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Question: why is borax a flux? What actually does a flux do anyway? (same question) G'day; I wrote something on these lines a few years ago in Orchid, but here we go again; A flux is a substance that cleans the surfaces of metals which are to be heated, and as a consequence of the heat, will combine with oxygen, sulphur and other materials present in the atmosphere or metal, thus keeping the surface fresh and clean. Fluxes do this because they have a special affinity for metallic oxides and other compounds. Borax has such an affinity for metal oxides etc and converts them into acid soluble borates which are in fact glasses and can be washed off heated work and dissolved in acids such as sodium bisulphate, citric acid, 10% sulphuric acid and even lemon juice after heating is finished. Borax is not the only material used as a flux. Plumbers used to use tallow when jointing lead for pipes and tanks. Electronics technicians use a special resin as a flux for tin/lead solders, as any acid left after soldering could cause electrical faults. Borax fluxes are used in some higher temperature work with uncompromising metals, often with the addition of ammonium or sodium fluorides which help with the dissolving of metal oxides. Brazing using brass alloy rods with some bronzes, iron, nickel, and so on works better with the presence of fluorides in the flux, and even silver and gold join more easily with fluoride fluxes. But fluorides are poisonous, and care must be taken not to inhale the fumes. But don't go overboard about fluorides - just be careful - remember that your natural teeth are coated with a fluoride complex. (Heat a tooth with sulphuric acid and the resultant vapour will etch glass) Now, given all this, what might one expect to happen if gemstones were to be coated with boric acid or borax and heated to red heat? As you now know, boron compounds have an affinity for hot metal oxides and other metal compounds, so you should expect them to etch gemstones like ruby, sapphire, tourmalines, agates, and so on. I am well aware that some books recommend this, but now you know why people like me say you should be careful to keep fluxes away from precious and semi precious stones. In other times, before instrumental chemistry became possible, one had to get the unknown chemical into water solution to enable subsequent analysis. This was accomplished by heating the unknown strongly with borax, then dissolving the result in acidified water so that other reagents could react and provide clues as to what the unknown contained. Any person over the age of 50 will tell you that this form of analysis was used to help teach chemistry in the sixth form. (Don't really know why they stopped that method of teaching) Here endeth the lesson. John Burgess Mapua, Nelson NZ |
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