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Expanding of the bottom piece over fire

expanding Hollowed bottom part is placed onto especially made structures like sawhorses over fire, so it can be easily moved and heeled for heating any desirable spot directly in the flame. Its inner surface must be kept wet with hot water water and steam, so it would soften and the inner layers would swell.

Expanding technique for pine wood was not known; plenty of experiments in smaller scale were done before it was found anew. Steaming alone does not soften pine wood enough to achieve the necessary expanding rate without cracks; two other factors were helpful:

  1. When wood is getting charred and starts to burn in a fire, it shrinks; that is why a stick of firewood would cover with a grid of small cracks. So, if the outer surface of a piece to be expanded is charred, the piece would turn outwards.
  2. High gradient of humidity can be created over the wall's thickness by keeping its inner side wet with hot water, and heating its outer side dry over fire. Then inner layers would swell with water and steam, while outer ones are dry and shrunk; this would also cause a strong effect, turning the piece outwards. This factor is probably the strongest with this technique, though temporary: the humidity gradient disappears as soon as expanding procedure was finished. Serious measures have to be taken to prevent a freshly expanded piece from quick drying -- it would crack then; a little water can be poured inside it, and some cover from direct sun provided.

expanding Outer surface of the bottom piece has to be kept over fire until it gets dry and starts to smoke and char; at this moment outer layers of wood shrink quite a lot.

All these factors are combined easily and naturally within this technique, and the site of a piece getting completely flat from initial diameter of 30cm without applying any external force is exciting enough. Bottom piece was expanded from 45 to some 120 cm diameter, becoming 100cm wide, while initially its edges were only 20cm apart. Sometimes one can see a firewood stick or some chip twisting and bending quite a lot in fire -- this is basically the same effect, just under conscious control. Expanding technique for aspen dugouts described in literature, does not seem to differ much from this one, too; at least heating with fire is always done from outside, and humidity gradient must exist anyway. Also, the outer surface always get more or less charred.

inside Inside view of the boat. Shape of the expanded bottom part is clearly visible.

For drawbacks and bottlenecks of this technique several things are to be mentioned:

Generally, the expanding procedure is the hardest and most stressful stage with this boat. It has to be taken quite seriously, everything must be prepared beforehand, till the smallest firewood stick; even then the bottom expanding took more then seven hours without breaks. It would make sense to have a team of 2-3 people for this job, for instance, the Finnish aspering bottom was said to be expanded in 3 hours by 3 men, by the way, one of them had the only duty to watch the outer surface and keep thinner spots from burning too much.

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