Non-conducting substance with a glass transition temperature around 400 to 600 °C?

Am looking for a substance to seel two pieces of ceramic that will have molten plastics flowing between at 300+ °C however silver solder circuit traces inside the device prevent me from going much above 600 °C without destroying them – otherwise I would be using some form of pottery glaze but the lowest melting one I have found reference to is around 1000 °C.


The ideal would be something I could bring back up to the glass point and separate the pieces for servicing.


Can any one either suggest a compound or recommend a resource that would allow me to search based on constraints like I outlined? 


