Ernie,
Fross here, I think I replied to your post on another forum. There is no reason you cant run 220v extension cord... Amperage remains the same regardless of the voltage and that is what will defeat an extension cord. If it was me running an extension cord to power a heat treat oven I would use a dedicated 10awg extension cord, 50 feet of 12awg running close to 85% if u use a 15 to 17 amp element is pushing it, in my opinion. just an opinion though. I really admire your attitude, what ever the case dont let anything get you down, keep it up...
Also yah, the shipping on those firebrick are going to kill you, lol its like $10 per 3 bricks to ship. I got mine locally for $3 a brick.