60 



HARDWOOD RECORD 



January 10. 1922 



End Checking of Glued Up Stock — Cause and Remedy 



(Continued from parte 24) 



in the final sampling for moisture content when the lumber is in- 

 tended for the class of product stated. 



The moisture percentage must be computed from careful weighings 

 on accurate scales (metric preferred for ready figuring). Scales 

 which have percentage tables with indicators attached to same for 

 finding percentages without figuring, can not apply, and should not 

 be used, excepting as scales. The tables show percentages only as 

 related to the first or wet weight, and not as related to the dry 

 weight, and therefore are misleading, unless this fact is fully under- 

 stood and allowed for. There are no short cuts, and the calculations 

 are really very simple. Weigh carefully, then oven dry carefully 

 and subtract the oven dry weight from the first or wet weight. 

 Multiply the difference by 100 and divide by the oven dry weight. 



Now, oven dry weight means so dry that no more moisture is 

 given up, and the weight remains stationary, no matter how long 

 the wood is continued in the oven. With sample strips %" thick 

 and with proper oven temperature, 24 hours in the oven is just about 

 right. Less time is not safe. The oven temperature should be be- 

 tween 175° and 200°, no less and no more. Live steam heater coils 

 are best for the oven. Electric heaters, unless thermostatically con- 

 trolled are not desirable. Unless samples are carefully weighed and 

 as carefully dried, the results will be misleading. 



Samples must be weighed as soon as cut, if accuracy is expected, 

 and the scale should be right at the saw for that purpose, but if this 

 is not possible and the samples must be carried to the scale at all, 

 then same should be carried in a tin box, shielded from air currents. 

 This injunction also applies, but with greater emphasis, to the 

 weighing of samples taken from the oven. By all means carry the 

 scale to the oven rather than the hot samples from the oven to the 

 scale. It makes a difference. 



Adopt a System 



Eigid adherence to a well-defined operating system around the 

 kiln, and clearly placed responsibility alone, will insure best results. 

 Let the operator report in writing on a suitable form, the test results 

 of final moisture found in each of a number of samples, belonging 

 to a Uln charge, and then decide, upon due consideration of the 

 lumber and its intended use, whether it is or is not ready for removal 

 from the kiln. This decision may be somewhat influenced by the 

 condition of the dry shed, and the probable length of time which the 

 lumber may have to remain there before going to the cut-up saw. 



And this brings us to the dry shed or tempering shed. Noah Web- 

 ster, had he seen the many so-called "dry" sheds which pass under 

 this name, would have defined it as an enclosed space (more or less 

 enclosed), into which lumber is placed after kiln drying, to keep it 

 dry, but in which frequently the reverse takes place. 



A dry shed or tempering shed must be reasonably airtight, shut- 

 ting out outdoor atmosphere as much as possible, and it must be 

 heated to that temperature which will reduce the existing at- 

 mospheric humidity which may leak into it to a point slightly below 

 that which is in balance with the moisture content to which the 

 lumber has been kiln dried and which it is desired to hold the 

 lumber at. 



If this means anything, it means that it is necessary to know and 

 watch the relative humidity of the dry shed, and that a good 

 hygrometer is just as important an instrument in the dry shed as 

 it is in the kiln, and in fact more so. 



Humidity Is the Important Element 

 In the dry storage shed humidity is all-important, and temperature 

 is of secondary consideration and is used only as a means of regulat- 

 ing the humidity of the air surrounding the lumber. If high humid- 

 ity was necessary in the kiln drying of the lumber, so low humidity 

 is necessary in keeping the dry lumber in its dry state. 



The letter states that the lumber is placed into the storage shed 

 for a period of two or three days, for tempering. This is a shop 

 expression, meaning equalization of moisture between the center and 



surface of the boards, and more important, gradual release of the 

 stresses set up in drying. It is necessary to point out that three 

 days are not anywhere near enough to accomplish either purpose, 

 especially at the lower temperatures generally obtaining in these 

 dry sheds. By far better results can be obtained from a short 

 period steaming with high pressure steam about 24 hours before the 

 lumber is ready to come from the kiln. Such steaming will not put 

 any moisture into the surface of the lumber which can not be 

 removed in 24 hours' drying, but it will thoroughly relieve the 

 surface stresses gradually set up in drying, as can be readily deter- 

 mined by the simple casehardening tests recommended by Tiemann. 

 If several lots of lumber of varying state of dryness are simul- 

 taneously stored in the same dry shed, then the lot having the least 

 moisture content (if it is desired to hold it so), will control the 

 necessary humidity condition, to retain the lumber moisture in equi- 

 librium with the air moisture, and thus prevent reabsorption of 

 moisture by the lumber from the air. 



Moisture Balajice Fundamentals 

 Experiments conducted by a number of competent investigators 

 of the Forestry Department have disclosed the following facts 

 about moisture balance between the air and wood near the tempera- 

 ture range usually obtaining in dry sheds (60° to 100°): 



Eelative humidity Counter balanced 



of the air moisture percent 



surrounding lumber in lumber 



10% 3 % 



20% 4y2% 



30% 6 % 



40% 7%% 



50% 9%% 



60% 11%% 



70% 13 % 



80% 17 % 



90% 221^% 



100% 32 % 



These figures are but approximate and they will vary with the 

 species, but the same are an excellent guide for practical results. 



Any lumber, therefore, which has been dried down to 3% and 

 thereafter is exposed for several days to dry shed air which has a 

 relative humidity of more than 10% will reabsorb moisture from 

 the dry .shed air, faster, the greater the air humidity, and vice versa. 

 If, for instance, the temperature of this shed atmosphere were at 

 60°, and the relative humidity of this air at 40%, as it could easily 

 become on a rainy day, if doors were left open for some time, then 

 this lumber would continue to absorb moisture from the air, until, 

 if these conditions were .sufficiently prolonged, it would contain 

 7%% moisture. Under these conditions, if it were desired to prevent 

 any absorption, the air temperature would have to be increased from 

 60° to 103°, when the expanding air would cause the relative humid- 

 ity to drop to 10% and absorption would cease, the lumber remaining 

 at 3% moisture content. 



Or, to quote another example, if the lumber with lowest moisture 

 content in the dry shed were down to 5%, and it is desired to hold 

 It so, when outdoor temperature is 60° and relative humidity is 35%, 

 then the shed temperature must be increased to about 80°, in order 

 to drop the relative humidity to 21^.%, which will balance the 

 moisture content of 5% in the wood. 



Again referring to the subscriber's letter, quoted at the head of 

 this article, it must be plainly seen from the foregoing that a 70° 

 shed temperature alone can not sustain a 3% moisture content in 

 the wood, unless accompanied by a relative humidity of but 10%, 

 which is hardly to be expected when outdoor and indoor temperatures 

 practically correspond, and there is no provision for artificial heat. 



A relative humidity of but 10%, when the shed temperature is 70°, 



