Yoghurt Pot

If you heat a yoghurt pot, it reverts to its previous shape - a small flat disc, and the design on the orginal pot is now anamorphic.

HOW TO DO THE EXPERIMENT

Take a clean, empty yoghurt pot, and put it into a pressure cooker with 2-3 cm of water. Boil for a few minutes, using the 7 kg pressure weight. This will give a temperature a bit over 100 degrees Centigrade. Cool the cooker, and take out the yoghurt pot which should now have turned into a flat disc. The writing and pictures on the original pot will now be distorted around the edge of the disc - an anamorphic image.

If you don't have a pressure cooker, you can boil the yoghurt pot vigorously in a covered pan of water. It will take longer if you do it this way, and may not change completely into a disc, since the material of the yoghurt pot seems to need a temperature of more than 100 degrees Centigrade in order to soften. However, you can add lots of salt to the water, which will raise the boiling point.

Many yoghurt pots are made of thermoplastic, which softens when reheated. (The other type of plastic is thermosetting plastic, which resists any further heat treatment.) The yoghurt pot was originally formed by being pressed out from a sheet of plastic, and still contains stresses in the plastic from that time. Heating the pot allows the plastic to return to its previous shape. Even a yoghurt pot has its memories!

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