Dear Ruth and other hard-boilers,
don't put fat back into bones! Fat in bones is a major problem in the long
run for comparative bone collections. Your problem is that you have
been "simmering" the bones. Avoid boiling. When you are heating tissue
that contains collagene to a temperature over around 75 degrees celsius
the collagene molecules shrink. This is a well known phenomenon that is
especially important during treatments of skin and leather. Bones don't
shrink like leather but you create tensions in the structure of the bone.
In fish bones and in the surfaces of many bird and mammal bones the tissue
is layed down in layers (appositional growth) and boiled bones tend to
erode on the surface in a flaky manner. Thin, flat fish bones twist and
get crumbled. Boiling bones in a solution the is weakly basic or acidic
will result in collagene being dissolved (you are cooking bone glue). If
you use heat to take off soft tissue you must only heat in order to
denaturate the flesh. Then use enzymes or bacteria.
Fat in bones decomposes into fatty acids which eventually make the bones
bittle. Fat migrates in the dry bone and dirt sticks to the surface. Fat
can migrate in the tissues of a frozen animal. In birds, that causes fat
to move from fat deposits outside bones to the inside. In mammals fat from
medullar cavities, brain and nerves goes into the bone walls. I guess you
are familiar with greasy ulnae and metacarpals in bird skeletons. Animals
should not be kept for long in the freezer. Deflesh and remove as much fat
as possible before storage in the freezer. Ideally, prepare skeletons from
fresh, unfrozen animals. Try enzymes that break fat, like "Lipolase" or
similar, or use some washing soap to emulgate the fat. As a last way out
use solvents like acetone or bensine (I dont recommend it). Try also the
compost method of Simon Davies.
To sum up: keep bones as intact as possible. Remove fat before processing
and don't help fat to migrate during stay in the freezer. Don't introduce
any foreign substance unless you know the effects. No glue boiling.
Leif Jonsson
Laboratory of Archaeological Science
Göteborg University
Sweden
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