The Big Smelt - Jonathan Hare's diary

Production team Day 32 - Smelting Gold
Feel really rotten today, have a cold and a very sore throat and feel horrible. Beechams and throat sweets keep me going.

Steve E (the Producer) and Martin and Sophie (the Directors) have told us that this last programme is really up to us what happens. We need to do something with the gold we have produced so what should we do?

We get together and discuss a little what we could do. We decide to try and make a few things with the gold, but in the meantime we all agree to help out making a furnace to smelt the gold into an ingot. We start on what will be one of the best Rough Science team efforts.

Day 33 - Smelting Gold
Still 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 meter 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 intact. Then I wire this to the meter 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 meter pointer certainly did move around as the bellows were pumped showing the temperature changing.

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°C. 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 break 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.