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Technical Tips Blog
The incredible plasticity of bentonite. It is the secret to win the ThrowDown!
The 20cm vase on the left is thrown from what I thought was a very plastic body, M370. I achieved close to the same thickness top-to-bottom (5mm). The one on the right was the same original height, 20cm. But it has dried down to only 18cm high, it shrinks 14% (vs. 6% for the other). The thinnest part of the wall is near the bottom, only 2mm thick! How is it possible to throw that thin? The body is 50% ball clay and 50% bentonite. Bentonite, by itself, cannot be mixed with water, but dry-blended with fine-particled ball clay it can. That bentonite is what produces this magic plasticity. But it comes at a cost. It took about four days to dewater the slurry on our plaster table. And one month under cloth and plastic to dry it without cracks.
Friday 23rd February 2024
Coarse body fires with smoother glaze
On the left is Plainsman M332, a sandy and coarse body dry ground at 42 mesh. On the right on a wet-processed body, sieved at 80 mesh and then filter pressed - it is porcelain smooth. Yet that glaze, GA6-C, on the smooth body is covered with blister remnants while the same glaze on the coarse body is glassy smooth (they were in the same firing). That smooth glaze is courtesy of the C6DHSC slow cool firing schedule. But why does it perform so poorly on the finer body? That body is being overfired. Pieces are warping. Although not bloating, it is beginning to decompose and generate gases, they are producing the blisters.
Tuesday 20th February 2024
Control gel using Veegum, brushing properties with CMC gum
This is G1214Z1 brushing glaze (with 5% titanium added). For a 340g powder batch (to get a pint) my initial target is 5g CMC gum and 5g Veegum. CMC controls drying speed and Veegum the amount of gelling. I first mix the CMC with the powder and shake the whole batch in a plastic bag. Then I add it all to 440g of water in the blender jar and mix it well (making sure no agglomerates remain (stage 1). Stage 2 is adding the VeeGum slowly, while high-speed blender-mixing, this is important because it enables tuning the degree of gel (which cannot be predicted). Because this recipe has little clay, it took all 5g of Veegum without overgelling (the entire mass moved freely in the mixer jar). But it did gel overnight (so 4g would be better next time). By contrast, it is the brushing behavior that demonstrates whether the amount of CMC is right. Not enough and coats dry too fast and go on too thick. Too much and it dries too slowly and too many coats are needed.
Sunday 11th February 2024
4-7% tin oxide in a clear glaze - is this better than Zircopax?
These samples illustrate 4, 5, 6 & 7% tin oxide added to a transparent. Tin is super expensive (Zircopax is five times less expensive at time of writing). But consider some advantages of tin. It creates a whiter glaze. A tin-opacified glaze may only need to be half as thick as a zircon-opacified one. Only half the percentage is required in recipes. Tin does not reduce the glaze's thermal expansion as much. On these samples, the higher percentages of tin seem to produce an even better glossy surface. Tin is less likely to cause crawling, a classic issue with high-zircon glazes (it impedes melt fluidity, that is what holds super thickly applied majolica glazes on the ware). Tin is the opposite; even though this recipe is high in strontium, and thus has a high surface tension, there is no indication of crawling with the tin addition. A final issue is cutlery marking, a common problem with zircon-opacified glazes. But not with tin oxide.
Context: Tin Oxide
Sunday 11th February 2024
This pottery glaze is not flaking off - the underglaze is.
Pure ceramic stain powders don't melt at typical pottery temperatures so they cannot bond with clay body surfaces. They don't suspend in water and don't harden on drying. Pure stains contribute only one ceramic property: Color! Commercial underglazes sold in jars dilute stains into a recipe of materials, a 'base medium', designed to impart the missing properties. Unfortunately, consider a problem: Blue stains are fluxes and potent at low percentages. Orange stains (and others) are refractory and require much higher percentages. But for convenience underglaze manufacturers will want to use the same medium and percentage pigment to make an entire line of underglazes. That causes the problem seen here - flaking is happening at the interface between the body and orange underglaze. This underglaze recipe needs more frit (or a more potent one) so that enough melting occurs to create a better bond. For some colors it can thus be an advantage to make your own underglazes.
Sunday 11th February 2024
First slip cast leather-hard full-sized beer bottle!
Left is our M370 + 10% raw umber, it is leather hard, just out of the mold. By the time it is fired it will be 12% smaller and will match the glass one. The pads are positioned firmly and correctly for the swing-top stoppers. The narrow neck can cause issues while draining the clay slurry from the mold, the flow creates suction that can pull the soft walls away from the mold face (a slower pour is thus needed). A large 3D printed pour spout is also needed to provide enough of a reservoir for the slurry during casting.
Tuesday 6th February 2024
Here is why porcelain engobe does not fit stoneware
This is how bad the fit can actually be. In the front is a bi-clay EBCT test strip of a grogged cone 10R sculpture clay sandwiched with a porcelain. After drying this bar was relatively straight. The back bar bent quite a bit even after bisque. But the bend on the front bar really shows the misfit. But this is not a thermal expansion issue where volume changes are measured in 100ths of a mm - these plastic bodies shrink 5-8% during firing, that is up to 8mm change in these 10cm long cars, that is the kind of volume change needed to make this happen. The porcelain has the higher fired shrinkage so it pulls the bar toward itself. The internal stress makes this bar a time bomb, waiting for a mechanical or thermal trigger to burst it into a hundred pieces. Admittedly, putting a thin layer of this porcelain onto a piece of heavy ware is not going to bend it - but the stresses of the porcelain being stretched-bonded will still be there, seeking relief (likely exhibited by cracking or flaking).
Monday 5th February 2024
Replace Custer Feldspar in a cone 10R glaze recipe in 2 minutes
Our G2571A and G1947U cone 10R glaze recipes are both in use in Ecuador. One user has been importing Custer feldspar, but cannot get it and needs to substitute a local feldspar. That material is much higher in SiO2 and much lower in Al2O3 and KNaO (so a pound-for-pound substitution is not going to work). Here is how I used my account at insight-live.com to figure out what to do. It turned out to be a matter of increasing the new feldspar to match the KNaO matched (which also matched the Al2O3) and then reducing the silica to match the SiO2 match. The amount by which I had to change the feldspar is a testament to how different the chemistry of these materials can be from country to country.
Thursday 1st February 2024
Glaze crawling on the rims, what caused this?
This problem only happens once in a while. On the rims of thick or thin pieces. The ware is glazed inside and out, sponge cleaning the rim at each stage. Finally, the rim is dipped (after a period of drying). Glaze thickness and layering did not affect the appearance of the issue. They control glaze rheology carefully (according to specs). In the end, the problem turned out to be a worker who was using hand cream, getting it on the rim as she was handling the ware (the touch points are the crawl points). A complicating factor was that bisque ware was not being kept clean and dust-free.
Context: Crawling glazes withdraw into..
Thursday 1st February 2024
High melt fluidity is required to achieve the visual effect of this glaze
This is G3948A, an iron red cone 6 reactive glaze (similar to several commercially available products). The reason for the variegated surface is the high fluidity of the melt. Adequate thickness is also important, enabling it to run downward to some extent. That means this is not actually over-fired. Using it thus requires consideration of the running behavior, accommodating it in the shape of the ware on which it is used. Obviously, using this on the insides of pieces would result in pooling at the base, which would likely produce glaze compression, cracking the piece during cooling. Use on the outsides may require a catch glaze.
Saturday 27th January 2024
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