DE-OXIDIZED VS. NON DE-OXIDIZED

Porosity in relationship to the type and karat of metal used is interrelated to the previous discussion in most are as except what is called gas porosity. Gas porosity occurs when gases enter the molten metal and are retained during solidification. This may appear as large or small holes and discoloration may occur. Most alloys used for karated gold contain copper, silver and zinc. Copper when exposed to the atmosphere will absorb oxygen, which produces cupric (black) and cuprous (red) oxide. Silver can absorb many times its volume in oxygen. Zinc will form zinc oxide on exposure to overheating. Non de-oxidized alloys which contain the above ingredients will absorb gases. When fresh metal is used little absorption will take place. When reuse occurs the absorption will increase, which is noticeable by a progressively darker appearance of the cast as well as an increase of porosity. When using non de-oxidized alloys, a protective atmosphere of forming gas, argon, nitrogen or. hydrogen should be used to prevent absorption of atmosphere.

De-oxidized alloys which use silicon, boron, or silicon-boron help reduce absorption of gases. These metals act as scavengers of oxygen, creating their own oxide formations, thus preventing absorption. Typically de-oxidized alloys are preferable for their cost effectiveness, due to better reusability, cleaner appearance and lower porosity rates.

NOTE: De-oxidized alloys will not correct for inadequate design or gate problems.

Solidification rates of different alloy formulas can affect the porosity rate. 1OK alloys have the highest melt point, yet will remain liquid longer than 14K. What this means is the higher the karat the faster it will solidify in the flask, regardless of melting point. Alloys higher in silver will solidify faster than lower silver alloys. Sterling silver solidifies even faster than the gold. Proper melt and flask temperatures in relation to the design and gate of the part will play a most important role in the level and type of porosity.