Burnout

Burnout removes residual wax and water from the mold.  It's fairly straight forward.  The furnace runs highly oxidizing to supply O2 to convert the carbon  from the wax to CO2.  We use the manufacturers recommended temperature range of 1400F to 1600F.

A shot of a mold ready for burnout.  Notice the cup is facing up so a chimney effect is created.  This helps remove residual wax and water more quickly.  It sits in a heavy stainless steel basket and it will be lowered into the kiln at heat.

This is the burnout temperature.  When the molds are place in the furnace, most of the wax is burnt off within 10 minutes.  By using a shield at the exhaust to force the hot gases back down into the furnace to combust, we have essentially smokeless burnout's.   We let them go at heat for about 45 minutes, depending on the mold.   I run up to around 1600F and then back it down to around 1500F.  Different foundries do different cycles, but this works for me. 

 
One hot little furnace.  The molds are in there being fired.  These molds do not vitrify like a standard ceramic.  It's mostly fused silica after all and it's vitrification temperature is well over 3000F.  We are really just drying them.   Actually the green, or prefired, strength of the mold is greater than the fired strength. 

A shot of two molds after burnout.  Notice they are now white.  There is some residual carbon in one of the cups, but that's OK, it won't harm anything.  Many foundries pour while the molds are still hot from burnout. we let them cool so we can clean them out with vacuum and compressed air.  This will remove small flakes and chips of mold material that inevitably occur.  These small chips might become imbedded in the bronze and are an annoying defect to repair.

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