Step-by-step, what it's like.I have a dolly for the furnace because it weighs about eighty pounds. I wheel the furnace out of the garage into a safe spot on dry ground, with nothing flammable nearby. I carry the lid out and put it nearby. I carry out the propane tank and burner, insert the burner into the furnace, and put a shim under the burner to support it. I put on my safety glasses and gloves, grab a handful of pine needles, drop them into the furnace. I open the valve on the top of the propane tank, and make sure the regulator reads. I light the needles on fire, and open the fine valve on the burner, and make sure I've got a nice hot flame. I put a handful of small (marble-sized) chunks of aluminum in the crucible and lower it in with the Big Iron Hook. The furnace is now ready to growl on its own for a while.
I put the wooden image of what I want to cast on a piece of plywood. Dust it with parting dust (like chalk or graphite powder.) The upper form is placed around the piece. I put a nice big ol' sprue riser beside it, that will be the pouring hole; this is a 2x2" that has a slight taper cut on it to make it easy to pull out of the form. (It has the opposite taper of the form because I pull it out the top and the form out the bottom but that's personal preference.) I use a fine riffle and brush the sand onto the top of the forms with a sort of butter-spreading movement with a trowel, pack it down and around the forms with my hands, add more, then lightly tamp it down with a hammer handle or such, fill, hammer, etc, and end up with a flat top (drag wood across it to flatten.) At several points during this, I go and check the furnace, rotate the crucible half a turn, and when there is a nice heel of molten metal in the crucible, I start adding in bigger chunks.
Back to the form: I put a piece of plywood on the top of the mold, so now I have a sandwich of upper plywood, frame full of sand, and lower plywood. I flip this whole thing and take off the top piece -- what WAS the lower board -- so that I can see the back of the wooden image. Put the lower form on, and dust the whole surface with some parting dust, then add sand using the fine riffle, pack more gently, add, pack, and so forth. When this is scraped off, the exciting part comes.
Oh, yeah, go check the furnace, add more metal.
Now, I lift the upper form off the lower, remove the wooden image (this is called rapping, because you smack it slightly to either side a little to set the sand around it, and maybe dribble some water at the edges of the form to help the sand maintain its shape, then smoothly draw it out of the sand, which I accomplish by having drilled a couple holes in the form and covered them with scotch tape, and I now thread wood screws into these holes and use them as handles) then remove the sprue riser, cut a sprue from the sprue riser to the form (make it thick -- as thick as the section of the form it contacts) then I use a long wire and push it through the mold from the inside to the outside in several places to let air vent out. I use a hacksaw blade for cutting sprues from the pouring hole to the form. It's also common to use things like steel conduit pipe, cut longwise, to form something like a very long, narrow trowel. After cutting the sprues, you might lightly tamp the sand back into place with your fingertips.
Then, the moment of truth, I lift the one form and put it back on the other, aligning the alignment pins and gently, gently letting them go together. Then, I INSERT THE FLASK PINS to hold the two together.
Now I've got the void space inside the form, and a hole leading into the void space.
I check the metal. Usually by this time it's hot to trot. You don't want to pour too hot, nor too cold. Cold leads to skulls and clogs in the mold because the metal doesn't flow through the whole mold space. Too hot leads to porosity. I usually pour when the metal is just barely glowing, in a dark space. Ideally it should be 1350 degrees F, I'm told. I certainly don't measure it. I probably should
This is where the really fun bit comes in. I put a muffin tin down beside the mold. I cut the flame to the burner off and pull the burner out of the furnace. In my left hand I have the Big Iron Hook. In my right hand I have the crucible tongs. I have my face shield on. I crouch beside the furnace (not kneel, no knees on the ground in case aluminum comes bubbling across the ground) and lift off the lid, setting it out of the way. I hook the crucible, lift it out, and lower it into the tongs, which are braced by the length of my arm and by my leg, both. When I know I've got a good firm hold on the crucible, I drop the iron hook and grab the tongs with my left hand as well, turn slightly, and pour into the flask. As soon as the metal has filled the flask sufficiently to have a nice big ingot sitting on top of the sand, I pour the excess good metal into the muffin tins, as much as I can get easily, and then I turn a little more and rap the crucible lightly on the ground to dislodge all the slag and dump it out on the ground.
I put the lid back on the furnace and put a rock in front of the burner orifice, and let the furnace cool down. It takes about six hours to cool. I turn the propane valve on the tank to off, vent the propane left in the line, and clean up the riffles and sand tools, put away anything that I've worked with, until the only things left outside are the furnace and the mold. The mold needs to sit for about half an hour before breaking it open. I punch it out over the main supply of casting sand and use pliers to pull the form, which is embedded in hard, dried sand, out of the sand box. Then I knock the dried sand off and take the casting to a hose and wash it off.
And that's all there is to it!
This page written on 12/15/00, last modified 23 May 2023.
Back to the casting index
Back to the main page index