obsidian aluminum

I would like to cast a piece of obsidian in place in an aluminum ornament. I can do lost foam and sand casting. Does anyone on the
list see any problems with doing this?
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Dr. Dan wrote:

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.
Seth
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Dr. Dan wrote:

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 softens.)
Dave
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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.
ScottV
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Let me try this again. I want to pour molten aluminum around a piece of obsidian, and end up with a piece with an obsidian window in it. Dr. Dan wrote:

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"Dr. Dan" wrote:

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.
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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.
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Dr. Dan wrote:

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 pouring.
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).
Dave
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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