The combination of your mold and metal pouring temperature of the
metal is way too high.
I once read the shape of the top of the sprue button can be used to
indicate if the combination was too hot or too cold. A convex shape
of the sprue button indicates the combination of metal and mold
temperature was too hot. The top surface of the sprue button should
be flat if the temperature combination is correct. If the sprue
button top is concave the temperature combination could be too cold.
Air bubbles that are inside the castings also indicated the
temperature combination is way too hot.
The surface roughness could be caused by not burning our long enough
at the top temperature. Carbon left in the mold due to incomplete
burnout will cause roughness on the surface of the casting and
possible holed in the casting.
I usually have a 16 or more 2.5 inch flasks in my burnout when I
cast. I stack short flasks two high. I pack as many flasks in the
oven as I can. I am not sure if it helps in burnout but I put the
taller flasks in the center of the oven and the shorter flasks around
the edge. I feel this produces a more uniform temperature in the
I take the temperature up to 1320 degrees in 300 degrees per hour
steps and remain there for 5 to 8 hours. That amount of burnout time
works for me. When I cast very large flasks the temperature increase
per hour is slower due to the mass of investment in the oven.
I drop the temperature to 860 degrees for silver and let the flasks
cool for at least 1 hour before I pour the metal. This allows the
flask to stabilize at 860 degrees.
I melt in an electro melt furnace to a temperature of 1860. I
usually melt enough metal for several pours at a time and allow 5
minutes between pours to allow the temperature of the top edge of the
crucible to reach the correct temperature.
Without a lot of experience it is impossible to know, when torch
melting, the exact temperature of the molten metal.
My recommendation is to remain at the burn out temperature longer
and reduce the combination of metal and mold temperature when