Silver Casting Problem

I have experienced problems with recent castings. The first is
apparently the result of breakdown of the investment surface
resulting in irregular flakey blotches. The second occurred on a
roughly spherical casting roughly 20 mm in diameter. The bottom half,
ie that at the opposite end from the sprue had a dark gray
encrustation that apparently dissolved in the pickle leaving a finely
textured casting surface.

The gray substance apparently is investment that somehow
deteriorated. It may be the condition known as porosity although the
texture is very fine. The condition did not occur above the equator,
but the flaking did.

The investment was burned out in a 3x3 flask at 300, 700 and 1250
degrees F. The 1250 period was 1.5 hrs. Was this period too long? Was
the temp too Low? Was the flask properly calcined? Some references
say do not exceed 1250 others say bump it to 1300.

I have a new digital temperature controller so the temps were
tightly controlled.

Orchid wisdom is requested.

Ben A Harris