At higher temperatures the melt is clear (transparent) but at lower than 700C is the dark-green. Is super quenching the solution?
The best way to prepare C12A7 glasses I can think of is to take the correct composition (referring to the binary phase diagram) and heat it to the temperature where C12A7 exists and then quench the slag by pouring it on a steel launder or water cooled Cu plate or by quenching in water. It would be ideal to use a Pt /Pt-Rh crucible in air/oxidizing atmosphere
Thanks Luckman, but Pt-crucible reacts with the compound which ends up contaminating the glass.
Pt should not react with CaO-Al2O3 glass systems under air or oxidizing atmospheres. These are relatively 'inert' glasses. What is the purity of the powders you use?. The only possibility I see is a reaction with some of the impurities (Fe, Mn oxides etc..if the powders are less than 99% pure), they can form eutectic mixtures with Pt.
I assume by C12A7 you mean 12CaO･7Al2O3? If this is correct then I agree with Luckman that under normal oxidising conditions one would not expect reaction of the melt with the crucible, that is unless you need extremely high melting temperatures which could melt the crucible.The CaO-Al2O3 phase diagram indicates you could expect the eutectic at around 1350oC so you won't melt your Pt at these temperatures. So - if you killed your Pt crucible it is likely (although not certain) this was caused by reducing melting conditions or your raw materials being contaminated.
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