EVALUATION OF WOOD PRESERVATIVES 427 



Shortening the Bioassay Test 490 



Toughness or Impact Tests for Determining Preservative Effectiveness. 491 



Other Accelerated Bioassay Tests 493 



Other Observations 494 



Conclusions 497 



Acknowledgments ■ 498 



Bibliography 499 



EVALUATION BY TREATED %-INCH SOUTHERN PINE SAPWOOD STAKES IN 

 TEST PLOTS 



Rating the Condition of the Stakes 



One of the general and unavoidable difficulties in experiments involv- 

 ing exposure of small specimens in test plots is arriving at a measure of 

 the inspector's judgment of the condition of the individual specimens at 

 each inspection period. Bell Laboratories' investigators have used a sys- 

 tem of numbers beginning with 10 as the highest, and running down in 

 single steps to 0, to define the various gradations of destruction shown in 

 the specimens as they pass from perfectly sound to the state of "failed" 

 units. This system has been considered by some as slightly cumbersome; 

 but it is a truly effective method of depreciation rating in a continuous 

 series of inspections. In such a series any minor errors of judgment in 

 one season can be corrected in the next. Slow depreciation can be recorded 

 in the upper ratings until progressive destruction becomes clearly evi- 

 dent. Most observers of test plot experiments on treated wood specimens 

 use a series of five numbers for five condition categories, about as follows : 



10.0 Sound — no decay. 

 7.5 Surface soft — suspicious of decay. 

 5.0 Shght — positive decay. 

 2.5 Severe — deep decay. 

 0.0 Failed — almost complete loss of strength. 



Some, like Rennerfelt^^ for example, use this system upside down, with 

 for no decay, and 10 for failure. The writer has proposed^^ the use of a 

 new 5-number depreciation system, with the same definitions, based on 

 the logarithms of the above 5 figures, and rounded off to 10, 9, 7, 4 and 

 respectively. This simplifies the Bell Telephone Laboratories' 11 divi- 

 sion, 10-0, system while retaining the advantage of slow depreciation at 

 first; and at the same time it avoids the sudden, and in the writer's 

 opinion, unjustified drop from 10 to 7.5 in the arithmetic series for 

 suspicious-of -decay specimens. In the following presentation and inter- 



