Day 34: Smelting Gold

Last night I was worried about the crucible that we put the gold into. I was told that it was a graphite crucible and I was worried that it might burn along with the coals!! This morning we found out that I needn't have worried. It was all still there.

Mike brought it out and I had the honour of turning the crucible upside down and tapping out the gold! What we had was a large lump of what looked like glass with some gold specks hidden within. I took a hammer and broke the glass to reveal the gold - very exciting. The gold looked great but unfortunately it was not in one piece to work with. We really wanted an ingot to be able to make something with but what we had was about 4 gold lumps. So we agreed that we could fire up the furnace again to try and smelt the blobs into one piece but that I would also think about another way of tackling the problem now that we had got the pure gold from the smelting.

One idea that had been buzzing around in my mind was to try and make a carbon arc to melt the gold. This consists of connecting up a car battery to two carbon rods. When the rods touch a very high temp spark is generated between them up to 3000^oC. The carbon does not melt or react with the gold and so it might be possible to heat up the gold this way and make a single blob.

I took a lantern battery apart to get a source of the carbon rods. These were cleaned up and then fixed onto copper tubes and then onto the car battery terminals. Striking the rods together made a wonderful bright white light arc. It also burnt away any of the impurities that were left in the rods. Then I put the gold pieces into a crucible and started to arc to melt them together. As the temp got higher the gold melted and through my welder's mask I could see the gold flowing together it was all very exciting!

When it had cooled and I had taken the mask off I was horrified - our wonderful gold that we had worked so hard to make had turned black!! But I was soon relieved as the black tarnish was only skin deep and it came off easily with a little filing/polishing.

The next step was to try and use the little time remaining to make something. I had wanted to make a Maori design and although jade is the usual material of their choice thought that something along these lines might be appropriate. I chose to make a sort of Maori fish hook as this was not only a lucky symbol but seemed to be appropriate to me as it represents my Rough Science efforts at making tools and machines. So I was filmed sketching out some designs and then the rather anxious moment to start cutting, filing and polishing the gold!

The basic gold shape was easy to cut out as the gold is soft (it's almost 24 carat) but I had a great deal of trouble drilling it. It seems that there must have been some tiny crystals of much harder material trapped within the gold because every so often I would drill a hole and the drill bit would brake off in the gold. This happened three times and in the end all I wanted to do was to make a hole for a thread so I gave up and used another technique. In the meantime I had wasted a lot of time and caused a large fracture in the body of the design. It wasn't going very well.

However I persevered and ended up with something that looked quite nice although it looked like a little shrimp more than a Maori fishing hook. I also used one of the garnets I had found as an eye for the design that looked very nice. The garnets added a sort of Celtic feel to the Maori design which was appropriate, I think. Mike L thought the final thing looked like whitebait, which is a speciality in this part of New Zealand, so maybe it was OK after all.

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