Silicious molds using aluminum phosphate (AlPO
4) and phosphoric acid (H
3PO
4) as a binder are heat resistant and acid-decomposeable. The silica sand mold resists up to 1,200°C when the molecular ratio of the binder Al
2O
3/P
2O
5 is 0.636, and up to 800°C when Al
2O
3/P
2O
5 is 0.304. The zircon sand mold resists up to 1,400°C when Al
2O
3/P
2O
5 is 0.636. The binder separates Al
2O
3⋅mH
3⋅PO
4nH
2O which subsequently loses water during drying, produces a viscous solution and forms a mixture of Al(PO
3)
3 and AlPO
4 at 800°C. When the Al
2O
3/P
2O
5 ratio of the binder is higher, silica sand particles bind with acid-soluble AlPO
4 preventing sintering of the particles during casting. This mold decomposes readily by dipping into a dilute sulphuric acid solution.
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