Gray inventment

Jewelry Discussion
1

I remember MANY years ago there was a manufacturer who produced
investment with carbon in it. Worked pretty well to eliminate
oxides but produced castings with a slight grainy surface. Lee
Epperson

2 
    I remember MANY years ago there was a manufacturer who
produced investment with carbon in it.  Worked pretty well to
eliminate oxides but produced castings with a slight grainy
surface.

It is a bad idea to have any carbon in the investment. The presence
of carbon reduces the temperature that the investment breaks down
at. When the molten metal hits the carbon rich investment the
investment surface breaks down and leads to poor surface finish due
to sulfur dioxide gas porosity. The gas is produced by the thermal
decomposition of the investment. You can get a similar effect by
burning out a too high a temperature, If I remember correctly the
thermal breakdown occurs at around 1400 degrees F when carbon is
present. This is why proper temperature and length burnout is so
important to good surface finish.

Jim