EVALUATION OF WOOD PRESERVATIVES 141 



evaporation, of the preservative. The feeder blocks and the %-inch test 

 blocks are usually made at the same time, from parts of the same boards. 

 The blocks are kept clean, and reserve stocks are carefully stored in a 

 dry room. The blocks in storage reach an approximate moisture equilib- 

 rium of 6 to 7 per cent, on an oven-dry weight basis. 



Test Block Selection for Density 



Random samples of the blocks are weighed and segregated into groups 

 at 0.1 -gram intervals, 4.10 to 4.19 grams and 4.20 to 4.29 grams, for 

 example. Blocks of practically equal weight can be chosen for the com- 

 parison within any given series of different concentrations of a preserva- 

 tive. The weighed groups of blocks are kept in convenient lot sizes in a 

 dry place. Since the blocks are accurately cut the segregation by weight 

 amounts to a segregation by density. 



It has not been found necessary or practicable to separate the blocks 

 into groups mth the same numbers of annual rings, although in some 

 instances an approximation to this ideal has been attempted. Further- 

 more, it has not been found practicable to separate the blocks on the 

 basis of the direction in which the rings run across their transverse faces. 

 From experience to date it does not appear that either ring direction or 

 ring count has any material effect upon the behavior of the blocks in 

 the culture as far as determination of preservative thresholds are con- 

 cerned; but experiments are under way at Madison to determine the 

 effect of density on the relative degree of decay. Inasmuch as all of the 

 blocks are placed in culture with the transverse surface down, so that 

 alternate spring- and summerwood layers are exposed directly to the 

 test organism, the latter can enter either springwood or summerwood 

 in accordance with its ability to resist the concentration of the preserva- 

 tive present in these two parts of the annual ring. 



Average Block Volume 



The average volume of the oven-dried blocks, determined from ran- 

 dom samples by a mercury displacement technique, was found to be 

 6.484 cc, with a standard deviation of 0.0831. This represents a coeffi- 

 cient of variability of 1.28 per cent. The minimum-maximum range of 

 volumes ran from 5.93 cc to 6.87 cc. These extreme deviations are nor- 

 mally detected in handling the blocks and both high and low volume 

 blocks are rejected. The variation in density and volume of the test 

 blocks will be discussed separately in the paragraphs dealing with the 

 treatment of the blocks. 



