Charcoal burning has been used since the beginning of the Iron Age. Ancient finds of tar include the “Guldhøje Klapstole”, approximately dated to 1400–1200 BCE. Ancient boats such as the “Nydamskeppet” show that treatment with tar to protect and preserve materials that otherwise would rot was in early use. Tar technology was so highly developed at the time of the first church constructions in the Nordic region that the results of the treatment can still be seen today in, among other constructions, the Norwegian stave churches, which have resisted the onslaught of time for nearly 1,000 years.
The Middle Ages gave rise to the profession of “greaser” someone who worked with tar, pitch and tallow. He occupied third place in the list of occupations ranked by prestige, after the shipwright and the blacksmith. Pine tar has been Sweden’s third largest export during several periods after the Middle Ages, and during the 17th century it was the largest export in certain periods, and the demand was particularly great during intense periods of shipbuilding in Europe.
A large part of this was to be used in rope-making and the treatment of ropes to protect them from the breakdown of natural fibres that is caused by the sun and damp. It was not until the end of the 19th century that the significance of tar as an export product fell, not principally due to “… a lower level of trade on the world markets, but due to other tar-producing countries have adopted better methods that produce an excellent product …” Resin, and resin acids, is a tree’s way of protecting itself and repairing damage. Terpenes are the tree’s own solvents for the resin, in order to facilitate, among other processes, the rise of the resin within the tree trunk, and to accelerate oxidation.
This is the process by which solid resin is formed when the resin exudes from the trunk in order to form a surface layer and in this way reduce the degree of moisture absorption, which provides good growing conditions for fungi, algae and insect pests.
The best raw material for tar is old pine stumps, broken from sandy, stony ground. The resin is concentrated in the stumps after the tree has been felled, and after 10–80 years, depending on climate zone, it may amount to a very large fraction of the dry weight of the stump. Furthermore, the sapwood rots away, leaving only a resin-rich core known as fatwood. The fatwood stumps are broken up, the wood is cut to size, split and dried, and then the burning process itself can begin. The dale or burning ground, is normally constructed as a cone of spruce branches that are covered with birch bark and finally covered with clay in order to make it airtight. The “shoe” is placed at the bottom where the tar collects, such that when the tar plug is removed the tar runs down a spout to the tar barrel The next stage is to place the wood in the dale following a certain pattern, and then to hammer the wood. When the wood forms a mound of the right size, it is first covered with twigs that help to start the burning. The dale is then covered with turf, moss or soil, in order to make it adequately airtight. Burning is started by the wood being ignited at the base of the mound, around the dale, through holes in the covering.
The rate of burning is then regulated by opening and closing these air holes, and hammering the dale as it burns. The fire should spread evenly through the outermost layer to the top of the mound, and then slowly spread downwards and inwards into the dale. The tar-master must continuously regulate the supply of air based on the signs from the dale, in the form of the colour and speed of the smoke, and later the colour and viscosity of the tar. Only when the fire has reached into the centre of the dale does the tar start to appear, first in the form of “tar water”, and then soon after as the finest pure light tar. The tar becomes darker towards the end of the process and becomes viscous and pitch-like.
Subsequent treatment of the tar consisted of “discarding” and sorting the tar. This was carried out at the “tar court”. One method of monitoring quality was to stick a polished iron bar into the tar barrel, and then to assess the quality of the tar based on the appearance of the various parts. Classifications used included “fine”, “ordinary” and “coarse”, or “primary”, “secondary” and “tertiary”.
Pine tar has been extensively used in medicine, both for internal use and external application. The pamphlet “Siris: Philosophical reflexions and inquiries concerning the virtues of tar-water, and divers other subjects connected together and arising from one another” by Dr. George Berkley, the bishop of Cloyne, gave much advice in the use of tar. Pine tar today is still used as an active component of skin creams, and tar in its pure form is used in veterinary practice to treat wounds and as a preventative “skin cream”. Tar is said to act as a softener and in loosening phlegm. We would like to conclude this description with the words of Sweden’s greatest tar researcher, Hilding Bergström. He dedicated around 50 years to studies into charcoal products and summarises tar-testing as follows: “There are no generally used analysis methods for determining the quality of pine tar or turpentine oil. Thus, tar is often judged from its outward properties. One test that is often used by tar-burners is that of smearing the tar onto a planed wooden surface, after which the colour, consistency and drying properties of the tar can be observed, as can also whether the tar is free of water or not”. (Quote from around 1900).