ceramic_shell

casting#techniques: ceramic_shell

Ceramic Shell

ripped from hobbicast for future distillation
You will be much better off building a separate bake out oven for your
ceramic shells. A two chambered furnace is good in theory but impractical,
you want to be able to pull your shell out of the oven and set it right down
into the sand bed so that you can get the bronze poured quickly. Having the
two combined in my opinion would cause some interference between the two.
Ceramic shells cool rapidly at the rate of about 100 degrees per minute due
the porosity of the new style shell materials. By having a separate furnace
(bake out oven) you can bake out the wax and use the same oven to preheat
the shells for pouring. A bake out or preheat oven can be easily built
using fire brick and one of Michael Porter's burners. Our new bake out oven
will serve as an annealing oven, bake out oven and pre heat oven all in one.

When baking out your ceramic shells raise the heat slowly, unless you have a
proven wax with low or no expansion the first 100- 150 degrees. A
temperature of 385 degree will melt all the wax out and any remaining
residue will burn out when the shell is preheated to 1600 degrees. It is a
good idea to blow shell out after wax bake out to remove any loose particle
of shell that will invariably be there.

One note on ceramic shells, the shell pre heat temperature is not always the
same as the bronze pour temperature, solid pours with thick areas (3/4" or
more) will require that you only preheat the shell to around 1600 degrees
even though you are pouring at 1950, where as thin castings will require a
higher shell temperature much closer to the actual pour temperature. The
trick is to keep thin sections hot longer than thick sections so as to
minimize shrink areas that will occur where a thick section joins a thin.

David Harris

Link:
Ceramic shell tutorial. This guy lives right down the road from me!
http://www.whiteriverfoundry.com/makingbirdform/shellbuilding.htm