final steaming. Such final steaming following ordinary kiln-drying could be expected to produce a larger degrade saving.

As far as is known the present literature of kiln-drying does not record any other comparative tests of degrades with and without final steaming.

SAWMILL SHED STORAGE OF LUMBER

The equipment at a considerable number of large manufacturing plants includes lumber storage sheds for the storage of both rough lumber and dressed or worked product. The rough lumber may be air-dried stock repiled in the shed after drying in yard piles, or it may be kiln-dried stock. It may eventually be shipped in rough form, but usually it finally gets sent through the planer for dressing or working to pattern. It is, of course, advantageous in a sawmill planing department to keep the machines working on long runs without frequent changes of set-up, and at any given time the kilndried rough product will include some stock not suited to the purposes for which the machines are being run at the time; such product will be temporarily stored, and the kind of strips or other material then being run will be partly sorted out of the dry-kiln current production and partly drawn from reserve accumulations in the rough-lumber shed. There may be much shifting of stock in and out of rough storage, and this extra handling is sometimes avoided by selecting logs to supply the kind of stock on which the planing mill is operating most heavily, either on special orders or to supply broken-stock items.

It often happens in any important lumber manufacturing plant that planing-mill product does not get shipped out promptly when manufactured; a comsiderable working stock of planing-mill items must be carried, just as there must always be a well-rounded stock of yard lumber; and the tendency at the present time is to let the seasonal reserves or surpluses accumulate at the sawmill rather than in wholesale or retail lumber yards. Buyers rely upon getting prompt shipments of their orders, which puts the responsibility on the manufacturer of having the lumber on hand to fill orders whenever the seasonal rush comes on.

At large plants covered lumber storage is becoming highly specialized. Often the unit-package system is used throughout, and the storage space is all under overhead cranes which can drop a unit package on a pile anywhere and later pick it up again and carry it out to the loading platform where, because box cars are not yet available with removable tops, it must be set down for hand loading in the car. Sometimes lumber movement about the plant is by strings of trucks drawn by an electric or gasoline locomotive, or sometimes by a straddle carrier which can run over a package of lumber and quickly lift and move it to a new location.

A lumber storage shed is rarely a seasoning shed; most of its contents are at a lower stage of moisture when received than at any later time; and while air-dried stock may hold its condition well in shed storage, kiln-dried stock, either rough or dressed, tends to reabsorption of moisture, gradual in some cases, more rapid in others, depending upon conditions. Even air-dried stock housed in

fairly dry condition in the fall will tend to become heavier during the winter months, though much more slowly when bulk piled under cover than if left in outdoor piles on stickers. The reduced shipping weight and the satisfaction of customers repay the cost of the practice of some western mills in putting their dry outdoor lumber into bulk-piled covered storage in September.

Kiln-dried items calling for a limited moisture content on shipment can, of course, be carried in shed storage for a limited time before absorbing moisture beyond the limit; and while some margin can be allowed by lower original kiln-drying, there are rather narrow limits set for that. Kiln-dried stock can be carried close piled with very slow moisture pick-up during dry weather, though the ends will get more moisture than the rest of the piece.

HEATED STORAGE

Heated storage is the correct method of maintaining kiln-dried product in condition and is capable of carrying it for an indefinite length of time if the degree of heat used properly balances the average relative humidity of the unheated air. By this is meant that the unheated air, which is the only air supply for the heated storage, must be heated enough to reduce the relative humidity to the point where it is in proper balance with the desired condition of moisture in the lumber.

If, for instance, the outer temperature is 60° F. and relative hu midity of the air is around 70 per cent, heating the air to 70° in a closed lumber storage shed will lower its relative humidity to about 50 and produce good conditions for maintaining a moisture of 10 per cent in the wood. A practical method for control of heated storage conditions would be to expose in it various small moisturetest samples of the kind being stored, with their bone-dry weights recorded on them, and keep the storage at such heat as will maintain in these samples the moisture content which it is desired to hold in the lumber. The chief advantage of these small samples is their sensitivity; they show a difference in weight resulting from atmospheric changes much more quickly than would larger specimens or the lumber itself.

Heated lumber storage is in use to a limited extent at factories. and in some retail lumber establishments and by some manufacturers of flooring; it is not known to be anywhere in use by lumber manufacturers at this time. It, however, requires only a tight building supplied with a few lines of steam pipe heated by exhaust steam, and this report can confidently predict it for future development as a further very desirable refinement in the storage of kiln-dried product. In the past it has not been possible to determine just what heat was needed in heated storage under given conditions of air humidity in order to hold lumber at any given stage of moisture content, because there was no available tabulation of facts. A chart of the Forest Products Laboratory has been published on various occasions, with curves giving the relation between relative humidity in air and equilibrium moisture content in wood, at temperatures of 70 and 212° F., produced by actual tests, and also midway between these two curves, or 141° F., produced by interpolation; but the