The Shop > Metal Stuff
Foundry Furnace for the Tiny Shop
vtsteam:
And then taped it back together.
vtsteam:
These are the ingredients for the lining renewal: some water, sand, and plaster of Paris, plus the Sonotube and shell.
I taped a bit of PVC pipe in the Tuyere opening to serve as a plug. I stuffed both ends of the pipe with paper towels to prevent the liquid lining pouring out before it set.
vtsteam:
You can see the Tuyere plug here, set at an angle to give a tangential flow for the blower (charcoal) or burner (propane).
I taped around the outside of the plug also, to prevent leaks.
vtsteam:
The shell is 12" in diameter, and with an 8" form, I should end up with 2" of lining.
Plaster of Paris is quite a good insulator. It's used as sprue and riser liners in big time iron casting for this reason. My guess is that the insulative quality is partly a result it being quite porous when cured. To be honest I was surprised to first learn that plaster was used in iron work at all. Bronze statuary, yes. Anyway reading that was one of the reasons I thought it might work out long term for this non-iron melter.
This originally experimental furnace really did live up to the hope -- and if I hadn't let it sit wet over this last winter, the first lining would still be giving good use. I've easily melted brass in it, as well as the lower temp aluminum and zinc castings for my "new" lathe.
Anyway, here's the first pour of plaster and sand mix. This forms the bottom, and I waited for it to set up before putting the Sonotube form in.
vtsteam:
I mixed plaster of Paris by filling half a gallon bucket with cold (not warm) water. Then I poured plaster in to the center slowly until it took no more and formed a lasting "island" in the middle of the water, without collapsing. Like in the photo below. That's the normal mix rate.
Then I mix by hand. Literally. You put your hand in and stir and squeeze out all the lumps. Trust me, a spoon or any other utensil will not work as well, and you will get lumps.
Then sand is added. I just added hand fulls to roughly what felt right. I'd guess about the same amount as I'd added plaster. Again, the hand stirring and lump dispersal method is best.
Then just pour.
Plaster of Paris will take anywhere from 3 to 20 minutes to begin setting up, depending on temperature, quantity mixed, and what it's mixed with. Sand, temperature, and volume of mix all accelerate it. So cool temps, mixing no more than 2 quarts (liters) of water, and stirring and pouring quickly all help.
I've had it go off in summer in 3 minutes, which isn't even enough time to clean out your container. It cures by exothermic reaction -- in other words it generates heat in curing -- a bigger volume generates more heat more quickly.
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