tion typical of the early stages of drying. D illustrates typical casehardening. Resawing has relieved the tension in the interior and the compression in the exterior and shows both areas in compression. E shows the result of a too severe conditioning treatment which has reversed the stress in the outer layers. Unless this treatment is too severe the lumber will eventually assume the condition A. Stress sections must be thoroughly room dried before they show the condition the stock will finally assume in service.

Moisture distribution tests. For the purpose of determining the kind of treatment which should be given casehardened stock and to determine the moisture content of different layers of stock after kiln drying it is necessary to make moisture distribution tests. These tests may be made as illustrated in Figure 29. The sections should be cut approximately the same size as those used for determining

A

B

C

D

E

FIGURE 28. This sketch shows the usual method of cutting a casehardening test section and the various ways in which the prongs immediately bend as influenced by existing stresses. A shows the appearance of a section cut from green stock in which no stresses have developed. B illustrates the appearance of a section cut from stock which is in the early stages of drying and in which shrinkage has begun in the outer layers. C shows a later condition in which interior shrinkage has balanced that in the outer layers. D shows typical casehardening caused by further interior shrinkage which is resisted by the "set" condition of the outer layers. E illustrates the results of a too severe conditioning treatment which was given to relieve casehardening

casehardening and moisture content, as explained under "Moisture content tests."

Temperature, humidity, and circulation tests. Although accurate recorders are being manufactured which make it possible for the operator to calculate the relative humidity of his kiln without going inside to take readings, a frequent check on the accuracy of these instruments is advisable. It is also advisable to take readings in various parts of the kiln and in the interior of the kiln stacks.

Most instrument manufacturers are able to furnish dry-kiln thermometers which are accurately adjusted and which have a range of temperature suitable for kiln work. Many of these companies, particularly those which specialize in dry-kiln instruments, also make various types of hygrometers (a combination dry and wet bulb thermometer used for determining humidity). Homemade hygrometers, consisting of two dry-bulb thermometers fitted with protective cases and wired together, will give very good service. A wick is fastened to the bulb of one of the thermometers and a small bottle attached

for holding distilled water. Charts for the purpose of determining the relative humidity from the dry and wet bulb readings may be found in most publications on kiln drying or may be obtained from a dry-kiln manufacturer.

In making temperature and humidity tests it is necessary to place the thermometers in the kiln, close the doors, and allow sufficient time for the instruments to become adjusted to kiln conditions. When the thermometers have reached equilibrium with the drying conditions, the readings are taken with the aid of a flash-light while the kiln doors remain closed.

Uneven drying is usually caused by poor circulation. For this and other reasons it is advisable for the kiln operator to know the rate

FIGURE 29.-Methods of making moisture distribution tests on small dimension stock. A shows a core and a surface test, whi'e B shows a core, an intermediate, and a surface test

and direction of circulation in the kiln and through the piles. The method of obtaining this information depends on the rate of circulation.

An instrument called an anemometer may be used in testing kilns having a fast circulation. The anemometer consists of a small fan, suspended in a frame, together with a device for recording the number of fan revolutions. The anemometer is placed in the desired location and allowed to operate for a definite number of minutes. Instructions are given for converting the number of fan revolutions into air movement at feet per minute and for the application of correcting factors for various rates of circulation.

Smoke is used for learning the direction of circulation in various parts of the kiln and for determining the rate of air movement in kilns having a slow circulation. The operator may ignite punk sticks, pieces of rope, etc., and learn the direction and approximate

rate of circulation by following the movement of smoke with a flash light. The kiln door should be closed while the tests are being made, and the kiln should be in a normal operating condition.

There are some disadvantages connected with the use of ignited materials, such as the fire hazard and the fact that the smoke from such material may have a higher temperature than the surrounding air and tend to rise disregarding the direction of circulation. The Forest Products Laboratory has developed a "smoke machine which has neither of these disadvantages. Briefly, this apparatus consists of two glass containers, one holding hydrochloric acid and the other ammonia. The containers are connected by glass tubes, and the operator, by blowing through these tubes, mixes the fumes from the two liquids and produces a dense white smoke.

DEGRADE RESULTING FROM KILN DRYING

The waste and degrade resulting from improper kiln drying causes an enormous waste of high-grade material. If the loss from this cause could be decreased 5 per cent, the estimated annual saving in New York State alone would amount to over $3,000,000. Degrade in kiln drying is largely due to poor kilns and equipment and to inefficiency of the kiln operator. Any one of these three causes may be responsible for poor results.

Many operators believe that ready-cut stock can not be successfully air seasoned and kiln dried and therefore claim that the only satisfactory method of manufacturing this material is to cut it directly from kiln-dried lumber or flitches. Other producers maintain that most species of wood can be successfully dried as small dimension and that the advantages of closer utilization derived from cutting dimension directly from the log or bolt more than compensates for any additional drying degrade. These different contentions are based on practical experience and have been made by men who are successful in the industry. Both methods of manufacture and drying have their advantages and disadvantages, and the successful application of either depends upon such factors as species of wood used, size of log from which material is cut, thickness of dimension being manufactured, the requirements of the consumer, type and efficiency of drying equipment, skill and experience of kiln operator, moisture content of stock at time of manufacture, and many others.

Casehardening.-Casehardening is defined as " a condition of stress caused by unequal 'set' in lumber of uniform dryness." This unequal set is caused by the drying of the outer section of a piece below the fiber saturation point and the consequent shrinkage. The interior of the piece still has a moisture content which is above the fiber saturation point and can not shrink, therefore the surface becomes set in a condition of compression. Later the interior dries down below the fiber saturation point and begins to shrink. This shrinkage is limited by the fact that the exterior is set and can not shrink. As a result the interior in turn becomes set in a condition of tension.

Many of the seasoning defects both in air seasoning and in kiln drying are caused by casehardening, and therefore an understanding of the causes of this condition is essential. It is almost impossible to dry wood without the occurrence of a slight amount of caseharden

ing at some time during the seasoning process. However, it is possible to relieve this condition during the drying process and to eliminate it entirely before the stock is removed from the kiln.

Surface checking and honeycombing.-The tendency of stock to surface check depends on the severity of the drying conditions and also the strength of the wood in tension across the grain. If the strength of the wood is not sufficient to resist the severe surface tension caused by shrinkage during the early stages of drying, surface checking will occur. Later in the drying process the tension in the interior may cause interior checking or "honeycombing." Because radial shrinkage is less than tangential, and because a point of weakness occurs where the medullary rays cross the wood fibers or tracheids, surface and honeycomb checks usually run radially in the stock. End checking and splitting.-The ends of boards and dimension are most subject to checking and splitting. The reason for this is that most of the wood elements, such as pores, fibers, and tracheids, run longitudinally. Wood dries much faster from the ends than through the other surfaces, because the moisture is conducted to the ends through these elements.

Warping and twisting.-Warping and twisting may result from any one of several causes. Spiral or interlocked grain, difference in shrinkage between heartwood and sapwood, and uneven drying may all contribute to cause this defect.

Cupping. Cupping is a natural reaction of all plain sawed stock, due to the fact that radial shrinkage is less than tangential. The normal amount of cupping varies with the species, density, location of the piece in the log, and other factors. Cupping is always on that side of the piece which is opposite the heart of the tree.

Diamonding.-Diamonding occurs in kiln-drying squares of many of the refractory species and seems impossible to prevent, since it is a natural shrinkage reaction of the wood due to the fact that tangential shrinkage is greater than radial shrinkage. This condition occurs. most frequently in drying stock green from the saw. Some operators report that they have more difficulty with this defect in drying small squares than in drying 3 or 4 inch stock-possibly because the larger squares are usually cut from large logs. The location of the square in the log also influences the occurrence of this defect. Diamonding is most severe when the annual rings run diagonally in the cross section of the square.

Collapse. In drying some species, such as red cedar, red gum, swamp oak, and redwood, green from the saw, some boards have a ridged, corrugated appearance caused by the collapse of several rows of cells in the interior of the piece. This is probably caused by water being removed from the cells without being replaced by air.

THE PREVENTION OF DRYING DEGRADE

Prevention and elimination of casehardening.-Casehardening is not necessarily a serious defect during the drying process, although it may lead to undesirable results. However, casehardening in the finished stock may be serious. For example, casehardened material which is resawed will cup; if such stock is surfaced on one side only, or if the material is surfaced more heavily on one side than on the other, the same condition will result. Therefore a final relief of casehardening is often essential.

To relieve or eliminate casehardening it is necessary to give the stock steaming or high humidity treatments with increased temperature. The object of these treatments is to soften the surface layers so that they will yield to the internal tension, thus bringing the whole piece back into adjustment. The absorption of moisture by the surface layers also speeds up drying by reestablishing moisture transfusion contact between the surface and the interior.

The usual treatment given casehardened stock is the application of a high humidity at less than saturation. If the core of the stock is between 15 and 18 per cent moisture content, temperatures of 160° to 185° and humidities of 75 to 90 per cent may be used. When the core of the stock is below 15 per cent, the same temperatures may be used with humidities ranging from 65 to 75 per cent. When the core of the stock is above 18 per cent and there are no evidences of surface checking, steaming treatments may be given, with temperatures ranging from 160° to 185° and 100 per cent humidity. High humidity or conditioning treatments are given for from 10 to 30 hours, depending upon the severity of the stresses present. Steaming treatments are given for from one-half to three hours. Short, mild treatments given at frequent intervals are recommended rather than long, severe treatments given less frequently.

Prevention of surface checking.-Surface tension is the first de.velopment of stress found in the drying of green stock. This is a normal condition and unless too severe will not produce harmful results. However, severe tension is liable to produce surface checks. Extreme surface tension is caused by the establishment of a moisture gradient curve that is too steep. In other words, the evaporation from the surface has taken place more rapidly than the moisture can transfuse from the center.

The remedy for severe surface tension is the application of a higher humidity, which slows the rate of evaporation from the surface and decreases the steepness of the moisture gradient curve. In many cases it is also advisable to give air-seasoned stock a high humidity treatment before kiln drying.

Prevention of end checking.-End checking is caused by the too rapid drying of the ends of the stock, and the prevention of this defect entails the treatment of the material to slow down the drying rate at and near the ends. In cases where end checking is extreme an end coating with material such as 213° pitch is recommended.

Prevention of warping, twisting, and cupping.-Careful piling will do much to eliminate warping, twisting, and cupping. Placing stickers as near as possible flush with the ends of the stock, close stickering (8 to 12 inches), slower drying, and placing weights on top of the stacks will all help to eliminate these defects.

Prevention of diamonding.-Since this defect is a result of natural shrinkage stresses, there is no method of absolute prevention. Stock which tends to diamond may be cut over-size to allow for the distortion which occurs later.

Prevention of collapse.-Collapse is important in the drying of only a few species. Since this defect is caused by abnormal shrinkage in drying stock with a high moisture content, the best method of prevention is operation at a decreased temperature or air dryin before kiln drying,