Expanding of the bottom piece over fire
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
layers would swell.
Expanding technique for pine wood was not known; plenty of experiments in smaller scale were
before it was found anew. Steaming alone does not soften pine wood enough to achieve the
expanding rate without cracks; two other factors were helpful:
- 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
the piece would turn outwards.
- 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
while outer ones are dry and shrunk; this would also cause a strong effect, turning the piece
This factor is probably the strongest with this technique, though temporary: the humidity
disappears as soon as expanding procedure was finished. Serious measures have to be taken
a freshly expanded piece from quick drying -- it would crack then; a little water can
inside it, and some cover from direct sun provided.
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
completely flat from initial diameter of 30cm without applying any external force is
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 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
more then seven hours without breaks. It would make sense to have a team of 2-3 people for
for instance, the Finnish aspering bottom was said to be expanded
hours by 3 men, by the way, one of them had the only duty to watch the outer surface and
spots from burning too much.
- 1) When expanding procedure seems to be finished successfully, it is yet too early to
in fact, now the most dangerous time is coming. The humidity gradient will disappear, and
tend to recover its initial round crossection (at least to a certain extent). As spreading
now prevent it from doing so, it will acquire wave-like crossection shape, thicker and
causing thinner and weaker ones to turn still more outward. This is along these weaker
belts that cracks
will appear, making them even weaker, and, therefore, causing their further turning outwards,
and so on
-- sort of an avalanche-like process. The result is clearly
the picture of ready planks drying. Freshly expanded piece feels very tense, it must
covered with boiling tar, this makes wood more elastic and makes its drying more slow and
piece must be fixed into its final shape with all kinds of spreading bars, clamps and wedges,
to keep weaker
belts from bulging in, and stronger belts from bulging out, i.e. to keep the curvature as even
possible. Plank pieces were fixed flat by means of sticks on both
tightly tied together.
- 2) Thickness and humidity of a piece are never perfectly even, so some spots will get dry
over the fire sooner, and other spots later. As thinner and dryer ones burn on the surface,
still thinner and dryer with respect to the rest of the piece -- another avalanche-like
which might result into burnt-through holes, let alone thin weak zones and belts. So one
have to watch the outer surface constantly during the expanding, wet the thinner spots with
outside, too, or even apply wet rugs to them; sometimes they must be covered from direct
piece of plank or a big chip.