TECHNICAL
NOTE
FOREST
. Ur S. FOREST SERVICE ' MADISON.
PROOUCTS
LABORATORY
NUMBER
F-13
WISCON5IN
UOISTUBECONTENT
AT DIFFEBEI{THTIMIDITIES
OF WOOD
Woodkept for any length of tlne ln one locality
or under fixed atnoepheric condiiione tends to cometo
a doflntte moieture content, the percentage being dependent upon the hunidity conditions prevaillng.
For
instance, lumber placed out of doore but protected from
rain and Enow in tha northern gtates eventually retainE
about LZ per cent notature; at certatn plaoee in the
arid Southweat or in heated buildlnge it retalne onl.y
4 to 6 per cent molature.
Since any abaorptlon or Loaeofnoieture in eeaeoned
woodcauees ewelling or, shrinklng, wood should be brought
to nearly the proper noieture content before being manufactured into dollcate parte or put in any eervice
where the retention of exact ehape 1s tmportant.
The amounteof noisture retalned by woodat ordinery
temperatures and different huniditiee ars given in the
accompanytngtabLe. Theeereeulto wsre obtained in tests
at the !'orest Products Laboratory on thtn Bpecimensof
aeven different woods. It, will be noted that only ellght
differences are ehown between the noiature-retaining
quaLitiee of the eeveral woods teeted, and the average
values given should, therefore, bo roughly applicable
for all wooda. Further teate on larger pieces of wood
indicate that a vartatton of about L per cent or less
nay be expected in tho same apecles. These reeulte
accurately check the data glven in t,he table between 2O
Above ?O per cent values on
and 70 per cent humtdity.
larger blocks appear to be eomewhatlower than given in
the table.
At I0O per cent hunidlty wood takeE on
noisture unttl eaturated.
The data ehould ftnd applicatlon in drying lumber
and in controll,tng tbe hunidity ln workroons and con-
citttonlng cha,nbersto brtng stock to the proper moleturE
content for any glven use, where the atmoopherlc con;
dlttone are known. An example nay be cited ln the condltloning of alrplane pqopeller stock for use in Arny
and.Narrymachlnee. The object was to eetablishmoieture
condltione in the propeller tn equi.l.ibriun with thoee
Thetendency of the
most ltkely to be net in servlce.
propeller to abeorb or glve off molsture, and Goneequently to warp and loee lts balance,, was thuE reduced
to the nlniqrm
In eeiasontng lunber it le usuall.y beet to dry it
unttl lto nolsture content approaches more nearly thE
ntnlnun lt wt1l. have in uee than the maxlmum, becauee
ewelling uaual,ly le not so detrlnental as ehrlnkage.
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