EVALUATION OF WOOD PRESERVATIVES 459 



residue above 3o5°C. The blocks were very heavily treated by a hot and 

 cold soaking process with the straight creosote and each of the four 

 fractions to the following average retentions, respectively: 52.2, 45.6, 

 50.3, 50.2 and 23.7 pounds per cubic foot. These high retentions place 

 the experiments out of line with most of the others cited in this paper; 

 but the South African tests are unique in that they supply evidence on 

 the rate of creosote losses from such high retentions. 



The relative order of losses of the materials, with the one having the 

 highest losses first, was the same for the blocks that were hung on wire 

 outdoors and for the open Petri dish samples, namely; Fraction I, II, 

 whole creosote. Fraction III, and the residue. The latter actually showed 

 no significant loss in either block or dish tests, and in the blocks there was 

 a slight increase, possibly referable to oxidation, of a maximum of 2.2 

 per cent at the end of the three-year period. This gain was gone by the 

 end of the 53^-year test. Fraction III was lost more rapidly from the 

 blocks than from the Petri dishes; the reverse was markedly evident for 

 Fractions I and II ; whereas the pattern for the whole creosote was very 

 similar after the first year. The rounded figures for losses from blocks 

 treated with the whole creosote, at the end of 1, 2, 3 and 53^ years out- 

 door exposure, were 36, 42, 44 and 47 per cent ; and the losses from the 

 open dishes for the same periods were 34, 40, 43 and 48 per cent respec- 

 tively. One may conclude from these tests and conditions that a loss level 

 of about 50 per cent would have been reached in about six years in blocks 

 treated to a reported 50-pound per cubic foot retention with an undiluted 

 creosote. 



^lost of the laboratory toxicity tests on fractions have been run by the 

 agar or the agar-block method. ^^ ^^^' ^^^' "' ^^^' ^^ The results obtained by 

 Schulze and Becker^^^ are cited by Mayfield, who includes as his Fig. 1 a 

 copy of the summary curves prepared by the Berlin investigators. The 

 interested reader can profitably use the rather full excerpts of tabulated 

 results of other investigators given in Mayfield 's paper as an introduction 

 to the difficulties of testing creosotes and of interpreting the results of 

 such tests. Some of the main controversial points are brought out by 

 Peters, Krieg and Pflug in 1937^^ who challenge the results of Petri dish 

 agar toxicity tests with results of the German agar-block tests — one of 

 the first such broad comparisons to be made. Broekhuizen '' pubHshed 

 his findings the same year in a comprehensive paper covering agar-block 

 tests on creosotes and creosote relatives and creosote fractions. He dis- 

 cusses his results with, different preservatives in relation to their toxic 

 properties, their protective and preservative qualities, and the perma- 

 nence of such quahties, and the bearing of these qualities on practical 



