24 



HARDWOOD RECORD 



January 10, 1922 



YARD AND KI LN 



c^ Section Intended to Promote Efficiency^ 

 and Economy in Lumber Drying ^ 



End Checking of Glued Up Stock — Cause and Remedy 



By C. J. M. 



One of our subscribers writes as follows: 



' ' We have lately had some trouble in kiln drying 4/4 plain red, No. 

 1 eoinmon gum, and we will apjireciate it if you can throw any light 

 on the subject. 



"We have a ventilated type of kiln, and after charging, start 

 from atmospheric temperature and humidityj gradually increasing 

 conditions to 125° or 130° and about 100% humidity. 



"In course of drying we then gradually, during the six or seven 

 day.s following, increase the temperature to 150° or 160° maximum, 

 and also bring the humidity down to 30% minimum. 



"We run the moisture content of the lumber down to about S'i. 

 The lumber is then placed into a tempering shed, where the tempera- 

 ture is kept at about 70° for two or three days. Then the lumber is 

 brought into the mill room, where we use it for making table leaves, 

 shelves for china cabinets, etc. 



"The trouble is that the glned-up stock checks in the ends, while 

 it is in the cabinet and finishing rooms, but the glue joints hold well. 



"In our cabinet room there is practically no humidity, but there is 

 in the finishing room." 



Some of the statements in this letter sound familiar, and hoping 

 that the discussion of the possible causes and remedies may prove 

 of more general interest, we have selected this matter for the subject 

 of this article. 



As frequently happens, the communication omits to state some of 

 the information needed to a clear analysis of the entire matter, and 

 therefore some of the conditions involved must be assumed, rather 

 than clearly deduced. 



Beginning with the kiln drying of the 4/4 gum lumber, we assume 

 that it goes to the kiln from the yard, rather than from the car, 

 after some air drying, and since the initial moisture content is not 

 stated, we assuhie it to be near 30% when the lumber reaches the 

 kiln. If taken from different lots and yard piles, the moisture con- 

 tent may vary considerably, and with the moisture content would 

 also vary the drying time. 



This variation may be from 30% to 50% in moisture content, or 

 even more, and if then, through neglect or oversight, no moisture 

 tests had been obtained from the wettest lot, and the drying and 

 removal from the kiln were based on the 30% samples, grief would 

 surely follow in the final manufacturing operations or possibly the 

 finished product. 



The material being gum, preliminarj- steaming, ahead of the dry- 

 ing is earnestly recommended, in fact the lumber should be brought 

 to the initial kiln temperature (and about 20° higher) by direct 

 steam, and then, after the steam has been shut off the kiln should 

 be held (closed absolutely tight), without anj' ventilation what- 

 ever, until the temperature has gone back to the starting point for 

 drying (135° on the dry bulb). The length of time required for this 

 steaming will depend on the temperature of the lumber when 

 brought into the kiln, on the steam pressure and volume of dis- 

 charge. An extension or a recording thermometer with its bulb 

 placed well within the center of a jiile, as far from the door as 

 pos.sible, should be used as the guide in this steaming. 



Such thorough steaming will tend to equalize the moisture content, 

 and more important, it will serve to release any casehardening which 

 may already exist, due to yard drying. The gradual cooling, after 

 the steaming, with all ventilation shut off, and without opening of 

 the kiln doors, is most important, otherwise very much harm and 

 worse casehardening would result. 



(hull lunilier softens very much when steaun'il, and therefore must 

 111' carefully sniijiortod on sticks, which should be placed no more 

 lluin 18" ajiart, in neat vertical rows, and if so supported it. will come 

 from the kiln straight and in good condition. If cross-piled over 

 three trucks, about 5'i)" apart, a suitable foundation must be placed 

 upon the trucks, consisting of at least three lengthwise beams, and 

 over these suitable cross beams, one under each row of stickers (IS" 

 apart), otherwise the lower courses will surel.v warp. If gum lumber 

 is to come from tlie kihi in straight condition it must be so sup- 

 liortcd. 



The temperatures stated in the letter seem conservative for this 

 •s|)ecjes, but nothing is said about the method of application, which 

 may be altogether too rapid in temperature increase, as w-ell as 

 humidity decrease, and that would surely cause casehardening. The 

 leniperature increase and the humidity decrease should not be made 

 at will, after a certain number of days in the kiln, but only a.s and 

 when the reduced moisture content of the lumber (when ascertained 

 by careful testing of .several samples of the wettest lumber located 

 in various parts of the kiln), aclually warrants this change of 

 conditions. 



When ilrculation of air is by gravity only, absolute uuifurmity of 

 drying throughout each pile cannot be expected. The necessity for a 

 number of samples, therefore, is plain, especially at the time when 

 the material is supposedly ready for removal from the kiln, and 

 these should be judiciously selected. 



Temperature and humidity should never be changed, until the 

 operator knows, as definitely as possible, from his tests, that the 

 moisture content h;is, on its downward course, arrived at the point 

 where an increase in the severity of drying is warranted. Mere 

 assumptions, based on the length of time in the kiln, are ha/.:irdous 

 and may prove expensive. 



A safe schedule for red gum lumber, if ]iroperly used, as per mois- 

 ture content, and not merely by time in the kiln, is as follows: 

 Moisture Temperatures Relative 



in lumber Dry bulb Wet bulb humidity 



75% 135° 128° 80% 



50% 135° 128° 80% 



40% 140° 130° 75% 



30% 145° 133° 70% 



25% 150° 132° 60% 



20% 155° 131° 50% 



15%, 160° 124° 35%, 



10% 165° 112° 20%i 



5% 165° 112° 20%, 



The average time in kiln on basis of 24-hour operation per day, 

 when drying from an initial moisture content of 30% (as previously 

 assumed), to a final moisture content of 5%, when this schedule is 

 rightly used, would be about 16 days for 4/4 stock. 



Inner Moisture Is All-Important 



The sampling for final dryness should take into consideration the 

 inner moisture of the test pieces, rather than their average moisture. 

 Test samples should be cut at least %" thick, across the entire width 

 of the boards, no less than 18" from the end, the board surface 

 should then be cut away as well as the board edges about %" thick, 

 leaving only the internal wood, and this only should be considered 

 ((Continued on pttije GO) 



