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Jonahan Hare's Diary

Day 33: Smelting Gold

Photo: Mike L and Kate with bellowsStill felt bad today but not nearly as bad as yesterday. We start today with Mike B loading up the crucible with the gold powder and also some other bits and pieces to make sure it is purified and forms an ingot. Then the furnace is loaded with hot coals and charcoal and the bellows put in. We all take it in turns to man the bellows and spend the day trying to raise and maintain the temperature.

I think about how we could measure the temp, or at least show that the temp is rising. Try out a range of experiments to make a thermocouple, but although the ideas work the indication on our metre was very small and basically useless. I do, however, have a real Rough Science brainwave! I realise that I could use the tungsten filament from a standard house light-bulb as a resistance thermometer. So I carefully break a bulb and leave the filament in tact. Then I wire this to the metre set to read resistance. At room temp the resistance is low, about 20 ohms. But on heating with a cigarette lighter it goes way up to about 1000 ohms. So I wire the bulb up as well as I can and then put it into the furnace via the chimney. I wanted to see if we could measure the temp going up when we operated the bellows for a while. This we did and the metre pointer certainly did move around as the bellows were pimped showing the temperature changing.

Photo: Jonathan making a thermometerBut we want to know what temp it is as near as possible to the crucible. At least, we want to know if we are hot enough in principle to melt gold. So what we did was to put a copper ring (securely tied to an iron wire) onto the end of a piece of iron rod and push it into the hot coals near to the crucible. We removed the iron rod but had the wire to pull out the ring later on. Now copper melts about 20 degrees higher than gold. When we removed the iron wire a little later we found that the copper had melted it. This told us that we had indeed reached the melting point of gold in our furnace!!

Knowing that the furnace was hot enough we felt very pleased about our efforts. We kept on pumping the bellows till the end of the day and then left the furnace with our precious cargo of gold to cool overnight.

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Photo: Jonathan Hare
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