We add a slow-cool onto our Cone 6 firing which develops the sort of crystallized look typically created by a hard fire brick kiln, but many glazes designed for a free-fall cooling in an electric cone become quite ugly.
Reformulating all of our glazes to be compatible with this firing took intuition and a lot of work. Ultimately, it typically required the addition of 10% to 20% of a very fluxed frit like Ferro 3269 which adds additional glass and flux, so the the crystal formation remains below a coating of glass.
Now we're trying to generically add macro-crystalline glazes to our regular firing without upsetting the results of other glazes. So I took our six hour slow-cool firing which free-falls from Cone 6 to 1,800 F and declines by 50 F per hour until 1,500 - and made two minor changes.
I've been told zinc silicate forms especially between 1,850 and 1,800. So I am now starting the slow-cool at 1,850 and have added a 30 minute hold at 1,800. So I'm adding an extra 1.5 hours of partial heating at a higher temperature range where in my experience I've seen little crystallization of other glaze melt materials.
One of the choices I made in this studio was purchasing a highly powered but small 36 watt a smaller 3.5 cubic foot kiln. To keep up with our supply of ware we fire an average of 186 times a year - and our studio is only officially open Saturday, Sunday and Monday.
What this means, apart from replacing heating elements and relays every 14 months, is that we have a very quick turn-around time for artists and don't mind placing one or two items in a fast gilding fire. It also means I can break down the firings into an occasional Cone 6 firing without a slow-cool if someone wants to use certain commercial glazes which don't look well in a slow-cool.
A larger kiln would have been modestly more energy efficient, with lower maintenance costs, but only if it was equally filled which would have meant long waits to get your ware fired. I also could have fired large pieces - my landing duck sculpture could have had its wings fully extended, instead of being caught in the act of folding his wings. But I'd still make the same choice of kiln.
We have a couple of people who have spent a lot of time using complex macro-crystalline firings, but they do these at home in their own kilns. I'm hoping for a fairly significant coverage of macrocrystalline formation with my minor modification. I can't envision using more complex firings to create different patterns of crystals unless we have a large demand for it.