and then finish it with polyurethane.
Is there any way that you can do this
Or maybe a library or school has
come to you with a question about pre-
paring a disk from a very old tree that
will then be marked with pins showing
various historic events that occurred
while it was growing.
Certainly, we would all like to help,
but anyone who understands drying
knows that a disk will develop at least
one major, unsightly crack extending
from the outer perimeter to the center
Here is why a crack will always
develop. Consider a green, circular
disk that is 24 inches in diameter. As
this disk dries, its circumference will
want to shrink about 8 percent. So, the
original circumference of ( 24 x pi = )
75. 4 inches wants to shrink 8 percent
to 69. 4 inches.
Of course, this cannot happen
unless the diameter is reduced to 22. 1
inches (that is, 22. 1 x pi = 69. 4 circumference).
But, the bad news is that the diameter will naturally shrink only about 4
percent during drying. (Technically
speaking, the circumference shrinkage is called tangential shrinkage.
The radius shrinkage is called radial
So, the radial shrinkage is not
enough to prevent the development of
some substantial stress.
To avoid stress development and
cracking, we have several options:
; A hole could be cut in the center to
allow the radius to shrink more.
; Several disks could be cut and then
one used for repairing the others.
; A salt paste could be applied to
; For thin, porous wood, the disk can
be dried in alcohol.
; The porous wood can be treated
with a chemical that restricts swelling. This chemical is known as PEG
(polyethylene glycol 300 to 1000), but it
is expensive. ;
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