It has been used for the filters. But for simplicity of manufacturing processes, glass would have been a better option. And people like to watch the coffee brewing. Ceramic could have been used for the upper bowls but would have been heavier than the glass versions. Ceramic for a glass bottom would have been poor. High temp glazes will still crack and fall apart from the high heat and cooling cycles. Plus it may not have held up well from the stress of the pressure and vacuum cycles.
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