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 an 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.
*Boiling: I actually don’t know if this works, but boiling wood can reduce checking. I think it works by breaking down the cell structure of the plant allowing water to move from the inside of the wood to the outside. It seems to me that boiled bones have less tendency to check when drying, but that is a very causal observation and one I’m not willing to stand behind! Boiling a bone definitely removes some of the protein material that cements it together, so I don’t recommend long boiling for the most part, since it may weaken the structure, though I suppose it depends on what you are planning to use it for. Further experimentation is definitely needed. I would say that if other methods are used carefully boiling is unnecessary, but could be an alternative and is interesting regardless and who knows how knowledge might be handy.
If you only want a small piece of bone, just go out and find one and break it or cut it up. Examine it VERY CLOSELY for any checks if you are about to invest any significant amount of effort into making something nice. I speak from experience :-/ Bones, like wood, have a grain that runs longwise. If the bone is checked and it is cut across the grain, whatever you make might very well fall apart.
If I want a bone completely unchecked for making tools or jewelry or something like that, here is what I usually do and it seems to work very well.
Like wood, fresh bone can form checks very quickly in hot dry weather, and I mean within minutes, not hours, so don’t leave them in the sun, and don’t procrastinate too much. Again, this depends on the bone’s character and size, and the weather etc… but just be warned that it can happen very fast.
Saw off the ends if you don’t need them. Clean out the marrow with a stick. This allows the interior of the bone to dry along with the outside, which means more even moisture loss and less checking. Plenty of marrow oil will usually stay inside the hollow portion of the bone.
Scrape the outside of the bone clean with a knife or stone flake.
Oil heavily, preferably with a heavy tallow type oil such as that from deer, goat, elk, moose, sheep, antelope or cattle. Put it on really thick.
If using a lighter oil, like lard, bear, raccoon etc…, you may want to put the bones in a plastic bag for extra insurance. Don’t seal the bag. Leave it very slightly open, or poke some holes in it to let moisture escape slowly. This may not be necessary, but is good insurance and easy enough.
Keep out of the sun or very hot areas, to assure slower drying.
That’s it! I’ve seasoned many bones successfully this way and actually can’t recall any failures. By contrast, bones left lying about will generally form checks unless they are very small or thin. I have a small collection of seasoned bone that I keep around. When I run across a really nice thick walled bone that I might want to use for something later, I’ll season it out as above and store it for eventual use.
It’s also good to know that fresh bone is much easier to work that seasoned bone! If you just keep a little oil on the bone and work in the shade, you can make your item out of fresh green bone and then oil it to season out when you’re done.