EVALUATION OF WOOD PRESERVATIVES 



471 



In their roof exposure, outdoor weathering tests Duncan and Rich- 

 ards^^' ^^ have regularly found losses of creosote, by weight, in %-inch 

 blocks treated with creosotes having residues above 355°C of 20-30 per 

 cent, to run in the neighborhood of 45-50 per cent. All treatments were 

 made by a full-cell vacuum process with toluene creosote solutions. The 

 over-all exposure period consisted of three stages,^^ a three-week con- 

 ditioning period in a constant humidity and temperature room at 30 per 

 cent relative humidity and 80°F, a sixty-day outdoor exposure on a rack 

 on the roof, and a three-week reconditioning to approximately constant 

 weight in the same humidity room. The losses were figured from the 

 original conditioned weight of the blocks, the creosote retention at treat- 

 ment, and the calculated amount of creosote remaining as indicated by 

 the weight of the weathered and reconditioned blocks. A condensed and 

 simplified summary for two creosotes and two block shapes^^' ^^ is shown 

 in Table XXIX. 



As might be expected under identical weathering conditions, the losses 

 were higher in the case of the % x % x 13^ inch blocks — with twice 

 as much end grain exposed as the %" drilled cubes — which presumably 

 resulted in an accelerated longitudinal evaporation. In other words, a 

 given percentage loss of creosote is arrived at sooner in the case of the 

 block with greater transverse surface area. The loss increase amounts to 

 about 7 per cent at the 8 to 10-pound retention level. This loss is about 

 twice as great as it would be if calculated on the basis of the increased 



Table XXIX — • Creosote Losses from Longleaf 

 Southern Pine Sapwood Blocks 

 Outdoor weathering tests — Madison (Duncan and Richards) data. 



* Retention by full-cell treatment under vacuum with toluene-creosote solu- 

 tions. 



t %-inch cubes. 



XH x^xl-}i inches. 



1, 



