Sunday, 22 July 2012

Neolithic tin mirrors: Casting experiments

After smelting from its ore (cassiterite),  tin is a soft metal which melts at 232 degrees centigrade.

I tried several methods to make a casting for a tin mirror using only those materials known to exist in late neolithic Britain (just after the discovery of tin). Of the methods tested, the following process seems to produce the best results:

Do not try this at home without suitable safety equipment.

Stage 1:
Use a flat bottomed pottery dish placed level within hot embers. Carefully add filings of tin:

The tin will gradually melt into the bowl to form the initial shape. Because of surface tension, the shape won't form unless you have about 3-4mm depth of tin. After the tin forms a flat surface, pick out any surface impurities (using a sharp piece of stone) and then leave to cool.

Stage 2:
Turn the cooled tin plate over onto a polished flat surface, preferably something which will cool slowly (such as polished slate, pottery or obsidian). Re-heat the tin and then leave to cool again:

In the example above, we've turned the tin onto a baking tray and re-heated. As can be seen, if the tin is allowed to cool too quickly, deep ridges will form in the upper surface (this may not matter if you only need a one-sided mirror).

The casting is now complete. The next stage is polishing (see link here).

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