MIN SCIIIEDULE rFO PLACE WALNUT
Information Ievikucd and Ieaffirmed
I.o R 4 E, 9 5 PC. k-. t
UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST PRODUCTS LABORATORY
Madison 5, Wisconsin
In Cooperation with the University of Wisconsin
KILIT SCUDULTL FOR LACK 'IALITUT GUIMSTOCK BLANTKS
H. H. SMil:, As.istant Techrologist
0. W. TR;-ESC:T, Senior engineer
The :-m.d mobilization of a large ar.y lhas greatly increased the de;aL-d
for dry black walnut '-:,n stock blanks. Holdover supplies of dried blanks are
depleted and most gun:tocks are now manufactured from material direct from the
kiln, thus furtl.er taking the already overloaded kiln capa.,ity of the lumber-
producing and wood-using industries.
During "rorld War I, black walnut gunstock blink:.- green from the saw
were dried successfully in the water-sp-_ay type of kiln according to methods
worked out at the forest Products Laboratory, and with varying degrees of
success in natural-circulation kilns. Since then the natural-circulation
kiln has been used rather extensively for drying both green and air-dried
gunstock material. Drying in such kilns, however, requires considerable time,
often more than 3 months for fully green stock.
Meanwhile, the efficiency of the lumber dry kiln has been improved by
the application of mechanical refinements and better design.
The modern type of dry kiln with forced circulation and automatic
control of temperature and relative humidity has made possible considerable
reduction in the drying time of gunstock blanks. To determine what further
re..,,ction in drying time might be obtained by the use of accelerated kiln
schedules, and to study the feasibility of such accelerated schedules, the
F''est Products Laboratory during the past year or iuore has undertaken a
series of t':perimcntal kiln runs designed to study several factors of drying.
&no-.; these are (1) the effect and desirability of preliminary steamnin.;
(P) the efficiency of various end coatings in retarding end dryir., and thus
in reducing. end checking and end honeycombing; and (3) drying conditions,
incl-'din initial conditions of temperature and relative humidity conducive to
rapid. ir-inr without degrade, intermediate changes of temperature and relative
h.n1'iity, final temperatures and relative humidities, and final conditioning
tre-tLent for relief of drying stresses.
The results of these drying experiments indicate that faster drying
is possible than has been customary in the past and that certain definite
liL'ts of dryi-,g conditions exist beyond which drying cannot be acomplished
without t prohibitive degrade.
Report No. R1433
in the LnE.ufFLct'..re of black walnut ganstocks, ..reen flitches or blanks
soul' r.ot be sta..cd before kiln drying. The steaming of green black walnut
l-.Lbcr to 'Liar'ke., t' e lap'.,ood is standard practice in industry, and users 0of
-la'k wql...ut l,'jJbcr prefer t:.f st--aLed stock. Steaming of 4/4 stoc'.: does
darken t. s...p....'ood aterially, iakiAn, a :.ort, unifiiorm oolor throughoAut tie
piece. -.i.. color c-ar.,-e is .;-r vte'_t lt or near the surface, however, and
('n:.torks that arc' carved from L.c4k Ulauks either stea.ed or iunsteauwed
reu ,Are the i' lic'tiot. of staii. to t:.e sap'.:ood during the finishing optra,-
tion to ob*,'-:in a r-.:fficicntly 'x.ifori color.
"L... Itr of e."p ri...rnts -.licate t}.at preliminary stea.ainr does not
repd',c, t'A' final "'il-n dryin" tiE
Pi -st of ;at';..,. st.-:ucd a-A.d -nsten:4ed :_att rial indicated that steaL-
ir.- for 3 i.- ;.t 1-'0 1-0, aid 200' :. -rnder saturated conliticns 2'ro 'ocer
a sgnifi'ar.t .."reasu i to'o;' .r.css. T:-,; i( rease ranged from a!,out 6
rc.. ro: st Li'.,- at - to 19 percent in the wood steaat o rcen a ti oo teae at 2'..0u .,
In'.c tUn that to'. hn>.ss decreases with incr'-asxi In the preliminary stea-iA:.
rli!Li.r.ary strn.'ai..4 of green blanks alco contributes to the develop-
ment of ..Id ;...i1 ''irface check'< th.-; are often s'if'ficlently severe to warrant
-ffective _.-.d COR t ig .eq'.-ircd
_- .c- :-e of a suitt.ble er.d coatin:- to retard e:.d drying and resultar.t
e:,d '.ecl-li r.-, socifll,- for a refrac.tory species such as black walnut, has
lor:g been rc-o:-r.icd. C rnsidernLble attention was ,.-iven to this problem when
black w.l ':.t .?. r--tock blankr were dried during World War I. Th:e er.d coatings
of rosir. an'd la:i;)black ar.d mixtures of rosin L.rd coal-tar ritch tsed at that
time were effective with-i the tcmeoraiture liZAts GLd t'.rn, b':t, when h-^iLjher
tt:..purature were consiiercd, addltlor.nil experier.tal work on end. coating was
necessa.r. to determine the ability of various "oatiipgs to ahere readily, to
withst..,: ".r,:.nl' -.:A.. to n.aintalrn nr. effective moisti.re barrier during
,ir.l. 2.' rcuilts of s'.ch cxpI, riicats i-.iicate t'at hot coating are .ore
duLrlra. e for r3cr, blanks than are coatinF.ln applied colid. Hot coatinEs car.
be ,j lie sucrcsvfuAlly 1,:' holi.Arn the :., of the .'rctur, bla.- a0-,dnst a
power-driven roller pirtl:' su^L.crgcd i:-. a vat of zoltc:; er.d-coati.: .aterial.
-.'.! roller of this e ..irp..:nt Eo..)_l4i revolve at aroxirmately 'wj revolutions
per-. i-:te, '.iK. the tL.:zr.erature of the coatin: when applied should be between
1 .7" cC'0 ., 'tpo, denr -,.; -,, the .,rtlcular co-.tin6 used. VE.rious coal-
tar itches, roiny-, -i':.d -sp:-ph-'ltt ',c.r- tried, scparatoly nar.i in different
c -...'" "'.* itiouc,
A.:. erdA-coatmi r.l-.lt're was developed that has jrovcd effocCtlve and
h.-:G ':-.n 1:-ncd comnorciallo:. T.he ateriLl crsists of:
:-.t ort 'o, .a4.3
60 parts 213 F. coal-tar pitch
25 parts 155 F. coal-tar pitch
15 parts 210-2200 F. asphalt
Mixtures of either rosin or high-melting-point asphalts and high-
melting-point coal-tar pitches are also satisfactory, and some asphaltic
enamels have shown promise. The addition of rosin hns been advocated by sonie
operators to increase the adhesiveness of pitch coatings. Ordinary roofing
pitches and asphalts and 1550 F. coal-tar pitch, used alone, are not
An adequate end coating is necessary because of its ability to protect
stock against degrade due to end checking. There is an upper temperature
limit for every coating beyond which complete protection will not be afforded.
The initial conditions of temperature and relative humidity are gen-
erally limited to those ranges whicn do not causu surface checking. Black
walnut is not particularly susceptible to sur 'ace checking:; thus the initial
conditions are limited by the danger of end. checking, which in turn is con-
trolled by the effectiveness of the end coating.
Initial temperatures of 125 or 130 F. are believed to be safe.
Initial temperatures above 120 F., however, have been used only to a very
limited extent and cannot be recommended with assurance until experience
ir.dicates further that their use is desirable. Furthermore, in drying mate-
rial greenn from the saw, such low initial temperatures are not particularly
detrineutal to total drying time as increases in temperatures, especially
towards the end of the run, can be .Iade as the drying progresses.
Initil relative humidity is not particularly critical. Early experi-
ments designed to establish the minimum initial relative hun-idity that could
be used without producing surface checking failed because no checking
occurred even in initial relative humidities as low as 67 M recent. Appar-
ently, an initial relative humidity of 75 to 78 percent is entirely safe,
and is, moreover, conducive to rapid drying. Initial humidities as low as
70 percent are believed to be safe, but it is conservative to keep above
70 perernt because of the variations of drying conditions that are more or
less common in any kiln.
Report 17To. R1433
In several Lxp,.riMEntal kiln runs the relative humidity was lowered
from the initial condition to 25 percent during the first 6 to 9 days without
pror.uirin.; surfaco c!.ecl's or any apparent end checks, This result further
indic'-t.d thie ability of black w'alnut to withstand relatively severe; humaiity
conCi'tionn, ?rovld1'. it has adoniuate end protection. 2oth exp-.riiuntal anri
coiannercinl d: t;& i.'.ic tte that "}he initial relative huirmidity can be gradually
low, red to "1O or t *,!rctnt 4-urin,, t'i- first 1: or 14 lays of dryin-.
Lo.rir. t.L relative huuldity by reducing the wi'-t-bulb tuerponrture on a
tio basis tA.id3 rupid t'ryin,: without t rubjecting t.e rat,.rial to conditions
t.'.t .ro.luc. d''dng ert.rudu, Jn.i r thiic LrtthoL, Liost or all of th'-. rolativ'
huiaidity rh-:., es ;ar'c L,'tde before th'n tt,-.uLr-trtre is increasa.d.
IntLr .:Jiae Tc-^n-rutiAres
Int erm di ate tcperature c'.an, es shld be based on the ..eostiie con-
tent of tht :oo1., T Loe ue of ith t>ip> ratr,.r while t.h- wood is above tse
fir'.e_-s:uration .rint (ap roxizaately 30 percent aoisturo content) is .t to
re!,ult in serious > ex-r:i 'e.
T.'is d;n,.ez'r ws L.ale a;'. aren't by exressive de.grade that resulted .ne:ir
a ::oIL-ercial kiln lo" of green blanks was dried by a schedule wherein t.ae
desired. decreases in relative humility -;ere obtained tb;' maai-,-aninr1n a con-
stant wet-bulb tt::.:'.orture and rnisin; the dr:--bulb temperature froL- 130 to
1,55 1. before all the wetter stocks ha, *Jrieo. to the fiber-sat'iration point.
Fast :.r'is.,- without degra.do: was obt-ined in a siDil].rir c.x;erimnental sched',-le
e:lo;i.i even hl.,-hEr -.riitial nn., final te-pert'..-es. Inre method is
darnerous, however, and. La, cause exces-'ive degrade u.'hen used for -oaercial
dryin,: er.ecause of tho gr.ea1er variability of drvin; characteristics
er.countored in wooi obtained Iroa various localities having a wide range of
A tcPne.rature much above l3O F. is not recoL.e.nded before the wood
hlas dried below the fiber-saturation point. Th-ie. first teL-perature changes
should cce conservative, allowing for any; slow-drying or particularly wet
Cla .nk that are not as dry as the kiln sas .les indicate. After the blanks
have dried i-.rther, the te.Aperature can 1.e increased Wore rapidly. At the
time of intermediate dry-'ulb tei;:,pr;.ture chazr-es, the wet-bulb te.pnerature
should' n.so > cl'n to maintain the d6.sired ec-illbrium noisturo content
(the :..oi sure context that wood i.ll eventually attain at any given condi-
tion cf te..f,-ratu--rc an d relative humidity).
7inal TeLCper t-;-e! a.d Fulative "-ur ditles
l.c firn;.l +uperaturees a: relative humidities to be used in the kiln
iry.in of 'i.l'-" walnut ,linEtock blanks are -:overned largely 1-. the effect o:
.i)., tuei.:rr'tures on the strniiF-th of the wood ar.d the effect of low final
relative hc,:.1l'itiea on t.he fInal moisture content, The Laboratory consilerz
ac"ort ;c 17
1650 F. to be the maximum temperature d,3irpble from a strength standpoint,
and higher temperatures are not recommended, even though they do increase
the drying rate and tend to reduce the occurrence of wet cores. The use of
165 F. after the blanks hIdve been dried to an average cross-sectional
moisture content of 15 percent is believed safe. A final relative humidity
of 25 or 30 percent (equivalent to an equilibrium moisture content of 3.5
or 4.0 percent) is considered desirable. Higher humiditjies will retard the
final drying period, and lower relative humidities will produce excessive
shrinka-e iand dryness.
A final conditioning treatment is necessary to relieve casehardeaning
stresses that develop during the usual kiln drying procedure. Final condi-
tioning distributes moisture more nearly uniformly within the wood. Com-
binations of temperature, relative humidity, and time were tried experi-
mentally to determine thie best conditioning treatment for 2-1/2-inch blanks.
The experiments in.r-'cated that dryinL stresses can be satisfactorily
relieved at a relative humidity of 65 percent and a temperature of 165 F.
for 24 hours. T'i-y further showed that a final conditioning treatm.-nt at a
given temperature an. relative humidity will relieve the drying stresses
only of blanks that are within a limited ran.-e of moisture content.
As an example, the recommended conditioning temperature of 165 F.
and relative humidity of 65 per-ent (equivalent to an equilibrium moisture
content of about 8.5 percent) -:_ll not produce ti.e desired stress relief in
blank. l-av-r a. moisture consent higher than approximately 8 percent, and
will cause reverse cvseharLer,'r.g in blarnks having a very low moisture content.
The best relief of drying stress will be obtained if the blanks have
been dried to a unlforma moisture content throughout the kiln load. The
degree of uniformity of moisture content increases as the blanks continue to
dry. A kiln load dried below 8 percent moisture content will be more nearly
ur-iformly conditioned than a similar kiln load having a higher moisture con-
tent. A uniform moisture content of 6 to 7 percent in the dried blanks is
desirable to prevent shrin'-ing and end checking in cores exposed by end
trimming to the dry air of heated shops during the winter months. Tests
show that blanks having a core moisture content above 10 percent are apt to
check in the core area when exposed to dry shop conditions after being
trimmed to length.
Storage of the kiln-dried blanks before manufacture into gunstocks
also helps to equalize residual drying stresses and moisture variations.
Report Io. R1.433
Recommendations and Precautions
For the most effective protection against en& checking, green blanksu
should bn end coated immediately after sawing. An end coating should be
used that will adhere to green end grain and will form a continuous moisture
barrier capable of withstanding the ab'iae of repeated nendling of the blanks.
T'-. end coating should be abl to withstand the temperatures encountered in
tr3 kiln during dryi.ig.
Recommendations rega:iing a schedule for drying black walnut gunstock
blar.kna are subject to modifications in order to obtain the best drying in the
many differor.t types awA kinds of lumber dry kilns. The schedule in table 1
is lesi-ned for a moiern forced-circulation cor.partment kiln with automatic
control of temperature ay.d relative humidity.
Table l.-?ronosed co.merci'l kil'. schedule for drying black walnut gunstock
.oisture content at which ry-bulb
c'..nge. is to be rade tA.Tmerature
Percent O eF.
Aoove 35"............... 120
At 305. .. ,............... ..... : 1320
At 25 ......... ................... .. .: 1
At 20 .......................... ..: 153
At 15 and to final................ : 165
Relative hJm.id.ity schedd.13
Duration if iry-i`. :lati ve
1st qr.i r d. .. .......... .. . .. 77
3rd. and 4th ......................... 74
5tn -nrd 6th ..................... : 70
7th ar. d gth ..................... : 65
9th ;..... ....................... 60
i :, h :50
iI h .... ....... .... ....... 50
12 th to fi'1-,................. *.#** 30 or less
I.. .-r:ral, :li tly lo-er ini tial relative h=riIties, can be used
in nat:ral1-circ'l tion Kilns. Lo-er i tinl r-l-tive hurriditics ire de-
sirabl' to prv-r.t Told "n the slow Iryir; nr.-es of rnatural-ciral:lation
kilns. Pi-r^e of the slower dry.In r-te, ho.ever, the dr:.ying time for
each rJl-tivo h'.idit; step should be extenrod, possibly an Rdditional day.
r,) port No. 1`33
In using this scbed"-le the tcmpcrture is govern-d by the moisturA
content of the kiln stnrples. The r -Ative h,.-iditLes to be used with tnes3e
temperatures Pre shown on a tilm basis. Thus, blliJ:3 in the kiln for six days
may have dried from 70 to 50 percent moisture content and the tomT,.r-t.turo
should th'n be maintained at ]..OF. until tao blP-i:s have dried. to a moisture
content of 35 p.rrc3,.t Trh relative h'.-nudcity, ho.-, vr, wiill h-ve be.n
lowered on the tirn- basis front 77 to 65 pcrufLnt after 6 days of dry"if.
This schedule is cornsider.-d safe for dryir.- sound, well end-coted
ai-. prerfr?,bly unsftcamed bl,-jck T.!alnut gunstock blujrks. I"'cro severe drying
conditions should not be used unless experience indicates the practicabil-
ity of such a change.
Hi-hr tcropertures increase the dancer of de:rade, especially when
rp-nliod before the av-rn.:-e croso-s-ctional moisture content has reec..:i
the fiber sat,*raticn point. "-n afte-r the moisture content of the sarizles
iniicntes a moisture content b-20lo'- 530 psrc3nt, only moderate tempcraturo
changes are r^.: idc.d becnvx-s of occasional high moisture content cores
or Tet s-ots. .-s ',he coe Cibo.rs begin to dry and, in drying, become
stronger, taheo ... .... rures can be used with :neater safety.
In ..n... .. f inal c(on20n onin? to elIi.i'_-a,-) casohPrdenin1 should be
carri ,d or. for 2?' hours a -...p.ature c-.,l t! 'he final dry-bulb
tc..iparp.tiro tnd a rel-tiv humidity, .quu.valr.nt to an equilibrium
moisture content 2 percent .i'..r thar the 7v.relo crors-sectional nois-
ture content of the materially or exFplo, ii th; final teomrperature is
165F. ana "th.e bblziirs h eve (Lri.d to a moistui;. content of 6 to 7 per, nt,
they hou-o:. i) :cc.t cn f.c 1 ho2s A.t a t.-ratur.e of 1o50',. anr-d a
rea ntlv. jii y -f i rc,.. Iho m.re no-.i'J.j u-iiform the moisture
content x.:i irw.w-,iviGl b... l :ns, the L.- .. .aly -iiform will be the re-
lief of the c- rd;3An; stresses.
Report No. 1433
UNIVERSITY OF FLORIDA
II 11111111ttI1 PIH tll l I I t 1ttllttiIII
3 1262 08925 4311