14 BULLETIN' 286, U. S. DEPARTMENT OF AGRICULTURE. 



The shrinkage measurements on the steamed material with and 

 without vacuum showed less than 1 per cent decrease in volume dur- 

 ing treatment for all the species. After seasoning a shrinkage of from 

 8.4 per cent for Douglas fir to 10.6 per cent for longleaf pine was re- 

 corded. Steaming and vacuum followed by creosote showed a some- 

 what higher shrinkage for Douglas fir than for the pines, both in the 

 unseasoned and air-dry pieces. The creasote bath had little influ- 

 ence on the shrinkage, the reduction after seasoning corresponding 

 closely to the shrinkage of untreated pieces. The pressure treatment 

 following the creosote bath showed a somewhat higher shrinkage for 

 Douglas fir than for longleaf or shortleaf . 



While the weakening in the Douglas fir stringers is not explained 

 by the series of special tests, they indicate that the trouble has to do 

 with stresses in the full-sized stringers, probably caused by rapid and 

 unequal shrinkage during the process. A further series of tests is 

 now under way on 8-foot stringers 8 by 16 inches in section treated at 

 the Forest Products Laboratory, from which results that bear more 

 directly on the problem are expected. 



DEDUCTIONS. 



(1) Timber may be very materially weakened by preservative 

 processes. 



(2) Creosote in itself aoes not appear to weaken timber. 



(3) A preservative process which will seriously injure one timber 

 may have little or no effect on the strength of another. 



(4) A comparison of the effect of a preservative process on the 

 strength of different species should not be made, unless it is the com- 

 mon or best adapted process for all the species compared. 



(5) The same treatment given to a timber of a particular species 

 may have a different effect upon different pieces of that species, 

 depending upon the form of the timber used, its size, and its condition 

 when treated. 





