I’ve had excellent results casting pure (9999 fine) gold in the past. My understanding is that because it’s non-reactive, it requires much less clean-up after casting, which means that very fine surface details can be preserved.
For a current project, I’m prototyping small sculpture-sized pieces that have very fine details (think lattice-work, filigree, internal structures, etc.). I’m trying to explore the limits of the process, and will eventually cast in pure gold once I settle on a design through prototyping. But part of what I’m exploring with prototyping is investment removal, polishing, etc. of very intricate details.
I need to get my process ironed out. Are my polishing tools fine enough to get into the nooks and crannies to my satisfaction? Etc.
But I don’t want to be dealing with firescale and such that will “clog” the fine details more than gold will eventually.
I’ve thought about trying aluminum, but I don’t imagine that it would polish well.
Any leads on a non-reactive, high-polish casting metal for prototypes that isn’t expensive?
This is for a lost-wax vacuum casting setup with an electric melting furnace.
You might consider using fine silver (999). The downside is, you can’t get a good casting using ordinary investment methods because molten fine silver dissolves loads of atmospheric oxygen, which then bubbles out when cooling and setting. That usually destroys the surface finish.
I have cast a great deal of fine silver (maybe as much as 100kg) over a period of years when teaching ceramic shell ‘bomb’ casting, which I learned from David Reed. The fine silver granules are held in a totally enclosed ceramic ‘bomb’ + mould, together with charcoal which absorbs all the oxygen during heating. The ceramic investment is sufficiently porous to gases not to explode on heating, yet sufficiently fine grained (the interior detail surface is a graphite layer) not to allow leakage of any molten metal. Silver is sufficiently dense to give excellent castings.
P.S. I’ve been retired a few years, and have merely assumed that silver is still classed as ‘affordable’!
I have some experience with 999 fine silver, from back in the day. It came out of the investment white instead of silver… I always thought that was a silver oxide, but I just looked it up, and silver oxide is white. But it did polish up nicely, and it wasn’t a crusty firescale like what I saw on copper.
Here’s a pic from back in the day, silver in the middle, pure copper on the right (and of course, glorious 9999 gold on the left):
I’m wondering if that “white” is actually just a “matte” finish.
They all polished up nicely, as you can see here:
However, there is some pitting in the copper that isn’t present in the others. Deep firescale, I’m guessing.
Yes, fine silver always cast pure matte white; it’s so soft (compared to most metals) and the depth of ‘matte’ so fine that it could be burnished to a bright shine with a finger nail. Burnishing (rather than polishing) had the advantage of work hardening the surface to a degree.
I frequently had to do a rough-and-ready check on found samples of silver by heating it to red heat; if it cooled to pure matte white, it was fine silver, if to dark it was sterling or Britannia.
Dang. I have to be a Debbie Downer but…
If you want to cast your pieces in pure .999 gold, silver, or platinum you might want to reconsider your designs that will have “that have very fine details (think lattice-work, filigree, internal structures, etc.)”
Pure metals are so soft that any intricate lattice-work and filigree will be so weak that it will collapse with handling. Also finishing very soft pure metals is an absolute bitch. It clogs files and abrasives. It drags rather than polishes. It usually has to be burnished with highly polished agate or steel. It can be put into a tumbler with stainless steel shot, but any fine details and latticework will be badly mangled in the process. Pure metals need just as much cleaning up after casting as any other metal. Also fire scale does not clog fine details. It is merely a surface stain.
Also if you are planning on using aluminum for test, do not ever use the same crucible files or emery with any other metals. It will contaminate them.
Good luck with your project, and get this book by James Binnion. Jewelry Metals: A Guide to Working with Common Alloys. It’s available on Amazon. Every non ferrous metalsmith should have a copy. It’s worth it’s weight in gold.
Yeah, pure gold is very soft… I will definitely have to avoid any “thin ears” on the exterior of the piece that will get bent when handling.
However, here’s my thinking:
If I have a bunch of intricate internal structures, firescale or the need for lots of polishing to show the color of the metal, is going to be a no-go. I’m not going to be able to reach those internal structures entirely. Some of them will have to be left “matte” as they come out of the casting.
So imagine a thicker “skeleton” on the outside, and then you can look through the ribs of the skeleton and see lots of more delicate internal structure. The finer structure will be impossible to handle, because the thicker skeleton will protect it.
I can’t find any perfect examples of this, but here’s something in that vein… where the sturdier “skeleton” has been polished, but the inner structures have not been:
I imagine that if I were to cast something like that in pure gold, it would work out okay. I could use ultrasound and a water-pik to get the investment out, polish the thicker, reachable parts, and then just leave the “matte gold” visible on the inner reaches.
And “Pure Gold” is also necessary for my project, for philosophical reasons (it’s a treasure for a treasure hunt).
