﻿32 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE. 



END COATINGS. 



When tho moisture sections have been weighed and placed in the 

 oven the samples should be end coated. It has already been shown 

 that wood dries out much faster from the end grain, and if the end 

 sin faces were not protected in some suitable manner the samples 

 would dry out from the ends, and since they are comparatively 

 short they would soon become drier than the rest of the stock and 

 would not represent an average. 



A number of materials are being used to prevent or retard end 

 drying under various conditions, and while some are excellent for the 

 low temperatures encountered in air seasoning, comparatively few 

 have proved suitable for kiln work. The most satisfactory end coat- 

 ing so far tested is a 213° coal-tar pitch. There are probably other 

 pitches, asphalts, and similar materials which would serve the pur- 

 pose, but additional research will be required to determine the rela- 

 tive efficiency of the many grades available. Materials with very 

 high melting points are barred, since they can not be applied to the 

 wood, and those with low melting points are unsuitable because they 

 would flow off at the temperatures used in the kiln. Rosin and lamp- 

 black mixtures have been used with success, but their efficiency is 

 not so great as that of coal-tar pitch, and their cost is considerably 

 more. No coatings, liquid at ordinary temperatures, have proved 

 so satisfactory as the hot dips. 



The ends of the moisture samples are dipped into the melted pitch 

 to a depth of about one-half to three- fourths inch. The pitch should 

 be hot enough to produce a smooth coating approximately one-six- 

 teenth inch thick, but not hot enough to cause any of the moisture in 

 the wood to flash into steam and blow holes in the coating. A very 

 thin coating is undesirable on account of lack of imperviousness, and 

 a thick one is wasteful of pitch and at the same time causes an error 

 in the current moisture determinations. As soon as a sample lias 

 been dipped it should be weighed immediately and the weight re- 

 corded. The average moisture content of the two moisture sections 

 is assumed to be the moisture content of the sample. The oven-dry 

 weight of the sample is found by multiplying the original weight of 

 the sample by 100 and dividing by 100 plus the moisture content 

 expressed in per cent. Thus, assume that the sample originally 

 weighs 3.75 pounds and that the two moisture sections average 25 

 per cent moisture. Then the oven-dry weight of the sample equals 



ion J- 9" ' 01 ^ pounds. If the moisture content Avere expressed as 



a decimal instead of in the form of percentage, this formula would 



3 75 

 be still simpler; oven-dry weight equals ^^-=3. The kiln samples 



are placed in convenient parts of the various truck loads or piles of 

 lumber and allowed to dry with the rest of the stock. 



Whenever a current weight is taken, the current moisture content 

 is always calculated on the basis of the calculated oven-dry weight, 

 just as if the sample were a regular moisture section, and the mois- 

 ture content of the load is assumed to be the average of the moisture 

 contents of the various samples. If the work has been accurately 





