1979 

UNITED STATES 

DEPARTMENT OF 

AGRICULTURE 

FOREST SERVICE 

FOREST 

PRODUCTS 

LABORATORY 

MADISON, WIS. 

Research Paper

FPL 345 

Drying Proceduresfor 

Bacterially Infected 

Northern Red Oak 

Lumber 
ABSTRACT 

A series of tests were conducted to 

determine an optimum procedure for 

drying green 4/4 northern red oak 

lumber containing bacterially infected 

heartwood. This type of lumber is more 

likely to develop serious honeycomb, 

surface checks, and ring failure during 

kiln drying under normally mild 

schedules than is green lumber with 

normal. noninfected heartwood. First, 

bacterially infected lumber should be 

dried green from the saw by a lowtemperature, forced-air schedule to at 

least 25 percent moisture content and 

preferably 20 percent. The forced-air 

dried lumber can then be kiln dried to 6 

to 8 percent moisture content with good 

results by using a milder kiln schedule 

than normally recommended for airdried oak. 

Honeycomb will be eliminated except 

in boards with advanced, rancid stages 

of bacterial infection. but even these 

boards will have minimal honeycomb. 

Severe surface checking was not 

reduced as effectively in bacterially 

infected oak as was honeycombing. 

Ring failure was not greatly minimized 

because it is an incipient form of ring 

shake that begins in the living tree. For 

all tests in this study, shrinkage was 

greater for bacterially infected 

heartwood than tor normal noninfected 

heartwood. 
UNITED STATES 

DEPARTMENT OF 

AGRICULTURE 

FOREST SERVICE 

FOREST 

PRODUCTS 

LABORATORY1 

MADISON, WIS. 

INTRODUCTION 

Bacterially infected heartwood of oak 

presents special drying problems when 

the wood is to be kiln-dried green from 

the saw. Infected oak is more prone to 

develop honeycomb and ring failure 

than healthy oak, even though dried 

under conventional or normally "safe" 

kiln schedules (16 2 , 18) . 

Kiln schedule T4-D2, recommended 

by the Forest Products Laboratory for 

4/4-inch-thick red oak (13 ), was found 

on one study to be safe for green lumber 

with healthy heartwood. but not for 

infected lumber (18 ). Healthy oak could 

be dried faster under a schedule more 

severe than T4-D2 without honeycomb 

and ring failure, but the severity of these 

defects increased in bacterial 

heartwood. 

This paper gives the results of more 

recent research to derive satisfactory 

procedures for drying infected 4/4 red 

oak under conditions milder than kiln 

schedule T4-D2. 

Background 

This research followed principles of 

kiln schedule development in response 

to drying strain outlined by McMillen (9) 

whereby three factors affecting drying 

stresses must be considered: 

Temperature, relative humidity. and 

time. Successful development of a 

schedule depends upon determining 

the optimum combination of the three 

factors during drying lumber from green 

to final moisture content (MC). Surface 

and end checking. honeycombing, and 

ring failure result from shrinkage 

stresses caused by drying wood below 

the fiber saturation point. Prolonged 

exposure to excessive temperature 

while any of the wood is above the fiber 

Drying Procedures for 

BacteriallyInfected 

Northern RedOak 

Lumber

By

JOHN M. McMILLEN, 

JAMES C. WARD, and 

JOSEPH CHERN, 

Wood Technologists 

saturation point weakens the wood and 

increases its susceptibility to interior 

checking and collapse. 

Until recently. it was generally felt that 

the ability of wood to withstand 

shrinkage stresses without rupturing 

depended largely on its MC, drying 

temperatures. rate of moisture loss, and 

the combined effect of time and 

temperature on reduction of strength. 

Now it appears that the extent to which 

bacterial tree infections weaken the 

wood must also be considered. 

California black oak and southern 

lowland oaks are notoriously difficult to 

dry without defects. Ward and Shedd 

(17 ) found many of the kilndrying 

defects in black oak lumber from 

Northern California could be traced to 

bacterial tree infections. Overcup oak 

and several species of southern red 

oaks growing on poorly drained 

bottomlands are often heavily infected 

with anaerobic bacteria, while oaks 

growing in the adjacent better drained 

uplands are more likely to have healthy 

heartwood (J.C. Ward, unpublished 

work). 

Other investigations 

Many kiln operators have been unable 

to successfully kiln dry green lumber 

from southern lowland oaks and 

California black oak. Consequently, 

common industrial practice is to first air 

dry green lumber from these species to 

at least 30 percent MC and preferably 

20 percent. The air-dried stock can then 

be kiln dried under a conventional 

schedule with good results, providing 

the airdrying conditions were mild. 

Kiln-drying experiments with a species 

such as California black oak are 

concerned with reducing drying times 

from the green condition while avoiding 

the degrade. 

Ellwood ( 4 , 5 ) was quite successful 

with kiln drying 4/4 California black oak 

green from the saw by using a schedule 

he designed from an analysis of drying 

stresses similar to that described by 

McMillen ( 7 ) and Reitz (14 ). However, 

when this schedule was employed by 

Ward and Shedd (17 ) to dry a green 

charge of 4/4 California black oak that 

was heavily Infected by anaerobic 

bacteria, 52 percent of the charge was 

degraded because of excessive 

collapse and checking. 

Low-temperature forced-air drying 

has been intensively investigated as a 

compromise solution to me problem of 

shortening the time required to air dry 

green oak lumber. Cobler (1 ) found that 

accelerating the air drying of southern 

oaks by forced-air circulation will give 

trouble if the humidity cannot be 

controlled. By controlling the humidity 

and keeping dry-bulb temperatures 

under 100° F. Smith (15 ) was able to 

successfully reduce the required initial 

air-drying period for California black 

oak by about 4 weeks. Gatslick (6 ) dried 

8/4 northern red oak from green to 20 

percent MC in 19 days and 9/4 in 55 

days in a forced-air dryer that had 

temperature and humidity controls. 

D. G. Cuppett3 was able to dry 4/4 

southern swamp red oak from green to 

6 percent MC in 53 days with good 

quality results. Cuppett used a 

1 Maintained at Madison, Wis., in cooperation with 

the University of Wisconsin. 2 Underlined numbers in parentheses refer to 

literature cited at end of report. 3 Personal communication from Cuppett at the 

Northeast Forest Experiment Station, Princeton, 

W.Va. 
combination of forced-air drying 

followed by conventional kiln drying. A 

matched sample of the same lumber 

required a total drying time of 109 days 

(green to 6 pct MC) when dried by a 

combination of conventional air drying 

and kiln drying. Forced-air drying times 

for the southern swamp red oak were 

significantly longer than the forced-air 

drying times for 4/4 Appalachian red 

oak reported by Cuppett and Craft (3). 

EXPERIMENTAL 

General Procedure 

In the first phase of this study, six 

different drying procedures were 

screened to determine an optimum 

method for drying bacterially infected 

red oak. The screening phase utilized 

29- to 30-in.-long sections crosscut 

from 8-to 12-ft boards for experimental 

drying samples. Other boards, also 8 to 

12 ft long, were dried in the second 

phase of the study. 

The second phase employed a 

combined procedure of forced-air 

drying and kiln drying that was judged 

to be most successful from the results 

of the screening phase. 

Sample Material 

Drying samples used in both phases of me study were taken from 600 bd ft of 

normal, rough 4/4 northern red oak 

(Quercus rubra L.) lumber and a like 

amount of rancid, bacterially infected 

red oak lumber. The material came from 

winter-cut-treesand was collected at 

two sawmills, one in Wisconsin and one 

in Illinois. Before the lumber was 

collected. logs suspected of being 

bacterially infected were marked on the 

log yard so that they could be followed 

through the sawmill and related to 

sample boards on the green chain. 

Identification of logs with bacterially 

Infected heartwood was based on the 

recommendations of Ward et al. (18) 

where the presence of shake is a 

principal criterion. At the time of 

selection, Infected logs at Mill Source A 

generally contained more extensive and 

advanced stages of bacterial infection 

than infected logs at Source B. Heavily 

Infected logs will contain rancid 

heartwood that extends almost to the 

sapwood and emits strong, rancid, 

goat-type odors. Oak heartwood with 

early types of infection will generally 

emit strong vinegar-type odors (17 , 18). 

Both mill locations had ample supplies 

of logs with normal. healthy heartwood. 

Sample boards were selected at the 

green chain where Infected material 

tom infected logs, end normal material 

from healthy logs. were stacked in 

separate piles. Logs preselected as 

Infected often yielded boards with a 

mixture of normal and Infected 

heartwood, so boards judged to have 30 

percent or less of normal heartwood 

were placed in the Infected material 

piles. For many boards with mixtures of 

heartwood, it was difficult to Judge the 

amount of infected heartwood on the 

basis of odor because of freezing winter 

conditions at both mills. Normal and 

infected wood designations had to be 

changed sometimes after the green 

lumber thawed. About 35 percent of the 

infected material at each mill graded as 

Select and Better lumber. 

The lumber samples were trucked to 

the Forest Products Laboratory in 

covered piles and then each pile was 

wrapped in polyethylene film and stored 

at 36° F. 

Screening Tests 

This initial phase involved two 

approaches (1) kiln drying directly from 

the green condition and (2) kiln drying 

after preliminary forced-air drying. Six 

drying procedures were used, three in 

each approach. The screening tests are 

outlined in table 1together with the 

number of normal and infected samples 

used in each procedure. 

Drying Sampler

The samples listed in table 1 were cut 

tom relatively clear sections of 120 

boards. Individual drying samples 

measured 29 to 30 in. In length with 

widths that ranged from 6 to 10 in. The 

average green thickness of samples 

from Mill Source A was 1-3/16 in. and 

from Source B 1-1/8 in. During sample 

preparation, matched wood sections 

were taken for comparative evaluation 

of green wood MC. growth rate, specific 

gravity, and type of bacterial infection. 

The type of bacterial infection was 

based largely on odor where the 

presence of rancid (butyric and valeric 

acids) and goat (caproic acid) odors 

indicated advanced or severe bacterial 

infections. Strong vinegar (acetic acid) 

odors usually indicated early or less 

severe bacterial infections. The majority 

of samples were all heartwood, but a 

few samples. particularly those 

designated as normal, contained small 

areas of sapwood. Each group of 

samples was wrapped separately in 

polyethylene film and stored at 36° F. A 

small tray of paradichlorobenzene 

crystals was used within each package 

to inhibit mold, but some mold did occur 

on sapwood. 

When starting an experimental drying 

run, the appropriate groups were 

removed from cold storage and warmed 

to 80° F. A 6- by 12.5-in. zone for 

surface check measurement was laid 

out on a clear, flat-grain portion of each 

face of each specimen (fig. 1). In this 

zone of each specimen, four lines were 

drawn to indicate where to crosscut the 

piece so as to measure honeycombing 

(internal checking). One of these lines 

was extended to the full width for 

shrinkage measurements. Each 

specimen was end coated with two 

coats of fast-drying chlorinated rubber 

coating. After thickness and width was 

measured. each specimen was 

stickered in the rack in the drying 

chamber (fig. 2). The material was kept 

2 
from drying from the time of original

sawing until shortly before the run 

started. 

Drying Methods

For kiln drying green boards, we used 

three experimental procedures with a 

modified form of hardwood kiln 

schedule T4-D2 ( 13 ) as a control 

schedule. This control schedule, KDSTD, has been used extensively by

industry for normal, upland red oak and 

has proven satisfactory in regard to 

drying time and quality. The 

modification is common for high-MC 

stock and consists of making the first Figure 1.–Schematic showing layout of study measurement zones on the surface 

two changes in equilibrium moisture of drying samples for the screening phase and kiln samples for the second phase 

content (EMC) at higher than with 8- to 12-ft boards: 

prescribed wood moisture levels. Mild A, B, C, D–Cross-section sawing lines to measure honeycomb for both study 

kiln schedules KD-I and KD-II were phases. modifications of hardwood kiln E, F, G–One-inch-wide sections for measuring final MC's (av. and shell + core), schedule T2-C1 ( 12 ). ScheduleT2-C1 is and stress relief for both phases. recommended for kiln drying southern

lowland oak green from the saw ( 13 ). (Key: Surface checks measured in 6- by 12.5-in. area far screening phase only.) 

Mild schedule KD-I has a lower initial M 148 096 

dry-bulb temperature than KD-STD but 

approximately the same EMC values. 

After 48 percent MC was reached, 

EMC’s were lowered fast to avoid 

increasing total kiln time greatly. A 160° 

F dry-bulb temperature was used at 15 

percent MC to lessen chance of 

honeycombing. Mild schedule KD-II was 

devised to reduce excessive 

honeycombing that occurred when mild 

schedule KD-I was used. In mild 

schedule KD-II, the early portion was 

longer and milder, and the middle 

portion was lower in temperature than 

for schedule KD-I. 

The first forced-air drying procedure, 

FA-I(25), was an attempt to follow the 

procedure for 4/4 red oak described in 

(2 ), using a 17 percent EMC and a drybulb temperature of 75° to 85° F. 

Equipment limitations resulted in only a 

13 percent EMC being attained while 

temperature remained at 85° F. When 

the seven wettest specimens from 

Source A reached 25 percent MC, 

conditions were changed to those of 

kiln schedule KD-I at the 25 percent MC 

level. Excessive surface checking and 

honeycombing occurred with FA-I(25), 

so the second and third forced-air 

procedures, FA-lI(20) and FA-ll(25), 

were designed according to Cuppett 

and Craft's procedure (2) for 8/4 oak.

The rationale was that 8/4 oak is more 

difficult to dry than 4/4 oak;

commercially, infected 4/4 boards 

could be included in a dryer charge of 

8/4 noninfected oak if the experiment 

showed that such conditions would be 

satisfactory. Kiln schedule KD-II was Figure 2.–Method of exposure in dry kiln for sample board sections in the 

used at levels of 20 percent MC for the screening phase of the study. (M 145 864-9) 

3 
second forced-air tests, and 25 percent 

MC for the third tests. 

The average air velocities over the 

specimens were kept uniform by 

manipulating air bypasses in the main 

kiln baffle, using small end baffles on 

some shelves, and employing a 

balanced design of specimen 

placement. With both continuous and 

intermittent fan operation, air velocities 

across the drying samples averaged 

about 330 ft/min for kiln procedures 

KD-STD and KD-II. and about 290 ft/ 

min for KD-I. Average air velocity with 

continuous fan use throughout the first 

forced-air procedure. FA-I(25), was 415 

ft/min. With intermittent fan use during 

the first 3 days for FA-II(20), the average 

air velocity was about 300 ft/min and 

thereafter was 180 ft/min. Air velocity 

was kept at 180 ft/min for both 

intermittent and continuous fan use for 

FA-ll(25). Air circulation was reversed 

every 6 hours.

Equalizing of MC in the samples was 

started when the driest specimen 

reached 4 percent MC and was carried 

out using a 4 percent EMC. 

Conditioning started when the wettest 

Specimen was 6 to 7 percent MC and 

was carried out with an 11 percent 

EMC. 

Measurements 

At 2- to 5-day intervals, small groups 

of specimens were briefly removed from 

the kiln. Each specimen was weighed 

and examined for surface checking and 

ring failure. While one specimen was 

being examined, the others were 

protected from moisture loss and 

temperature change. The average MC 

of the seven wettest specimens in each 

group was plotted on semilogarithmic 

graph paper to predict when the next 

kiln condition change would be 

reached. All kiln condition changes 

were made within 5 hours of the time 

dictated by the schedule. 

Width of both the green and dry 

material was measured to the closest 

0.02 in. with a steel rule. Average green 

thickness was determined from similar 

measurements on both edges. Final 

thickness was measured by micrometer 

calipers at the thinnest place along the 

width measurement line. The effect of 

drying conditions and type of 

heartwood on shrinkage was evaluated 

on the basis of transverse shrinkage or 

the combined shrinkage in thickness 

and width. 

The cumulative length of surface 

checks 0.50 in. or more in length was 

measured with a steel rule in the 

designated zone (outlined on the flattest 

grain portion of both surfaces of each 

sample board). The total surface area of 

the two zones was 150 in.2. Depth Of the 

deepest surface checks was measured 

to the closest 0.02 in., using a probe 

0.015 by 0.052 in. wide at the tip. 

The width and length of areas that 

would be unusable because of ring 

failure, heart shake, and split were 

measured to the closest 1/4 in. in the 

12.5-in. length of the surface check 

zone. but considering the full width of 

the specimen. The maximum width of 

each ring failure was multiplied by its 

maximum length. 

Observations during the final crosssectional examination sometimes 

resulted in increases of ring failure 

dimensions. The shake or splits existing 

in a few of the green specimens were 

marked and not included in areas lost 

because of drying operations. Although 

surface and internal checks often 

occurred in ring failure zones, these 

checks were not included in the 

evaluation of ring failure defects. 

The lengths of honeycomb checks 

were measured from the four sections 

crosscut from the samples to the 

nearest 0.02 in. and the cumulative total 

reduced to honeycombing per 10 inch 

of width. In most cases, honeycomb 

checks were open and easily 

measurable. but a few barely visible 

checks extending down from the 

surface were included in 

honeycombing, 

Lumber DryingTest 

The remainder of the rough green 

lumber not used for the screening tests 

was dried in an experimental FPL dry 

kiln for this phase of the study. During 

the 4 months between tests, the boards 

were wrapped in polyethylene and 

stored at 36° F. Ninety boards, 8 to 12 ft 

long. were stacked in a 4-ft-wide kiln 

charge on 3/4-in. stickers spaced 16 in. 

apart. Fourteen kiln samples, 32 in. 

long, were used to determine moisture 

loss during drying. About half of the 

lumber volume and 12 kiln samples 

contained bacterially infected 

heartwood. 

The drying procedure used was 

similar to the forced-air procedure, FAII( 25 ), used in the screening tests, but 

was modified to terminate forced-air 

drying at 23 percent MC rather than 25 

percent. It was assumed the interior 

boards In the pile would be slightly 

wetter than the kiln samples on the 

edge. Kiln fans were run between 8 a.m. 

and 8 p.m. daily for the first 11 days, 

after which fans were operated 

continuously. Air velocity averaged 200 

ft/min during the entire run and 

uniformity across the load was achieved 

by baffling. Fan reversal was changed 

from 6 to 3 hours. 

For evaluation purposes, the boards 

were divided into two groups: (1) The 14 

kiln sample boards and (2) the uncut 

lumber. Each of the 14 kiln sample 

boards was marked off according to the 

method outlined for the 30-in. drying 

samples used in the screening tests (fig. 

1). The kiln samples were measured for 

shrinkage and drying defects by the 

same methods used in the screening 

tests. Before drying, ail full-length 

boards were examined for checks and 

splits and measured for thickness and 

width. After drying, these boards were 

measured for shrinkage, surface. 

checks. and end splits in the rough 

state. Then the boards were twice 

surfaced on two sides; first to 7/8 in. 

thick and then to 3/4 in. thick. Presence 

of honeycomb and ring failure was 

determined on the basis of open 

internal checks appearing on the 

surfaced faces of the boards. 

RESULTSANDDISCUSSION 

Wood Quality Factors 

Some characteristics of heartwood 

for the red oak used in this study are 

listed in table 2. The average green MC 

content values of infected wood from 

Source A are higher than the average 

green MC of normal wood and compare 

favorably with values from other studies 

and observations in industry. The 

material from Source B is an exception 

to the general rule, as the green MC of 

normal heartwood averaged higher than 

the MC of infected heartwood. Both 

normal and infected heartwood from 

Source B had higher average MC values 

than are generally observed for normal 

green oak heartwood. 

Although Source B material had 

higher green MC values, less advanced 

or heavily infected rancid heartwood 

was present than in the infected 

material from Source A. The advanced 

type of bacterial infection observed in 

Source A material could be related to a 

tendency of the heartwood to develop 

deep surface checks and honeycomb 

during the early and middle stages of 

drying under both conventional and 

forced-air drying procedures. These 

data suggest that presence of such 

volatile fatty acids as butyric, valeric, 

and caproic acids better indicate 

defect-prone oak heartwood than do 

4 
green MC values averaging above 85 drying curves and times are shown in infected wood. Red oak with advanced 

percent. figures 3, 4, and 5 for each group of bacterial infection is subject to 

The specific gravity of heartwood drying procedures. excessive honeycombing even with the 

from Source A was higher than for Changes in drying conditions were moderate temperatures of experimental 

Source B. These higher specific gravity based on the average MC of the seven kiln schedule KD-I. Under the kiln 

values can be related to the faster rates wettest infected specimens from Source drying conditions of KD-STD, 

of growth for the B material. The A. This group was the slowest drying of honeycombing and considerable 

presence of bacterial infections could the four groups in each run. Generally, darkening of the wood occurred in the 

not be related to a lowering of specific the other groups in each run had infected material. Forty percent or more 

gravity for study material from either slightly milder kiln conditions than the of the infected Source A specimens 

source. infected material from Source A after dried by KD-STD, KD-I, and FA-I(25)

the first step of each schedule. had measurable internal checking or 

Drying Conditions and Rates Surface checking and honeycombing honeycombing. This defect can be 

were the basis for judging the six drying controlled by withholding use of higher Drying conditions are listed in table 3 procedures in the screening phase of temperatures until internal MC values for the three kiln procedures, in table 4 the study. High EMC's and low air are below the fiber saturation point for the three combined forced-air plus velocities in the early stages of drying (8 , 14 ). Less honeycomb and greater kiln procedures, and in table 5 for the help to limit surface checking in brightness was attained in infected full-sized lumber phase. Similarly, wood dried by forced-air procedures 

FA-ll(20) and FA-Il(25). Table 2.–Characteristics of normal and bacterially infected northern red oak used in this study All procedures were generally 

satisfactory for infected Source B Specific gravity Moisture content Infect ion material. Total drying times shown 

of green heartwood Growth rate 

ovendry weight and type2 include equalization to about 5 percent 

Range rings average MC and conditioning for 14 to 

Source Mean Minimum Maximum per inch1 Mean Range Early Advanced 17 hours. The total time of 32 days for 

green volume 

Pct Pct Pct Min Max Pct 

NORMAL HEARTWOOD 

schedule KD-STD was slightly longer Pct than the 26 to 28 days generally 

required commercially for normal red 

A 80.0 64.1 - 96.9 9.7 0.576 0.444 - 0.736 - - oak of this density and thickness. 

Schedule KD-I required only 2 days B 93.7 67.5 - 108.9 11.8 .540 .494 - .595 - - longer than KD-STD. Although 

INFECTED HEARTWOOD schedule KD-I was very mild at the start, 

A 86.2 68.7 - 105.9 10.6 .568 .489 - .632 29 71 conditions were made more severe than 

those of KD-STD in the intermediate B 89.3 56.1 - 103.4 11.0 .553 .503 - .614 75 25 part of the schedule. Schedule KD-lI 

was considerably less severe than KD­ 1On cross sections of specimens after drying. 

2Estimated as a percent of total samples with infected heartwood from each source. Early infection indicated by wood with strong vinegar or STD and took 9 days longer. 

acetic acid odors while advanced infections indicated by rancid and goat odors of butyric, valeric, and caproic acids. The first forced-air procedure FAl(25) took 37 days. Although this was 

Table 3. – Drying conditions for kiln drying of green 4/4 northern red oak samples in the screening phase of the study 

KD-STD, modified kiln 

schedule T4-D2 KD-l KD-II 

Control Temperature, Control Temperature Control Temperature 

MC1 Dry-bulb. Wet-bulb EMC MC1 Dry-bulb Wet-bulb EMC MC1 Dry-bulb Wet-bulb EMC 

Pct °F °F Pct Pct °F °F Pct Pct °F °F Pct 

Green 110 106 18 Green 104 100 17.5 Green 105 102 18.5 

54 110 105.5 17 58 105 100 16 51 106 102 17.5 - - - - 48 105 96.5 13 46 106 100 15 

42 110 101.5 13 40 110 93.5 9 40 106 97.5 13 

35 110 96 10 34 115 84 5 35 110 92 8.5 

30 120 88 5 30 120 79 3 30 115 84 5 

25 130 80 2 25 130 85 3 25 120 86 4.5 

21 140 90 2.5 20 140 92.5 3 20 130 91 4 

15 180 129 3 15 160 110 3 16 150 95 2 

11 180 145 5 11 180 136 4 11 170 119 3 

9 180 137 4 - - - - - - - -

EQUALIZING 

8 175 131 4 8 170 122.5 3.5 10 170 130 4.5 

CONDITIONING2 

7 180 171.5 12 6 176 166 11 6 175 165 11 

1Average MC of 7 wettest specimens of material from Source A. 

2Conditioning times approximately 14 to 17 h. 

5 
only 5 days longer than the control, 

drying conditions were too severe. 

Forced-air procedure FA-II(20) was very 

conservative. Total time was 52 days. 

Using forced-air procedure FA-II(25) 

reduced drying time to 43 days. When a 

kiln charge of full-sized lumber was 

dried by forced-air procedure FA-II(23), 

similar to FA-II(20) and FA-II(25), total 

drying time was extended to 58 days 

(fig. 5). 

There was no practical difference in 

final MC or drying stress relief among 

drying procedures. sources, or types of 

material, Final MC average between 6.0 

and 7.6 percent, except for 7.8 percent 

for both normal and infected material 

from Source A dried by KD-STD. 

Minimum MC values for ail specimens 

were 5.6 percent, and all specimens 

were fully relieved of stress 

("casehardening") except three dried 

by KD-STD, two of the screening 

forced-air runs FA-II(20) and FA-II(25). 

and one of the final forced-air run FAII(23). The treatments were considered 

entirely satisfactory and not conducive 

to honeycombing of infected material. 

Shrinkage 

Average shrinkage values for all 

samples dried In thisstudy are listed in 

table 6. Shrinkage of the heartwood 

from Source A was greater than 

shrinkage of material from Source B 

6 
Figure 3.–Drying curves for normal and infected heartwood of northern red oak M 148 099 

board samples kiln dried in the screening phase. 
Figure 4.–Drying curves for normal and infected heartwood of northern red oak 

board samples dried by combination forced

-air and kiln-drying procedures in the 

screening phase. M 148 100 
Compression set develops in the interior 

of boards during the intermediate 

stages of drying when the MC of the 

core is still above the fiber saturation 

point. The higher temperatures of kiln 

drying will cause greater compression 

set with significantly greater shrinkage 

than the lower temperatures of air 

drying ( 8 ). A possible explanation for 

shrinkage differences between normal 

and infected heartwood is that infected 

wood is weaker in compressive 

strength. 

Figure 6 shows that during the course 

of drying the core of infected heartwood 

being kiln dried will be wetter at a given 

average MC than the core of infected 

heartwood being forced-air dried. At the 

same average MC, the cores of both 

normal and infected heartwood being 

air dried will have about the same inner 

MC's. Yet, infected heartwood that is 

forced-air dried shrank more than 

forced-air dried normal heartwood. 

Although not shown in figure 6, the core 

MC of kiln-dried normal heartwood is 

less than the core MC of infected 

heartwood at the same average MC and 

close to the core MC of the air-dried 

material. It does not appear that 

differences in core MC under different 

drying procedures contributes as much 

to magnitude of shrinkage as do drying 

temperatures and compressive strength 

of the wood. 

Surface Checking 

infected heartwood has a greater 

tendency to surface check under all 

drying procedures (table 7). The 

tendency to surface check cannot be 

completely blamed on bacterial 

infections. and such wood quality 

factors as specific gravity and growth 

ring orientation must be considered 

also. Boards from Source A with both 

normal and infected heartwood 

checked more than comparable 

material from Source B. and this can be Figure 5.–Drying curves for normal and infected heartwood of northern red oak In attributed to differences in growth rate 

the full-size lumber phase. M 148 097 and specific gravity. Flat-sawn boards 

with normal heartwood were more likely 

and can be partly explained by the These observations agreed with other to surface check then quarter-sawn and 

higher specific gravity of the A material. studies where kiln-dried and bacterially bastard-sawn boards with infected 

factors other than specific gravity are infected heartwood of eastern red oaks heartwood. However, a quarter-sawn 

evident in table 6 and need to be (J. C. Ward, unpublished) and California board with infected heartwood is prone 

to develop honeycomb, whereas a flat- considered with respect to shrinkage of black oak (17) shrinks more than airoak. dried and normal heartwood. sawn board with normal heartwood 

Table 6 shows that in this study The general explanation for does not develop honeycomb even after 

infected heartwood shrinks more than differences in shrinkage (given similar surface checking more. 

normal heartwood. Also, infected specific gravity and growth ring Bacterial infection does contribute to 

heartwood tends to shrink more when orientation) is that pieces with greater increased severity of surface checking 

oven-dried green than when first dried by shrinkage developed greater although it is not the sole factor 

low-temperature, forced-air procedures. compression set during drying. Involved. The data In table 7 indicate 

9 
any advantages that might be gained 

from using forced-air drying for 

reducing surface checking will not be 

realized if bacterial heartwood is 

present. During the early stages of 

drying, the surface zones of green oak 

are subjected to very high 

perpendicular-to-grain stresses (19) 

and bacterial infections obviously 

weaken the wood in this direction 

without necessarily reducing the 

specific gravity. 

Initial surface checking developed by 

the second to fourth day of drying. and 

infected heartwood usually started 

checking earlier than normal 

heartwood. The MC at the time of last 

surface check extension was lower for 

infected heartwood than for 

noninfected wood. The MC's Of infected 

material from both sources averaged 37 

percent at the time of last checking. For 

normal material surface checking 

stopped sooner at 52 percent MC as 

compared to normal Source A material 

which ceased checking at 44 percent 

MC. Gatslick (6 ) found that surface 

checking in both 4/4- and 9/4-in.-thick 

red oak stopped at approximately the 

same MC (45 pct) when dried under 

low-temperature forced-air conditions. 

In this study the MC at last checking 

depended on differences in type of 

heartwood rather than drying 

procedures. 

Intermittent fan operation is 

recommended by Cuppett and Craft (2) 

lor forced-air drying batches of oak that 

tend to surface check more than 

normal. The fans should be turned off 

during the day and on only at night. For 

oak being forced-air dried in 

installations without humidity control, 

Cobler ( 1 ) found that fans must be 

turned off when the relative humidity 

falls below 50 percent. Probably some 

flattening of the moisture gradient 

occurs, resulting in lower drying 

stresses than is possible with full-time 

fan operation. Use of intermittent air 

circulation during the early stage of 

forced-air drying in procedures FAII(20) and FA-II(23) might provide a 

partial explanation for low checking 

values in infected boards. These low 

checking values compare with those 

achieved by presurfacing normal red 

oak ( 10 , 11 ). 

Honeycombing 

Honeycomb was always confined to 

bacterially infected heartwood. Rancid 

heartwood with advanced stages of 

Figure 6.–Core MC as a function of average MC at intermediate MC infection was much more prone to 

determinations. M 148 098 develop honeycomb than the vinegar10 
type heartwood in earlier stages of 

Infection. Bacterially infected boards 

from Source A contained proportionally 

more rancid heartwood and tended to 

develop more honeycomb than infected 

boards from Source B. 

The Occurrence and seventy of 

honeycomb is greatly reduced but not 

always eliminated with low

-temperature, 

forced

-air drying of green oak with 

infected heartwood (tables 8 and 9). 

Best results were obtained when 

infected lumber was forced

-air dried to 

an average MC of 20 percent with 

procedure FA

-II(20). Bacterial 

heartwood in early stages of infection 

(Source E) was successfully dried 

without honeycomb with conventional 

kiln schedule T4

-D2 as shown in figure 

7. Comparable reduction of honeycomb 

in rancid heartwood from Source A was 

obtained only by mild forced

-air drying 

of green material as shown in figure 8. 

We assume. from results of previous 

studies ( 17 , 18 ), that kiln schedules 

moresevere than T4-D2 will cause 

honeycombing of oak lumber with all 

types of bacterial heartwood. It is also 

possible that, with initial kiln 

temperatures in excess of 140° F, 

honeycomb will develop even in normal, 

healthy oak heartwood but this Is yet to 

be studied. 

Honeycomb can originate in three 

ways: Deepening of a surface check 

followed by closure at the surface 

(bottle

-neck checks): extension of an 

end check; and spontaneous formation 

within the board (8 ). All three varieties 

of honeycomb were observed in the 

bacterial heartwood of this study from 

material dried under what is generally 

considered mild temperature conditions 

(table 9). It is well known that the raising 

of temperatures can cause 

honeycombing of oak that has been 

partially air dried to 30 percent MC or 

above, but when the MC is less than 20 

percent, temperatures of 180° F will 

cause no internal checking even in 

bacterially infected oak. 

We must assume then that bacterially 

infected oak

-especially the rancid 

type–is intrinsically weaker than normal 

oak even at the lower drying 

temperatures. This is supported by the 

curves in figure 6 showing no significant 

difference between the core MC of 

bacterial and normal oak boards when 

dried under low

-temperature, forced-air 

conditions. In this respect, it is 

noteworthy that Youngs and Bendtsen (19 ) found a significant consistency in 

thelocation of maximum stress zones in 

the cross section of red oak boards 

dried at temperatures of 80°, 110°, 

11 
Figure 7.–Cross sections of specimens from Sources A and B kiln dried by T4-D2 

schedule (KD-STD), showing honeycomb and some ring failure and some 

darkening. 

(M 145 764-2, M 145 764-3) 
Figure 8.–Cross sections of specimens from Sources A and B dried by second 

forced-air procedure to 25 percent MC, then kiln dried by schedule KD-II. 

(M 145 764-12, M 145 764-1) 
125°, and 140° F. They observed that 

the consistent location of maximum 

shearing stress at all temperature levels 

is coincident with the location of 

honeycomb and collapse. 

Ring Failure 

The occurrence and severity of ring 

failure is shown in table 10. Ring failure 

developed only in bacterially infected 

heartwood, but not in as many 

specimens as surface checking and 

honeycomb. On the other hand, Me 

severity of ring failure could not be 

decreased with mild drying conditions 

to the extent that honeycomb was 

decreased. This supports the 

proposition by Ward et al. (18 ) that ring 

failure is an incipient form of ring shake 

which is first initiated by stresses in the 

living tree. 

CONCLUSIONS 

1. Red oak lumber containing rancid 

or bacterially infected heartwood 

should first be dried green from the saw 

by a low-temperature, forced-air 

schedule to at least 25 percent MC and 

preferably 20 percent. The forced-air 

dried lumber can then be kiln dried to 6 

to 8 percent MC with good results by 

using a milder kiln schedule than 

normally recommended for air-dried 

oak. 

2. Forcedair and kiln-drying 

combinations will greatly reduce. but 

not always eliminate, honeycombing Of 

oak heartwood in the advanced, rancid 

stages of bacterial infection. Red oak 

heartwood in early stages of bacterial 

infection can be kiln dried green under 

both mild and conventional kiln (T4-D2) 

schedule with minimal and sometimes 

no development of honeycomb. 

3. Severity of surface checking is 

substantially increased by bacterial 

Infections. Low-temperature, forced-air 

drying is not as effective for reducing 

surface checking in bacterially infected 

oak as it is for reducing honeycombing. 

4. Bacterially infected oak is subject 

to surface check extension at lower MC 

values than normal oak, precluding fast 

lowering of relative humidity in the 

intermediate stages of kiln drying. 

5. Ring failure was not minimized by 

using the milder drying conditions of 

this study as were surface checking and 

honeycomb. This is because ring failure 

appears to be an incipient form of ring 

Shake that begins in bacterially infected 

wood of the living tree. 

6. Shrinkage of bacterially infected 

heartwood is greater than the shrinkage 

of normal heartwood. Kiln-drying 

bacterially infected oak from the green 

condition generally results in greater 

shrinkage than if the wood was first 

dried under low-temperature, forced-air 

conditions. 

7. If kiln dried from the green 

condition, northern red oak in the 

advanced stages of infection has higher 

core MC values than forced-air dried 

normal or infected oak at any specific 

average MC value between 14 and 32 

percent. 

8. Satisfactory relief of drying stresses 

was attained with both normal and 

bacterially infected wood by equalizing 

at 1 percent lower MC and EMC than 

are normally recommended, and then 

conditioning for 14 to 17 hours at the 

EMC values usually prescribed. 

14 
LITERATURECITED 

1. Cobler, W. E. 

1962. Accelerated air-drying by forced-air circulation. So. Lbrmn. 

205(2561):97-99.

2. Cuppett, D. G., and E. P. Craft. 

1975. Low-temperature, forced-air drying of Appalachian hardwoods. 

USDA For. Serv. Res. Pap. NE-328. NE For. Exp. Stn., Broomall, Pa. 

3. Cuppelt, D. G., and E. P. Craft. 

1971. Low-temperature drying of 4/4 Appalachian red oak. For. Prod. J. 

21(1):34-39. 

4. Ellwood, E. L. 

1959. Drying stress patterns in California black oak. Calif. Forestry and 

For. Prod. 9. 

5. Ellwood, E. L. 

1959. Kiln drying green 4/4 California black oak. Calif. Forestry and For. 

Prod. 17. 

6. Gatslick, H. B. 

1962. The potential of forced-air drying of northern hardwoods. For. 

Prod. J. 12(8):385-388. 

7. McMillen, J. M. 

1955. Drying stress in red oak. For. Prod. J. 5(1):71-76.

8. McMillen, J. M. 

1955. Drying stresses in red oak: Effect of temperature. For. Prod. J. 

5(4):230-241.

9. McMillen, J. M. 

1958. Stresses in wood during drying. U.S. For. Ser. For. Prod. Lab. 

Rep. 1652. 

10. McMiilen, J. M. 

1969. Accelerated kiln drying of presurfaced 1-inch Northern red oak. 

USDA For. Serv. Res. Pap. FPL 122. 

11. McMillen, J. M., and R. C. Baltes. 

1972. New kiln schedule for presurfaced oak lumber. For Prod. J. 

22(5):19-26. 

12. McMillen, J. M., and E. M. Wengert. 

1978. Drying eastern hardwood lumber. USDA Agric. Handb. No. 528. 

13. Rasmussen, E. F. 

1961. Dry Kiln Operator's Manual. USDA Agric. Handb. No. 188. 

14. Rietz, R. C. 

1970. Accelerating the kiln drying of hardwoods. So. Lbrmn. 

221(2741):19-22, 24. 

15. Smith, H. H. 

1961. Recommendations for drying California black oak. U.S. For. Sew. 

Tech. Pap. 62. Pacific SW For. and Range Exp. Stn. 

16. Ward, J. C. 

1972. Anaerobic bacteria associated with honeycomb and ring failure in 

red and black oak lumber. Phytopathology 62(7):796 (abstract). 

17. Ward, J. C., and D. Shedd. 

1979. California black oak drying problems and the bacterial factor. 

USDA For. Serv. Res. Pap. FPL 344, For. Prod. Lab.. Madison, Wis. 

18. Ward, J. C., R. A. Hann, R. C. Baltes. and E. H. Bulgrin. 

1972. Honeycomb and ring failure in bacterially Infected red oak lumber 

after drying. USDA For. Sew. Res. Pap. FPL 165, For. Prod. Lab., 

Madison, Wis. 

19. Youngs. R. L., and B. A. Bendtsen. 

1964. Tensile, compressive, and shearing stresses developed in red oak 

as it dries. For. Prod. J. 14(3):113-118. 

2.0-8-79 

U.S. GOVERNMENT PRINTING OFFICE 1979-650-028/12 

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