Low temperature alloys

Mounting natural crystals to form an 'ohmic' (normal) connection while holding them securely to allow a cat's whisker to be positioned is a tricky problem. In the 1920s they were either held by a metal ring with screws (which can be problematic as it can break the fragile crystals) or using a conductive low temperature alloy.

The reason it needs to be low temperature is because heating natural minerals too high - e.g. soldering them normally, can cause changes to the crystals. I tried dipping pyrite in molten solder to mount it in the past to see what happens (in a well ventilated place!), and seemed to work at first, but slowly crumbled away over time.

The original method was to use Wood's metal which was patented in 1860 and contains bismuth, lead, tin, and cadmium and melts at 70 degrees. I think this is the alloy used on a lot of the vintage crystal detectors still obtainable, the problem is that the cadmium and lead make it highly toxic to handle.

A safer (although more expensive) alternative is Field's metal which melts at 62 degrees, does not contain any lead or cadmium, but instead is made from bismuth, indium and tin. The indium is more expensive than silver, and makes it expensive. The material in the photograph cost £60.

Pyrite mounted in a low temperature alloy, presumably Wood's metal.