Mould growth on kiln-dried and air-dried timber

ORIGINALS ORIGINALARBEITEN

Mould growth on kiln-dried and air-dried timber

Pernilla Johansson • Thomas Wamming •

Gunilla Bok • Marie-Louise Edlund

Received: 26 September 2012 / Published online: 19 May 2013

� Springer-Verlag Berlin Heidelberg 2013

Abstract The problem with discoloration, due to fungal

growth, of wooden outdoor constructions seems to have

increased in recent years. One reason for this increase

might be an impact of new drying methods of timber.

Modern kiln drying methods use high temperatures in an

effort to shorten the drying process, which leads to fast

capillary water transport and subsequently redistribution

and accumulation of dissolved substances at the surface.

These can then be used as nutrients by fungi. In this study,

wood was dried according to different simulated drying

schedules. The mould resistance of the timber was then

tested. Wood dried at room temperature was used as a

reference. No differences could be confirmed at the end of

the test; mould growth was extensive on all the samples.

However, mould growth started earlier on the kiln-dried

samples than on air-dried timber. As for the discolouring

fungus, there was a clear difference between wood dried at

room temperature and kiln-dried wood, though no differ-

ence could be established between the two artificial

methods.

Schimmelwachstum auf technisch getrocknetem und

luftgetrocknetem Schnittholz

Zusammenfassung In den letzten Jahren ist eine Zu-

nahme des Verfarbungsproblems bei Holzkonstruktionen

im Außenbereich aufgrund von Pilzbefall zu verzeichnen.

Als ein Grund dafur wurde der Einfluss neuer Trock-

nungsmethoden von Schnittholz genannt. Moderne tech-

nische Trocknungsverfahren verwenden hohe

Temperaturen, um die Trocknungszeit zu verkurzen. Dies

fuhrt zu schnellem Kapillarwassertransport und dadurch zu

einer Verlagerung und Akkumulierung geloster Stoffe an

der Oberflache, welche dann den Pilzen als Nahrstoffe

dienen. In dieser Studie wurde Holz mittels verschiedener

Trocknungsablaufe getrocknet und anschließend auf

Schimmelbestandigkeit gepruft. Bei Raumtemperatur get-

rocknetes Holz diente als Kontrollprobe. Bei Versuchsende

konnten keine Unterschiede festgestellt werden; alle

Prufkorper wiesen starkes Schimmelwachstum auf. Aller-

dings begann das Schimmelwachstum bei technisch get-

rockneten Prufkorpern fruher als bei luftgetrocknetem

Schnittholz. Bei den holzverfarbenden Pilzen gab es einen

deutlichen Unterschied zwischen Holz, das bei Raumtem-

peratur getrocknet wurde, und technisch getrocknetem

Holz. Jedoch konnte kein Unterschied zwischen den beiden

technischen Trocknungsprogrammen festgestellt werden.

1 Introduction

Wood is a natural, biodegradable building material and

may therefore act as a substrate for fungi. Species of mould

fungi are early colonizers on wood as they principally use

simple sugars that are soluble in the wood and therefore do

not cause major deterioration of the wood itself. The main

factor that controls whether mould can grow or not is the

availability of moisture. However, when conditions are

favourable for growth, different wooden materials may

have different capabilities to withstand mould growth.

Even boards of wood from the same species can be affected

differently. One example of this can be seen in outdoor

constructions where some boards may be heavily discol-

oured by microfungi while there is no visible growth on

other adjacent boards (see Fig. 1).

P. Johansson (&) � T. Wamming � G. Bok � M.-L. Edlund

SP Technical Research Institute of Sweden, Box 857, 501 15

BORAS, Sweden

e-mail: [email protected]

123

Eur. J. Wood Prod. (2013) 71:473–481

DOI 10.1007/s00107-013-0699-y

One proposed explanation for the differences between

wood materials in their susceptibility to mould growth is

the difference in concentrations of available simple car-

bohydrates on the surface of the timber (Terziev 1996;

Theander et al. 1993). The sugars originate from the

metabolism in the growing tree and are dissolved in free

water in the wood. These sugars are then available as

nutrients for surface moulds after the tree has been felled

(Schmidt 2006).

During the drying of manufactured timber, the free

water in the wood cells is relocated from the inner to the

outer parts, mainly by capillary action (Long 1978). The

dissolved carbohydrates are transported with the water and

accumulate on the surface of the timber. This accumulation

can sometimes be seen as a brown border directly beneath

the surface of the timber (Sehlstedt-Persson 1995) (see

Fig. 2). An increase in the temperature in the kiln speeds

up drying leading to higher concentration of sugars on the

surface (Terziev 1996).

Extended mould growth may be visible to the naked

eye, especially if the spores and hyphae are pigmented.

But if the hyphae and spores are not pigmented, this

growth can usually not be seen without a microscope even

though a well extended growth is present. On outdoor

constructions such as facades and fences, this invisible

growth does not cause any major problems as it does not

affect aesthetics and usually does not affect strength

properties either. However, inside building constructions,

growth that cannot be seen with the naked eye may be

significant as there is a potential risk of a negative effect

on the indoor environment, which can pose health risks to

users of the building.

In this test, mould growth on wood that had been dried

using different drying methods was studied. Side boards

and centre boards that had been dried artificially at dif-

ferent rates and temperatures were compared; wood dried

at room temperature was used as a reference. The drying

schemes used were intended to be similar to previous and

current commercial processes. Not only is the modern

drying scheme faster than older methods but it also uses a

higher final dry temperature. Discolored (pigmented

hyphae/spores) and uncoloured (hyaline) mould growth

was studied on the surfaces of the samples after they had

been incubated in a climate that was favourable to

microorganisms.

Several partial hypotheses were tested in the study, all of

which were based on the overall hypothesis that artificial

drying of wood leads to greater concentrations of sugar on

the surface of the wood, and that this in turn affects the

extent of mould growth. One of the hypotheses was that

artificially dried wood exhibits mould growth sooner than

wood dried at room temperature. Another was that wood

that has been dried quickly and at high temperature in a

kiln is expected to be more susceptible to mould growth

than wood that has been dried at lower temperature and

more slowly. Furthermore, it was expected that if the sur-

face layer was planed off, the mould growth would be less

extensive as the sugars concentrated near the surfaces

would be removed.

During drying, there were stickers between the samples,

as in all industrial kiln drying, so that the air could circulate

over the surface. One of the hypotheses was that the sur-

faces that had been under stickers during drying were

expected to have lower extent of mould. The water trans-

port, and thus the transport of sugar towards the surface, is

expected to be less there, as the presence of stickers slows

down the evaporative drying from this surface since air

circulation is reduced.

Samples that were split were expected to have less

growth on the split surface, as the concentration of sugars

was expected to be less inside the sample than on the

original surface layer.

Fig. 1 Variation in discoloration caused by microfungi on adjacent

boards

Abb. 1 Unterschiede in der Verfarbung benachbarter Bretter auf-

grund von Pilzbefall

Fig. 2 Accumulated sugars seen as a brown border directly under the

surface of the timber. From Sehlstedt-Persson (1995)

Abb. 2 Akkumulierte Zucker erkennbar als dunkler Rand direkt

unter der Schnittholzoberflache. Aus Sehlstedt-Persson (1995)

474 Eur. J. Wood Prod. (2013) 71:473–481

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2 Materials and methods

2.1 Wood material and preparation of test samples

for drying

The wood material was selected at a sawmill located in

northern Sweden. The material originated from winter-

felled pine (Pinus sylvestris) and spruce (Picea abies) trees

with a maximum diameter of 175 mm. Centre boards

(50 9 150 mm2) and side boards (25/22 9 100 mm2)

were taken directly from the saw line and transported to SP

Tratek’s laboratory in Skelleftea. Samples of 98 cm in

length were cut from each board starting from the butt. A

20 mm slice was cut from each sample to determine the

moisture content and density. The ends were sealed with a

one-component adhesive sealant (Sikaflex-221) before

drying.

2.2 Drying process

Artificial drying was performed according to different

drying schedules, as shown in Table 1, in a laboratory kiln

with an 18 kW steam boiler, 7.5 kW electrical heating, a

water spray system (16 l/h), 3 m/s airspeed with a full load

and a WSAB control system for the control of wet and dry

bulb temperatures. For each dimension (50 9 150 mm2

and 25/22 9 100 mm2), there was one schedule for fast

drying and one for slow drying above the fibre saturation

point. The time for the heating phase and total drying

differed among those groups according to Table 1.

A third drying schedule was natural drying indoors for

about 4 weeks in a room climate (20–25 �C and RH

35–45 %). In the following text, this will be referred to as

air-dried.

During drying, there were stickers to separate the timber

in the stack, as in all industrial kiln drying, so that the air

could circulate over the surface and facilitate the move-

ment of moisture from the surface.

2.3 Preparation of test specimens

Five samples of sapwood from each dimension, species and

drying schedule were chosen for the mould growth study.

Each sample was split and cut into two parts along the

fibres (see Fig. 3).

One part of the sample was planed (2 mm for the side

boards and 1 mm for the centre boards), and one part

retained the original surface. From each of the prepared

samples, knot-free test pieces of size 50 9 100 mm2 were

prepared. Test pieces were also prepared from boards that

had been re-sawn. The positions of the stickers during

drying were marked on the original surface of each sample.

The number of test pieces is summarized in Table 2.

2.4 Mould growth tests

2.4.1 Fungal strains

Freeze-dried strains of Aspergillus versicolor (CBS

117286), Aureobasidium pullulans (CBS 101160), Clado-

sporium sphaerospermum (CBS 122.63), Eurotium her-

bariorum (CBS 516.65), Penicillium chrysogenum (CBS

401.92) and Stachybotrys chartarum (CBS 109.292) were

obtained from Centraalbureau voor Schimmelcultures,

Utrecht, The Netherlands. The strains were kept on malt

agar in petri dishes with Malt Extract Agar for 7–14 days

until sporulation occurred.

Among the species used, there were fungi with and

without pigmented hyphae and spores.

2.4.2 Preparation of spore suspension

A suspension of spores was prepared, mainly according to

MIL-STD-810F, Method 508.5. Distilled, autoclaved water

was poured into each subculture and the surface of the

growth scraped to liberate spores into the water, which was

then poured into a flask containing glass beads. The flask

Table 1 Summary of drying schemes

Tab. 1 Angaben zu den Trocknungsprogrammen

Drying

schedule

Heating

phase (h)

Start climate

Tdry/Twet (�C)

Final climate

Tdry/Twet (�C)

Total drying

time (h)

Centre boards of spruce (50 9 150 mm) Fast 1 1.2 70/65 77/60 69

Slow 1 12.5 52/50 63/50 92

Centre boards of pine (50 9 150 mm) Fast 1 2.1 66/60 72/55 72

Slow 1 12.3 52/50 65/50 102

Side boards of spruce (22 9 100 mm) Fast 2 1.7 67/60 74/56 21

Slow 2 1.1 46/45 60/45 45

Side boards of pine (25 9 100 mm) Fast 2 1.1 67/60 74/56 26

Slow 2 1.1 46/45 60/45 45

Eur. J. Wood Prod. (2013) 71:473–481 475

123

was shaken to liberate the spores from the fruiting body

and to separate any spore clumps. The suspension was then

filtered through glass wool, centrifuged and washed three

times in distilled, autoclaved water. The spore concentra-

tion in the final residue was determined microscopically in

a counting chamber (Burker) and diluted so it contained

approximately 106 spores/ml. The final spore suspension

was prepared by mixing equal volumes of suspension from

each species.

2.4.3 Inoculation, incubation and microbial analysis

One surface of each test specimen was sprayed with 0.4 ml

of the spore suspension using an airbrush (Clas Ohlson

Model AB-119, Insjon, Sweden) attached to a minicom-

pressor (Cotech, Clas Ohlson) with a pressure regulator and

water separator. The working pressure was 2 bar. This

device setup atomized the suspension through a nozzle.

During spraying, the airbrush was swept along at an even

speed.

Following inoculation, the test specimens were incu-

bated in the dark in climate test chambers (CTS C-20/350,

CTS GmbH, Hechingen, Germany) at 95 % RH and 22 �C

for 28 days with moist air streaming over the test pieces at

an air speed of 0.3–0.5 m/s.

The pieces were removed from the climate chambers

after 6, 12, 22 and 28 days and were analysed for mould

growth with a stereomicroscope (6.7–609 magnification).

The extent of growth was assessed according to Table 3.

2.4.4 Statistical methods

The ratings for mould growth consist of ordinal data; non-

parametric statistical methods were therefore used. As the

scale consists of a few numbers (six), and the number of

samples in each group was relatively low, Fisher’s exact

test was chosen for comparisons between the groups

studied. Bonferroni correction was used to calculate the

adjusted significance levels in the multiple comparisons of

the effect of surfaces on mould growth. The results from

the pine and spruce samples were used together as one

group as the effects of drying were expected to be the same

for both species.

3 Results

3.1 Moisture content before and after drying

There was great variation between the moisture contents of

the different samples before drying, but this variation was

significantly reduced afterwards (Fig. 4). Hence, the

amount of free water dried off from the each piece varied

greatly between different test pieces.

3.2 Mould growth

The extent of total fungal growth increased with time for

all groups. Initially (after 6 and 12 days), there was a

significant difference (p \ 0.05) between kiln-dried and

air-dried pieces (see Fig. 5). After 22 days, this difference

no longer existed as the growth on all pieces had reached

the maximum value on the rating scale. There was no

significant difference (p [ 0.05) in mould growth between

the test pieces from the different drying schemes (see

Fig. 6).

Discoloration did not appear until after the second

analysis (12 days). At the time of the analysis on day 22,

there was a difference, as perceived with the naked eye,

between the air-dried and kiln-dried test pieces while there

was no obvious difference between the different kiln dry-

ing methods (Fig. 7). There was no increase in discolor-

ation when the test pieces had been incubated for another

6 days (28 days of incubation).

On many of the test pieces, there was a visual difference

in discoloration in the areas that had been under the stickers

compared with areas that had been between the stickers

(Fig. 7). The surfaces that had been under the stickers were

not free from mould growth, however, since all of those

surfaces had heavy growth of hyaline fungi. The extent of

this mould growth was initially higher on the surfaces that

had been between the stickers than those that had been

under the stickers on kiln-dried test pieces (Fig. 8). On the

air-dried test pieces, the development of mould growth was

the same for both types of surfaces.

On kiln-dried test pieces, mould growth was initially

less extensive on the surfaces that had been planed and re-

Fig. 3 Illustration of how each board was treated and divided into

smaller test pieces with different surfaces

Abb. 3 Auftrennung von Brettern und Aufteilung in kleinere

Prufkorper mit unterschiedlichen Oberflachen

476 Eur. J. Wood Prod. (2013) 71:473–481

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sawn than on the original surfaces (see Fig. 9). After

22 days of incubation, these treatments no longer had an

effect on the mould growth. The same effect of planing and

re-sawing was not visible on the air-dried test pieces.

4 Discussion

The results of this study show a difference in mould

resistance between kiln-dried and air-dried timber after less

Table 2 Test specimens in the study

Tab. 2 Untersuchte Prufkoper

Wood material Drying method Surface treatment

after drying

Number of test pieces

Between stickers Under stickers

Centre boards of spruce (50 9 150 mm) Fast 1 Planed surface 5 4

Re-sawn surface 5 0

Original surface 5 4

Slow 1 Planed surface 5 2

Re-sawn surface 5 0


Air-dried Planed surface 5 4

Re-sawn surface 5 0


Centre boards of pine (50 9 150 mm) Fast 1 Planed surface 5 5

Re-sawn surface 5 0



Re-sawn surface 5 0



Re-sawn surface 5 0


Side boards of spruce (22 9 100 mm) Fast 2 Planed surface 5 4






Side boards of pine (25 9 100 mm) Fast 2 Planed surface 5 1






Table 3 Rating scale for the assessment of mould growth at 409 magnification

Tab. 3 Bewertungsskala zur Beurteilung von Schimmelwachstum bei 40facher Vergroßerung

Rating Description

0 No fungal growth

1 Initial fungal growth consisting of scattered hyphae on the surface

2 Still scattered growth, but more apparent than for rating 1. Seldom visible to the naked eye

3 Patchy distributed heavy growth. Not always but in most cases visible to the naked eye

4 Heavy growth over the entire surface. In most cases visible to the naked eye

5 Very heavy growth over the entire surface. In most cases visible to the naked eye

Eur. J. Wood Prod. (2013) 71:473–481 477

123

than 1 week of incubation in climate conditions very

favourable to mould growth. This supports the hypothesis

that nutrients accumulate on the surfaces when wood is

dried at high temperatures in the kiln. The differences in

mould growth between the groups did not remain after

22 days in the same climate.

The hypothesis that wood dried on a fast-drying sche-

dule is more susceptible to mould growth than wood dried

on a slower-drying schedule due to more nutrients on the

surface was not confirmed. One possible explanation for

the lack of difference is that the samples varied signifi-

cantly in their initial moisture content. The amount of free

water that dried out from the samples consequently varied

greatly and, hence, the simple carbohydrates on the surface

were also expected to vary. The variation in mould growth

between different test pieces is sometimes bigger within

than between the two groups of fast- and slow-dried test

pieces. Another explanation is that the industrial drying

schemes chosen were not different enough in terms of

water transportation to show the effect on mould growth.

In addition to analysing the samples with respect to the

overall growth of fungi, the visual discoloration caused by

the fungi was studied. The colouring fungi were established

Fig. 4 95 % confidence interval for the mean of the initial (before

drying) and final (after drying) moisture content for test samples with

different drying schemes

Abb. 4 95 % Vertrauensintervall fur den Mittelwert des Feuchtegeh-

alts am Beginn und am Ende der Trocknung der unterschiedlich

getrockneten Prufkorper

Fig. 5 Mould growth according to Table 3 on air-dried and kiln-

dried test pieces after 6, 12 and 22 days of incubation at 95 % RH and

22 �C. The lower part of the box is the 25th percentile, the upper part

the 75th percentile and the thick horizontal line the median (50th

percentile). The whiskers show the highest and lowest values, and the

dots and stars represent outliers and extremes

Abb. 5 Schimmelwachstum gemaß Tabelle 3 auf luftgetrockneten

und technisch getrockneten Prufkorpern nach 6-, 12- und 22-tagigem

Befall bei 95 % relativer Luftfeuchte und 22 �C. Der untere Rand der

Box gibt das 25 %-Quantil an, der obere Rand das 75 %-Quantil und

die dicke Linie gibt den Median (50 %-Quantil) an. Die Whiskers

zeigen den hochsten und den niedrigsten Wert, die Punkte und Sterne

Ausreißer und Extremwerte

Fig. 6 Mould growth according to Table 3 on kiln-dried test pieces

after 6, 12 and 22 days of incubation at 95 % RH and 22 �C. The

lower part of the box is the 25th percentile, the upper part the 75th

percentile and the thick horizontal line the median (50th percentile).

The whiskers show the highest and lowest values, and the dots and

stars represent outliers and extremes. There were no significant

differences between the fast- and the slow-drying schedules

(p [ 0.05)

Abb. 6 Schimmelwachstum gemaß Tabelle 3 auf technisch ge-

trockneten Prufkorpern nach 6-, 12- und 22-tagigem Befall bei 95 %

relativer Luftfeuchte und 22 �C. Der untere Rand der Box gibt das

25 %-Quantil an, der obere Rand das 75 %-Quantil und die dicke

Linie gibt den Median (50 %-Quantil) an. Die Whiskers zeigen den

hochsten und den niedrigsten Wert, die Punkte und Sterne Ausreißer

sowie Extremwerte. Es gab keine signifikanten Unterschiede zwis-

chen den schnellen und den langsamen Trocknungsprogrammen

(p [ 0,05)

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early in the experiment, but it was not until after the

analysis at 12 days that they grew to such an extent that

they caused a discoloration that was visible to the naked

eye. When the samples were analysed after 22 days of

incubation, there was a higher extent of discoloration on

the kiln-dried test pieces than on the test pieces dried at

room temperature. As in the case of total fungal growth,

there was no difference between the fast- and slow-dried

test pieces.

One reason for differences in discolouration may be that

different nutrient status, due to different drying methods,

favour different species of fungi. The dark production of

pigment by fungi can also be dependent on which nutrients

are available, or the growth phase of the fungi (Eagen et al.

1997; Fleet et al. 2001; Gadd 1980). Aurobasidium pullu-

lans, Stachybotrys chartarum and Cladosporium sphaero-

spermum represent discolouring mould fungi in the spore

suspension used. These species are secondary colonizers on

building materials and may therefore have a limited ability

to compete with the primary colonizers Aspergillus versi-

colour, Eurotium herbariorum and Penicillium chrysoge-

num on the surfaces with higher nutrient level (Grant et al.

1989). Since the growth to fungal species was not identi-

fied, no conclusion of the reasons for the differences found

can be drawn.

Water evaporation from the surfaces under the stickers

was lower and therefore the amount of sugars transported

to the surfaces was expected to be lower. In this study, it

Fig. 7 Discoloration due to fungal growth after 22 days on test pieces from different drying schemes. The test pieces on the left of each picture

are pine, and the pieces on the right are spruce. On some pieces, there are surfaces that were under stickers during drying. These areas are placed

on the right of each test piece and are marked with dotted lines

Abb. 7 Verfarbung von unterschiedlich getrockneten Prufkorpern nach 22-tagigem Pilzbefall. Auf der jeweils linken Seite der Bilder sind

Kiefernprufkorper und auf der rechten Seite Fichtenprufkorper abgebildet. Bei einigen Prufkorpern war die Oberflache wahrend der Trocknung

teilweise durch Latten abgedeckt. Diese Bereiche befinden sich auf der rechten Seite der Prufkorper und sind durch gestrichelte Linien markiert

Eur. J. Wood Prod. (2013) 71:473–481 479

123

was found that the surfaces under the stickers during drying

were colonized by fungi to a lesser extent early in the study

than the surfaces between the stickers on kiln-dried test

pieces, but not on test pieces dried at room temperature.

There was a clear borderline of discolouring growth

between the surfaces under the stickers during drying and

those between the stickers. This, again, may confirm that

the rate of transportation of water and nutrients to the

surface does affect mould growth.

On the planed surfaces of kiln-dried wood, the extent of

mould growth was initially not as extensive as on the ori-

ginal surfaces. This effect could not be seen on the air-

dried test pieces. The results are consistent with earlier

studies (Terziev 1996; Viitanen and Bjurman 1995). One

explanation for the differences is that planing removes the

outermost layer where the concentrations of sugars are

highest (Theander et al. 1993), leading to lesser amounts of

nutrients for mould growth. On the re-sawn surfaces of the

kiln-dried pieces, the colonization of fungi was even slower

than on the planed pieces, which further confirms the

hypothesis, as the amount of sugars was expected to be

lower on those surfaces that were deeper in the wood

material during drying. Planing alters the surface structure

in a way that changes the microclimate for the fungi, which

may be an explanation for the lesser extent of mould

growth. If this was the only reason for reduced growth, the

air-dried pieces should show the same pattern, though this

was not the case.

After 22 days in constant optimal conditions, the factors

studied here no longer had any influence on the mould

growth, and all the timber could then be considered equal

in terms of total fungal growth. A similar pattern was found

by Fruhwald et al. (2008).

No increase in discoloration was noticed after 28 days

of incubation. If the study had proceeded for longer in the

same climate, the discoloration might have increased and

the differences between the groups might have disap-

peared. It is important to notice that even though it looked

like some test pieces did not have any growth, there was

heavy mould growth on all the test pieces after 22 days of

incubation.

The individual differences among boards exposed to the

same climate conditions may be explained by individual

differences due to varying moisture flows in the material

during drying as a result of a high variation in the initial

moisture content rather than the result of different drying

schemes.

The authors’ experience from other tests is that there is

less differences in mould resistance among different

materials in a warm and humid (optimal) climate compared

Fig. 8 Total fungal growth on surfaces that were under stickers

during drying compared with fungal growth on the surfaces that were

between the stickers

Abb. 8 Vergleich des gesamten Pilzbefalls von Oberflachen, die

wahrend der Trocknung durch Latten abgedeckt waren, mit dazwi-

schenliegenden freien Oberflachen

Fig. 9 Effect of planing or re-

sawing the surface on mould

growth

Abb. 9 Einfluss der

Oberflachenbeschaffenheit

(gehobelt, sagerau) auf das

Schimmelwachstum

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123

to when they are exposed to less optimal conditions. The

different drying methods might have exhibited greater

differences if a less optimal climate, both in terms of rel-

ative humidity and temperature had been used for the

incubation of the samples.

In this paper differences in mould growth were dis-

cussed on the basis that it is the redistribution of sugars that

is responsible for the differences, based on previous stud-

ies. However, there is also the possibility that there are

differences in other substances that affect mould growth,

such as fatty acids or nitrogen that may have contributed to

the differences found.

The laboratory tests used for studying mould growth in

this study are an accelerated form of testing. A constant

high relative humidity and temperature optimal for mould

growth for an extended time are not common in natural

circumstances. Instead, there is fluctuation in both tem-

perature and humidity levels. Also, in the study the test

pieces were inoculated with spores in a spore suspension.

Penicillium chrysogenum, Aspergillus versicolor and Eu-

rotium herbariorum are regarded as primary colonizers

with ability to quickly establish and start a growth on

surfaces when the moisture conditions are suitable (Grant

et al. 1989). This might lead to earlier growth compared to

natural infestation. The results of this study can therefore

not be used to predict how long timber can be exposed to

moist situations without risk of mould growth.

5 Conclusion

According to the results from this study, there will be no

difference in mould resistance of wood depending on

which wood drying method has been used. However, with

respect to discoloring growth only, kiln-dried wood is more

susceptible than timber-dried at room temperature. This

might be due to different concentration of sugars, which

might favour the species of fungi that discolored the test

pieces. Drying of timber at room temperature, however, is

not recommended because of other disadvantages. It was

used in this study only as a reference material to the arti-

ficial drying schedules.

The difference in the extent of mould growth was

sometimes bigger between individual test pieces than

between the groups of different drying schedules. This

indicates that the pattern of different discoloration of

external wood in practical use may be a consequence of

traits in the original wood rather than due to different

drying methods.

Changing the surface by planing increased the time

before mould growth appeared, but did not have any effect

at the end of the test. Planing can therefore not be used as a

measure to prevent mould growth.

Extensive mould growth can occur on wood without

being visible to the naked eye. The mould fungi in this type

of growth are the quickest to establish on wood. The fungi

used in this study are representative of fungi found natu-

rally on wood and, if the conditions are favourable, mould

can therefore be expected to grow before the wood

becomes discoloured.

Acknowledgments The present research was part of the research

programme WoodBuild co-ordinated by SP Technical Research

Institute of Sweden. The research was financed by VINNOVA, the

Swedish Federation of Forest Industries, and a number of companies

in the forest and building sector.

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Documents

Mould growth on kiln-dried and air-dried timber