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April 1944
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¡C All) ¡l-IL NATION'S WAI? I)I?COI?AM
FOREST RESEARCH LABORATORY
LIB RArY
EOrQSI
Iatholoy
Spccial
lQ1sQ No. 16
UNITD STATES DEPARTMENT OF AGRICULTURE
AGRICULTURAL RESEARCH ADMINISTRATION
BUREAU OF PLANT INDUSTRY,
SOILS, AND AGRICULTURAL ENGINEERING
DIVISION OF FOREST PATHOLOGY
NEW HAVEN, CONNECTICUT
IN COOPERATION WITH THE
FOREST PRODUCTS LABORATORY
FOREST SERVICE
MADISON, WISCONSIN
ST.IMMLRY
Toughness tests were made on discolored sud olear specimens of .Amerian beech
rotarr-.cut veneer.
.LrmyJavy Aeronautical Specification AT..T...P-5llb does not enumerate all acceptable and. nonacceptable defects or blemishes in veneer and. these tests were made
to aid in inspection under this epoctli.cation.
Numerical data presented. in table i show the toughness values for wood, with diecolorations of the severaltyea. expressed as.peroentaes of similar values for
sound material, These d.ata are the baei for the following classification of
discolorat ions:
A.
Discoloratioris that are not prohibited. by the specification.
Sap stain.
4.ppearaxu,e
-..
brwnieh
to bli&i*h
discoloration of
sapwood
streaking in from ends of logs before they are cut into venoer,
Occurrence -- occasional,
Brownish-yellow heartwood. Occurrence -- occasional.
?ink heartwood,
Occurrence -- common.
an heartwood..
Occurrence -- common.
orm streak. Appearance -- brownish-gray streaks originating at worm boles.
Occurrence -- common on 4.11egheny Plateau but raro elsewhere.
Mineral streak. ADpearance -- reddish-brown to grayish-brown streaks.
Occurrence -- rare.
Pith flocks. Appearance -- sizall brownish f lccks
Occurrence -- rare.
B.
Discolorations that are prohibited by the specification.
Sap stain with decay. Appearance -- uniform brownish darkening of
heavily stained. sapwood.
Occurrence -- raro.
bown incipient
Wood
Appearance
broad. brown zones in heartwood.
brash but not noticeably softened. Occurrence
óominon.
Decay in heartwood. Appearance -- wood perceptibly soft displaying yellow
spots, zone linos, rot pockets, or streaks.
Occurrence
common.
rot.
--
-
,
-
Toughness of specimens with discolorations enumerated under paraaph 4, conpared to pieces of loar wood from the same sheets of veneer indicato that such
discolored materia], is only moderately, if any, lower thafl olear material.
Other strength properties, sueb as extremo fiber stress in bending and. maximum
crushing strength in compression parallel to grain, would. be expected. to be
reduced even less, Such discolored material should prove satisfactory for
airplane
use1
Material with disc*loratios, as listed under paragr$ph
and which is indica..
ti've of decay had. very low toughness values comparedto clear material from the
sains sheets of veneer,
Such material would also be expected. to be low in other
strength properties ad should. not be use fôr airplane construction.
complete descriptions of these discolorations and decays, and photographe
of the more important ones, are given in the body of the paper.
More
J'ø.r.
Path. Spec. Release No. iG
THE SIGNIFICkNCE OF TFE DISCOLORATIONS IN AIRCRAFT VENEERS:
AMERICAN BEECH
By J. R.
ANSBROUGH,/Assocìate Pathológist
A. M4 WATEBM&N, Assistant Pathologist
R. L. KRAUSES
and
Assistant Fatholgist
DivisIon of Forest Patholog.
American beech (Fagus gratidifolia) is now, and probably will b,
increasingly used to repla5e yello birch for aircraft construction. Even
though it is one of the principal components of the northern hardwood forests
of North America, study of the characteristics and properties of its wood has
been neglected tb the extent that relatively little is known of the normal
color variation and correspondingly less of the common discolorations.
NORMPL COLOR VARIATION IN BEECH VENEER
In beech veneer the sapwocd varies from whitb to a light reddish brown
with intermediate shades represented, and the heartwocd varies from white
through th same shades as the sapwood but to a deeper reddish brown
In
addition, normal heartwood is often a distinct tan ïn which no hint of red is
apparent,
Even though beech is a diffuse-porous wood, the pores or vessels are
smaller and less numerous in the late or summer wood than in the early or
spring woo',
This makes the summer wood denser and darker in color than the
spring wood, and in sorno beech Teneer a ribbon or band figure is producod
The writers acknowledge the cooperation of all the veneer producers visited
while this study was being made; and, in particular, cf the Pearl City
Cattaraugus,
Veneer and Ply-wood Co., Falconer, N. Y. Setter Bros.,
Inca,
N. Y.; Pilgrim Plywood Corp., Waterbury, Vt,; Deerfield Plywood Corp.,
Wilminpton, Vt, and National Veneer and Lumber Coi, Indianapolis and
Seymour, lud,, for their generosity in providing ample material for testing.
Bureau of Plant Industry, Soils and Agricultural Engineering, Agricultural
Research Administration, United States Department cf Agriculture, th cooperation with Yale University, New Haven, Connecticut, and the Forest Products
Laboratory, Forest Service, Madison, Wisconsin,
ror. Path, Spec. Release No. 16
-1-
1ISC0L0RATI0NS IN BEECH VENEER
The discolorations in beech veneer observed in this study are worm
streak, mineral streak, sap stain, pith flecks, brown incipient rot, and
various types of recognizable decays
In addition, and most common of all,
there are three distinct variations in the color of sound veneer from th
central part of the 1og pink heartwood, tan heartwood, and bröwnish-yellw
heartwood.
STUDY PROCEDURE
All of the beech veneer tested was rotary cut and was 1/24 or 1/45 inch
in thickness. All test pieces were 5 inches long and 1/2 inch wide and were
taken as nearly parallel to the grain and fiber direction as could be determined visually. Each discolored test piece was matched against a control piece
of clear wood from the same sheet of veneer.
All tests were made on the
Forest Products Laboratory intermediate-capacity toughness-testingmachine.
The
pecifio gravity and relative toughness values for all discolorations and
their matched controls are shown in table 1.
WORM STREAK
In beech veneer, worm streak is one of the most noticeable discoloratiens.
It is rare in the beech of New England, northern New York, and the
Lake States, occasional in that of southern Indiana and Ohio, and common in
that of the Allegheny Plateau region of Pennsylvania and southern New York. As
indioated by the name, the streaks have their origin at worm holes made by a
borer infesting living trees. The holes are small in cross section, usually
about 1/16 to 1/8 inch in diameter, and occur singly or in groups of two or
three at the center of each streak.
The streaks commonly vary from 1/4 to 1/2
inch in width and from 4 to 16 inches in length.
They are light brownish gray
in color, bordered at the ends particularly with successive bands uf very
light gray and dark brownish gray respectively, and finally terminate in light
red streaks that merge into normal wood.
features
Figure 1 shows the gros
of worm streaks.
Microscopic examination of the wood in worm streaks shows moderate to
heavy deposits in the véssels and ray cells. Fungus myceliurn is occasional
but never abundant. Effervescence occurs frequently when dilute acid is
applied, hut the specific gravity of the test pieces was the same as that of
the controls, suggesting that not much mineral is deposited in the streaks.
.
In 115 tests, the veneer from worm streaks was identical in toughness
with the controls. The only noticeable variation from normal wood, except
for color and the presence of the worm holes, was an added stiffness that showed
best when worm-streaked veneer was bent in the direction shown in figure 2,A.
For.
Path. Spec. Release No.
16
-2-
Instead tf bending to an even curvature throughout, the streaks retnained flat,
resulting in a tendency to split r check at the edges. In bending in the
direction shovm in figure ZB, no iregularity was observed, Veneer containing
worm streaks should b acceptable if the size and number of the lNorm holes is
w&thin
allwable limits.
MINERAL STREAK
Mineral streak occurs spare1y in beech veneer. In an occasional log
there may be considerable of it, but ort the average it may he classed as one
of the
re rare dicoloratiens As in yellow birch, mineral streaks in
beech are narrow elongated discolorations, sharply distinguished from bright
wood at the sides but merging gradually with it at the ends (fig. 3, A). In
color, mineral streaks ¿ro nearly niways reddish brown or grayish brown and
darker than the surrounding wood. Rarely, tiny white flecks are visible near
the ends, Addition of dilute acid to a hjte flecked zone causes vigorous
efferrescence, indicating that the flecks are minute concentrations of
carbonate,
Mineral streaks were 1 pereent lower in toughness and were 2 percent
higher in specific gravity than the matched controls. They wore slightly
harder and stiffer than adjoining clear wood and showed the same tendencies
in bending as those described for worm streak.
SA? STAIN
beech, sap stain occurs almost entirely as a brownish to bluish
sapwood, running inward from the ends of the log a distance
of a few inches to two or more feet. Figure 3, B shows the general effect on
n sheet of veneer, The background is uniformly light brown with darker blue
flecks scattered throughout. On infrequent occasions the blue or the brown
discoloration will occur alone,
Ini
stain of the outer
was 5
Saptained beech veneer was the same in toughness as the controls but
percent higher in specific gravity.
SAP STAIN WITH DECAY
Ini two
logs,
some
decay was observed in the sap-stained veneer.
It
from sap stain alone in that it was darker brown and more uniform in
color and was less streaky ini general appearances Such veneer was 15 percent
weaker in toughness and was 5 percent lower in specific gravity. This indi-
differed
cates that the principal hazard with sap-stained veneer is that it may mask
decay. Whenever any sap stain Is observed, special attention should be given
to the inspection of it to assure that decay is not also present.
For, Path, Spec. Release No,
16
PITR FLECKS
Very uncommon in beech veneer are the discolorations known as pith
flecke; when the flecks are present, they are much less conspicuous than in
most other hardwoods.
They are small, ovate to oblong brownish patches caused
by insect larvae that, in feeding, make tunnels in and through the cambium of
living trees. Callus tissue soon fills the tunnels and normal growth is
resumed.
Beech veneer containing pith flecks was 7 percent weaker in toughness
and was 2 percent higher in specific gravity than the matched controls.
So
far as is known, there is no 'weakness associated with pith flecks other than
this moderate decrease in toughness.
In beech veneer, pith flecks occur so
rarely, and when present are usually so widely scattered, that they are of
slight importance.
PINK HEARTWOCD
Sound wood from the central part of beech logs is usually uniformly
pink in color and is bright and lustrous.
In this study it was classified as
pink heartwood. Microscpicafly, the only difference between it and clear
sapwood is that tyloses ate present in varying degree, usually abundantly,
definite pigmentation is visible in the cell walls, and deposits of unknown
composition occur in the vessels and rays,.
In comparison with clear sapwood
controls it was 2 percent weaker in toughness and was 5 percent higher in
specific gravity. Pink heartwood ìn beech is normal,
TAN HEARTWOOD
Occurring commonly in beech veneer, but less frequently than pink
heartwood, is tan heartwood.
It is usually drab and lusterless, and is most
common in logs with thick white sapwoocl. Microscopically, it is indistinguishable from pink heartwood, nor does it differ much in toughness.
In comparison
with clear sapwood controls, it was 6 percent weaker in toughness and was 7
percent higher in specific gravity. This moderate reduction in toughness
should he considered as of no significance, for in the beech veneer examined
and tested there was no evidence of any abnoiiality in tan heartwood.
For
practical purposes, both tan and. pink heartwood may be regarded as normal
heartwood of the species.
For4 Path. Spec.
Release No. 16
-4-
BROVThTISHYELLOW HEARTWOOD
Occurring much less frequently in beech veneer than pink and tan heartwood is brownish-yellow heartwood. As indicated by the name, it is predorninantly light brownish yellow in color and is frequently figured with
longitudinal bands or striations that show considerable variation in color
but are usually some shade of brown or reddish brown, occasionally with a
greenish or purplish cast. In comparison with clear sapwood controls it was
identical in both toughness and specific gravity.
The most noticeable characteristic of brownish-yellow heartwood, other
than its color, is Its consistently lower moisture content at the time of
cutting into veneer than any other type of beech wood, This leads to serious
difficulties in drying.
It is said that if the drier schedule is such as to
bring the moisture content of normal veneer down to abôut 10 percent,
brownish-yellow hoartwood is apt to be down to 5 percent or lower, sometimes
almost oven-dry. This òauses it to check severely and leads to rejection of
the veneer.
The preferable method of drying veneer containing much brownishyellow heartwood is to run the drier at a speed and temperature that will
bring the moisture content of the brownish-yellow heartwood down to 6 to S
percent and the normal wobd-down to 12 to 14 percent. All the veneer should
then be bulk-piled under fairly uniform temperature and humidity conditions
to allow it to come to moisture equilibrium. If properly cut and dried,
brownish-yellow heartwood in beech makes high quality veneer.
BROWN
INCIPIENT ROT
Toward the central part of beech logs, hoad brown zones of incipient
rot occur frequently. According to Ridgway1s color standards/these zones
vary roughly from buckthorn brown to mummy brown or from tawny olive to sepia,
but in everyday language they are always some definite shade of brown and are
always dull and lusterless.
'Iicroscopic examination usually discloses some
evidence of decay, such as slight erosion of cell walls, bore holes, or
fungus mycelium, but to the unaided eye the only definitely distinctive
character is the brown color.
The 88 samples of brown incipient rot tested in this study were 64
percent weaker in toughness and, oddly enough, were 2 percent higher in
specific gravity than the matched controls. All samples tested broke brashly
with little or no splintering. The wood was not perceptibly softened, but
its texture, color, and brashness made the rot recognizable.
Broad brown zones in beech veneer should be rejected for they
definitely indicate an early stage of decay. Manufacturers of beech veneer
for aircraft use should make every effort to clip out brown incipient rot.
Narrow, well-defined, brownish streaks in beech veneer should usually be
JRidgway, R., Color Standards and Color Nomenclature. 43 pp., illus.
Washington, O. C., 1912.
For, Path, Spec. Release No. 16,
-5-
considered as mineral streaks or worm streaks and, as such, are acceptable.
With practice no inspector should have much trouble in distinguishing mineral
streaks or worm streaks from the broad general brown discolorations that
denote decay,
OBVIOUS DECAY
Sharply opposed to brown incipient rot, where the color is the only
visible index of decay, are various other features in beech heartwood veneer,
by which the more obvious types of decay may be recognized.
In later stages
of decay, brown incipient rot often is marked with small yellowish spots
(fig. 4,A) that are usually elongated parallel with the grain.
A reduction
of 65 percent in toughness was associated with such spots.
Occasionally,
brown or black zone lines (figs 4,B) may be observed.
Toughness was reduced
70 percent when they were present
Not uncommonly a yellow, streaky rot was
encountered. The affected veneer showed no particular discoloration other
than the rather indistinct light yellow streaking (figé 4,C).
In tests of
such material, toughness was reduced by 77 percent.
A greenish-yellow rot
that is sometimes present was associated with a reduction of 65 percent iii
toughness.
All obvious decay is accompanied by marked changes in the color,
hardness, and texture of the affected veneer and should always be rejected.
For. Path.
Spec, Release No. 16
-6-
Table li--Relative toughne$s and specific gravity of rotary-cut beech veneer!'
Type of
Logs
:Veneer:Tests
discolora±ion :sampled.:sheets:
:
:
:
:
:
:
:
:
:
:
:
:
:
:
:
:
:
stain
Sap
:
Brownish-yellow:
heartwood
Mineral streak
:
Tan heartwood
:
Pith flecks
:
Sap stain with
:
6
:
10
:
Pink heartwood
9
:
decay
Brown incipient:
9
8
9
8
Brown rot with
yellow spots
Brown rot with
zone lines
Yellow streaky
:
rot
:
:
4
:
:
:
1
Greenish-yellow:
rot
0,62
t
100
12
:
24
:
100
:
.69
:
.66
:
105
21
:
83
:
:
37
48
24
:
178
28
:
178
12
:
88
:
:
7
:
100
:
.61
102
105
:
98
:
.61
.58
.62
.58
:
107
.57
:
102
94
.58
85
:
48
¡
36
:
35
:
22
:
:
59
95
:
33
:
:
.58
:
102
.55
:
100
98
:
.55
:
:
35
.68
.63
:
:
46
:
:
:
:
.59
:
.60
:
.63
:
.61
:
103
.51
:
.55
:
93
:
23
:
:
.60
.62
:
:
5
:
:
:
:
11
:
29
:
.60
99
53
:
12
100
:
:
2
:
Percent
:
:
2
:
0.62
:
:
:
:
:
:
:
100
:
2
Percent
:
:
3
:
115
8
:
:
:
:
:
:
:
2
:
:
:
:
30
:
:
rot
:
:
:
:Number :Number:Number:
Worm streak
Average/
Specific gravity./
ratio of
:tougfless of: Average values :Ratio f disdiscolored ------------- ---:colored sped:specimens to:Disool-:Matched :mens to matched
:matched con-: ored
:controls:controls
:spedl
trois
mens
:
:
:
15
:
:
:
!Average toughness
values, are not given because the test pieces were not uniform
in thickness and averages of the absolute values have little
meaning.
The ratio of the toughness of each discolored
specimen to its matched control
was computed and all ratios for each discoloration were averaged
geometrically.
Based on volume at test and oven-dry weight.
For. Path.Spec. Release No. 16
Figure l.--Worm streaks in American beech veneer. A, General
appearance -- about one-half natural size. B, Natural size
with color variation indicated.
Z M
53836 F
!A1
Figure 2.--Bending worm-streaked American beech veneer in
direction A produces a flat spot across the streak and a
This Is
tendency to split at the edges of the streak.
not a cause for rejection as the flat spots are merely an
indication of greater stiffness in the worm streak than
Bending in direction B reveals
in the surrounding wood.
no irregularity.
Z M
5363? F
Figure 3.--A, Mineral streak in Anierican beech veneer, natural size.
B, Sap stain in American beech veneer, about one-half natural size,
showing its streaky appearance. The darker areas are bluish but
elsewhere the discoloration is brownish.
Z M
5368
F
Figure 4.---Three types of obvious decay in American beech veneer, all
natural size. A, Light yellow spots in brown rot; B, zone lines;
C, yellow streaky rot.
Z
53639 F