Day 33: Smelting
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.
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.
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.