Hi Danny, thanks for the input, yes I have a bag of Herculite also and if I find the temperature required is any higher than 120-130 degrees then your probably right although good old fairy liquid as a release barrier has remarkable properties which we used in my old refractory engineering days, however I have no intention of risking the plug unduly and will probably cast a plaster plug at the very least first as a 'just in case' spare. Linds