|Monthly Tech-Tip |
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.
Shown is a tube hand extruder die (with a floating core). It was 3D printed using ordinary PLA. These posts weigh 23g/in (vs 81g/in for the commercial one shown). The posts were cut square-ended using the length patterns shown. Commercial regular posts will support the weight of a city bus but we are supporting pottery - so a thinner more energy-efficient version seems appropriate. This new drawing has been uploaded (visible in the Files panel in your Insight-live.com account). This drawing is more parametric than version 1, it enables setting the post width, wall thickness, draft of the float and position and angle of the float supports. This clay is normal pottery stiffness and contains ~20% grog and sand. We are experimenting with various refractory mixes (e.g. L4543).
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.
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.
Supply chain issues during Covid made it difficult to get posts. I wondered if I could make my own. I have access to the same ceramic materials that post manufacturers use. Firing to the highest temperature they will ever experience should make them dimensionally stable, for me that is cone 10. In the past it was always a hassle to make the extruder die, but not anymore. An ordinary 3D printed PLA template will easily withstand the pressure in a hand extruder (even if printed with infill). The precision tapering possible is amazing. Notice that the floating centre has a tapered bridge to help clay knit together as it flows around it. What about a refractory plastic body? I started with L4543, it is cheap to make and dries quickly with minimal shrinkage. To cut posts to length with a square end I print sleeves to slide them into. I you would like the 3D drawing it is in the Files manager in Insight-live.com, it is parametric and easy to change.
A densification process occurring within a ceramic kiln. With increasing temperatures particles pack tighter and tighter together, bonding more and more into a stronger and stronger matrix.
In the ceramic industry, refractory materials are those that can withstand a high temperature without deforming or melting. Refractories are used to build and furnish kilns.