April 



1922 



HARDWOOD RECORD 



25 



be remembered that the per cent of shrinkage shown is the total 

 from green to oven dry. It is a well known fact that oven dry lum- 

 ber is unfit for commercial use, perhaps as much so as lumber con- 

 taining too high a moisture content. 



Moisture is contained in the tree, both in the actual fibrous struc- 

 ture and in the open spaces or pores between the fibres. The former 

 is known as "fibre moisture" and the latter as "free moisture." 



The total shrinkage from green to oven dry generally takes place 

 after the free moisture has been eliminated and during the process 

 of reducing the fibre moisture. The fibre moisture point is known 

 to be between 25 and 30 per cent, although this does not apply to 

 all species. Some species contain more than 200 per cent (by dry 

 weight of the wood) in their green state. The more common 

 would be around 100 per cent moisture in the green. 



For example any species which contain 100 per cent moisture in 

 the green, and which have a known fibre saturation of 25 per cent, 

 could be reduced in moisture either by air drying or kiln drying 

 to the extent of 75 per cent without shrinkage. If the total shrink- 

 age by volume were 10 per cent, as may be determined from avail- 

 able tables, then this entire amount would shrink in reducing the 

 balance of the moisture. 



Must Leave Some Moisture In 



Since a certain per cent of moisture must be left in the lumber 

 to retain it in near balance with average atmospheric E. M. C. 

 (equilibrium moisture content) conditions, (commonly 5 per cent 

 moisture, although not necessarily correct) the total shrinkage by 

 volume in the perfectly dried board would be four-fifths of 10 per 

 cent or 8 per cent. 



From the shrinkage table it will be seen that Tennessee chestnut, 

 for example, shrinks 3.4 per cent radially and 6.8 per cent tan- 

 gentially or plain and quartered sawn, respectively. As only four- 

 fifths of the total shrinkage would occur in reducing the moisture 

 content to the proper point this would leave 2.72 per cent in the 

 radial direction and 5.44 per cent in the tangential direction. 



It is, therefore, obviously impossible to save the same amount in 

 thickness as in width, because whatever the shrinkage in the radial 

 (or plain sawn) direction, it would be approximately twice the 

 amount in the tangential or quarter sawn direction, and the total 

 shrinkage to the correct moisture content would be only 8.18 per 

 cent, -none of which could he reduced without injury to tlie lumber. 



Best .Drying Shrinks Most 



Of most importance is the fact, well known to those who have 

 made a study of the subject, that the best possible drying prac- 

 tice, unfortunately produces the greatest amount of shrinkage. 



There is one certain way of reducing the per cent of shrinkage 

 on the outer dimensions of the boards and that is — case-hardening 

 and honey-combing. In this case the outer surfaces are reduced 

 to a dry state too rapidly, moisture transfusion from the inner 

 fibres does not keep pace with the rate of moisture evaporation 

 from the surfaces and consequently the outer surfaces set rigidly. 

 When the inner fibres finally have given up their moisture and 

 inevitably shrink their energy is not sufficient to shrink the hard 

 outer shell and therefore all the later shrinkage results in internal 

 ruptures; in other words, while the shrinkage of the outer surfaces 

 has been reduced the lumber has been ruined by internal shrinkage 

 or honey-combing. 



Savings in woodworking plants can certainly be effected through 

 the elimination of waste, and lumber conditioning no doubt offers 

 one of the most conspicuous possibilities. It is, however, a case 

 where a little knowledge often proves dangerous. 



In conclusion, executives are advised to write the Directors, U. S. 

 Forest Products Laboratory, before making any serious changes 

 in their lumber drying and conditioning methods. Dependable 

 advice will gladly be given without obligation and to the extent 

 of the scope of work of that organization. 



the Sturtevant fan circulatioin 



^irinciple. 



"VellowviOod, being the most abun- 

 dant indigenous woiDd, has been placed 

 foremost on the seasoning program, 

 A quantity of 212 cubic feet of falcate 

 xellowwood was received from the 

 Transkei forests. The green logs were 

 sawed into IVfi-incli boards an im- 

 mediately afterwards placed in the fan 

 kilns. The load was subjected to a 

 drying temperature of 130° F. to 175° F. 

 with humidity varying from 30 to 100 

 per cent and was steamed at intervals 

 to relieve case-hardening stresses. In 

 ]ij days the wood dried from 70 to 7 

 per cent mo stiire content. Since then 

 a further quantity of yellowwood has 

 been seasoned and no matter how the 

 hoards are cut, flat or deep, there is no 

 warping and cracking. This is most, 

 satisfactorv 



A South African Report 



Note that the green lumber was im- 

 mediately placed in the fan Kilns. 



Note also that in sixteen days it was 

 ready to ship; no tied up capital here. 



And further, "no matter how the boards 

 are cut, flat or deep, there is no warp- 

 ing and cracking." 



Catalog 282 is full of interesting informa- 

 tion and data on lumber drying; a copy will 

 be mailed you free on request. 



Slurfeva 



HIGH HUMIDITY 



,.._ PCTs^^B^^^x^K.^^^ DRY KILN 



THE KILy WITH A CIRCULATION YOU CAN UNDERSTAND 



HYDE PARK, BOSTON, MASS. 



