A number of years ago I talked with the then retired director of
research at Ransom and Randolph. His comments are relative to this
discussion:
First, he suggested putting a flask into a hot (1350 F) kiln for
rapid burnout. I replied that I had seen discussion indicating that
this would cause the investment to crack during burnout. His reply
was that only a partially dried flask would crack and that the flask
should be moist before putting it into a hot (1350 F) kiln. If a
flask has been setting for several hours or days, it should be
immersed in water to re-hydrate the investment before placing in the
burnout kiln.
It is well known that investment hardens by a chemical reaction with
water not by the process of drying. Like concrete it will harden
when totally immersed in water. A partially dried flask is in danger
of having the investment crack and should be re-hydrated before
burnout, especially if it is placed in an oven already at high
temperature for rapid burnout. Slowly ramping the flask to burnout
temperature would probably make re-hydration less critical.
Hope this helps,
Fred