Another possible burnout problem is the lack of ventilation during burnout.
If there is not sufficient ventilation during the burnout process then
there will still be foreign matter in the cavities of the molds and will
most certainly cause porosity. Oxygen combines with the carbon residue and
is driven off as a gas.
Improper spruing can also cause porosity. If the sprues are too small in
diameter they will freeze before the items being cast have cooled. The
cooling pieces of jewelry need to have a source of molten metal to draw
from as they cool and "shrink". If there is no molten metal to draw from
then the metal is forced to shrink anyway sometimes causing a spongy
porosity or depressions.
Flask and metal temperatures have alot lot to do with porosity. The trick
to any casting situation is to cast using the lowest possible temperatures
for the flask and the molten metal that is possible to assure a complete
cast. There are no rules, only documented experimentation.