Soldering Flux. This substance is applied to a metal surface to remove oxides or prevent their formation. When the oxides are removed, the solder is free to wet and spread over the clean metal surface.
Borax glass (Na2B4O7) is frequently used with gold alloys because of its affinity for copper oxides. An often-cited soldering flux formula'-' is borax glass (55 parts), boric acid (35 parts), and silica (10 parts). These ingredients are fused together and then ground into a powder.
Fluxes are available in powder, liquid, or paste form. The paste is popular because it can be easily placed and confined. Pastes are made by mixing the flux powder with petrolatum. The petrolatum excludes oxygen during heating and eventually carbonizes and then vaporizes.
New fluxes are available for use with nongold-based alloys. Their formulas are not generally published. At present, none of the new fluxes are totally capable of preventing oxide formation during heating of the base metal or nonnoble alloys. An exam-
ple of a rapidly forming oxide on a base metal occurring during a simulated postceramic application soldering can be seen in Fig. 28-14. Soldering of base metal alloys is still unpredictable .'6
All fluxes should be kept from contacting porcelain-veneered surfaces. The contact will cause pitting and porcelain discoloration.
Soldering Antiflux. Antiflux is used to limit the spreading of solder. It is placed on a casting before the flux application to limit the flow of molten solder. When the metal surfaces are clean, any excess solder introduced into the work gap will tend to flow into unwanted areas. The antiflux helps prevent this.
Graphite (from a pencil) is often used as an antiflux. However, the carbon easily evaporates at higher temperature, leaving the workpiece unprotected. A more reliable antiflux is iron oxide (rouge) in a suitable solvent such as turpentine, which can be painted on the casting with a small bristle brush.
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