United States 

Department of 

Agriculture

Forest Service 

Forest

Products

Laboratory

Research

Paper

FPL 344 

California Black Oak 

Drying Problems and 

The Bacterial Factor 
Abstract

It is often difficult to kiln dry California 

black oak lumber green from the saw 

without developing excessive degrade 

from honeycomb, ring failure, and 

collapse. Results from this study 

indicate that defect-prone lumber 

contains heartwood that was infected 

and weakened by anaerobic bacteria in 

the living tree. Green, bacterially 

infected boards should be segregated 

and dried to percent moisture 

content (MC) under mild air-drying 

conditions or by low temperature, 

forced air-drying schedules. The 

percent MC lumber can then be kiln 

dried to end-use MC without additional 

defect. Boards with normal, noninfected 

heartwood can be successfully kilndried green from the saw under 

conventional schedules. 

California Black Oak 

Drying Problems and

the Bacterial Factor

By

James C. Ward1

and

Del Shedd2

economic impetus on the west coast to

utilizeCalifornia black oak sawtimber in

a region predominately covered with 

softwood sawtimber. The bacteriarelated drying problems are easily

obscured by more obvious processing 

and marketing problems with west coast

hardwoods. Forest surveys indicate

California black oak is represented in

sawtimber volume by approximately 

billion board feet in California and 

million board feet in southwestern 

Oregon (Smith Ellwood 1958). 

Even so, west coast oak timber 

resources are but a fraction of those in 

the eastern United States where the net 

merchantable volume of oak sawtimber 

is billion board feet with select red 

and black oaks comprising 39.5 billion 

feet (Forest Service 1973). 

Consequently, eastern lumber 

producers are more experienced in the 

utilization and marketing of oak lumber. 

West coast producers are more familiar 

with softwoods which in California and 

western Oregon comprise about 

billion board feet of merchantable 

sawtimber on commercial timber lands 

(Forest Service 1973). 

California black oak is the only oak 

species considered potentially 

important for sustained forest 

management on commercial forest 

lands of the west coast (Roy 1962). 

However, lumber processing problems 

resulting from improper seasoning and 

excessive drying defects were 

considered a serious obstacle to

successful management and utilization 

of California black oak (Edwards 

McDonald and Sundahl 1967). Bois 

(1975) found that the entire philosophy 

of kiln drying on the west coast must be 

INTRODUCTION

A strong demand for kiln-dried oak 

lumber is evident on the west coast, 

especially by the furniture, millwork, and 

cabinet industries in southern 

California. Most of the oak lumber 

consumed there is imported from the 

eastern United States, but a ready 

market exists for kiln-dried lumber from 

native western oaks. One Los Angeles 

lumber dealer says he could easily sell 

one-half million board feet of 4/4-inch

California black oak lumber every 

month of the year, but is unable to 

locate a mill that can consistently 

furnish this amount of kiln-dried

material.

lumber is in short supply is that the 

green lumber is difficult to dry without 

costly degrade. 

This paper reports on a cooperative 

study between the U.S. Forest Products 

Laboratory (FPL) and the KimberlyClark Corporation to explore the causes 

and extent of drying degrade in 4/4 

black oak lumber from several different 

saw timber stands in Shasta County. 

California. Most drying degrade in this 

study was traced to heartwood that was 

weakened by anaerobic bacteria in the 

living tree. We believe bacterial tree 

infections may be responsible for most 

of the unexpected drying degrade in 

California black oak lumber in general. 

Oak Sawtimber in a 

Softwood Region 

Aside from the furniture, cabinet, and 

millwork industries, there has been little 

One main reason California black oak 

reexamined if costly degrade in oak is to

be avoided at mills accustomed to

drying softwoods. It was felt that a 

successful program of drying research 

would greatly increase the economic

potential of California black oak. 

A Persistent Drying

Problem

For a single oak species, the drying of 

California black oak was studied more 

intensively than were similar problems 

in some of the more plentiful species of 

eastern oaks. From the late 1940's until 

the early 1960's, a series of drying 

studies was carried out to promote the 

utilization of California black oak. The 

earliest drying studies were conducted 

by the U.S. Forest Service's California 

Forest and Range Experiment Station 

(now Pacific Southwest Station) in 

cooperation with the DiamondInternational Corporation at Chico, 

Calif. These studies, reported by Smith 

(1949, 1961). evaluated both the 

air drying and the kiln drying of green 

3/4- and 4/4-inch boards. Additional 

studies reported by Ellwood (1959a, b) 

were conducted at the University of 

California Forest Products Laboratory, 

Richmond, to develop suitable 

schedulesfor kiln drying 4/4 lumber 

green from the saw. 

Drying problems with California black 

oak lumber were caused by four 

defects: (1) surface checking and splits, 

(2) honeycomb or internal checks, (3) 

Madison. Wis. 

Anderson, Calif. 

1Technologist. U.S. Forest Products Laboratory, 

2Quality Control Supervisor, Kimberly-Clark Corp., 
collapse or excessive shrinkage, and (4) 

warp. All studies indicated that degrade 

from these defects would be at a 

minimum with a combination of careful 

air drying followed by kiln drying. First it 

was necessary to air dry the green 

lumber below 30 percent average 

moisture content and preferably to 20

percent. Air-dried stock at 20 percent

moisture content could then be safely 

kiln dried to 8 percent moisture content 

with no additional defects under 

schedules similar to those conditions.” 

recommended by the FPL (Rasmussen 

1961) for eastern red oak and black 

oaks.

Warp can be caused by severe drying 

conditions, but Wahlgren (1958) found 

that excessive warp could be related to 

the presence of tension wood. Ellwood 

(1959a) was able to keep warp to a 

minimum when kiln drying green lumber 

by spacing stickers 18 inches apart and 

weighting the piles. Initial kiln 

temperatures were then kept 

comparatively low until the boards 

reached 25 percent moisture content. 

Ellwood (1962) developed a 

reconditioning treatment to remove 

much of the collapse and the warp from 

cup in California black oak and other 

California hardwoods. 

Air drying of green 4/4 California 

black oak varies from 63 days to 5

months depending on the time of year 

(Smith 1950, 1961). Much shorter drying 

times can be realized by kiln drying the 

lumber green from the saw, but the risk 

of economic losses is also greater. By 

west coast softwood standards an initial 

dry bulb temperature of 120° F is 

considered mild, yet it was severe 

enough to completely spoil most of a 

charge of 4/4 California black oak from 

collapse and excessive warp (Smith 

1950). The recommended kiln schedule 

for drying green 4/4 California black 

oak developed at the California Forest 

Products Laboratory by Ellwood 

(1959a) specifies that the initial dry bulb 

temperature must not exceed 110° F 

with a wet bulb depression of 4° F. The 

wet bulb temperature should not be 

reduced until after the lumber has dried 

below 54 percent moisture content, 

otherwise surface checking will be 

excessive. To control development of 

honeycomb and warp, the dry bulb 

temperature should not be substantially 

increased until at least 29 percent

moisture content is attained. 

West coast lumber manufacturers 

agree that past research developed 

sound drying techniques, yet the drying 

problem persists. According to sawmill 

operators producing California black 

oak lumber. this species will generally 

yield about 30 percent No. 1 Common

and Better grades of lumber. Yet only 

about 20 percent of the lumber may be 

used for factory lumber, while 80

percent is used for pallets and other 

lower value products. This 10 percent

loss (from 30 to 20 pct) is attributed to 

drying degrade; as one mill operator 

puts it “California black oak is a tricky 

species to dry even under the best 

The Kimberly-Clark Corporation at 

Anderson, Calif., has considerable 

volumes of black oak sawtimber on its 

timber lands in northern California and 

is aware of the drying problems. Don 

Prielipp, production manager at 

Anderson, was familiar with the 

utilization of eastern oaks from the 

Company's sawmill operations in 

Georgia and wondered if bacterial tree 

infections might not be responsible for 

some of the drying problems in 

California black oak. Ward, at the FPL. 

had found that heartwood infections by 

anaerobic bacteria in red and black oak 

trees can result in development of 

honeycomb and ring failure in 4/4 

lumber under normally mild kiln-drying

conditions (Ward 1972, Ward et al. 

1972). When Kimberly-Clark initiated a 

study to utilize its California black oak 

sawtimber, FPL evaluated the 

sawtimber and lumber for bacterial 

infections.

harvested late the following September 

and the lumber kiln dried by November. 

The final sample was harvested in 

December and the lumber kiln dried 

during late January and February. All 

kiln charges were dried to a final 

moisture content of 5 to 6 percent.

were kiln dried according to the 

California Forest Products Laboratory 

schedule for 4/4 flatsawn California 

black oak (Ellwood 1959a). This 

schedule calls for an initial dry bulb 

temperature of 110° F with wet bulb 

depressions of 3° to 4° F. It was 

impossible to maintain the required 

initial drying conditions for the first kiln 

run. Dry bulb temperatures rose to 120° 

to 125° F during the day despite being 

set at 110° F. These high temperatures 

are due to the hot, dry summers in the 

Anderson area and to adjacent kilns 

drying softwood lumber at temperatures 

of 170° to 190° F. It was much easier to 

follow the California Forest Products 

Laboratory schedule for the second run 

in October and November. 

The final kiln charge was dried under 

a milder schedule that can be 

maintained during the winter months. 

This schedule outlined in table was

designed according to the 

recommendations of Smith (1961) for 

low-temperature drying and the FPL 

schedule for lowland eastern red oak 

(Rasmussen 1961). One reason for 

using a milder schedule was that, under 

the California Forest Products 

Laboratory schedules, Ellwood reported 

rough lumber degrades of 4.8 to 6.3

percent while Smith (1961) found a

corresponding degrade of 3.6 percent

when the initial drying temperatures did 

not exceed 100° F. It was apparent that 

bacteria were responsible for much of 

the degrade in the first two kiln charges 

and it was decided that a kiln schedule 

for southern bottomland oaks would be 

useful for California black oak. 

After kiln drying, each charge of oak 

lumber was regraded and special note 

made of the reasons for degrade. Any 

boards that dropped below the No. 1 

Common grade were reclassified as 

dried pallet stock. 

Microbialand Wood Quality

Evaluations

Three phases were involved in the 

evaluation of California black oak for 

bacterial heartwood infections. The first 

phase was directly connected with the 

grading of lumber for the sawmill study. 

The second and third phases were 

initiated after the sawmill study and 

The first and second lumber samples 

PROCEDURES

Milling and Drying

A major portion of the study was 

concerned with sawing woods-run

grades of sawlogs into 4/4 lumber at 

the Kimberly-Clark sawmill in Anderson. 

The logs came from black oak timber 

growing in Shasta County, California, at 

elevations of 2,400 to 3,800 feet. All 

green lumber was graded under the 

rules of the National Hardwood Lumber 

Association (1971) after sawing. Boards 

grading No. 1 Common or Better were 

piled on stickers for kiln drying while the 

lower grade boards were bulk piled and 

sold for green pallet lumber. The No. 1

Common and Better lumber was kiln 

dried green from the saw in kilns at the 

Anderson mill. 

Three samples were involved in the 

milling and drying part of the study. The 

first sample of oak sawtimber was 

harvested in May and kiln drying of the 

lumber was completed by July. A 

second sample of sawtimber was 
Table 1.–Mild kiln schedule used for drying 4/4 California black oak lumber at 

Anderson, Calif. during the winter months 

Kiln conditions Wood moisture 

content

Pct °F °F Pct 

Above 50 100 4 17.5 

50-45 120 15 9.7 

45-40 125 15 9.7 

40-35 135 20 8.0 

35-30 140 25 6.9 

30-20 145 30 5.8 

20-15 150 30 5.8 

15- 6 160 40 4.3 

Dry bulb Wet bulb Equilibrium moisture 

temperature depression content

rancid odor-2 boards 

bacterially infected heartwood 

with a strong vinegar odor1 board. 

(3) Mixed normal heart and 

Immediately after sawing, the boards 

were wrapped in plastic and shipped to

FPL in Madison. At FPL the boards were 

surfaced on two faces from a rough 

green thickness of 1.5 inches down to 

1.2 inches. They were also edge 

surfaced so that each board had a 

uniform width. About 1.5 inches were 

trimmed from the ends of each board, 

and each board cut into three sections. 

From the ends of each board, two 

sections 24 inches long were cut, 

leaving a center section 8 to 9 inches 

long.

The matched 24-inch-long samples 

were used in small-scale drying 

experiments; one was air dried while the 

mate was kiln dried. All matched 

samples were end coated to retard 

longitudinal moisture loss. Air drying 

samples were on stickers in a weighted, 

covered pile placed outdoors on the 

shaded north side of FPL. 

When these samples had air dried to 

15 percent average moisture content, 

they were transferred indoors to a 

controlled temperature-humidity

chamber and dried to a final moisture 

content of 6 percent. The matched set 

of samples was kiln dried from green to 

7 percent moisture content in a small' 

experimental drying chamber with 

controlled temperature, humidity, and 

air flow. Initial dry bulb temperature was 

120° F with a 5° F wet bulb depression. 

These initial conditions were used 

because they represented the mildest 

conditions in which black oak could be 

kiln dried at Anderson. Calif., during the 

summer.

Two cross sections about 1.5 to 2.0 

inches along the grain were cut from 

the center board section to be used in 

determinations of moisture content and 

specific gravity. The remaining green 

section about 5 to 6 inches long was 

tested for resistance to a pulsed electric 

current with a portable ohmmeter called 

the Shigometer Model 7950 (Shigo and 

Shigo 1974). This section was then split. 

Splinters asceptically removed from the 

core were cultured under both aerobic 

and anaerobic conditions to test for the 

presence of micro-organisms.

entailed sampling living trees and 

selected board samples to establish a 

more precise relationship of bacteria to 

drying degrade in California black oak. 

have sound but very rancid-smelling

heartwood, and the third tree appeared 

to be in the early stages of bacterial 

infection.

Intensive tree sampling consisted of 

asceptically removing wood cores from 

the lower trunk at 1.5 to 4 feet above the 

ground line with a sterilized increment 

borer. These cores were systematically 

removed in separate sections; first from 

the sapwood, and then from the outer, 

middle, and inner heartwood. Each core 

section was divided with sterilized 

forceps into four subsections. 

Three core subsections were placed 

in tubed culture media-one medium for 

incubating and culturing the wood 

under strictly anaerobic conditions and 

the other two media for aerobic 

conditions. The fourth core subsection, 

which would eventually be examined 

under the scanning electron 

microscope (SEM) at FPL. was fixed at 

the tree site in 2 percent buffered 

gluteraldehyde for about 4 hours. Then 

it was dehydrated through increasing 

concentrations of ethanol for a total of 

48 hours, at which time it was in 100 

percent ethanol. Total time for 

preparation of the four coresubsections 

after removal from the tree did not 

exceed 4 minutes. 

Details of this matched increment 

core method for evaluating the 

microbial condition of standing trees 

are described by Sachs, Ward, and 

Kinney (1974). 

Wood Quality Phase

Wood quality and related drying 

factors were evaluated from four board 

samples, 5 feet long, that were sawed 

from three freshly cut logs. Three 

heartwood conditions were represented 

by these boards, namely: 

Sawmill Phase

During the grading of green lumber, 

special attention was given to 

heartwood that emitted strong odors of 

volatile fatty acids which are 

characteristic of oak infected by 

anaerobic bacteria (Ward et al. 1972). In 

addition. green boards containing ring 

shake, or from logs containing ring 

shake, were considered to be partially 

or completely infected by bacteria. 

When the kiln-dried lumber was 

regraded, a comparison was made 

between the incidence of drying defects 

and the presence of fatty acid odors 

and shake in the logs and green lumber. 

TreePhase

Standing trees growing on KimberlyClark forest lands in the Cascade 

mountains northeast of Anderson were 

sampled and evaluated for the presence 

Of bacteria in sound heartwood. The 

procedure first entailed a survey of 

black oak tree stumps and logs on 

stands being logged. This survey 

indicated the quality of wood to be 

expected in the standing trees on 

adjacent stands not being logged. 

Thirty standing oak trees were first 

sampled by examining cores taken with 

an increment borer. These increment 

cores, taken at 2 to 4.5 feet above 

ground. indicated by odor and 

appearance which trees contained 

normal. noninfected heartwood and 

which were probably infected by 

anaerobic bacteria. Three trees were 

then selected for more intensive 

sound with no apparent microbial 

infection, the second tree appeared to 

RESULTS AND 

sampling. One tree was considered (1) Normal noninfected heartwood- DISCUSSION

1board

(2) Bacterially infected with a strong We found that the heartwood of living 

3
Figure 1.–Scanning electron micrographs of noninfected sapwood (A-E) and bacterially infected heartwood (C-D) from living 

California black oak trees. Spore of Clostridium sp. within the bordered pit in D. (M 147 430, M 147 428, M 147 429, M 147 428)

California black oak trees can be

infected by anaerobic bacteria in a

manner similarto that reported for

eastern red and black oaks (Ward 1972,

Ward et al. 1972). Results from both the

laboratory study indicate that the

bacterially infected heartwood can be

related to the unexpected occurrence

Ofdrying degrade from honeycomb,

ring failure, and collapse.

MicrobiaPopulations in Trees and

Logs

Anaerobic bacteria were consistently

isolated from sour- and rancid-smelling

4

heartwood of California black oak trees

and less often from logs and green

lumber. Apparently the survival of these

microorganisms depends upon

environmental conditionswithin wood

cells, conditions disrupted when the

tree is cut. Sapwood and heartwood

with a typical oak odor were found to be

generally sterile. The scanning electron

micrographs (fig. 1) demonstratethat

our tissue isolates represent actual

conditions within the living tree and are

not the result of contamination.

Laboratory cultures from the isolates

indicate that a mixed population. or

more than one species of bacteria, will

occur together within sour or rancid

heartwood. A favorite niche for these

bacteria appearsto be in the bordered

pits between vessels or between a

vessel and an early wood tracheid (fig.

1-C).

Anaerobic bacteria could always be

associated with the so-called wind or

ring shakefrequentlyencountered in

the butt logs of California black oak

trees. Culturing of heartwoodadjacent

to shake zones, together with

examination under the SEM, always

revealedthe presenceof bacteria and

sometimes fungi. Ring shake was never

found in normal, noninfected

large-scalemill study and the FPL
heartwood of the California black oak in 

this study. 

Fungi were neither isolated from, nor 

observed in. the bacterially infected 

heartwood that was sound and clear of 

dark discolorations. It was not 

uncommon, though, to isolate fungi 

from discolored heartwood adjacent to 

ring shake. Presumably, the anaerobic 

conditions of sound, bacterially infected 

heartwood are unfavorable for fungal 

growth, but the resultant shake may aid 

the progress of heartwood decay. 

Heartwood decay from fungal 

infections was observed in many of the 

logs sawed for the mill study, but green 

boards containing decay were generally 

not included in the No. Common and 

Better grades that were kiln dried. 

Mill Study and Manufacturing

Losses

California black oak logs from three 

different timber stands were sawed into 

4/4 lumber for a total green volume of 

almost 580,000 board feet. The oak 

timber was generally mixed with 

ponderosa pine and both species were 

harvested in the same logging 

operation. Table 2 shows that the yield 

of green No. 1 Common and Better 

grade lumber from these threestands 

follows the general grade yields 

reported by other sawmill operators in 

northern California. Our green lumber 

yields also agree with the 30.15 percent

yield of No. 1 Common and Better 

grades reported by Malcolm (1962) for 

board feet of 4/4 California 

black oak lumber sawed from logs 

ranging in grade from No. 1 to No. 4.

The No. 1 Common and Better lumber 

was kiln dried green from the saw while 

No. 2 Common and lower grade boards 

were surfaced and sold for green pallet 

lumber. When the kiln-dried lumber was 

regraded, there was an average loss in 

total volume of 15.9 percent, which can 

be attributed mainly to shrinkage. There 

were also losses in lumber grade which 

resulted largely from the formation of 

collapse, surface checks. splits and ring 

failure, and to a lesser degree from 

warp. Kiln-dried boards falling below 

No. 1 Common grade were sold for dry 

Pallet lumber. Kiln-dried boards with 

collapse and ring failure usually 

revealed the internal presence of 

honeycomb when the boards were 

crosscut, but honeycomb did not 

contribute directly to degrade 

evaluation.

inspection of the kiln-dried

lumber revealed only 1 to 3 percent of 

the degrade in each of the three 

samples could be attributed to warp and 

end splits. Most degrade resulted from 

defects associated with bacterially 

infected heartwood. It was possible to 

detect the characteristic rancid and 

strong vinegar odors in dry wood 

sections crosscut from boards 

containing collapse, ring failure, and 

honeycomb. Lumber in the kiln charge 

of sample 2 contained the least amount 

of bacterially infected heartwood and 

sample 3 the most. Table 3 shows that 

percent of the lumber volume for 

sample 2 fell below No. 1 Common

grade because of kiln-drying defects 

while sample 3 lumber with the most 

infected heartwood suffered a 52

percent loss in grade. 

Undoubtedly less degrade would 

have developed in the infected 

heartwood of the first sample of logs if 

initial drying conditions had been less 

severe. Hot weather conditions forced 

daytime kiln temperatures up to 120° to 

125° F during the first 10 days of drying 

and much of the collapse and ring 

failure developed within this period. 

Nevertheless, the greatest amount of 

degrade observed in the third sample 

developed when the lumber was dried 

during the cool winter months under a 

mild, easily controlled kiln schedule. 

Monetary losses from drying degrade 

with bacterially infected lumber can be 

calculated in various ways, depending 

on whether the manufacturer owns and 

5

Table 2.–Yield of green 4/4 California black oak lumber from woods-run logs at

Anderson, Calif. 

Sample Lumber grades

No. 1 Common and Better 

Volume yield 

Board foot Percent 

Green pallet stock 98,650 

Total

No. Common and Better 

Total

Green pallet stock 

No. Common and Better 

Total

Green pallet stock 

All No. Common and Better 

Total

Green pallet stock 

Table 3.–Yield and value of 4/4 California black oak No. Common and Better

rough lumber kiln dried green from the saw

Dry by drying

Value added Samples Green

Sample 1 (Kiln time = 29 days)

No. 1 Common and Better (MBF) (0.65)

Pallet stock (MBF) 113.362( .35) 

$/MBF (Dollars) 

Sample 2 (Kiln time = 36 days)

No. Common and Better (MBF) 68,850 ( .90)

stock (MBF) 17,570( .10)

$/MBF (Dollars) + 63.64 

Sample 3 (Kiln time = 39 days)

No. Common and Better (MBF) ( .48) 

Pallet stock (MBF) ( .52) 

$/MBF (Dollars) 2328.68

Samples combined 

No. Common and Better (MBF) 

Pallet stock (MBF) 

( .73) 

( .27) 

$/MBF (Dollars) 3344.38 1Dry pallet degraded from green NO. and Better lumber during kiln drying by collapse, ring 

2Based on green lumber values of: FAS = $450/MBF. Selects = $435/MBF, and NO. Common = $275/

3Based on dry lumber values of: FAS = $531 /MBF, Selects = $516/MBF, NO. 1 Common = $356/MBF, 

failure, and splits.

MBF(Jan. 1978 basis).

and dry pallet = $150/MBF (Jan. 1978 basis).
harvests the timber or buys logs or 

rough green lumber. For the purposes 

of this paper, we assigned average local 

lumber values for green and dry grades 

of oak to the yields from this study as 

shown in table 3.

These values show that the company 

lost from to per thousand 

board feet by not selling all of the 

boards in samples and as green 

lumber. Because of the low incidence of 

bacterial heartwood in the logs of 

sample a value of per

thousand board feet was added by kiln 

drying the No. Common and Better 

lumber. The added value from drying 

sample did not offset the losses from 

drying samples and

These results indicate that any plans 

to commercially produce kiln-dried

lumber from California black oak should 

consider whether or not the timber 

contains bacterially infected heartwood. 

There should also be an awareness that 

selection of the upper, more expensive 

grades of green lumber for kiln drying 

will not guarantee successful drying 

results if the wood is infected. 

figure 4.

Collapseor excessive shrinkage in

thickness is evident in all samples

containing bacterial heartwood when

compared with cross sections from

normal, noninfected heartwood

samples.

These laboratory drying experiments 

demonstrate that the normal, 

noninfected heartwood of California 

black oak can be kiln dried green from 

the saw without defects even under an 

initial temperature of 120° F. This 

compares with the first test of the largeLaboratory Drying Experiments 

Experimental drying conditions and 

moisture loss curves for the green 

boards shipped to FPL are shown in 

figure for simulated kiln drying and in 

figure for air drying. Under both drying 

conditions, surface checking started in 

about hours in bacterially infected 

heartwood of the rancid sample and the 

mixed wood sample. The initial air 

drying conditions were quite mild but 

not enough to prevent surface checking 

in bacterial heartwood. Normal, 

noninfected heartwood did not develop 

surface checks under either air drying 

or kiln drying. Ellwood (1959b) 

observed that most surface checking 

occurred in green flatsawn California 

black oak on the first day of kiln drying. 

Ring failure was present in cross 

sections cut from both the kiln- and airdried samples of the rancid board, but 

ring failure was more severe under kilndrying conditions (fig. 4). Ring failure 

did not develop in normal, noninfected 

heartwood or in the infected heartwood 

of the mixed sample. Honeycomb 

developed in the kiln-dried rancid 

sample, but could not be detected in the 

air-dried sample. The mixed heartwood 

sample developed a few internal hairline 

checks in the bacterial heartwood 

under kiln drying but not air drying. 

Neither the surface checks which 

closed during the final drying stages nor 

the hairline internal checks are visible in 

Figure 2.–Experimental kiln conditions and drying curves for 4/4 California black

oak.

(M 146 707)
scale mill study where boards with 

normal-appearing heartwood remained 

defect-free even though initial kiln 

temperatures exceeded 120° F. 

The absence of honeycomb in airdried boards containing bacterial 

heartwood substantiates Smith's (1961) 

recommendations that California black 

oak in general should be predried under 

temperatures that do not exceed 100° 

F. However, our laboratory experiments 

also indicate that some bacterially 

infected heartwood, particularly the 

rancid-odor type, cannot be dried 

defect-free even under the mildest 

commercial conditions. This is 

substantiated by the third test of the mill 

study where over half of the kiln charge 

was degraded from checking and 

collapse, even though drying was done 

in the winter with initial temperatures 

that did not exceed 100" F.

Wood Quality 

Considerations

The Occurrence of drying defects in 

bacterially infected heartwood is partly 

partly due to alteration of the physical 

and chemical properties of the wood by 

the bacteria within the living tree. But 

what are the adverse effects of 

anaerobic bacteria on the properties of 

California black oak heartwood? How 

can some of the salient features of the 

bacterially infected wood serve to 

distinguish such defect-prone wood 

from normal, healthy wood? 

Honeycomb and Ring Failure 

Examination of boards from both the 

mill study and the laboratory tests 

strongly indicates that honeycomb will 

Only develop in the bacterially infected 

heartwood of California black oak under 

conventional kiln-drying conditions. 

Bacterial heartwood that emits strong 

rancid odors appears to be more prone 

to develop honeycomb under the milder 

drying conditions than infected 

heartwood with a pronounced vinegar 

odor. This may be attributed to certain 

types of anaerobic bacteria found in the 

microbial population of the infected tree 

trunk.

genus Clostridium are prominent 

members of microbial populations in the 

heartwood of California black oak and 

eastern red oaks that emit rancid odors. 

Clostridia produce spores (fig. 1-D) that

can remain viable in the soil and oak 

heartwood for many years. All strains of 

Clostridium spp. isolated from California 

Figure 3.–Atmospheric conditions and air-drying curves for 4/4 California black 

oak.

the result of drying conditions and (M 146 705) 

lumber from logs containing ring shake 

than in bacterially infected heartwood 

from logs without ring shake. 

Ring shake in logs and ring failure in 

dried lumber both have characteristic 

wood separations between cells. This 

suggests that pectolytic enzymes 

produced by anaerobic bacteria in the 

tree are also responsible for weakening 

of the wood that results in ring shake 

and ring failure as well as honeycomb. 

Shrinkage and Collapse 

The shrinkage of California black oak

will depend both upon drying conditions 

and type of heartwood (table 4).

Transverse shrinkage was greatest for 

kiln-dried bacterially infected

heartwoodand least for air-dried

noninfectedheartwood. Specific gravity 

values for these samples (table 5) show 

the interdependence of shrinkage and 

density with drying conditions andtype

of heartwood. These data are 

apparently in agreement with similar 

data published by Ellwood (1959a) and

Schniewind (1958, 1960). 

The type of heartwood and drying 

conditions apparently can obscure the 

contribution Of annual ring growth to

wood density (table 5). For oaks and 

other ring-porous hardwoods, Paul

(1963) found that a faster growth rate 

generally resulted in higher specific 

gravity and an increase in mechanical

black oak and eastern red oaks 

produced pectindegrading enzymes in 

culture tests at FPL, These bacteria 

were not able to cause visible 

disintegration of wood tissue under 

laboratory conditions, Microscopic 

examination of bacterially infected 

heartwood from living oak trees does 

not reach a visible disintegration of cell 

walls as can be seen with wood 

decayed by fungi. 

Until further tests are undertaken. it 

can be postulated that the compound 

middle lamella which holds the wood 

cells together (and is relatively rich in 

pectic compounds), is weakened by 

enzymes produced by anaerobic 

bacteria, particularly in the genus 

Clostridium. When subjected to 

shrinkage stresses during drying. this 

bacterially weakened wood will rupture 

and split apart between cells rather than 

across cell walls as can occur in wood 

degraded by fungi. 

Observations from this study, 

together with those from a previous 

study (Ward et al. 1972), suggest that 

ring failure is an incipient form of ring 

shake that was initiated in the tree but 

did not actually rupture until the wood 

was being dried. This may explain why 

ring failure developed under both airand kiln-drying conditions in the 

laboratory tests, whereas honeycomb 

developed only with kiln drying. Ring 

failure was more likely to develop in

Strictly anaerobic bacteria in the

strength properties. If density is 

compared with other wood properties 

for California black oak, the microbial 

condition of the heartwood should be 

considered and specific gravity must be 

based on green wood volume. 

The greater shrinkage of bacterially 

infected California black oak is largely 

across the thickness of the board. Air-dried board 

Collapse seems to be more prominent in Kiln-dried board 19.85 20.68 

California black oak than in most of the 

eastern red and black oaks. The deep Board samples with noninfected 

collapse zones in California black oak heartwood followed a conventional 

can be attributed to relatively severe pattern of shrinkage. where shrinkage 

drying conditions, bacterially weakened in a direction tangential to growth rings 

wood. and plugging of the vessels or or in width was greater than the radial 

pores with tyloses. Moisture movement shrinkage in thickness. Except tor the 

through the vessels is retarded by the air-dried board with mixed heartwood, 

tyloses and bacterial debris. Tensile all samples containing bacterial 

stresses that are set up are sufficient to heartwood shrank more in thickness or 

cause collapse of the weakened wood radially, than in width or tangentially. 

tissue. These shrinkage data in figure 

The presence of tyloses is generally suggest a comparsion with Ellwood’s 

considered a diagnostic characteristic (1959a) drying data for flatsawn 4/4 

of white oaks rather than the red and California black oak kiln dried green 

black oaks. But California black oak is from the saw. Ellwood's data were 

an exception. All heartwood, both grouped in table according to sample 

normal and infected, that was examined boards with honeycomb, without 

in this study contained an abundance of honeycomb but with surface checks, 

tyloses. Furthermore, a comprehensive and boards with neither surface checks 

study Of the anatomy of American oaks nor honeycomb. With this arrangement 

by Williams (1942) found that tyloses are his average shrinkage values exhibit a 

the rule rather than the exception in pattern similar to ours in figure 

California black oak. Possibly the excessive radial shrinkage 

and the formation of honeycomb 

(fig. 5) show that tyloses form in reported by Ellwood may also be the 

noninfected wood during the result of bacterial weakening of the 

transformation from sapwood to heartwood. (Swiss cheese odor), valeric (putrid. 

heartwood.

The influences of bacterial weakening Wood Characteristics for 

of the wood and of drying conditions on Lumber Presorting the dimensional shrinkage of California

black oak are illustrated in figure The best solution to the drying 

Table 4.–lnfluence of drying conditions and type of heartwood on the shrinkage of 

California black oak (flatsawn 4/4-inch thick) 

Transverse shrinkage1-percent loss 

from green to ovendry dimensions 

Normal, Mixed: Normal Rancid, 

noninfected and bacterial bacterial 

heartwood

Drying conditions 

heartwood heartwood 

associated with collapse, especially Oven-dried section 11.84

1Combined shrinkage of thickness and width. 

problem is to grow California black oak 

trees without bacterially infected and 

defect-prone heartwood. Until this can 

be accomplished, the next best solution 

is to identify and separate lumber with 

infected heartwood from noninfected 

material. Results from this study 

indicate that segregating bacterially 

infected logs and lumber offers reduced 

drying times and energy costs and a 

reduction in drying degrade. Bacterially 

infected lumber can be dried under the 

mild conditions recommended by Smith 

(1961), while lumberwith normal 

heartwood can be safely kiln dried 

green from the saw under relatively fast 

schedules. Effective methods for 

presorting oak have as yet not been 

developed.

bacterially infected California black oak 

are the most consistent and salient 

characteristics of the green wood. 

These odors are due to a combination 

of volatile fatty acids which include 

butyric (rancid butter odor), propionic 

rancid odor), and caproic (goat odor) 

acids. Normal oak heartwood will 

contain small amounts of acetic acid. 

but infected heartwood will have 

elevated amounts and emit a strong 

The rancid or sour odors Of

The scanning electron micrographs

Table 5.–Effect of drying conditions, sample volume moisture content and growth 

rate on the specific gravity of California black oak 

Specific gravity 

sample Specific gravity values 

Markwardt and 

Wilson (1935) California

Products

This study2

Heartwood condition Moisture 

and growth Volume content Normal Mixed Rancid

bacterial

Forest

heart- heart- heart- Oregon California Laboratory2

condition at 

weighing wood wood wood type type type

Pct

Green Green 0.537 0.538 0.491 0.462 

Air or low-temperature

dried from green Ovendry 0.599 0.594 0.675 – – 0.503 

Kiln dried from green Ovendry 0.666 0.689 0.704 – – 0.636 

Growth rate – rings per 

inch (green wood) 1Data from this study based on matched specimens from the same board sample within each of the types of heartwood. 2From Ellwood (1959a) for kiln-dried samples and Schniewind (1958, 1960)for green and air-dried samples. 
vinegar odor. The rancid odors in 

California black oak are identical to 

those described by Zinkel et al. (1969)

in bacterially infected heartwood of 

eastern red and black oaks. These fatty 

acid odors were reproduced in potato 

mash medium with cultures of 

anaerobic bacteria isolated from 

California black oak. 

Rancid fatty acid odors are a reliable 

wood characteristic for presorting 

because they persist in the heartwood 

long after the bacteria have died and 

their cells have disintegrated and 

disappeared. One drawback is that a 

practical and objective method for 

detecting these fatty acid odors in green 

lumber has yet to be developed. 

Bacterially infected heartwood can be 

accurately distinguished from normal 

heartwood by analyzing steam distillates 

from the wood with a gas 

chromatograph. but this procedure is 

time consuming. Rancid and sour odors 

are most rapidly identified with the 

human nose, but the accuracy varies 

with individuals. Accuracy of odor

Figure 5.–Scanning electron micro

- perception diminishes even with 

graphs of tyloses in living California experienced mill workers when many 

black oak. A. Vessel from sapwood to oak boards must be evaluated within a 

heartwood transition zoneshowing in

- few hours time. 

itial tyloses development from ray pits. Electrical resistance of bacterially 

B. Vessel from noninfected heartwood infected heartwood tends to be lower 

showing abundant tyloses growth. C. than the resistance of normal 

Closeup of tyloses in B showing: (1) heartwood. but the use of electrical 

Crystal inclusion inside one tyloses resistance as a detection characteristic 

and (2) Secondary burgeoning from does not appear to be promising. There 

membranes within adjacent tyloses. is considerable overlap in the range of 

resistances for normal heartwood with 

the resistance ranges for infected 

heartwood (table 7). We have measured 

electrical resistances in additional oak 

boards and find that normal and rancid 

boards cannot be distinguished with 

certainty, although rancid heartwood 

tended to have lower average 

resistances.

Ellwood (1959a) reported the best 

indicators of which California black oak 

boards were most likely to check during 

drying are: (1) wood with a high initial 

moisture content and (2) relatively dark

-

colored wood and wood containing 

dark streaks or patches. 

With our study material we found that 

green moisture content of bacterially 

infected heartwood will usually be 

higher than percent and can range in 

average from to percent. The 

green moisture content of normal 

heartwood averages around percent.

but can be as high as to percent.

No moisture meter currently available 

can accurately measure green moisture 

contents much above percent.

(M 147 433, M M 434)

10
The color of bacterially infected oak 

may or may not be darker than normal, 

noninfected heartwood. When the 

heartwood is darker, it is likely to occur 

in the lower grades and not in the upper 

grades dried in this study. 

Ring shake in California black oak 

trees and logs can be a clue to the 

Presence of bacterially infected 

heartwood. Not all logs with bacterially 

infected heartwood will have ring shake 

but if ring shake is present, it invariably 

occurs in bacterially weakened 

heartwood. A considerable reduction in 

drying degrade can be achieved if logs 

from trees containing shake are 

presorted. and the subsequent lumber 

is dried under mild conditions. 

An earlier FPL study found that all of 

the honeycomb and ring failure in 4/4 

No. Common and Better northern red 

oak occurred in boards sawed from 

trees containing shake (Ward et al. 

1972). Neither of these defects 

developed in No. Common and Better 

lumber from shake-free trees with 

noninfected heartwood. Possibly this 

correlates with the observations of 

Smith (1961) who found that five logs 

from only two trees (out of a sample of 

47) accounted for percent of the 

drying degrade in 4/4 rough lumber. 

SUMMARY

The heartwood of merchantable and 

outwardly healthy trees of California 

black oak can be infected by mixed 

populations of anaerobic bacteria. The 

composition of these populations has 

not been completely defined, but strictly 

anaerobic bacteria in the genus 

Clostridium were consistently isolated. 

Fungi were rarely encountered in 

bacterial heartwood that appears visibly 

clear and sound. Bacterially infected 

heartwood emits characteristic rancid 

or sour odors of volatile fatty acids 

resulting from bacterial metabolism 

within the wood. 

Figure 6.-Effect of drying conditions and bacterial tree infections on the dimensional shrinkage of flatsawn 4/4 California black oak boards. All heartwood contained tyloses. (M 706)

Table 6.–Compilation, according to drying defects, of sample board data from the California Forest Products Laboratory 

Study (Ellwood 1959a) on the kiln drying of green 4/4 California black oak 

Average shrinkage to 

from green dimension2

Mean Range Width Thickness 

Total surface percent moisture content Specific Part of Green moisture gravity sample content mean mean1

Defect group checking

Pct Pct In. In. Pct Pct 

Both honeycomb and surfacechecks 0.686 15-46

Only surface checks 0.650 8.7

No defects 0.584 –

Total sample 0.636 18 1Based on ovendry wood volume and weight. 

2AIl flatsawn stock. 
Table 7.–Resistance to a pulsed electric current of green heartwood from sample 

boards of California black oak 

Resistance–thousand OHMS1

defect. Collapse and honeycomb can 

be greatly reduced by first air drying 

bacterial heartwood, but ring failure is Number of Range an incipient form of ring shake and measurements Mean cannot be adequately controlled with Type of heartwood

Minimum Maximum milder drying conditions. 

Because of the bacterial infections in 

340 species cannot always be profitable for 

168 255 kiln-dried factory grade hardwood 

Normal-not infected 

Mixed: California black oak heartwood, this Normal wood zones 

Bacterial wood zones 

1Measured with the Shigometer Model 7950 using needle electrodes, spaced in. apart, Which were 

Rancid, bacterially infected Tyloses were the rule rather than the 

exception in both the normal and the 

infected heartwood, but were not 

observed in sapwood. Collapse did not 

occur in noninfected heartwood 

containing tyloses. 

can be kiln dried green from the saw 

with a minimum of degrade from 

collapse, honeycomb, and ring failure if

bacterial heartwood is not present in the 

boards. Normal heartwood can be 

safely dried under conventional kiln 

schedules and possibly with initial dry 

bulb temperatures of 120° to 125° F. 

be dried from green to a percent

average moisture content under mild 

air-drying schedules recommended by 

H.H. Smith. The percent material can 

then be kiln dried without additional 

205 lumber. Presorting green lumber with 

bacterial heartwood from lumber with 

noninfected heartwood before drying 

would greatly enhance the prospects 

for economic utilization. Effective 

presorting under commercial conditions 

cannot be undertaken until rapid and 

accurate methods are developed for 

measuring volatile fatty acids and 

moisture content in green lumber. 

Rancid-bacterial 

driven in into the surface of the boards. 

heartwood that is sound and clear can 

occur in the No. Common and Better 

grades of California black oak lumber. 

Despite the sound appearance, rancid 

oak heartwood will be weaker than 

noninfected heartwood and is prone to 

develop ring shake in the tree and 

honeycomb, ring failure, and collapse 

when the lumber is dried. The anaerobic 

Clostridium is apparently responsible 

for weakening thechemical bonds in 

the compound middle lamella; this 

results in ruptures between wood cells 

when the boards are subjected to 

shrinkage stresses during drying. 

Collapse is a common defect in 

California black oak lumber that has 

Collapse is attributed to a combination 

of bacterial weakening of the wood and 

plugging of the vessels with tyloses. 

One-inch California black oak lumber 

ACKNOWLEDGMENT

The assistance of Dr, l. B. Sachs, U.S. 

Forest Products Laboratory, with 

preparing the scanning electron 

micrographs is deeply appreciated. We 

are also grateful for the information and 

assistance provided by J. R. Long, Vice 

President and Division Manager, J. E. 

Higgins Lumber Company, Union City, 

Oak Lumber Company, Oroville, Calif. 

Bacterially infected heartwood should 

been kiln dried green from the saw. air-drying conditions or by the forced Calif., and by Guy Hall, President, Cal
LITERATURE CITED

Bois. P.J. 

1975. Some considerations in drying oak. Proc. Western Dry Kiln Clubs.

26th Annu. Meet. Oregon State Univ., Corvallis, Oreg.

Edwards,M.B.

1957.California black oak-its management and economic possibilities. 

J. For. 55(7):506-510.

Ellwood, E.L.

1962. Reconditioning-a practical method of removing collapse and 

warp from California hardwoods. Calif. For. and For. Prod. Note No.

p. Mar. Univ. Calif., Berkeley, Calif. 

Ellwood, E.L. 

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

Prod. Note No. p. Nov. Univ. Calif., Berkeley, Calif. 

Ellwood, E.L.

Drying stress patterns in California black oak. Calif. For. and For. 

Prod. Note No. p. Jan. Univ. Calif., Berkeley, Calif.

Ellwood, E.L.

1958. Problems and prospects in drying California hardwoods for high 

quality use. Calif. For. and For. Prod. Note No. p. Dec. Univ.

Calif., Berkeley, Calif. 

Forest Service, U.S. Department of Agriculture.

1973.Theoutlook for timber in the United States. For. Resour. Rep. 

Available from Gov. Print. Off., Wash., D.C. 

Malcolm, F.B. 

California black oak-a utilization study. U.S. For. Prod. Lab. Rep. 

2237. p. For. Prod. Lab., Madison, Wis.

Markwardt, L.J., and T.R.C. Wilson.

1935. Strength and related properties of wood grown in the United 

States. U.S. Dep. Agric., Tech. Bull. No. 479. p.

McDonald, P.M. 

1969. Silvical characteristics of California black oak (Quercuskelloggii

Newb.). USDA For. Serv. Res. Pap. PSW-53. p. Pacific Southwest 

For. and Range Exp. Stn., Berkeley, Calif. 

McDonald, P.M., and W.E. Sundahl.

1967.California black oak-a general bibliography on an increasingly

valuable species. U.S. For. Serv. Res. Note PSW-134. p. Pacific

Southwest For. and Range Exp. Stn., Berkeley. Calif. 

National Hardwood Lumber Association. 

1971. Rulesfor the management and inspection of hardwood and 

cypress lumber. 116p. Chicago, Ill.

Paul, B.H.

1963. The application of silviculture in controlling the specific gravity of

wood. U.S. Dep. Agric., Tech. Bull. p.

Rasmussen, E.F.

1961. Dry kiln operators manual. U.S. Dep. Agric.. Agric. Handb. 

p. Gov. Print. Off., Wash., D.C. 

Roy, D.F.

California hardwoods: management practices and problems. J. 

For. 60(3):184-186.

Sachs. I.B., J.C. Ward, and R.E. Kinney.

1974.Scanning electron microscopy of bacterial wetwood, sapwood,

and normal heartwood in poplar trees. Proc. 7th Annu. Scanning 

Electron Microscope Symp. (Pt. II):453-460.

Schniewind, A.P.

1960.The strength and related properties of California black oak II.

Shrinkage and strength properties in the air dry condition. Calif. For.

and For. Prod. Note No. 21. p. Univ. Calif., Berkeley, Calif. 
18. Schniewind, A.P. 

1958. The strength and related properties of California black oak I.

General description and strength properties in the green condition. 

Calif. Forestry and For. Prod. Note No. p. Univ. Calif.. Berkeley, 

Calif.

19. Shigo. A.L., and A. Shigo.

1974. Detection of discoloration and decay in living trees and utility 

poles. USDA For. Serv. Res. Pap. NE 294. p. Northeast For. Exp. 

Stn., Upper Darby, Pa. 

Smith, H.H. 

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

Tech. Pap. 62. 19 p. Pacific Southwest For. and Range Exp. Stn., 

Berkeley, Calif. 

Smith. H.H. 

1956. Improved utilization of western hardwoods by modern drying. For. 

Prod. J. 6(3):121-124.

Smith, H.H. 

1950. Further experiments in seasoning California black oak. U.S. For. 

Serv. Res. Note p. Calif. For. and Range Exp. Stn. 

23. Smith, H.H. 

1949. Seasoning California black oak. U.S. For. Sew. Res. Note p.

Calif. For. and Range Exp. Stn. 

Wahlgren, H.E. 

1958. Occurrence of tension wood and related seasoning defects in 

California black oak. U.S. For. Prod. Lab. Rep. p. For. Prod. 

Lab., Madison, Wis. 

Ward, J.C. 

Anaerobic bacteria associated with honeycomb and ring-failure in 

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

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 kiln drying. USDA For. Serv Res. Pap. FPL 165. p. For. Prod. 

Lab., Madison, Wis. 

27. Williams, S. 

1942. Secondary vascular tissues of the oaks indigenous to the United 

States-Il. Types of tyloses and their distribution in erythrobalanus 

and leucobalanus. Bull. of Torrey Botanical Club 69(1):1-10.

1969. Odor problems from some plywoods. For. Prod. J. 19:60. 

Zinkel, D.F., J.C. Ward, and B.F. Kukachka. 

U.S GOVERNMENT PRlNTlNG OFFlCE 1979-650-028/19