Homemade Foundry - Construction
My furnace was based off plans bought here. I made my furnace slightly wider, to fit a larger ceramic crucible, and used commercially made refractory rather than the homemade one recommended in the plans.
Furnace Body: The shell of my furnace is made from two pieces of 7" ductwork put together for an outer diameter of 14". Other items required for the body were the legs, which were cut from 3/4" NPT galvanised steel pipe, and the lifting handles which are made for garage doors and came from Home Depot.
The Bricks: The bricks are 2½" thick firebricks, and were bought from a local masonry supplier for about $2.50 each. I cut the grooves in the bricks using a cheap wet-saw, intended for tile cutting, with the brick sitting on a piece of ½" plywood so that the fixed blade wouldn't cut all the way through. I would make a pass through each brick, then move the rip fence on the saw over a little bit, and repeat the process. Each groove took about 6-7 passes, so the process was very time consuming. The results, however, were quite satisfactory. I had trouble doing the brick with the angled grooves, so I ended up drawing the lines on using a crayon then cutting the grooves using an abrasive wheel in my angle grinder, which worked quite well.
Me cutting the grooves into the bricks. I put the saw on a stand right after this picture was taken, since cutting the bricks took me a couple full evenings, and even I can't kneel that long!
Refractory: I tried making the homemade refractory that Dan raves about, starting with just the base of the furnace. I must have mixed it incorrectly as it was so crumbly when it dried that it had to be replaced. I used a large $20 bucket of fireplace cement in the process, plus a couple bags of perlite. For the amount of fireplace cement needed to do the whole furnace, it didn't cost much more to buy a commercial, castable refractory, and with that you know how it will turn out.
The refractory I used is Super Kastite 3000, which is rated for 3000°F. It has a "K Factor" (in BTU·in/sq.ft.·hr·°F) of 7 @ 1500°F. I'm not sure what that means really; it has to do with the insulating properties of the refractory. It weighs 143lbs per cubic foot, and I used three 50lb bags for my furnace. I purchased it from a local supplier.
The refractory mixes up like cement, so you want it wet enough to be useable but not so wet that it gets sloppy. I did the three sections of the furnace separately from each other, sitting on a plastic bag on a flat surface, and I mixed the refractory in a wheelbarrow. Wooden forms were cut on my dad's wood bandsaw and put around the pieces of the furnace shell, which kept them from bending out of shape while the refractory was being poured.
For the vent in the lid, I made the mistake of using a glass jar to form the hole. I couldn't pull it from the refractory later, so had to smash it out. I strung wire across the bottom of the base before pouring the refractory, rather than adding it afterwards as Dan suggests. I also welded washers to the ends of the leg bolts, and strung reinforcing wire between them to further strengthen the refractory.
The three parts of the furnace, before the refractory was poured. Note the wood forms.
The furnace right after I finsished pouring the refractory.
Refractory Curing: After the refractory had set, about a week after pouring, I carefully cured the refractory by heating the furnace slowly. This was easy in an electric furnace, as I hooked it up directly to 110VAC, which warmed the element slightly; just enough to make steam come out of the furnace for about 4 hours. When I couldn't see any more steam, I hooked it up to the full power, but put little pieces of firebrick between the lid and furnace body. This created a gap which kept the furnace from heating up too rapidly. A couple hours later, I put the lid directly on the furnace and let it go to full temperature. The refractory turned out very well, as no significant cracks have formed after several heats.
Curing the refractory. The gap under the lid keeps the furnace temperature from rising too quickly. The temperature probe is reading 245°C here.
Lifting Loops: I also bent a couple pieces of 1/8" x 1" flat bar into hoops and attached them so that they were sticking out from the lid. This allows the furnace lid to be removed by putting a steel rod through the hoops and lifting it off that way. I was concerned that the handles would get too hot, however I usually just use them wearing my gloves.
The lifting loops installed on the furnace lid.
Part 2: The Element and Power Controller
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Last updated 2-Feb-09
Copyright ©2009 Alexander Sutherland