- Fungal Stains
There are several different kinds of stain caused by fungi.
Sapstains.
The most common form of sapstain is a blue colouration. With all sapstains the
staining is caused by the presence of fine dark-coloured fungal hyphae within
the wood cells. Some types of sapstain can have a disfiguring effect on
certain lighter coloured hardwoods.
Sapstain usually infects and spreads within timber between the time it is
felled and dried. With timbers like ramin and rubberwood, which are
susceptible to sapstain, it is therefore important that drying is carried out
as soon as possible after felling and that the timber is not left exposed for
long periods to damp conditions which encourage development of the staining
fungi.
To restrict the spread of sapstains during drying, any sawdust left on the
wood should be brushed off and, if it is necessary to dry susceptible timber
species at low kiln temperatures, a special treatment should be applied.
Sapstain does not affect the strength properties of timber but by making it
unfit for decorative and certain other purposes, this type of stain can result
in considerable financial loss.
Mould.
Under certain high humidity conditions, mould growth may become extensive on
timber being dried. Its growth is confined to the surface of wood and although
superficial staining may result, this can be removed during normal processing
of the timber after drying.
One of the problems with extensive mould growth is that it could at certain
stages impede the drying of timber. Additionally, a severe outbreak of mould
could release large numbers of spores into the air and under some
circumstances this may expose kiln operators to a health risk. It is advisable
when drying timber which is already heavily colonised by mould, to apply a
high-temperature sterilisation treatment as recommended for sapstain.
Other Stains Caused by Fungi.
Wood affected by dote (incipient decay) may change colour quite markedly
during kilning and this can be mistaken for a blemish caused by the drying
process. Dote reduces the strength of timber and although kiln drying will
prevent its spread the original defect cannot be corrected.
- Chemical Stains
Uniform change of colour.
Most woods permanently darken in colour to varying degrees when exposed to
elevated temperatures during kiln drying. Frequently the discolouration is due
to oxidation of some constituents of the wood and at the temperatures and in
the standard kiln drying schedules H,J,K,L and M darkening is also caused by
the combined effect of high temperature and moisture on the wood.
Sticker Stain.
On dismantling piles of certain species of timber after drying, it is
sometimes found that where the piling sticks have been in contact with the
wood, there are distinct bands which are different in colour from the rest of
the surface. The occurrence of these stick stains can sometimes be attributed
to the use of dirty, mouldy and wet sticks or sticks of more acidic species.
When kiln drying some species, well defined stick stains may develop even
when they are piled with clean dry sticks. These stick stains may penetrate so
deeply into the material that they are not removed by normal planing. Staining
of this type is caused by the comparatively slow loss of moisture from the
areas under the sticks during initial drying. This results in chemical changes
under the sticks which are different from those occurring on the fully exposed
parts where moisture loss is more rapid.
Although very little work has been done on the problem of stick staining,
it would appear that a possible way of avoiding it is to obtain a quick
surface drying of the timber before it is piled. This can be done by
end-racking the boards or planks.
Water Stain (dark brown surface stain).
This is commonly found on air dried timber which has not been adequately
protected from rain. It should not occur when timber is kiln dried but
occasionally it may be caused by inadvertent wetting of parts of the charge in
a kiln. The stain can usually be removed by planing and is not generally a
serious problem.
MTC’s
