456 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1953 



the soil-block tests at Madison^^ must be used to secure gradient reten- 

 tions down to and below the threshold level. 



The Distribution of the Preservative in the Block 



The opinion has been expressed by a number of investigators^^' ^^* ^^' ^°° 

 that it is difficult to secure uniform distribution of the preservative in 

 the test block, and that evaporation would cause concentration of certain 

 preservatives on the block surface, where the toxic material might influ- 

 ence the behavior of the blocks in the test cultures. If such a concentra- 

 tion occurs the result would be to fix the threshold at lower over-all 

 treatment retention than it would be if the preservative were not con- 

 centrated, for example, at the transverse faces. One is likely to agree that 

 uniform distribution of some creosotes might be difficult without the use 

 of a diluent, particularly in the low retention groups. Rhodes et af ^ were 

 apparently satisfied that they got a fairly good distribution with a 16.7- 

 pound retention. Experience at Madison and at Murray Hill indicates 

 that the blocks are saturated with the toluene-creosote solution or with 

 the toluene-pen ta-petroleum solution; and this confirms the ideas of 

 Schulze, Theden and Starfinger^ ^^^ on the way oven-dried test blocks 

 take up the treating solutions. 



Data on the distribution of residual creosote in weathered blocks is 

 presented in Table XXX, and discussed in the section on weathering. 

 Preliminary assay of blocks treated with pen ta-petroleum, (a) just after 

 treatment, (b) after weathering and (c) after testing show that the 

 pentachlorophenol concentration is slightly lower at the transverse faces 

 than it is in the middle section of the blocks. The same seems to hold true 

 for the copper metal in blocks treated with copper naphthenate in 

 toluene-petroleum. 



It should be pointed out here that preservatives are rather fortunately 

 distributed in treated wood in such a way that the outer fibers or annual 

 rings normally contain much higher concentrations of the toxic material 

 than is found in the inner fibers. (See Tables XV and XVII and Fig. 29). 



Heat Sterilization of the Treated Blocks 



In pure culture test experiments some form of sterilization must be 

 used to avoid contamination by other organisms than the test fungi. 

 Most frequently such sterilization, for the minimum necessary time, is 

 accomplished by flashing the treated block through a hot flame or by 

 steaming at 100°C and atmospheric pressure. Either of these procedures 



