Bone is a beautiful and useful material, but if you pick up any random bone from the yard, or one that has been buried, it may very well be cracked. That is because bones contain quite a bit of water and, like wood, when drying bone is subjected to stresses caused by shrinkage. Something has to give if the stress is high, and the bone will start to come apart along the grain forming “checks”. Rules similar to those for drying wood without checking can be applied to bone.
Size matters: Like a large piece of wood, a large bone is more liable to crack than a small one. Small bones will often dry without cracking regardless of how they are dried. If I bury a leg and dig it up a year later, none of the small toe bones will be cracked, but most of the larger leg bones will have checks in them. Speed matters: Drying things fast causes more stress than drying things slow. That is because when things dry they shrink. As the outside, which is drying faster, shrinks, it has to shrink around the plumper, slower drying interior and cracks are liable to form in the outside. It helps quite a bit that bones are hollow. One way to decrease checking in wood is to bore a hole through the center. But, see next… Bone is very dense: Dense materials tend to check more easily than less dense materials. Very heavy dense woods are more liable to cracking in general than light porous woods for instance. So, even though bones generally have the advantage of being hollow, they still have a strong tendency to check if not dried in a controlled way. If a bone was at thick as a tree or split piece of wood, I doubt there would be much that you could do to prevent checking, or at least it would take extreme measures. Control drying: The best way to avoid checking is to control the speed of drying, and there are several ways to do this. *Humid environment: Drying in a humid environment slows moisture loss, and that’s what it’s all about. If the moisture loss is gradual, moisture from the interior of the bone has time to redistribute throughout the bone, resulting in more even moisture loss, which translates to less stress on the bone’s structure. *Slowing drying of the exterior with a coating: Coating the bone with something to slow the drying of the exterior will also allow the whole bone to dry at a more even rate, greatly reducing the likelihood of checking. Using animal fat is easy and effective. Fat can also seep into the bone replacing some of the water. (more…)