|Monthly Tech-Tip |
This homemade kiln shelf (left) for our test kiln was fired at cone 10. This is a third the weight (and thickness) of the cordierite one on the right. However it does not have the thermal shock resistance of cordierite, uneven heatup can crack it. It is made from a body I slurry up consisting of 96.25% calcined alumina and 3.75% Veegum. It rolls out nicely and dries flat between pieces of plasterboard, taking about three days (if you try this and the body is not plastic then your alumina is not fine enough or you did not blender mix the slurry well enough). Alumina produces a lighter shelf than Zircopax and shrinks much less than refractory bodies we have tried (e.g. L4543), I cut the slab only 1/4" larger and it has fired to the same size.
This is a ConeArt 119D, 0.57 cu ft, 11"x9" cone 10 test kiln. While there is 120v model, don't take a chance, go with 220v (actually ours is 208v). Ours fires 1000 times on a set of elements, mostly in the cone 4-7 range. The old BX controller is shown here, it is $300 cheaper, but don’t even think about getting that! Do not use your electric like a pop up toaster, make it a technological enabler of custom firing schedules, get the Genesis GX. Having good control of firing is a key to success and this is superior for that. These kilns are economical to fire. Big enough for 5 mugs, but I typically fire a dozen clay and glaze test specimens. We make our own super-thin shelves. The controller holds about 20 schedules, even controllable remotely (it is Wi-Fi connected). We can fire cone 04 up and down in three hours! Of course, since this type of kiln can enable so much more testing you also need a code numbering system and a place to record and search all the results: An account at insight-live.com.
I am making my own low-temperature kiln shelves by rolling 3/16-inch thick slabs from plastic clay (from a 50:50 mix of Pyrax and Kaolin with 20% grog added). How can I dry them flat? Between two sheets of plasterboard. You might think that the weight of the board above would impede the shrinkage of the slab and crack it, but that does not happen. Even if the clay is quite soft or less plastic these still slabs still dry within two days without splitting or cracking.
It is 5 mm thick (compared to the 17mm of the cordierite one). It weighs 650 grams (vs. 1700 grams). It will perform at any temperature that my test kiln can do, and far in excess of that. It is made from a body I slurry up (80% Zircopax Plus, 16.5% 60-80 Molochite grog, 3.5% Veegum T). The body is plastic and easy to roll and had 4.2% drying shrinkage at 15.3% water. The shelf warped slightly during drying (I should have dried it between sheets of plasterboard). Firing at cone 4 yielded a shrinkage of 1%. Notice that cone on the shelf: It has not stuck even though no kiln wash was used! Zircopax is super refractory! This is sinter bonded, so the higher the temperature you can fire the stronger it will be. Although it would be very hard to make full 18 or 22-inch shelves for larger kilns, smaller ones designed to "network" would enable a tighter load of ware with a much lower shelf-to-ware weight ratio (especially using my own lightweight posts). Like alumina, this does not have the thermal shock resistance of cordierite, uneven heating can crack these.