Low melting-point alloys

Alloys of Indium* and Gallium** can be made that melt as low as room temperature. One such alloy is used when grinding glass spectacle lenses to shape. The round "blank" of the chosen dioptric is held on a kind of copying lathe by a cylindrical plug of the alloy which is cast onto the surface of the lens. Wetting the glass, like a solder, this sticks tightly. A shaped plastic pattern, easily adapted with hand-tools to the size and shape of the frame, is mounted on the same shaft, and as pattern and blank turn slowly together a wet diamond grinding wheel shapes the glass. This is fascinating to watch. When the lens is the correct size and shape the pattern prevents it from further touching the grinder. It is then placed in boiling water ; the alloy melts off and is recuperated.

The Optometrist taught me a good trick, which was most useful for replacing odd-shaped watch "glasses". When they had to make lenses for metal frames, they would heat the empty frames and press them onto a piece of Perspex™. This left a mark, which could then be sawn around with a piercing-saw to make the pattern, and kept for reference - they had hundreds...

Many ladies "cocktail" watches had peculiarly-shaped glasses - I've even seen a shamrock / lucky clover. When the glass was lost the watch became useless. Adapting the Optometrist's trick, I found that by pressing the empty watch case onto a thin (1.2mm), warmed sheet of plastic on a cushioned pad, not only would it make the mark but also push the sheet up into a nice curve. Flat glasses look naff, out of place on 50's watches, and often do not leave clearance for the passage of the hands.

* MP 156ºC
**MP 30ºC - melts in your hand, not in your mouth...