I don't know a ton about this, but in art glass casting, the standard is
usually using plaster of Paris. I would worry about using sand casting
if you want a smooth surface. You have to be more careful about your
annealing times right after you cast as well. It can be tough enough to
cool at the proper rate without including the different coefficients of
thermal expansion between the obsidian and the aluminum. If you are
working with a smaller amount of Al (not enough to dissipate plenty of
heat), you may also want to take a look at what your casting temperature
will be so you don't get any Al surface melting. Good luck.
Cast obsidian directly into aluminum?
The effective melting temperature of the obsidian will likely be much
higher than that of the aluminum.
Maybe you can consider casting the aluminum over the obsidian?
Also remelting obsidian can be very difficult due to outgasing causing
foaming (if the obsidian was formed at above atmospheric pressure,
supersaturated trapped gas will cause bubbles to form when the obsidian
You would have to heat up the aluminum, pour the glass in and then get it
into an annealing oven/furnace to slowly cool.
Very likely melting the aluminum in the process.
The slag we get from our small iron cupola looks just like black obsidian
and we had a student anneal a blob.
If it cools quickly outside of the oven it shatters into small pieces.
Any reason why it must be cast? How about
hot pressing a powder around the obsidian?
Or building up the aluminum by thermal metal spray?
Or electrodeposition? Deposition from a gas?
Why aluminum? Other metals might be easier
to work with.
Cast aluminum because I have the equipment to cast aluminum, and a shed
full of aluminum alloy scrap. I guess that pewter or some lower
temperature alloy would be easy.
The deposition methods would require a lot of equipment that I don't
have. I'm thinking of objects 4 to 8 inches wide or long and less than
1 inch thick (20X20X2 centimeters or smaller) with a 1 to 2 inch window.
There'll be some thermal stress and thermal shock involved. These
stresses can be minimized by preheating the obsidian and mold as high
as is practical.
The melting temperature of pure aluminum is 1220F; the maximum
temperature that obsidian can withstand before foaming or warping will
depend on its composition and origin, but will be similar.
I recommend heating a piece of your particular obsidian until some
badness happens; your best chance of a successful casting is to slowly
preheat the mold and obsidian insert to almost this temperature before
Make sure the aluminum is low-alloy so it is soft as it cools; it'll
try to shrink more than the obsidian as it cools & it'll help if it can
easily deform to accomodate the shrinkage difference. Slowly cool the
casting (pile fiberglass, vermicullite or something like that on it if
it isn't in a furnace).
PS you might get by with a fairly low preheat; there's really no way to
know in advance without a lot of detailed info.
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