Environmental Certifications

SULFUR GAS

The effects of sulfur dioxide gas and the porosity it causes has been researched with definitive reactions identified. The most common reaction in all alloys is where calcium sulfate &carbon are in a neutral or reducing atmosphere at 700 degrees C and above. The reaction can and often does occur in electric burnout ovens as well as gas ovens.

The side effects of sulfur gas have some particular reactions on silicon deoxidized alloys. In 14 karat yellow gold the effect is seen as a rough dendritic surface. Finished castings may show a cloudy surface texture with small round holes. In some extreme cases cracking has been seen. The sprue (tree) will show a very rough dendritic structure on the inside button, and may be dendritic farther up the sprue (tree).

The effect of sulfur gas on nickel white gold with silicon, appears to promote a silicide of nickel and silicon, NiSi,. This conversion seems to correlate directly with sulfur gas porosity.

Conditions that may cause conversion of sulfur gas:

• Flames too high in gas burnout oven
• Never connect oven exhaust vent directly into vent going outside.
• Oven exhaust vent restriction
• Overhang of flasks in direct contact with flame
• Exhaust air too strong for make up air in room
• Back-draft or lack of exhaust
• Over packing oven
• Introducing investment powder through remelt
• Severe overheating of metal
• Burnout temperature above 1450 degrees F.

Elimination of sulfur gas conversion:

• Adjust flames lower in oven, add air through venturi air gas mixer. If flames do not reduce in height, it may be necessary to add additional gas.
   
• Never connect oven exhaust vent directly into vent going outside.
   
• Never restrict exhaust from oven. The main problem in electric ovens is inadequate exhaust.
   
• Never place flasks over edge of hearth in gas oven (if designed with open sides). Always stack flasks so that air flow can be received at flask opening,
   
• When exhaust is too strong, a lack of make up air may be present at venturi mixer. This can cause the oven flames to rise. Negative pressure can be very negative indeed. Use an air flow engineer when necessary.
   
• A back-draft or lack of exhaust can cause the smoke to suffocate the oven, preventing fresh air from circulating properly.
   
• Over packing of oven can cause sulfur gas reaction when any other airflow condition is marginal. If normal conditions allow for proper burnout with 12 flasks per oven even the possibility of 1 extra flask will cause conversion of sulfur gas. The variable that changes is the amount of carbon (wax) produced in the given oven.
   
• Introducing investment powder to a remelt through poor removal of investment can cause a direct gas reaction. The best method of investment removal is:
   
  • Enclosed water jet cabinet with high pressure spray.
  • Mild devest solution/rinse.
  • Tumble trees to be remelted in stainless steel shot.
     
• Over heating of metal can cause a direct gas effect even if all burnout reactions are perfect.
   
• A burnout temperature above 1450 degrees F can cause conversion to sulfur dioxide gas due to a chemical breakdown of the investment.