Steaming black walnut lumber to darken the sapwood


Material Information

Steaming black walnut lumber to darken the sapwood
Physical Description:
Forest Products Laboratory (U.S.)
U.S. Dept. of Agriculture, Forest Service, Forest Products Laboratory ( Madison, Wis )
Publication Date:

Record Information

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All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 29234804
oclc - 237787095
System ID:

Full Text

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By Forest Products Laboratory-
SForest Service U. S. Department of Agriculture

"coirreroial practice, black walnut lumber is usually steamed before it is
5 soned, mainly to darken the sapwood. Some manufacturers think that it
also improves the color of the heartwood. The sapwood of steam.ied lumber
seldom becomes fully as dark as the heartwxood, however, and additional
staining is often required in finished products* The bright sarw.vood of
unstear.-d lumber, especially air-dried lumber. requir-es greater skill and
care in staining. For this reason buyers usually demand steamed walnut
lumber for general use.

The exact reason why the sapwood is darkened during steaming is not defi-
nitely known, but it is probable that most of the darkening is a result of
chemical action within the sapwood due to high temperature and high rela-
tive humirdities, and that leaching of color from the heartwood is relatively

The general practice is to construct a vat or pit of suitable size, with
an open or perforated steam supply p-pe leading into the bottom. The pipe
should be extended '.nri frmu.r-d or boxed in, so as to obtain good distribu-
tion of steam through the pile. Planrl:ing or concrete may be used in making
the vat. Concrete is preferable, because wood deteriorates rapidly under
the conditions of temperature and humidity that prevail.

The lumber is cormmonly solid piled in the vat and covered with sawdust,
after which steam is admitted into the bottom area. Some openings between
layers of the pile, such as may be obtained by using thin stickers or lath,
may be desirable to help bring about more uniform circulation of steam
throughout the pile to produce more uniform darkening, The usual practice
for common thicknesses of lumber is to steam for periods of tvo to four
days. Longer periods seem to have no appreciable advantage, and w.;ould
only contribute to loss of strength and a greater tendency to honeycomb
in subsequent kiln drying. Seasoning degrade and los1 in strength proper-
ties, especially toughness, may also be serious if the aver'rv, temperature
in the steaming vat exceeds about lO F., and for this reason low-rs sure
or exhaust steam should be used. Such seam has more moisture per of
heat liberated and, therefore, higher humidities -:r.d lower temperatures ,re

After the steaming treatment, the stock is allowed to cool in place or is
cooled by spraying it with water before it is removed from the vat and

-Waintained at Madison 5, Wis., in coopertLtior. with the Uni. ity of

Rept. No. R1673 -1-

piled for air soasonin- or kiln drying. Tredi.ate e-rosure of the hot, wet
lumh'br to r latively cold air may initiate s-rfaco che:kirng or end splitting.
c-ch defects occur because of the low relative humidity conditions produced c
at the board surface as the heat loaves the lumber. <-=

To save handlinF cost-,, the green lumber nay be piled on kiln trucks or
pallets an, steamed ii the dry kiln or in a special steaming co-',rt'-.eit
by usingr a temperature of about l60 to 180' F. with a very high relative
hunia ty. This procedure, however, is somewhat less effective than oteam- --
1n2- in a vat. Yost kilns are unable to maintain sufficiently high relative -
hu'; ...ties at such hi-h temperatures to darl'en the sapwood to the desired
deTroo v:ihout e-cessive loss of steam.; therefore, che practicality of(
ct-a. in-- a kiln is doubtful. moreover steaming at elevated tern r-ra.-
tules rr.ay t Imn ,e t kiln if regularly done. For these reasons, ster.;ir
of Et ekered stock should be done in a separate compartment built specially and of durable material. Following steaming and cooling in this
compartment, lumber on kiln trucks or pallets can be moved directly into
the kiln or to the air seasoning yard for drying.

April 1947

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