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The small, 4" & 6', molds have not slumped I reset the temp. to 1275 degrees. Hold time is 10 minutes

Just how hot can the mold go before they crack? The glass is only a single layer of fusing glass.

Small molds take longer to slump than large ones.

In general, you should keep the slumping temperature below 1300F (some people never go above 1200, but for small items 1275 is fine). You want to vary the TIME you spend soaking at the temperature. It's not unusual to have to soak 1/2 hour or longer -- you'll need to keep an eye on the piece to know when it's slumped satisfactorily.