Impregnating agents, and how to avoid them
As has been illustrated in several chapters in this book, wood is, for
most purposes, a good choice from the environmental point of view.
However, where possible, wood should be used in a way that ensures a
long lifetime and, as far as possible, not be treated with toxins or
additives.
Wood and other organic materials are easily attacked by insects
and fungi in damp conditions. In central and Northern Europe, six
types of insects are especially attracted to timber buildings (see
Table 19.1)Table 19.2. Fungus is a type of lower plant species that
lacks chlorophyll. Fungi that attack buildings can be divided into two
main groups, discolouring fungi and disintegrating fungi. Discolouring
fungi give timber a superficial discoloration, without decreasing
its strength. Disintegrating fungi attack the cell walls in timber and
destroy the wood.
Spores from disintegrating fungi are ubiquitous. They spread with
the wind in the same way as pollen, and attach to anything. These fungi
belong to nature’s renovating corps, their main operation being the
breakdown of dead organic material, which regrettably includes many
building materials. The optimum conditions for this phenomenon relate
to dampness, temperature and acidity. Dampness in organic material
needs to be from 18 to 25%. Humidity above or below these figures is
not attractive to these spores. The majority of fungi, however, survive
long dry periods. A temperature between 20 and 35 C makes an attack
possible, but there is no activity below 5 C. Disintegrating fungi do not
strike in environments with a high alkaline content, i.e. with a pH over
6.0. One exception is the Merulius lacrymans.
There are four principal ways to avoid attack from insects and fungi:
1. Use of high quality material in exposed locations.
2. Structural protection of exposed materials.
3. Use of non-toxic treatments: passive impregnation.
4. Use of toxic substances: active impregnation.
The toxic preservatives are usually divided into insecticides and fungicides. The concept behind them is the creation of biological toxins
that kill, which frequently has unforeseen consequences for other
animal species, not least humans. The main task of this chapter is to
show how these substances can be avoided.
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Table 19.1 Vermin
Type
Comments
House longhorn beetle (Hylotrupes bajulus)
Does not attack heartwood in pine
Carpenter ants
(Camponotus herculeanus)
Does not live on wood, but uses it as its home and lays eggs,
even in pressure-impregnated wood
Common furniture beetle
(Anobium punctatum)
Prefers a temperature of 20–25 C and a relative humidity of 50%,
only found in coastal areas
Woodworm (Dendrobium pertinax)
Attracted to wood that has already been attacked by fungus
Violet tanned bark beetle (Callidium violaceum)
Dependent on bark left-overs for its survival
Bark borer (Ernobius mollis)
Dependent on bark for its survival
19.1 CHOOSING HIGH QUALITY MATERIAL
In old trees of most species, a large part of the trunk consists of
heartwood, which has a strong resistance to fungi and insects. Not
even the house longhorn beetle can penetrate the heartwood of pine.
Heartwood was traditionally used in log construction and external
panelling and, until the nineteenth century, in windows and doors.
Initially, pine was thought to be more durable than spruce, but this
conclusion has been modified. The core of pine has almost no
moisture absorption capacity, whereas the sapwood has a moisture
absorption capability (lengthwise in the cells) 10 times greater than
that of spruce. Pine cladding from the young core is therefore less
protected than spruce. Birch cladding is even weaker; its permeability is about 1000 times greater than that of spruce. Generally speaking, the absorption of moisture increases in relation to the breadth of
the growth rings.
Table 19.2 Minimum slope of roof to prevent water seeping in
Type of roof covering
Normal situation ( )
Exposed location ( )
Corrugated metal sheeting
10
14
Concrete roof tile
15
22
Corrugated cementitious sheeting
14
18
Slate, single layer
22
30
Slate, double layer
20
25
Fired clay tile, interlocking
20
30
Bituminous shingle
18
22
Bituminous roofing felt, two layers, welded
3
3
Plastic membranes, welded
2
2
Timber, shingle and plank roof
22
27
The manner of sawing plays an important role. There is least warping
and cracking in planks with standing annual rings (see Figure 10.4).
This greatly reduces the likelihood of water penetration and fungal
attack.
Timber should be felled in winter, because wood felled in summer
has a much higher sugar content, making it more attractive to insects
and micro-organisms. By removing the bark from the felled trees,
attacks by bark-eating bugs are avoided. Sawn timber should be dried
to 20% moisture content before spring, and logs that are not going to
be sawn should be stored in water. In northern climates, pine for log
construction should be felled in September and profiled on both sides
during the spring. It should be dried during the summer and used as
building material in the autumn. See more about preventative sawing
and drying routines on page 169–170.
Material from a building that has recently been attacked by the
house longhorn beetle or the common furniture beetle should not be
re-used.
19.2 STRUCTURAL PROTECTION OF EXPOSED COMPONENTS
19.1
Protective principles for outdoor use of
timber (balconies etc.); a-b: Exposed tangential sides; c-d: Exposed radial sides,
but the pith and the juvenile wood will
make the wood shake and deform; e-h:
Exposed radial sides without pith.
Source: Tr€
ainformation, Sweden.
19.2
Protective splicing of vertical panelling.
Type (a) and (b) are highly protected while
type (c) will easily shake and crack, especially around the nails. Source: Tra€tek,
Sweden.
If buildings have been constructed with materials so that air circulates
easily, and keeps them dry, then fungus will not attack.
All types of timber should be used in a way that allows movement to
take place; otherwise splitting and gathering of moisture will occur. The
heartwood side, which is generally the least moisture absorbent,
should be on the outside (Figure 19.1). Moisture is usually most quickly
absorbed at the ends of the timber. The end grain must therefore be
protected. Exposed ends of beams can be cut at an angle or preferably
covered.
Panelling should be well ventilated. The more exposed a wall is to
driving rain, the wider the air gap behind the panelling should be; this is
usually 5 cm in very exposed areas, and about half that in normal inland
situations. Horizontal battens fixed directly to panelling should have a
sloping top side, or be mounted on a vertical batten system against the
wall. The distance of the panelling from the ground should be at least
20–30 cm.
The bottom end of vertical panelling should be sawn at an angle so
that drops are formed and let off on the outside face of the timber
(Figure 19.2). The root end should be pointing downwards as it contains
more heartwood. Water may collect in the joint between the two layers
of vertical panelling. Along the coast where there is plenty of driving
rain, this often results in rot, as drying periods can be very short-lived.
On the coast, panelling should therefore be horizontal. This also gives
the advantage of less exposed end grain. Rot usually occurs at the
bottom of the wall, and with horizontal panelling it is quite easy to
remove and replace a few planks; with vertical boarding all the planks
would be affected.
In particularly damp areas, the colour given to the surface can also
play a part. A dark ochre colour can reach a temperature of up to 40 C
higher than a white surface in sunny weather. This can be a significant factor for drying times. In damp places where, even during the
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summer, there are only short periods of sun between showers, the
drying time needs to be as short as possible. However, if the temperatures get too high, splitting or cracking can occur, which can also
increase the intake of moisture.
Combinations of wood with metal, lime and cement-based mortars
and concrete can cause problems. Condensation can occur around
metal components, while in combinations with cement and lime, alkaline reactions can arise which increase porosity and moisture absorption in the timber.
19.3 METHODS OF PASSIVE IMPREGNATION
Methods of passive impregnation are partly aimed at enhancing a
timber’s natural capacity to withstand insects and fungi, as well as
partly reducing access to nutrients and humidity.
Self-impregnation is a well-known traditional method used in most
cultures. The usual procedure is to lop off the top of a pine or similar tree
and remove a few stripes of bark from the bottom to the top. Three or
four of the highest branches are left to ‘lift’ the resin. After a few years,
the whole trunk is filled with resin and the timber will gain a quality
similar to heartwood.
Cleaning out the cell content makes the timber less attractive to
most insects and fungus. This can be achieved by storing it under water
for a time. In salt water the results will be even better due to the salt
absorbed, since this has an antiseptic effect.
In the past, it was common practice in Scandinavia to boil wooden
shakes before they were used for roofing. Boiling is a very effective
way of washing out the content of the cells.
Heat treatment. Burning the outer layer of wood is a traditional way of
increasing the durability of any part of the wooden piles that were going
to be placed underground. The carbon coating that forms lacks nutrients and is almost impenetrable to insects and fungus. The heating also
enriches phenolic resins and aldehydes in the outermost part of the
pile. These are natural biocides. The greatest impact occurs with pine,
which is rich in resin, while burning spruce and deciduous trees is not
so effective. During burning the timber can easily split, and it is easy for
fungus to get access through the splits, so burning must be carefully
controlled, preferably by using a blow lamp. The depth of the burning
should be 1–3 mm, after which the surface is brushed with a bronze
brush. This process takes a long time. Julius Caesar described the
technique in his book De Bello Gallico in connection with setting
up fortifications in the Roman Empire. This method has also been
used for centuries in Portuguese and British timber warships, as it
not only increases resistance to rot but also makes the surface less
water-absorbing.
A modern version of heat treatment is done by heating the timber,
usually pine or spruce, in a closed chamber at 200 to 250 C for
24 hours. The process is quite energy intensive, but in addition to
increasing the timber’s resistance to fungi, it gives the wood a golden
brown patina which has made it an attractive competitor to tropical
hardwoods. This method seems to give the timber the same resistance
as heartwood (Edlund, 2004).
Chemical modification is a process that normally uses acetylation
through a process whereby the timber reacts with acetic anhydride –
which comes from acetic acid (see page 177). This effectively reduces
the ability of wood to absorb water. The result is a highly stable timber
that is no longer digestible. The resultant resistance against fungi is
greater than for heat treated timber and about the same as for timber
impregnated with copper (Edlund, 2004).
In a second type of chemical modification, furfuryl alcohols produced
from biomass waste such as sugar canes, corn cobs and sunflower
are used. The furfury alcohols are driven by pressure into the cell
walls of the wood and heated with steam (80–140 C) for 6 to 8 hours
to achieve polymerization. The impact strength of the wood is thereby strongly reduced, but at the same time a higher stiffness is
achieved, and the treated wood is no longer attractive to fungi
(Figure 19.3).
In a third type of chemical modification, the cells in the wood are filled
up to 90% with synthetic monomers. After heating and gamma radiation a plastic polymer is formed that is virtually inaccessible for fungus
and insects.
Seen from the environmental point of view there is no doubt that
acetylation and furfurylation are preferable to this last method.
As with normal timber these treatments involve few environmental
doubts. The chemicals are based on renewable resources and
the timber will be suitable for energy recovery. If modified with
synthetic polymers the waste will have to be treated like other
plastics.
Saturation with linseed oil is done by pressure treating the timber
with linseed oil, in amounts of about 90 litres per m3 of timber. The cell
pores are then filled and thus become inaccessible to insects and
fungus, as in heartwood. Linseed oil treatment is also water repelling.
The resistance against fungal attack is considered equal to treatment
with copper based fungicides (Edlund, 2004).
pH-regulating substances can be used both as a preventive measure against fungal attack and for remedial treatment. Mould will not
grow if the pH level is higher than 6.0. Treatment with pH-regulating
substances is also effective against insect attacks. Exceptions are
the fungus Merulius lacrymans and the longhorn house beetle,
which are not affected. The pH-regulating substances available are
primarily alkalis such as clay, cement, lime, waterglass and lye.
Treatment with lye also brings the resins and tar to the surface
of the wood in the same way as burning. The pH-regulating substances are not poisonous in themselves, so they do not cause
problems in the indoor climate of the building or for the surrounding
environment.
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19.3
Exterior woodwork modified with chemicals from biomass waste. Source: Kebony products.
TREATMENT WITH WATERGLASS
Waterglass(seepage 90)isveryalkaline.Inaddition, it formsacoatsohardthatinsects
cannot penetrate it to lay their eggs. It is, however, susceptible to leaching when
exposed to rain, and can therefore only be used indoors or on protected parts of the
building.Waterglass needs a rough surface; it does not bind well to a planed surface.
It is dissolved in boiling water and applied to the wood with a brush. It can also be
applied to straw materials, using a solution of one part waterglass to two parts water.
Waterglass is often used as a £ame retardant and it is very opento water vapour.
LYE MADE FROM SODA AND POTASH
The soda solution is made by boiling 1 litre of water with 50 g of soda powder. The
liquid is applied when still warm. Potash solution is either based on pure potassium
carbonate or on wood ashes, which contain about 96% potassium carbonate. A
potashsolutionismadeupbyboiling1 litreof water with 0.5 litresofpineashandletting
it simmer for 15 minutes. The solution is sieved and applied while still warm. The
treatment has to be repeated every two to three years.
Soda and potash lye have been used for surface treatment in many Swiss villages
for hundreds of years, and the buildings have kept well. A drier climate is, of course,
partly responsible for their success.
19.4 METHODS OF ACTIVE IMPREGNATION
Experience has shown that timber with a high content of tar and resin
lasts longer than timber with a low content of either. This is partly
because the timber is harder and partly because these substances
have ingredients that are poisonous to fungus and certain insects.
Traditional types of timber protection aim to increase the quantity of
such materials by covering the timber with tar. Extract from bark has
also been used to impregnate oak, birch and spruce, with good results.
This method was once so popular that bark extract became a major
Norwegian export. Over 2000 years ago the Chinese used salt water as
an impregnating agent. Wood containing more than 5% of table salt
(sodium chloride) is not susceptible to fungus. The modern version of
this is the use of metal salts. Wood tar has mostly been replaced by
derivatives of fossil oil (Table 19.3).
Since forestry was industrialized, the general quality of timber has
deteriorated considerably, and the need for biocides has rocketed over
the last decades. New fashions in architecture, which include highly
exposed exterior timber structures, have accelerated this trend.
For active impregnation the following functional qualities are
desirable:
*
*
*
*
*
Enough poison to prevent attack from fungus and insects.
Not be poisonous to people or animals.
The ability to penetrate into the material.
Resistant to being washed out or vaporized from the material.
Free from damaging technical side effects such as miscolouring,
corrosion of nails, etc.
Unfortunately, an impregnating substance with all these qualities
does not exist. There is generally a clear relationship between toxicity
and effectiveness. Effective poisons such as metal salts have particularly damaging effects on the environment, including humans. Less
damaging substances such as bark extract and ferrous sulphate are
at the same time less effective and susceptible to rapid leaching.
Preventive impregnating agents must be differentiated from treatments that are used after the material has been attacked. The same
substance can, however, often be used in both cases. To make the
mixtures fully effective, both fungicide and insecticide may be needed
in the same mix. They are dissolved in water or organic solvents.
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PART 3
Table 19.3 Active substances in fungicides and insecticides
Type
Fungicide
Insecticide
Toxicity
Arsenic salts
x
x
Very high
Aluminium sulphate
x
Boric acid, oxides and salts
x
Copper and copper salts
x
Medium
Ferrous sulphate
x
Low
Fluorine salts
x
Medium
Sodium chloride (table salt)
x
Low
Zinc and zinc salts
x
Mineral based
Low
x
x
Medium
Medium
Oil and coal based
Creosote
x
Endosulphane
Hexachlorobenzene (Lindane)
Very high
x
x
Very high
Very high
Parathion
x
Very high
Pentachlorophenol
x
Very high
2-Phenylphenol (Preventol)
x
Pyrethrin (Permethrin)
Medium
x
High
Triazole (Propiconazole)
x
Medium
Tributyl tin
x
Very high
Bark extract
x
Low
Tar from softwood
x
High
Tar from beech
x
Low
Wood vinegar
(x)
Wood based
(x)
Low
The substances are applied to the timber by pressure/vacuum treatment or by dipping/brushing on.
Apart from creosote (composed of polycyclic aromatic hydrocarbons), pyrethrine (Permethrine) is the most common oil derivative
and has superseded such derivatives as pentachlorophenol, which
were phased out during the 1980s and 1990s because of high environmental and health risks. The most important metals used for impregnation are arsenic, chrome and copper where copper is regarded as
least toxic.
There are different classes of impregnating substances; when in
contact with the ground, timber requires strong substances in large
doses, but in well-ventilated, outdoor cladding, a much weaker mix will
be effective enough. A strong salt impregnation agent usually contains
a mixture of copper, chrome and arsenic salts (CCA) where arsenic and
copper are the biocides and chrome acts as fixative, however still highly
toxic. For timber above ground level it is quite adequate just to use
copper compounds.
Both metal and fossil oil products are based on very restricted
resources.
Production of impregnating substances and the work at manufacturing workshops can result in emissions of strong biological toxins to
earth, air and water. Heavy metals are highly toxic and have large
biological amplification capacities. Also frequently used fluorine salts,
zinc salts and borates have serious toxic effects. From the impregnation industry based on fossil oils, vaporized solvents can be released as
well as a range of chlorinated hydrocarbons. Many of these will, in the
same way as heavy metals, have a capacity for biological amplification.
In the house, solvent-based tributyltin, zinc and copper-naphthenates volatize, potentially exposing the occupants to their toxic fumes.
Water-soluble metal salts are usually stable in buildings. They are,
however, released from exterior surfaces exposed to rain and may
contaminate ground water and soil. Normally about 30% of the metal
salts will leach out in the course of a 30-year period (Hansen, 1997).
Acid rain increases the rate of leaching. These substances will quite
easily combine with earth particles which delays the drainage and
spread of the substances to some extent. Pure sand will not have this
effect. Leaching will also occur with the oil-based products. These also
have gaseous emissions to air. In creosote impregnated buildings,
considerable concentrations of naphthalene have been registered
inside buildings even when the application has been outdoors
(Gustafsson, 1990). Creosote combined with solar radiation can also
cause rapid and serious burning of the skin.
If creosote-impregnated timber is combusted at temperatures under
350 C, the entire contents of polycyclic aromatic hydrocarbons (PAH)
will be emitted with the chimney gases. Much the same is the case
with products impregnated with CCA; for example, about 80% of the
arsenic compounds are released. Alternatives are either disposal at
strictly controlled tips, where the substances will eventually return
to nature, or combustion at very high temperatures, as in cement
factories.
TRADITIONAL WOOD-BASED TREATMENTS
Wood tar is usually extracted from the parts of pine which are rich in resin: the bole
and the roots. It can also be extracted from other coniferous and deciduous trees.Tar
from beech is widely used in mainland Europe.
Modern extractiontechniquesgiveavery clear tar.Traditionalextractionof woodtar
took place in charcoal stacks and high levels of pitch and particles of carbon were included.Thestack wasdugoutonaslopingpieceofgroundwiththebottomshapedlike
a funnel and covered withbirchbark. A pipe made outofa hollowed trunk was placed
in the bottom of the funnel. The timber was split into sections about 18^20 cm long
and1 cm thick which were stacked radially around a strong central log.The stack was
thencoveredwith earthand turf, andlitat thebottom.The stack was allowed tosmoulder forupto 24 hours,dependinguponitssize.Thetargatheredinthefunnelandcould
be drained o¡through the wooden pipe.
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Wood tarcan be used pure or mixed with boiled or raw linseed oil in a proportion of
1:1; pigmentcanalso beadded.Wood tarextractedfrompinetrees contains considerable amounts of polycyclic aromatic hydrocarbons (PAHs), for example as benzo(a)
pyrene, which is awell-knownmutagenand carcinogen.Tar from beech is almost free
from these substances.
Bark extract isslightly toxictoinsectsandfungus,eventhoughsomewhat weak.Itis
not dangerous to humans. Bark extract has little water fastness and is most useful on
exposed materials indoors. Extract based on birch bark has the best impregnating
properties (see recipe on page 405^406).
Wood vinegar is corrosive and is not used preventively but for treating materials
that have already been attacked by fungus and insects.Wood vinegar is extracted by
distillation from deciduous trees, although even coniferous trees contain wood vinegar, but in smallerquantities.
REFERENCES
€ndighet hos Miljo
€ anpassat Tra
€skydd, Sveriges ProvnEdlund, M.-L. (2004) Besta
ings- och Forskningsinstitut.
Godal, J.B. (1994) Tre Til Tekking og Kledning, Landbruksforlaget, Oslo.
Gustaffson, H. (1990) Kemisk Emission Fran Byggnadsmaterial, Statens Provning
sanstalt, Boras.
Hansen, O.C. (1997) Træbeskyttelsesmidler og Imprægneret træ Miljøstyrelsen
Arbeidsrapport, nr. 57, København.
Johannsson, G. (1990) kvalitotskrav pa byggnodsvinke, Byggforsknings radet Rapport 105, Stockholm.
Townsend, T. et al. (2000) Environmental Impacts of Treated Wood, CRC Press,
Boca Raton.