There are several problems with electric pot furnaces
If there are any acid residues in the material you are melting (silver crystal, gold powder, etc.), the fumes can attack the elements. Also, the fumes from melting other things, such as ore concentrates, can do the same.
I am assuming the furnaces are lined with soft insulating firebrick, which is readily attacked by molten slag and/or metal. A small slag spill can eat a hole all the way through the brick. Also, if molten slag or metal gets on the elements, it can damage them. Most slag spills occur when the material foams over. This is usually due to using borax or other fluxes that contains waters of hydration. Standard grocery store borax has a formula of Na2B4O7.10H2O, which is 47% water. The water converts to steam, which expands and foams over. Borax is part of most all flux mixtures. Especially in an electric furnace, I would always use anhydrous (meaning no water) borax - Na2B4O7. The other flux ingredients, such as soda ash, should also be anhydrous
When things like the conversion of silver chloride with soda ash or the oxidation of base metals with niter are performed, the crucible can be severely attacked and can develop holes in it and leak slag and/or metal into the furnace. Also, normal melting will attack the crucible and, after a number of melts, it can get too thin and sprout holes.
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I agree with etack and would convert one of these furnaces to gas. I prefer natural gas over propane. Instead of glass wool, though, I would cover the insulating brick on the bottom and sides with about a 1 or 2 inch layer of refractory cement. If the inside is hexagonal or octagonal, I would pour the bottom and then use a plywood form to pour the sides in the same shape. You could use a round form but it would take more refractory, which isn't cheap.You would need a hole for the gas inlet and most gas furnaces have a slanted hole at the bottom to drain out molten slag spills. I would also cut down the soft brick about an inch at the top and, after doing the sides, fill it level with the shell with refractory. You might be able to use the original lid for awhile if you cut about a 3 inch in the center to act as an exhaust. Were it me, though, I would probably cast a new lid out of refractory, inside a metal ring with several pieces of rebar support welded to it. All in all, you could end up with a nice gas furnace.
That is what I would do if I had that furnace. As is, the electric furnace might be somewhat serviceable if you (1) are selective about what you melt, (2) are selective about what fluxes you use, (3) never put any wet material in the crucible (4) don't fill the crucible too full, and (5) don't try to get too much usage from a crucible. Also, if you get it too hot, it can splatter material out of the crucible. Just remember that one good flux spill could wipe out the furnace. I have seen many operating gas furnaces in my life and every one had some slag built up in the bottom of it. Also, even the bottom of a good refractory lid will tend to waste away.
