Category The Ecology of Building Materials

Green vitriol

Green vitriol is a relatively harmless impregnating substance based on ferric sulphate. In liquid form it can irritate the skin and is slightly damaging to organisms living in water. A good impregnating solution consists of 10-13 g/litre of water, with a little alum added as a fix. Green vitriol is also a fire retardant and gives timber a shiny silver surface. It is often called acid treatment. Such a treatment can last up to 15 years but will in time be washed out of the timber.

[1] Loss factor is the percentage of material that is usually lost during storing, transporting and mounting of the product.

(2) The figures in brackets under combustion value show the value that is no longer available due to its poisonous character or the structure of the material.

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Bor salts from borax and boracid

These impregnating substances combine effectiveness against vermin with rela­tive harmlessness to humans. The emission period from an impregnated surface is as short as 10 hours, so the interior of a building will be risk-free after a couple of days.

In Germany borax is the only one of the more effective poisons used indoors. It is also used to impregnate cellulose insulating materials where it also acts as a fire retardant. It is, however, quite easily washed out of materials. Borax is bought as powder, and usually used as a 5-10 per cent solution in warm water applied in two coats. Very dry timber is moistened first so that the borax will penetrate better.

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Soda and potash lye

These have been used for surface treatment in many Swiss villages for hundreds of years, and the buildings have kept very well. A drier climate is, of course, part­ly responsible for their success, but this treatment deserves discussion. Impregnation with lye brings the resins and tar to the surface of the wood in the same way as burning. The lye also has an antiseptic effect. The treatment has to be repeated every two to three years. Gloves and glasses should be worn during the treatment, as the material is very alkaline.

Recipe for lye made from soda and potash

The soda solution is made by boiling 5 litres of water with 250 g of soda powder. The liq­uid is applied when still warm...

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Bark extract

Bark extract often has borax and soda salt added to increase its antiseptic effect. The extract is poisonous to insects and fungus, even though somewhat weak. It is not dangerous to humans. Bark extract is not waterproof, and is most useful on exposed materials indoors. Extract based on birch bark has the best impregnat­ing properties. (See also ‘Recipe 2: Bark stain’ p. 425.)

Wood vinegar

Wood vinegar is corrosive and is not used as a preventative but for treating materials that have already been attacked by rot and insects. Wood vinegar is extracted by distillation from deciduous trees, although even coniferous trees contain wood vinegar, but in smaller quantities.

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The least dangerous impregnating substances


Wood tar is usually extracted from parts of pine that are rich in resin: the bole and the roots, which are burned to charcoal. It can also be extracted from other coniferous and deciduous trees. Tar from beech is widely used in mainland Europe. Modern extraction techniques give a very clear tar – previously, when burning took place in a charcoal stack, high levels of pitch and particles of car­bon were included.

Extraction of wood tar in a charcoal stack

The stack is dug out in a sloping piece of ground. The bottom is shaped like a funnel and covered with birch bark. A pipe made out of a hollowed branch is placed in the bot­tom of the funnel. The timber is split into sticks about 18-20 cm long and 1 cm thick and they are stacked radially round a strong central log...

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Poisonous surface-coats or impregnation

Experience has shown that timber with a high content of tar and resins lasts longer than timber with a low content of the same. This is partly because the tim­ber is harder and partly because these substances have ingredients which are poi­sonous to fungus and certain insects. These natural fungicides and insecticides consist of, or are similar to, different types of tannic acid. 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...

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PH-regulating surface-coat or impregnation

Substances which regulate pH are an effective way of preventing or removing mould attack. Mould will not grow if the pH level is higher than 6.0. The same can be said of insect attack. Exceptions are the fungus Merulius lacrymans and the longhorn house beetle. The pH-regulating materials to use are alkalines such as clay, cement, lime and waterglass. They are not poisonous in themselves, so they do not cause problems in the indoor climate of the building.

Waterglass as a pH-regulating coat

Waterglass is very alkaline and in addition forms a coat so hard that insects cannot pene­trate it to lay their eggs. Waterglass is, however, not waterproof when used on timber, and can therefore only be used indoors or on protected parts of the building...

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Non-poisonous surface coats

The application of a non-poisonous layer on the surface is mainly to protect the timber from mechanical wear and tear and direct solar radiation. Exposure to these can lead to large or small cracks which, in a damp area, can lead to fungus attack.

Many different paints give a timber wall this sort of protection. The best of these are probably the pure linseed oil paints, which penetrate well into the wood. The effect of this layer depends upon reapplication at regular intervals.

A positive parameter in this treatment is the fact that the paint is considered water-repellent...

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