creation of metal oxides

While some oxidation is desirable for controlling the solution of gases in molten metal, excesses of oxygen will combine with the molten alloy to form undesirable oxides. or dross.

It is quite common to find the oxides of copper, zinc, tin and lead in the slag of the 85-5-5-5 type alloy, for example. Silicon bronze alloys are apparently protected against harmful oxidation by a film of some silicon oxide that forms on the bath. Aluminum bronzes easily form oxides of aluminum, which' complicate the han­dling of these alloys. The yellow brasses or manganese bronzes frequently form large quantities of zinc oxide.
In a reducing melting atmosphere, such oxidation is negligible, but when an oxidizing melting technique is used, it is necessary to closely control the melting atmos­phere so that only a slight amount of free oxygen is pres­ent in the products of combustion. Some foundry men use cover fluxes to prevent the formation of undesirab1e oxides. While it is true that certain chemicals will parti­ally protect the metal from exposure to air, improper usage of such covers may exclude too much oxygen and thus Permit hydrogen gas content to build up.
Deoxidizers are helpful in reducing the amount of metal oxide present in the bath except with silicon, man­ganese and aluminum bronze. Since oxides will impair the castability of the alloy, the deoxidizer should be added immediately after the molten rnetal has been skimmed of dross. It will then inhibit the additional pickup' of oxygen during pouring operations.
One of the most harmful common practices in brass and bronze foundries is stirring or agitating the molten metal' while it is .in the furnace or in the crucible on the foundry floor. Stirring will thoroughly mix the oxides into the metal, and when the casting is finally poured, it will contain .an unusual amount of dross. Of course, when adding deoxidizers, gentle mixing is required, but excessive or violent stirring must be avoided.