The subject of oxide accumulation is important to the quality of cast, rejection rate and reusability factor. Oxides are formed when the alloys used for gold are exposed to:
- Air (Oxygen)
- Chemical compounds (investment powder)
- Calcium sulfate reactions (from investment powder)
- Graphite (from crucibles)
- Overuse of metal (not enough fresh gold)
All alloys are subject to oxide accumulation. Non silicon alloys, when oxidized, will usually produce copper oxide. This appears as red or brown oxide and when in abundance will cause cracking and heavier than usual oxide on the metal surface. The oxide accumulation we will be discussing here deals with silicon deoxidized alloys and their reactions.
Silicon alloys cast with a clean and bright surface appearance. When exposed to any of the above reactions, silicon oxide is produced as the metallic silicon fights off the attacker. When silicon oxide appears it is important to skim the metal surface with a carbon or quartz rod to remove this accumulation. Fluxing will be necessary if the buildup is large. When accumulations become too great for the remaining silicon to resist, the silicon oxide and other contaminants will become trapped in the cast parts, with either cracking or porous dirty castings being the result.
Air or oxygen is the least troublesome of the reactions due to the silicon's ability to withstand the air. Only a slight skin will develop which usually presents no problem. if possible use a reducing atmosphere at al I areas the molten metal is exposed to air. We recommend forming gas (75% nitrogen 25% hydrogen) for electric melters. Argon or pure nitrogen are also adequate. When gas melting, the natural gas is a reducing atmosphere which prevents absorption. If possible also add gas cover to tongs for hand pouring and over vacuum machine when casting.
Chemical compounds are difficult to avoid when metal is reused. Investment powder contains calcium sulfate which will attack the metal. Chemical solutions for devesting will also leave residues that will have some reaction with the alloy. We recommend remelting old metal into shot before using for casting. This allows the ability to flux and burn off any residues.
Overheating will create excessive silicon oxide to surface and increase the likelihood of problems. In automatic machines such as the Memco or lnresa be careful to avoid overheating. The lnresa seems hotter than its temperature control after the first few melts. The Memco overshoots temperature after casting, so if any metal is left in crucible, it will be heavily oxidized. Remove any metal left in these crucibles before recharging. Gas melters run slower, making overheating less likely. Make sure the air-gas mix is proper, if too much air is used it can cause heavy oxidation to take place. This is usually noticed on sterling and I 8K before problems arise with I OK or 14K.
Calcium sulfate which is used in all the popular investment powder scan cause reactions with all gold alloys. The appearance is either a gray color on the investment wall or a yellow reaction when metal is poured into flask. A rotten egg smell may also be noticed. When proper burnout temperatures are used and metal is not overheated it is unlikely to have any noticeable reaction. Beware if metal is all fresh and the last inch of the tree is heavily oxidized.
Graphite, which is the most common crucible used in electric melting, is very susceptible to breakdown. Fine particles of graphite will shed onto the melt causing contamination. When the accumulation becomes excessive the graphite particles may become trapped in the cast; with possible cracking taking place. The cracks will show graphite inclusions. The graphite may interact with the silicon which will compound the problem.
Overuse of metal will gradually spend the metallic silicon content of the alloy. It is advisable to use a constant percentage of new gold for consistency. High fluidity silicon alloys (United's ULTRALOY and 200 Series) have the highest reusability factor. Depending on conditions, these alloys when used in 14K may not generate any scrap. If conditions are inconsistent, or exposed to the other conditions listed previously, some scrap will be generated. When the trees become dark, pitted or oxidized, it would be recommended to scrap them. We have heard many remarkable stories of how these alloys were used for days without adding fresh. This however will cause an excessive accumulation in the trees, with a higher rejection rate following shortly. When an excessive amount of oxides are present, just adding new gold will not remove these oxides. They will be spread throughout the new mix with random rejection. United now makes an alloy replenisher #99, for refreshing old buttons and trees.
What has been discussed are the major factors in determining rejection rate. Control of equipment, clean shop habits and regular rejuvenation of metal will help insure a quality cast.
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