I've built myself a Gingery-like charcoal furnace. Here I present my infinitely adjustable spacers used to center my internal form. It's just 2 sticks of scrap screwed together with drywall screws and then separated with a chisel. Advance or retreat the screw to increase or decrease the space between the sticks. The short sticks attached to the sides of the spacers maintain the spacers' vertical position.
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For my refractory I mixed 2:2:1.5:1.5 portland:finely sieved sand:clayerlite (all by weight). I mixed it tight, barely moist, and rammed it into place after which I set a 60 watt incadescent bulb for a long weekend while we went away.
I've also produced an infinitely adjustable airflow using a thrift-store hairdryer, a 1 1/2" steel pipe, and a piece of extruded Al tubing into which I made a cut which accepts the plastic support from a foam brush I used to shellac my flask. I cut a port to allow the blocked airflow an escape. Naturally, duct tape hold it all together.
For green sand I fine sieved some play sand achieving about a 50% yield. I mixed the sand with pounded and blendered clay at 11% clay by weight.
I can get the borax red, but I don't have a pyrometer. I'm melting cast Al, no cans or extruded. I'm guessing I get about 1300 *F no problem. So far no apparent damage to the furnace or major crack in the refractory.
Without question the art is in the pattern making. I'm brand new to this, now having 4 melts under my belt. My plan is to get to bronze eventually. That would mean a new furnace!