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TECHNICAL NOTE NUMBER 213
UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE
FOREST PRODUCTS LABORATORY
MADISON 5. WISCONSIN REVISED December 1958
DETECTION AND RELIEF OF CASEHARDENING AND FINAL
MOISTURE CONTENT TESTS IN KILN-DRIED LUMBER
Nonuniforrnly dried, casehardened wood will usually distort during ma-
chining. The distortion may be due to a change in moisture content,
to unbalance of drying stresses, or both. Moisture content and the
presence of drying stresses cannot be determined by visual inspection
of the surface of the wood. They are easily determined, however, by
simple tests, These tests are made from the kiln samples or from
fairly representative boards selected from the kiln charge following the
From the samples or boards, three c `srl4 ar l (fig. top,
left). The sections should be cut frort ab t I :fIII c f
a kiln sample and 20 inches or more fro the en s o .
Test for Moisture Content
Use one section (fig. 1, top, center) t U.Sf D-ape TQMature c n-
tent of the test piece. To do this, weigh the section, irnmedia e ter
cutting it, on a balance or scale calibrated in grams, to 1/10 gram.
Then dry it in an oven at 2140 to 221 F. (101 to 105 C.) until it
reaches constant weight. The weight lost during this drying is the
weight of the moisture that was in the section. Divide this weight by
the weight of the ovendry section and multiply by 100. This value is
the percentage of moisture that was present in the section at the time
l ple: Weight of section when cut ......... 27 grams
.. .- _.- -,/. Weight of section after ovendrying. 25 grams
L' A",\ Weight of water in section ......... 2 grams
M-0 j Moisture content = T' x 100 = 8 percent
MAR 10 9 ,
Another section is cut, as shown in figure 1, top row, right, to deter-
mine how uniformly the moisture is distributed. The core and shell
are weighed separately, ovendried to constant weight, reweighed, and
their moisture content calculated.
Test for Casehardening
Preparing Test Sections
Use the third section for casehardening tests. This section is sawed
parallel to the wide faces of the original board to form prongs (fig. 1,
center and bottom). The saw cuts should extend from one edge to with-
in 1/2 to 3/4 inch of the other edge of the section. Stock less than 6/4
inch should be sawedto produce 3 prongs of equal thickness, and stock
6/4 inch and thicker should be sawed into 6 prongs of equal thickness
(fig. 1, left specimen, center and bottom rows). In a 3-prong test sec-
tion the center prong shouldbe broken out. Prongs 2 and 5 are removed
from a 6 prong test section.
Evaluating Casehardening Test Sections
When the kiln operator believes casehardening has been relieved, the
kiln should be shut off and some of the kiln samples or other boards
removed and tested. If, at the time of sawing, the outer prongs of the
test section turn awayfrom the saw a distance about equal to the thick-
ness of the prong or slightly more, the stock is usually free of case-
hardening and the charge can be pulled. If, however, the outer prongs
remain straight or pinch the saw, the stock is still casehardened and
the conditioning treatment should be resumed until subsequent tests
show satisfactory relief.
After the preliminary evaluation is made, the test sections should be
room dried for about 24 hours. The following conditions may then be
(1) The outer prongs have turned in considerably (fig. 1, center speci-
men, middle and bottom rows). This indicates that the stock is still
casehardened and the conditioning treatment on subsequent charges of
the same material should be extended.
(2) The outer prongs are straight (fig. I, left specimen, middle and
bottom rows). This indicates that the lumber is free of casehardening
and subsequentcharges of the same material should have the same con-
MfETHO O ) CuTrTING
fINAL MOi.STu qf CONTENT
SCTIOSVJ FROf KILV
,SAMPLe AFTER KILN DRYING6
3TOCA 4'ANVD ri-frcreR
5tOLD BL .SAW F AS SHOWN
50 AS TO PRODUCE SIX PRONGS
Of .'OUAL THCtVf.sJ FOR
RONBS 2 AND I SHALL BE
T7OCK LESS T4nA, ".'ICr
SHOULD BE S.AWE D AS SHOWN
0 A5 TO7 PRODUCE THREf.
RCdvG5 OF EOJAL THiCINL55
FOR CA.H ARCL-VIN. TESZ
-e. C f'Tirq PRONC,
5"ALL 8E BROKEN OUT
__ "' -'-
m-OIS TWR.EC 7T10 'IY O'5 TUR-I DIrSTRI/UFi
FINAL 1I105TURL& 5ECTO/VO5
.7 JUf *
NO 5 VfR X V RL
C45mARDtfN'"vG CA5HA'OfNINARNVG YEVRf
CA5EHARDEVING 5ECT/ON 5LCT/Ofv TO BE R. 001"
3fO"ORE CON/CiUS/ON 15 IADE A5 TO CA5HIPDfNCNG
Figure 1. --Method of cutting sections for
stress determinations in kiln-dried lumber.
(3) The outer prongs have turned out considerably (fig. 1, right speci-
men, middle and bottom rows). This indicates that the stock is reverse
casehardened. Subsequent charges of similar material should be con-
ditioned at a lower relative humidity or for a shorter time.
If the prong tests are made on stock that has cooled 24 hours or has
been stored for some time, the outer prong reactions may be the same
as those obtained on sections that have been room dried for 24 hours.
Final evaluation, however, should not be made until after 24 hours of
Relief of Casehardening
Casehardening canbe relievedin a dry kiln by a conditioning treatment
at the end of the drying. To be assured of good relief of casehardening
inall of the stock, the differencein moisture content between all boards
in the kiln charge should not be too great. The desired uniformity of
moisture content amongboards can be obtained by an equalizing treat-
The procedure for equalizing a kiln charge of lumber, using the values
given in table 1, is as follows:
(1) Start equalizing when the driest kiln sample in the kiln charge has
reached an average moisture content 2 percent below the desired aver-
age moisture content. For example, if the final average is 8 percent,
start equalizing when the driest sample reaches 6 percent.
(2) Whenthedriest sample reaches the moisture value stated in step 1,
establish an equalizing equilibrium moisture content (EMC) in the kiln
equal to that value. For the example given in step 1, this would be 6
percent. During equalizing, use as high a dry-bulb temperature as the
drying schedule permits.
(3) Continue equalizing until the wettest sample reaches the desired
final average moisture content. In the example given in step I, the
wettest sample should be dried to 8 percent.
The conditioning procedure, using the data of table 1, is as follows:
Table l.--Kiln sample moisture content and equilibrium moisture
content (EMC) values for equalizing and conditioning
a charge of lumber
Desired : Equalizing Conditioning
average : Moisture : EMC :Moisture content: EMC values
moisture: content : for : of wettest --------- .----
content : of driest: equalizing : sample at end : For : For
: sample : : of equalizing : softwoods : hardwoods
: at start :
: : --- - - -- -- - --
(1) As soon as the wettest sample reaches the moisture content stated
in step 3 of the equalizing procedure, conditioning should be started.
The conditioning EMC shouldbe 3 percent above the desired final aver-
age moisture content for softwoods and 4 percent above the desired
final averagemoisture content forhardwoods. For example, if a hard-
wood is involved and the final desired moisture content is 8 percent,
the conditioning EMC is 12 percent.
(2) Continue conditioning until satisfactory stress relief is attained.
This time may vary between4and 48 hours, depending upon the species
and thickness of the stock.
Itis advisable tohold conditioning time toa minimum to decrease steam
consumption and avoid excessive moisture regain.
Equilibrium moisture content (EMC) is defined as the moisture content
at which wood neither gains nor loses moisture when surrounded by air
at a given relative humidity and temperature. It is also used to desig-
nate a set of relative humidity and temperature conditions which corres-
pond to a certain equilibrium moisture content.
UNIVERSITY OF FLORIDA
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3 1262 09216 74