Craig Lindsay, of Totara Park, asks :-

We have been told by scientists that glass takes one million years to break down. Then how come there are bone fossils which are organic/carbon and are up to four hundred million years old? Don't say they are fossilised by silica, because if so they would be long broken down.

Ian Freestone, a geochemist at University College London's Institute of Archaeology who specialises in archaeological materials, responded.

There are several aspects of this question, each of which requires an answer.

First, bone is not only carbon, but is mainly composed of an inorganic mineral, apatite, a form of calcium phosphate along with collagen, an organic material. While collagen breaks down relatively quickly like many other organic components of the human body, apatite is more stable than glass and can last millions of years in the right conditions.

However, many of the fossil bones found are not simply the apatite of the original bone. The pores (spaces) in the bone structure may be filled up by other minerals, deposited when the bone is buried, or be slowly replaced, molecule by molecule, so that the fossil bone is a completely different composition. Because the process takes place very slowly, the shape of the fossil bone can look exactly the same as the original, which allows us to identify the animal that it was part of. These fossils can last hundreds of millions of years.

Another issue to consider is the rate of decay of the glass or bone. This depends upon two main factors: first the environment in which the material was buried and second the chemical composition and structure of the material. For example, acidic soils are particularly corrosive to bone, and when people have been buried under such conditions, their skeletons may disappear in just a thousand years.

The preservation of glass is particularly dependant on its chemical composition. A natural glass, obsidian, which is formed by some volcanoes has a very stable chemical composition and may last for millions of years. However, some stained glass in the windows of European cathedrals has dissolved away in the rain after only a few hundred years - the people who made it needed it to melt at low temperatures and the recipe they used contained a lot of alkali, which makes glass soluble in water.

The organic material in bone decays relatively quickly, so that we usually cannot extract DNA from bone if it is more than a few thousand years old. However, if the bone (or the flesh) has been preserved in a special environment, such as ice in a glacier, then the organic material will not decay so quickly, and last longer.

There is therefore a complex mix of factors affecting the preservation of bone, glass or any other material, and it is difficult to compare one with the other without defining these more closely.

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