This type of dry heating tests may be alternative or complementary to flame tests. These tests are based on the ability of some salts, such as sodium tetraborate decahydrate, commonly known as borax, and ammonium sodium phosphate, called microcosmic salt, of melting and forming a colourless, glass-like bead. When kept over the flame of a Bunsen burner, borax swells up and loses its water of crystallization; then, being always kept heated, the anhydours sodium tetraborate will release boric anhydride and sodium metaborate. Burning above an oxidizing flame, boric anhydride is able to dissolve some metal oxides that can come from the thermal transformation of other compounds: thus they are transformed into metaborates characterized by particular colours according to the metal included. Burning above a reducing flame, (better, with little coal dust), metaborate can be reduced releasing a flame of different colour. Going on burning above a reducing flame, metaborate can be reduced furtherly to metallic copper giving an opaque, brown-red bead. When melting above a Bunsen burner, microcosmic salt loses water and ammonia transforming into a bead of metaphosphate. If sodium metaphosphate comes into contact with metal oxides when burning above the oxidizing flame, it will react forming the corresponding double orthophosphates giving the beam characteristic colours for each metal. Also melting microcosmic salts above reducing flame will lead to a further reduction of some metal oxides with different colours of the bead. In some metals, their bead can change colour after being cooled.
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