THE ARTIFICIAL SEASONING OF TIMBER 417 



wetting the surface with hot vapour, and then resuming the normal 

 drying process, but the surface wood will be found to be deteriorated, 

 and a microscope shows that the cells are deformed and broken. Case- 

 hardening is a fault that is due to careless and hurried working, occurring 

 chiefly where the timber is being partially dried to reduce freight. It 

 need not occur if the process is carefully conducted. 



The whole success of the drying depends on intelligent working, and 

 the proper use of technical data. The difference between rule of thumb 

 and scientific working shows in the quality of the dried timber, and lack 

 of proper attention is false economy. 



Tests. — It has not formerly been customary for those who use timber 

 to experiment upon it, to determine either its strength as a species in 

 general, or the individual qualities of some particular parcel. Yet data 

 of much practical advantage can be obtained by such experiments, 

 provided that a sufficiently large mmiber of tests are made, and that the 

 specimens chosen are thoroughly representative of the species or parcel 

 under consideration. It is most important, however, to use tests which 

 represent as nearly as possible the actual working conditions under which 

 the timber is to be emploj^ed. It is, for instance, of httle use to make 

 compression tests only on timber which is to be used for aeroplane 

 propellers, where the chief stress vrill be in bending, or bending tests in 

 wood for wheel spokes, where resistance to compression is needed. In 

 aU cases, however, tests for dryness are essential, particularly where 

 artificial seasoning processes are employed. 



There are two kinds of tests : laboratory tests and working tests, 

 which take place either in the factory or at the dryer. The former are 

 divided into two categories ; botanical, which deal with the structure of 

 the wood, and mechanical, in which the material is subjected to trials 

 such as endurance in compression, bending, torsion, tension, the deter- 

 mination of Young's modulus, and the modulus of rigidity. In connection 

 with all these tests, the water content of the specimen is assayed, and 

 the density, or weight per cubic foot, is calculated. 



Working tests are more simple, the principal object being to determine 

 the percentage of moisture in the wood, in order to ascertain, in the case 

 of green timber, what degree of drying is necessary, or in the case of 

 partly seasoned wood, to find out the exact stage which has been reached 

 in that process. Simple breaking tests can also be made on small 

 specimens, which will show approximately whether the material is capable 

 of the usual breaking load. Examination of the fracture thus made 

 is of great use in determining whether the wood is tough or brittle. 



The alternative to making moisture determinations is the exercise of 

 private judgment, which is notoriously faUible. It is therefore preferable 



2E 



