WarmGlass.com

I would like to cast some glass using glass pieces I find at my local thrift shop. I'm not worried about compatibility (I won't be mixing it with any other glass) just wondering what annealing temperature to use.

There are ways to tell, but not practical for your situation. Assume the anneal range begins somewhere between 1000ºF and 900ºF, unless it is lead crystal, and then it could be lower. A slow ramp down should catch the possibilities.

For sheet glass, you can cantilever a strip 1" x 12"between 2 fire bricks and place another brick past the end to visually gauge when the glass moves. Heat the strip to a temperature not hot enough for it to move. Soak 30 minutes, then bump the temp up 5ºF and hold 30 minutes. Note the temperature at which the glass first moves. Anneal soak around 100ºF beneath that point. For blowers, you can pull a cane about 1/4" in diameter and do the same test. I did these tests in the mid to late 80's and found that float moved at 1100ºF and Bullseye moved at 1020ºF. The cool thing about this test is even if your thermocouple /pyrometer are out of calibration, the test will still work, relative to your reading.