Product of lead and tin alloying
I think Ivan Neretin has given the right answer. 

Any alloy between Sn and Pb melts at relatively low temperatures. You stated that the compound you fabricated did not even melt at 2300 &deg;F. I have been looking at the stable lead oxides and tin oxides. The only compound of this set that has a melting temperature above 2300 &deg;F is tin dioxide at 2970 &deg;F. 

Please note that I cannot be certain of this, as I only checked a small set of compounds I found to be the best candidates. It could be the case that you have created something more complex. (beautifully phrased by DrMoishe Pippik as a 'toxic miasma' :-) )