It might be the burnout temp but I don't think so. We're ramping to 1350 degrees F (732 C) and holding for 2hrs. Approaching 1500 degrees F (815 C), the investment will start to break down. The material turns black and starts to burn at 800 degrees (see picture above). In previous tests, extending the time at max temp didn't really improve the casting. Look at the letters in the picture; you can see the rough edges where the letters broke off on the inside of the mold as the material was expanding. The porous outside surfaces are most likely caused by residual ash residue in the mold.

I have to get this job done and out the door pronto so I'm moving to phosphate-bonded investment. I'll experiment with gypsum investments after I finish this one.