Day 33: Smelting
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 metre 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 in tact.
Then I wire this to the metre 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 metre pointer certainly did move around as
the bellows were pimped showing the temperature changing.
we want to know what temp it is as near as possible to the crucible. At
least, we want to know if we are hot enough in principle to melt gold.
So what we did was to put a copper ring (securely tied to an iron wire)
onto the end of a piece of iron rod and push it into the hot coals near
to the crucible. We removed the iron rod but had the wire to pull out
the ring later on. Now copper melts about 20 degrees higher than gold.
When we removed the iron wire a little later we found that the copper
had melted it. This told us that we had indeed reached the melting point
of gold in our furnace!!
Knowing that the furnace
was hot enough we felt very pleased about our efforts. We kept on pumping
the bellows till the end of the day and then left the furnace with our
precious cargo of gold to cool overnight.