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The formation of a given phosphate depends not only on the composition of the melt but also on the rate of cooling. The further cooling of the remanent melt depleted of high-melting oxides results in the formation of more mixed orthophosphates; when cooled rapidly (quenched) a melt containing an adequate proportion of SiO 2 is converted to the glass phase. In long-term firing (over 20 h) at 1600~ or higher the sodium ions are volatilized out of the melt. The complete vaporization of the sodium and the crystallization of the liquid phase are followed by the formation of mixed calcium and magnesium orthophosphates, e.g., Ca3Mg3(PO4) 4. A further increase in the firing temperature and time results in the gradual volatilization of the P205 from the phosphate bond until the latter is completely decomposed. LITERATURECITED