Surprised to see the result from only 1650 watts. If you could get this thing on a 20 amp circuit and add a little more heat I'd be really curious to see where that leads.
One safety note. Many, if not most cartable refractories are electrically conductive at temperature. With the element connected to the casting via nichrome you have created a potential path for electricity. I don't know what refractory you used, and the risk is probably low, but it is there none the less.