Now the wax pattern is covered in yet another mold, called the "investment." This mold reproduces both the outside of the sculpture as well as filling the hollow interior of the wax pattern. Also, this mold has to be able to withstand the high temperature of molten bronze.
Ceramic, made from good old sand, is just what's needed for the new mold. Workers dip the wax pattern in liquid colloidal silicon (ceramic slurry).
Then they sift on more fine silica sand to build up the thickness of the layer coating the wax inside and out. They repeat the dip and sifting steps several times to make the investment strong enough to survive the casting process.
After air-drying, the ceramic is solid. The unfired ceramic and wax are put in a kiln and heated to melt the wax, which can be recovered and
reused. Then the ceramic is fired at a higher temperature to harden it and burn out the remaining wax.
The wax is now "lost" and the remaining ceramic is the second "negative" version of the original sculpture. It is now ready
to be filled with bronze.