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.