A Bone Is Two Materials at Once
You already know that bones are made of living cells and minerals, and that a broken bone can knit itself back together.
Read that again, because it is a strange pair of facts to hold at the same time. A rock is hard and it is not alive. A living thing is soft and it does not hold up a roof. Your leg bone is doing both jobs at once, and the first article named the two ingredients without saying why you need two.
You need two because neither one works on its own. And you can prove that at the kitchen table in about a week.
What you will need
- One clean bone from a cooked chicken — a leg bone is ideal
- A clean jar with a lid, big enough for the bone to lie flat
- Enough white vinegar to cover it
- A calendar, or somewhere to write dates
Before you start
Take one ingredient away
- Test it first. Hold the clean dry bone at both ends and try gently to bend it. It will not bend. Remember exactly how that felt, because that is the measurement you are taking.
- Put it in the jar and pour in enough vinegar to cover it completely. Screw the lid on.
- Look at it after an hour. There will be tiny bubbles clinging to the bone. That is the vinegar getting to work.
- Change the vinegar after three days for a fresh lot. The first batch has done as much as it can.
- After about a week, take the bone out, rinse it under the tap, and try the same gentle bend.
It bends. Depending on how thin the bone is, it may bend right round, and a thin one can be tied into a loose knot.
Nothing was added to that bone. Nobody softened it or heated it. One of its two ingredients was dissolved out and carried away, and what stayed behind is the other one.
What was taken, and what was left
The vinegar is a weak acid, and acids dissolve the mineral in bone — the hard, chalky, calcium part. Given a week it takes out most of it.
What survives is a mesh of long, bendy protein fibers, and it is those fibers you are holding when you tie the knot. They were always in there, running through the bone in every direction. You just could not tell, because the mineral packed in around them was stopping the whole thing from moving.
Now do the opposite in your head. Suppose you got rid of the fibers instead and kept only the mineral. There is a way to do that — a very hot fire burns the protein away — and it is not an experiment for a kitchen, so take this one as read. The result is a bone that keeps its exact shape, goes chalky white, and becomes so brittle that a light tap shatters it. It cannot bend at all. It just breaks.
So the two experiments fail in exactly opposite directions:
- Mineral, no fibers: stiff. Holds its shape. Snaps like a biscuit.
- Fibers, no mineral: tough. Will not snap however you fold it. Holds nothing up.
Neither one is the strong one
Here is where most people's instinct goes wrong.
The mineral is the hard part, so it feels obvious that more mineral means a stronger bone. It does not. Try it.
Slide from a bar made of pure protein all the way to a bar made of pure mineral, and load each one.
Both ends of that slider fail. That is the thing to hold on to, and it is not what anyone expects.
Strength here is not one ingredient turned up as far as it will go. There is no setting marked "strongest" at either end. The useful bar is the one in the middle, and it is better than either pure version at the job — stiff enough to carry the load because of the mineral, and tough enough not to crack because of the fibers running through it.
A material made of two things on purpose, where the mixture beats both ingredients, is called a composite. Your skeleton is one.
Roughly two thirds of the weight of a bone is mineral, and most of what is left is a protein called collagen — the fibers you tied a knot in. That is the recipe, and it has not needed changing in a very long time.
People kept reinventing it
Once you know to look for this trick, it turns up everywhere humans build things that must not break.
- Concrete with steel rods running through it. Concrete is the mineral half of the story: brilliant under a load, hopeless at bending, and full of cracks waiting to run. The steel rods are the fibers. Almost every bridge and tall building you have ever stood in is a bone, at a different scale.
- Fiberglass. Fine threads of glass, set in plastic. Glass on its own shatters and plastic on its own bends, so boats and helmets get made out of both.
Nobody copied the skeleton on purpose. They arrived at the same answer, because there is really only the one.
A bone soaked in vinegar for a week can be bent double, and one baked in a fire snaps at a tap. What does that pair of results show?
- Each ingredient on its own fails, in opposite ways, so the strength comes from having both
- Vinegar and fire both damage bone, so the experiment tells you nothing about what bone is made of
- The mineral is the strong part, and the fibers are only there to hold it together
- Bone is strong because it is alive, and both treatments killed it
Next time you knock your elbow on a doorframe, notice that it hurt and nothing broke. Two ingredients that would each have failed on their own, doing the job together.