Burn-out time

The final part of the Winter courses casting component meant that I carefully packed 20 ceramic shells in expanded polystyrene chips inside a couple of large square plastic boxes (both originally held Turkish dondorma!), and carted them off in the train to the workshop near Wimbledon.The weather was very cold, somewhat breezy but clear when I got the furnace dome set up and the task of burning out the waxes underway. To my dismay, of the 60 or so items on top of the shells, around 9 came to some kind of grief, although later about half of these were to be repaired at least to some extent. Unfortunately one of the casualties was the wax of a 12cm pig, intended to be cast in silicon bronze. This gave an audible loud ‘pop’ inside the furnace dome, breaking into four or so large pieces.My feeling is that the increased failure rate (about double the usual) is probably related in some way to the temperature at which the waxes had been stored, and that at which the burn-out took place. However in at least one case, the failure was due to the fact that a collection of items on a shell had insulated the outlet wax stalk from the heat; the expanding wax had nowhere to go but out through the top, bursting it off. This particular item was later given a repair.I wondered, as on several occasions in the past, if the number of failures could be reduced with strategic additional 'sprues' intended to allow leakage of wax during burn-out.