Old jewelry scrap being weighed in upon receipt.
The scrap in front of the crucible and furnace.
The scrap being loaded into the crucible that has been placed
into the furnace.
Once melted the scrap is stirred to ensure that is completely
melted and well mixed.
A pin sample that was just taken being removed from the
furnace. A pin sample is obtained by
inserting a thin evacuated glass tube into the molten metal. The hot metal melts the end off the tube and
the vacuum sucks the molten metal up into the tube. The temperature of the tube above the melt
being much less than the molten metal causes the metal sucked up to solidify
immediately. The pin is then withdrawn
which is what you see here.
Here are three pins, two that have already been
cleaned. The third is having the glass
tapped off. Later small pieces (about a
gram or so) will be cut from the pin and will be analyzed in an X-ray fluorescence
machine and/or sent for an even more accurate fire assay.
The gold is poured into a form. After pouring the crucible is examined
carefully (and scraped out if necessary) to make sure all the gold has been
removed.
The form is flipped over and the bar removed.
The bar is cooled in a bath of water and slag clinging to
the bar is chipped off. The slag comes
from borax which is added as a flux and other things such as glass and stones
that might have been in the jewelry.
The bar is further cleaned after removing from the water
bath.
It is cleaned against a wire brush to ensure all extraneous
material is removed.
Finally the bar and sample are weighed. Once an assay
(per cent of gold present) is obtained, it will simply be multiplied by the
weight of the bar and the amount of gold present is known. If the process
is done properly, the amount of gold so determined will be accurate to within
better than a 1/10 of 1 per cent. From here the bar will be
combined with many similar bars and when a sufficiently large amount of
material is obtained (usually well over 1000 oz t) it will be again melted and
poured into a water bath in a thin stream to obtain small broken up little
pieces called “shot”. This shot will then be put into a bath of aqua regia
acid, a comination of nitric and hydrochloric. Most metals are soluble in
this acid and dissolves into it. Another
chemical is added that causes the gold to precipitate out of the solution
forming a layer at the bottom the refining vessel. Basically the chemical that is added only
precipitates the gold leaving all the other metals in the solution. The solution is filtered leaving the gold in
the filter paper. The gold is then
washed to remove any residual acid and dried.
At this point it is a brownish/redish powder called sponge. It is then melted and turned into solid form
where its color becomes the familiar “golden” color of pure gold. You might want to view our “
Fine Gold Sponge
Melt” slideshow and our “Final Gold Refining Process - Accent Diamond Removal."