What is an Igneous Rock?
I’m not trying to pretend to be an expert so I am taking this directly from Encyclopedia Britannica. “Igneous rock, any of various crystalline or glassy rocks formed by the cooling and solidification of molten earth material.”
So, I guess, the first question I had to tackle with this piece was, how do I make an authentic rock? Let alone a type of rock that originates from “molten earth material”. Luckily, ceramic kilns reach the same temperatures the mantle of the earth does. I fired this piece to cone 6 which is around 2232 degrees Fahrenheit or 1222 degrees Celcius. The mantle of the earth, where magma resides, can range from 1832 F to 6692 F.
The second factor I had to figure out was what materials were authentic and would give me similar results to pumice (a type of igneous rock I was aiming to replicate). I made a triaxial blend of Red Art clay (low fire) and silicon carbide (a powder used in glazes to make them gas and bubble). Fired to cone 02, I didn’t see the results I was after. Some research led me to believe that the silicon carbide needed to interact with a more liquid state at top temperature for the bubbles to be able to form and escape without being weighed down. After a lot of research, I stumbled across this Youtube video, outlining a process of making Supersol (synthetic pumice) in Japan.
Glass and Silicon Carbide
Glass is mainly made of silicon dioxide, limestone, and sodium carbonate. Silicon dioxide is the main ingredient in almost all igneous rocks. I had finally found my authentic material to try to gas and crater! So I did a line blend test, of different amounts of silicon carbide mixed with the same amount of crushed glass. The best result was a 1:1 ratio.
Igneous Rock Process
I kept the mixture dry and poured it into a bisqued bowl that was lined with kiln wash and alumina silicate. This was to ensure that the rock would be able to come out of the mold and not destroy the inside of the kiln.
Then once the igneous rock was cool enough I took it into a sandblaster and synthetically eroded it. The rock ended up on a pedestal with scraps of recycled clay that were bisqued. These were made to look like rocks as well but are purely decorative.