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Can someone explain why boric acid prevents zinc from vapourizing out of the brass alloy when it is being melted? What is the actual chemistry occurring and why? Thanks

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It depends on the temperature, specifically the 1700 F +/- boiling point of zinc , not the flux. At 1700 F the zinc will vaporize regardless of any flux . I never saw a single component flux, likely the the flux also contains soda ash, silica, etc. and it does protect the molten metal from atmosphere by forming a physical layer. Footnote: many years age I worked with copper alloy fluxes and found that for some alloys, the boron promoted the formation of spinels in the metal ; These are exceptionally hard materials that essentially prevent machining of the bronze/ brass.

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In the melting of copper alloys (as well as other alloys), various auxiliary molten metal processing steps are necessary other than melting and alloying to improve meltquality through the control of impurities. Fluxing and metal refining is one among these processing steps. One of fluxing process is neutral cover flux, which is used to reduce metal loss by providing a fluid cover. Fluxes of this type usually used are based on borax, boric acid, or glass, any of which melts at copper alloy melting temperatures to provide a fluid slag cover over the melt surface. For example, borax melts at approximately $\pu{743 ^\circ C}$ ($\pu{1369 ^\circ F}$), compared to the lowest melting yellow brass, C85800, which melts in the range of $\pu{871-899 ^\circ C}$ ($\pu{1600-1650 ^\circ F}$). These glassy fluxes are especially effective when used with zinc-containing alloys (such fluxes are also used by goldsmiths, etc.), preventing zinc flaring and reducing subsequent zinc loss by $3$ to $10\%$. The glassy fluid cover fluxes also help to refine alloys by accumulating and absorbing nonmetallic impurities from the melt such as oxides (e.g., $\ce{CuO}$), molding sand, machining lubricants, etc.

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