The set up for the ‘open distillation’ is shown in Figures 7.1 & 7.2. A hollowed- out stick from elder (Sambucus nigra) leads from the bottom of the ‘oven’ through the ground whereas the other opening ends above a vessel. The base of the ‘oven’, as well as the birch bark are covered by a sealing layer consisting of a mixture made of mud, sand and clay and in which the fine grain part of the clay is highly reduced. The mixture should not only hold the layer together, but it should retain its integrity while fired. The layer is only 2 to 3 cm thick, in order to let the heat enter the inner ‘oven’ area easily. The layer around the bark is then smeared well with watery fingers to close all holes, so that the bark is completely sealed. A ring of stones support the structure (Figure 7.1).
A thick pack of dry grass covers the ‘oven’ to create a layer of ash around it to possibly fill little cracks that might appear while firing; and then a fire was laid around the structure. Then the oven was fired immediately for two to three hours to reach as high temperature as possible. The wood for the experiments was gathered and stacked beforehand so that the fire could be sustained continuously for this period.
Observations
In general there was first ‘smoke’-which is a mixture of gas and steam – and after about 10 minutes the smoke continued until the firing stopped. After about 20 minutes from the start of the firing, a somewhat clear watery liquid was recognised (wood acid, according to Wikipedia). It was at this stage, that occasional dark brown drops – the tar – began dripping out of the pipe. After the fire burnt down to ashes, the oven was allowed to cool down for half an hour. When breaking open the oven the birch bark had transformed to charcoal. The vessel was then put close by or above either a fire or hot ambers so that the fluid liquid evaporated and left a little bit of black, sticky pitch. The firing process in general consumed a full wheel barrel load of wood for each experiment, sometimes a bit more, when there was a good wind.
With one experiment, the mud/sand/clay sealing layer around the bark cracked slightly during firing and smoke was observed coming out of these cracks. It had contained a higher concentration of fine pottery clay than the other experiments. Filling the cracks with fine sand during firing did not close them properly. In this case it happened in a late stage, so that most of the tar was already extracted and it did not really seem to effect the distillation process.
For comparative purposes we made one distillation firing experiment with a raw (unfired clay pot) and one with a metal pot. The clay pot turned red during the distillation firing process and after cooling down, it was observed to be well-fired. When tapping it with a fingertip it gave a high tone, which indicates a well-fired clay pot and high temperatures during firing. Interestingly, the colour of the pot outside is reddish to light brownish, but inside it was completely black, indicating a burning process without oxygen (Figure 7.2).
Conclusions
All these experiments yielded only a very little pitch but our control distillations with the clay and the metal pot gave very similar results. Thus, it is likely that the birch bark used was not of good quality. It was very, very thin and harvested from young birch trees. There was still a lot of wood on it too and the pieces were small, so it was not possible to pack them in a high density. However, as we still received some Figure 7.1: Diagrams to show the ‘open’ distillation and ‘closed’ distillation arrangements. Source: Werner Pfeifer and Marco Plaussen.
118 the life cycle of structures in experimental archaeology
pitch each time, the method of construction worked well. The same method was Figure 7.2: The ‘open distillation’ method. Source: Werner Pfeifer and Marco Plaussen.
a. b.
c. d.
e. f.
demonstrated a few weeks later in Kierikki, Finland, with thick good quality birch bark was available, and it produced a lot of pitch. These later results strengthened our conclusions regarding the role played by the quality of birch bark.
The sealing layer around the birch bark needs to consist of a high percentage of mud and sand and just a very little bit of fine clay. It must also be very thin (2-3 cm only) in order to let the heat enter the centre of the structure. A previous experiment a year earlier used a thick layer of about 8-10 cm of mud fired for several hours: The experiment was unsuccessful as the bark inside was not burned at all proving that a thick layer prevents the production of tar.
The remains of the clay/mud/sand sealing layer after the firing process are quite soft and brittle, especially when taken in the hand and squeezed a bit. This means, that the remains of the oven will have disappeared within a year or two. It will look just like any normal fire place. Thus as such, recognizing a pitch distillation feature in an archaeological context after thousands of years will be extremely difficult, if not impossible. There might be some remaining microscopic tar traces in the sand, but there needs to be some more investigations done in this matter. We did one experiment without any pipe or pot under the oven; instead we placed a 3 cm thick layer of sand on the base – where the pipe or pot would have been – to test if the tar will be visible in the sand afterwards, but we could not find any visual traces of tar at the end. The comparative experiment without the unbaked clay vessel showed that some fragments of fired clay might remain, but identification of such remains might be problematic in the archaeological record if they are off site.
As we needed a lot of wood for gaining very little tar, it is suggested that the pitch distilleries in the Stone Age might not have been in the settlements but rather in the woods where all raw material was in-situ and no extra labour was needed to carry everything into the settlements; except, of course, for the pitch at the end. Thus, finding these places will be very difficult nowadays. This might be a reason why no Stone Age pitch distilleries have ever been found.
The main result from the clay pot distillery experiment was that the pot was black inside but did not smell at all of tar or pitch! It was filled with water after cooling down and there were no indications of oily or other substances on the water surface. The water tasted clear as well. This indicates, that all tar had been removed during distillation, or some might have entered the clay itself, but leaving no visible traces.
A colleague, Mrs Erika Drews in Albersdorf (Germany) had made a small clay pot distillery, which yielded no tar or pith in her experiment, but she claimed that after the experiment her clay pot was waterproof, which is normally not the case with this type of clay. So it might be, that the tar entered the clay during firing and sealed it. This means for archaeologists to look for traces of tar in the clay of late Stone Age vessels itself, especially when they are black inside (Erika Drews pers. comm).