| Monthly Tech-Tip | No tracking! No ads! |
My first zircopax kiln shelf
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.
Related Pictures
Thin walled kiln posts using a hand extruder
This picture has its own page with more detail, click here to see it.
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 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).
Drying a flat slab of clay between sheets of Gyproc
This picture has its own page with more detail, click here to see it.
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.
Making your own hexagonal shelves using calcined alumina
This picture has its own page with more detail, click here to see it.
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.
Making your own kiln posts, is that even practical?
This picture has its own page with more detail, click here to see it.
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 (click the link below to go straight there).
Super kiln wash that can be applied with a paint roller
This picture has its own page with more detail, click here to see it.
Traditional 50:50 kaolin:silica kiln wash can be a real bummer to use. It flakes, both on drying and after every firing. Pieces of it stick to the feet of ware (plucking). It is not refractory enough either. Shelves need cleaning and rewashing often. Three outside-the-box ideas make this a better recipe.
#1 No raw clay! Strangely, calcined kaolin is better than raw kaolin, it imparts multiple advantages.
#2 No silica. We use zircon or alumina instead, they are more refractory. And we use 80%, not 50%.
#3 We add CMC gum. It is the hardener, it enables a high specific gravity, imparts awesome brushing properties and slows down drying on cordierite and alumina shelves.
The low water requirement and slow drying make this behave more like paint. It can be applied by roller or brush. Coverage is much more even and it does not shrink and crack on drying. Normally the raw kaolin in 50:50 kiln wash suspends the slurry, makes it brushable and hardens it on drying. But CMC gum is way better for the latter two. It is so nice to be able to apply a thin layer of wash even on highly porous shelves (like these alumina ones we make ourselves). Unfortunately, we can't have everything - a downside of this recipe is settling (more information on the recipe page linked below). Fortunately, if used every few days it won't be a problem.
This is not available as a product, I just like it so much I made a label for it!
Videos
Links
| Materials |
Zircopax Plus
|
| Materials |
Zircopax
Zirconium silicate, its principle use in ceramics is as an opacifier in glazes. It is an expensive material, but less so than tin oxide. |
| Glossary |
Sintering
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. |
| Glossary |
Refractory
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. |
Got a Question?
Buy me a coffee and we can talk 
https://digitalfire.com, All Rights Reserved
Privacy Policy