Fields metal cassiterite experiment

An experiment using Field's metal with some samples of cassiterite from the Great Flat Lode exotic semiconductor field trip. This material is tricky to use in it's raw form as the cassiterite, which while being well crystallised - is quite evenly distributed through the matrix and forms small clusters that are electrically insulated from each other. While it's possible to connect cat's whiskers to the crystal surfaces, it's very hard to use the crocodile clips in my cat's whisker setups to connect them, using the usual method of aluminum foil.

Stage 1: Preparing the material

The first thing to do was to isolate some relatively pure specimens from the matrix. This took a bit of time breaking down the rock into small chips, and carefully collecting intact crystals that fell out of the matrix. I ended up with a small collection with differing quantities of matrix left on them. I selected fragments that were as electrically connected as possible to make the next stage easier.

Stage 2: Preparing the field's metal

I obtained some Field's metal, imported from Germany - 100g for 60 EUR, costly because of the indium content of the alloy. I was worried this wouldn't be enough for the high price, but in fact it's plenty to do many crystals. It melts at 62 degrees, and it took a bit of playing around to figure out how to use it. Firstly I needed to divide the largish 'ingots' into smaller fragments for the relatively small crystals I was dealing with.

The best way I found was to use boiling water and the corner of a plastic bag. If you leave it long enough the whole ingot melts and you can squidge it into smaller quantities which then solidify. You can break them easily before they are fully solidified.

Stage 3: Mounting the crystals

The next stage is to select a fragment of Field's metal for each crystal to mount, you put it in the corner of the plastic bag and bring it into contact with the hot water. When it has fully melted the crystal can be dipped in and carefully held while you pull the bag out and leave it to cool. The molten metal will not stick to the matrix, and it can be hard sometimes to get it work. The trick is to pull the crystal slightly out so the surface tension can be seen pulling on the crystal.

Once solidified and checked, I used a multimeter to check the resistance between the metal and the shiny parts of the cassiterite. This proves that there is a connection between them.