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Hello Richard, Lee covered the health and mixture of the investment.
I can think of some other possible problems. Your burnout schedule
looks fine as long as you are holding the 200, 500, and 700 (F) where
damage can be done if hurried through. With too high of a climb rate,
steam can build up enough pressure inside the flask to produce
similar symtoms. Was the flask bench hardened for an hour before the
burnout? 1350 should be the top of the burnout. Investment can break
down with temperatures higher than that causing the same symtoms.
When is the last time you calibrated your kiln? The needle (or LED)
can read "1350" but can actually be much hotter. Another possibility
is air pockets in the wax near the surface that explode and fill with
investment during vacuuming. This occurs most often on one of a kind
waxes that have been carved, added to and recarved. It's always the
details isn't it ?!
John, J.A.Henkel Co., Inc., Moldmaking Casting Finishing,
Producing Solutions For Jewelry Artists