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October 2008
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corvi [userpic]
metal and music

Metalwork is such a strange, fantastic thing. I'm used to metal being solid, the bones of cities, buildings with rebar skeletons and bridges strung with coiled steel, steam pipe and railway and ship hull. So there's some part of me that's always surprise at the way scale flakes pearly and grey, like eggshell, away from a blade on the anvil, or the way welding heat feathers along metal in sunset-colored fractal tributaries, the rainbow scars of tempering upon a new-hardened tool, at copper left in by vapor deposition in thin butterfly-scale lines in translucent new-formed marble. Bone, yes, but sometimes breath and heartbeat too.

Yesterday, I attended a casting party. I hadn't done any casting myself yet, but it was neat.

Here is a crucible full of molten metal, inside a furnace. Shiny!

Making a clay container that will hold molten metal is an art. You can't stick seams together. You can't fold the clay. You can't smoosh random patches of clay on to thicken thin areas. You can't twist or torque or cut the clay. Molten metal will find any cracks, seams, or areas where the grain is under torque, and probably leak all over everything. You have to start with a lump of clay and grow it into a crucible, like unfolding stone. The descriptions remind me of some of the martial arts I've done, direction and redirection, spirals and sweeps, no sharp corners.

I would really like to try making my own crucible someday, but I am not sure it is the sort of thing I would be good at. Luckily, they're not hard to buy, either.

Crucible full of molten bronze being lifted out of the furnace. The furnaces are the same general design as our forge - a propane tank augmented with an insulating refactory lining, and a burner. Though they open up in the middle. I successfully resisted the urge to make the furnaces sing muppet songs at passerby.

There are a lot of ways to get a shape to pour metal into. The one I was fooling around the most with is "petro-bond" sand - a very fine sand containing some mineral oil. It feels very odd to touch, soft and a bit sharp and a bit slick, like the way cat's fur drags over your skin. If you squeeze a bunch of it in your hands, it sticks to itself - you open your hand and there's this little stony shape with every wrinkle and scar recorded in it.

So the general idea is that you take the object you want to cast, or a model of an object you want to make, and put it at the bottom of a wooden frame, and pack the petrobond in around it, wham wham wham, and then flip it over, put another wooden frame on top, and back petrobond around the other side of the object, wham wham wham. Then take the frames apart, remove your object, and you should have two halves of a mold for it.

After that, you cut tunnels (called "gates") through the petrobond for the metal to flow from the top of the box all the way into your mold, like digging moats for a sand castle. The petrobond makes an odd sort of ringing note under a knife blade. When I was a kid, I used to love digging elaborate irrigation systems in beach sand, and it's much cooler to do it for molten metal - I think my favorite part may've been digging gates.

It's like pouring light, if light were thick and sweet as honey. There's music to it, you can hear it going down into the tunnels, a skeleton of light and flesh of stone.

Oops. Mmmm, fire.

This is a plaster mold, created by making a sculpture out of wax, casting plaster around the wax, and then heating the plaster until all the wax melts out. When the metal is cool, the plaster is smashed. This way seems to be very risky - a lot of the time, some wax remains behind, and boils when the metal hits it, leaving you with a sculpture of metallic foam, jagged and intricate as a snowflake. It looks neat, but is probably not what most people are aiming for.

Extra metal is made into ingots, which where, at this particular party, immediately re-melted and cast into other things.

Now I am left with the intense desire to think of something to cast, and also to hold another smelting party, though it is, perhaps, too rainy for that now.


ooo! pretty! got any pictures of the metal foam?

intellectually, i know that a crucible containing molten metal in a furnace will come reach the same temperature as the furnace, but it's still strange to see one glowing orange.

Alas, no, I didn't think to get a picture of the piece a guy was showing me.

making crucibles

Is it possible to start with a (necessarily big) block of clay and carve it into a crucible shape?

Re: making crucibles

I am not an expert, so what follows is merely my best guess, based on a never-tried-it vague understanding. :) I would say that as long as the final crucible was not scored at all, even scored-and-repaired, and had no interior corners or bubbles, you could do that. I think not-scoring is why you're supposed to use only your hands to do it.

I'm certainly not that good with clay-working tools, but someone (you?) might be.

Re: making crucibles

I tried to find instructions on making clay crucibles online -- I've found them before but can't now -- and I seem to remember it was mostly a slipcasting technique.

You might also be interested in this crazy guy using a microwave to melt his metal by taking advantage of silicon carbide's absorption of microwave radiation.

(no subject) - (Anonymous)

I seem to recall sunspiral does casting?

If not, come visit! I'm already scoping propane tanks to build a casting furnace from. :)

I built my crucible from a piece of steel pipe welded to some steel plate. It works well for aluminum. It would be very marginal for bronze.

I built the furnace from a 5 gallon paint bucket. It hasn't gotten hot enough, in six years of use, to burn the paper labels off the paint bucket.

Sorry that the pics are all Bryce: I had a crash and lost my digital pics and haven't gotten around to taking new ones.

Safer than lost wax and faster than full cope/drag mold casting, is Lost Foam Casting. It's also a great excuse to build that three-axis CNC styrofoam hotwire milling machine.

Holy crapola that looked cool -- You get to play with the neatest stuff!

Is this similar to the lost-wax casting method for jewelry? I always wondered what that looked like, and now have interesting and possibly associated visuals.

Yes, same idea, much smaller scale. Jeweler's wax is less flowy and flexible than sculptor's wax, so that you can get all your fiddly little details right, but otherwise it is much the same. The people I've know who did lost-wax casting to make little silver plaques heated up their crucible in a modified toaster oven.

The silver itself was melted with an oxy-acetylene torch, which didn't work particularly well. (Silver is a fairly tricky metal, liking to suck up spare oxygen into its impurities and get all weird and lumpy.) The toaster oven was used for burning out the wax and pre-heating the mold. Now that I have the forge, I'll probably use that for both purposes. Whenever I get back to the silver casting project, in my copious spare time. :P

Huh, I somehow thought you were using the low-melt stuff and an oven. Thanks for the correction. Yay copious spare time. :)

I've melted a fair bit of silver. Acetylene tends to be kind of sooty. Propane-oxygen is a much cleaner flame, and if slightly reducing, keeps down the oxidation loss. Propane-air seems to work just fine, though. It takes a little longer...

Yeah, it was a suboptimal choice, but I thought it was a good excuse to get an oxy-acetylene torch.


I found a lot useful info. Thanks.

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