

■^7 



Conditions in Casehardened Wood ^ 



IIV JAULS li IMUIi;. ASilSlAM I'inMCIsr. InUiST rilOULCIS l.AlioltAI'illV, MAI'lf-ciN, \Mh< iiNMN 



I- 



ON. 



H.-tirn Inrrh IiiiiiIht 



rci;uliitin|; kiln 



■ 1^.. llic |ilirnnniciion 



Uo({ro«. The iiihmI 



aii'liiii; of tlir nnturo nn<l 



>t. An rniwlmriloning in 



■- resulting from ini- 



While developing ii nieUic. 

 green from thr mw in the Kor< 

 «t the I" "^ liberator >. 



of eniu ' ■>t\H'i\ to nn 



for invi-j.ti^;itiiiii, li.ilui(; {•■ 

 eauao of tlii" <li'f<vi. vma ; 

 one of ' and hiuki 



proper - _ resoarrli ■ 



of some scrt'ice not onW in iiolving the larch 

 problem, but also to the solution of drying 

 problems in general. In an att4<mpt to de- 

 termine thr conditions rxistiu); in r;i^ili:ird- 

 cned wood, u »oric» of s|n'eifti' ox|i<'riiiu'iits 

 were jHTformcd at the Forest Products Lab- 

 oratory, whose results, together with inisccl- 

 laiicou<i investigations ami observations made 

 TThilc studying problems of kiln drying var- 

 ious species of wood, brought facts to light 

 which make necessary certain modifications 

 of the popular conception of the nature and 

 cause of cosohardening in wood. As we often 

 receive samples of wood in some one of the 

 various stages of casehardening, accompanied 

 by inquiries concerning the nature, cause, and 

 prevention of the defect, from people who 

 dry or manufacture lumber, we shall first de- 

 fine the term casehardening by discussing the 

 manifestations which should render its pres- 

 ence strikingly obvious to the operator. 



Visible MANirrsTAXioNs of Casehardeni.no 



A tendency to cascharden invariably results 

 from too rapid surface drying. This condi- 

 tion may or may not be evidenced by the 



appearance of surface checks during the first part of the drying 

 process. If this tendency pcrsist.o to the end of the drying period, 

 its presence may Ix? indicated still more conclusively by the forma- 

 tion of interior checks commonly termeil honeycombing or hollow- 

 homing. 



If surface checking and honeycombing occur during the drying 

 process, resawed material may not cup on the saw. Under similar 

 conditions boards mav \ye surfaced on one side with no resultant warp- 



ing. 

 iririiiMl 



PLATE 



I. SUKFACB CHEPKED A.\I> 

 HONEYCOXIBED OAK. 

 1— Surface checked durliig nlrdrylng ; 2 — 

 Hnnc.vcombi'd during alrdrylng : .'! — Honey-, 

 combed during kiln-drying. 



Ii'rmed e:i i, when 



:ig are sir < us that 



I iiie Hliigi' ul the seasoning. I'lutc I shows 



II, Mg. 



Certain siiecies, such oa cyprcn and western rod cedar, usually 

 show cnwhardening not by surface checks or honeycombing, but by 

 rcfuning free triinsfiision of the moisture from the center of the 

 wood through casehardened surface. This bottled up condition 

 of the water within the wood probably ac- 

 companies, to a certain degree, caschnrdon- 

 ing in all species although it« presence is 

 usually moro marked in certain woods. 



When casehardened lumljer is thoroughly 

 dried without checking and honeycombing, 

 it cups on the saw when resawed and warps 

 more or less when surfaced on one side. Pinto 

 II illustrates these conditions. It is .i mat- 

 tor of common knowledge that casehardened 

 wiiod is very hard as compared to properly 

 .'^e.'isoncd stock. This explains why knife and 

 S.HV trouble is often experienced in working 

 casehardened material. As the average kiln- 

 dried hardwood luml>er is more or less case- 

 hardened, the popular prejudice against kiln- 

 dried stock is readily accounted for. Cer- 

 tain of the more progressive operators are 

 adopting better kilns and methods of drying 

 so that in the future we may expect this dis- 

 crimination to diminish. 



Whether casehardening results in surface 

 checking and lioneyconibing or in warjiing oni 

 the resaw and planer, the loss of material is 

 usually great. Aside from the actual loss of 

 stock, perhaps the next greatest defect of ca.se- 

 hardened lumber is its unreliable action after it is manufactured 

 and in onlor to jirevent the occurrence of these defects it is neces- 

 sary that the causing factors be known. 



Manifest Conditions Existing in Casehardened Wood 

 Some of the conditions existing in casehardened wood are obvious^ 

 and have been acknowledged and successfully avoided in properly 

 ojierated kilns. The conditions manifest tliemselves simply and for- 

 cibly either by surface checking and honeycombing, or by cupping 



n.ATE II. 



1 2— Origin:!' < ■•■ '!■•' 



WESTERN I.ARCII CASEIIAnDEXED 



.: '• s_i-. . .,..,.1 Sectlon.s ; tf — One Side ."^nrfaccd. 



29 



m.i 



21.0 

 13. S 



APV.Z 



11.2 

 l+.O 



'0.0 



n 



w 



ri.ATE III. 



SECTIONS OF OAK SHOWING nEVKKSAL OF" 

 STRESSES niRING DRYING. 



