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Rough Science 3 New Zealand: Mike Bullivant's Diary: The Big Smelt

Updated Tuesday, 27th February 2007

Can Mike Bullivant provide the team with a working furnace to help them smelt their gold?

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Mike Bullivant mans the fire Copyrighted  image Icon Copyright: Production team

Day 32 - Smelting Gold I’m feeling a lot more mobile than I did at the end of the last challenge. Even with rest, it’s going to take 5 or 6 weeks for my ribs to mend, and I plan to take things easy in this final programme.

 

My final challenge is to purify all the gold that we’ve amassed - ‘amassed’ seems a rather grand word for the 6g that we’ve managed to extract over the last 5 weeks. Kate tells me that I’m in charge of building a furnace to smelt the gold. Once we’ve done that, Jonathan, Ellen and Kathy have the job of designing and making something from the pure gold that we manage to recover.

We all set about designing a furnace that we hope will give us the 1200 or so degees Celsius that are needed to melt the gold. In the kit for this challenge, we’ve been given a pile of firebricks, which will help enormously.

We decide that we need to insulate the firebricks with clay mixed with charcoal, so that as much of the heat that’s generated by the burning coals will stay inside the furnace and not be lost to the outside world.

There are enough firebricks to build a double-walled furnace, but we won’t be able to build the outer wall until the inner wall of firebrick (lined with clay/charcoal) has dried out.

We set about constructing the inner furnace, with Kathy and Ellen doing a great job of churning out loads of clay/charcoal ‘pizzas’.

It really is a team effort, with Mikey L making a wonderful pair of leather and plywood bellows, and Jonathan and Mike working on a way of ‘measuring’ the temperature inside the oven.

By the end of the day, we have a lined, inner furnace, which we charge with hot coals and leave gently simmering overnight to dry out.

This is a process that usually takes two to three weeks to do properly, so we’re all feeling a little apprehensive as we leave the sawmill at the end of Day 1 with our little furnace glowing away in the rear-view mirror.

What will greet us tomorrow morning? Perhaps drying the clay/charcoal liner so quickly will prove to be too much, and all we’ll succeed in doing is cracking the clay, which won’t help at all.

There are already strong doubts as to whether the furnace can generate the temperatures we need, even if all goes well tonight. We shall just have to wait and see.

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.

By two 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. H

e’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.

Come 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.

Day 34 - Smelting
We’re bursting to know whether we’ve actually got some purified gold from the smelting process. However, we have to be patient, as no one is allowed to reach into the cooled furnace to retrieve the crucible until both camera crews are ready to capture the moment on film.

I’m given the job of fishing the crucible out, and I can see as soon as it hits daylight that a green, glassy layer has formed in the crucible. That’s a good sign. I just hope that embedded in that glass is a nugget of pure gold.

Sure enough, when the crucible is cracked open, out fall three pellets of a beautiful, yellow metal, much more yellow than the gold we started with, which had a reddish tinge (copper impurity?). Our furnace has done its job.

Over to Jonathan, Ellen and Kathy now. Mikey L and I spend the rest of this final day of filming kicking our heels. Our job’s over. That’s it for Rough Science III I think. Within a day or two we’ll be back in the UK, and all this will be just a memory...

 

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