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Mike Bullivant's Diary

Day 33: Smelting Gold

We're all relieved to find that the lining of the furnace has survived virtually intact, apart from a large crack down one side. What's more, the clay/charcoal lining seems to have dried out quite thoroughly. This is just as well, as we have to go with what we've got. By this time tomorrow, we need to have in our hands a purified nugget of gold, so that Jonathan, Ellen and Kathy have time to fabricate it into jewellery or whatever.

We spend the morning building an outer wall round the furnace, using the remaining firebricks. I say ‘we', but everyone's so concerned about my condition that they don't let me do any of the heavy work that's involved.

Kate and bellowsBy 2 o'clock, we're ready to put our 6g of impure gold into a crucible, along with some broken bottle glass (a source of borax), and some fertilizer (an oxidizing agent that will remove most of the metal impurities in the gold). Once the charged crucible's in the furnace, we light the coals, and start bellowing away to force air into the furnace to get a higher temperature. We all take turns at the bellows, which seem to be working perfectly - until disaster strikes. What we hadn't realized was that, having forced air out of the bellows and into the furnace, we would be sucking very hot air into the bellows on the down stroke, despite Mikey L having cleverly built a simple, two-way valve into the bellows' design. The valve didn't work as efficiently as he'd hoped. The consequence is that the leather on one of the bellows catches fire. The only solution is to be more careful, and not bellow too forcefully. It seems to do the trick.

Mikey L has also come up with an ingenious way of estimating the temperature inside the furnace. He's constructed a device using a small firebrick. He's drilled small indentations in the brick into which he's placed some small pieces of metals of known melting temperature: lead (m.t. 328ºC), aluminium (m.t. 660ºC), brass (m.t. 1027ºC) and copper (m.t. 1083ºC). If we remove the brick from the oven from time to time and check which of the sample metals have melted, we'll at least be able to get an approximate idea of the temperature inside the furnace. For instance, if when we take the brick out we see that the aluminium is molten, we'll be able to say that the furnace temperature is at least 660ºC. We know that the m.t. of pure gold is 1 063ºC, but the impurities in our ‘gold' will lower that by a few degrees. We therefore need to create a temperature of at least 1100ºC for the smelting process to work properly.

filming kateCome late afternoon, after lots of burnt coal and more bellowing than we all care to remember, we find that the copper bead has melted. This means that, at worst, we must be very close to our target temperature. All we can do is to throw more coals on the fire and leave the furnace going overnight. Despite some initial reservations about whether our furnace would work, we all go home feeling very positive that we might just succeed.

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Photo: Mike Bullivant
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