G2934Y is a fabulous base glaze but it is not without issues. It has significant clay content in the recipe and high levels of Al2O3 in the chemistry, these make it susceptible to crawling. While it is normally fine as is, when you add certain stains to color it (especially at significant percentages) or opacify it using zircon (this has 10%), it can become more susceptible to crawling. On this mug, the glaze layer thickens at the recess of the handle join, that produces crawling during firing. Crawling can also happen on the insides of mugs, where wall and foot meet at a sharp angle. This happens, both because the glaze cracked here during drying and because the zircon stiffens the melt, making it less mobile. Rounding such contours will help. Even better, adjust the glaze recipe so it shrinks a little less on drying (by trading 5% of the raw kaolin for calcined). Adding a little CMC gum (e.g. 0.1-0.2%) will make it adhere better.

Why are these happening (on this piece by Paul Briggs)? It is not completely clear. The glaze has plenty of carbonates, including copper, enough for over 20% LOI. But these normally produce high populations of small blisters, this is the opposite. The melt appears to have enough surface tension that the bubbles survive and endure top-temperature-soaking. And they don't pop until the temperature has dropped so far that insufficient melt-mobility remains to heal them. The glaze has an unconventionally low SiO2 content, that makes it flow vigorously, well enough that the melt is moving and collecting in surface contours. The glaze recipe is quite unconventional, any effort to "improve" its adherence to limits would likely lose the visual aesthetic. A drop-and-hold firing schedule is likely the key to alleviating this.

Simulating a white-on-black oil-spot effect at cone 6 oxidation proved to be a matter of repeated testing (that got me past some misconceptions). Stopping to think about the results at each step and keeping a good audit trail with pictures, in my account at insight-live.com, really helped. I had three black glazes: G2934BL satin (G2934 with black stain), G2926BB super-gloss (G2926B with black stain) and G3914A Alberta Slip black. Going on a hunch, I mixed up a bucket of the G3914A first (with some gum to help it survive second-coating without lifting). Rather than just try any white, I created G3912A by substituting as much CaO and MgO as possible for SrO in the G2934Y base. I later learned this to be an error, SrO reduces the surface tension, I should have used MgO (the G2934Y is a high-MgO glaze so it would have been fine as-is)! As you can see on the far right, this white still worked (at cone 5, 6, 7, 8). Why? There is another factor even more important. The effect only works on the Alberta Slip black. But its LOI is not higher than the others. And it worked even after ball milling. So I need to continue to work on this to learn more about why this works.

No. Because you will face a whole array of problems. This is the first, poor mold release, or more correct, impossible mold release! Plates will be too thin walled. If you cast them longer wall thickness will be uneven. Edges will crack like this (because of poor plasticity). They will warp during drying. They will lack dry strength for handling. They will warp during firing. You won't be able to get a good rim. You won't be able to cast a foot ring without an indent showing on the inside. Note here that another issue is at play: The clay is either not plastic enough to cut cleanly at the rim, without tearing. Or, it is being cut too late or with a dull knife. These tears provide places for cracks to initiate. Plates are much better made using the jiggering, ram pressing or dust pressing processes. Or by throwing them on plaster batts.

Why? Glaze fit. These are available on Aliexpress (as Drip Pottery) and they are made by a manufacturer that has close control of body maturity (and thus strength) and a dilatometer to precisely match the thermal expansion of the glaze. The glaze has to fit better than normal because of the absence of an outside glaze. Too low an expansion and it's compression (outward pressure) will fracture body (because these are thin-walled pieces). Too high and it will craze. And that thick glaze? It will shiver or craze with far less forgiveness than a thin layer. And how did they get the glaze on this thick? They deflocculated it, up to 1.7 or more, glazed the inside, let it dry, then glazed the outside. These pieces are a visual and technical achievement. If you are a potter you had best think twice before attempting the same.

I found seven secrets with recipe, process, glaze and firing. 1. A lot of Zircopax, in this case 20% (for whiteness, opacity). 2. The whitest burning materials: New Zealand Halloysite as the kaolin and nepheline syenite as the feldspar. 3. 3% Veegum to gel the slurry (enabling low specific gravity for thin and even coating). 4. The recipe, L3685Z2, has 55% kaolin, that will certainly produce drying cracks. But 1% CMC gum stops that and makes it brushable. It even works on on bisque, I pour-applied it to the insides of these two slip-cast pieces, it drained to perfectly even coverage (in a very thin layer). 5. A terra cotta casting/throwing body to fit the engobe to (has the same fired shrinkage at a target temperature, e.g. cone 04): Initially I am using L4170B. 6. A clear glaze that fits and is transparent: Notice how much whiter the left one is, G3879. At the same thickness as the G1916Q on the right, it is more transparent, better transmitting the white of the engobe. 7. The right firing curve: The 04DSDH drop-and-hold schedule for defect free surfaces.

Louise Solecki Weir working on one of her large sculptures. Sculptors can be passionate about the clay they use. For good reason, they have a lot to lose. While it might seem that Louise would be most concerned about drying shrinkage and drying performance (resistance to drying cracks), not so. To her, the ability to re-wet sections that dry out is paramount. And she has learned to overcome drying challenges posed by the high plasticity to benefit from the smooth texture, workability and rewetability it offers. How plastic is this clay (Plainsman F96)? It is a five-equal-parts-mix of silica sand/grog, ball clay, Lincoln fireclay, a low fire red clay and a medium fire red clay (there is no feldspar or silica). All four of the clays are highly plastic to super plastic. The body's drying shrinkage would be 8% if it was not for the 20% aggregate (a mix of fine 75 mesh sand with a small complement of fine 40 mesh grog) that reduce it to 6.5%. These offer a far higher surface area than coarse grog and provide channels for water to re-enter. If you would like the recipe of this body (non-production) please contact us.

Then they may need a week to dry! This plate had a one-inch-thick base (while the rim is a quarter of that). During the first few hours a thin rim like this will dry quickly, leaving the base far behind. But as soon as it would support the weight of a cover-cloth I put it into a garbage bag and sealed and left it for several days. Even after that it did not detach easily from the plaster, even though the bat had been dry. When I did get it off the base was still quite soft but the rim was stiff enough to enable turning it over and trimming it (I endeavoured to create a cross section of even thickness). Then I dried it under layers of cloth for several more days. It took at least a week. Had I allowed the rim to dry out during the first few hours it would likely have cracked later on.

While colorful glazes on the outsides of pieces get lots of praise and glory, the transparent or white providing the functional surface on the insides of pieces often gets little attention. Really, what good is an attractive piece if the food surface is crazed, pinholed, blistered, leached, cutlery marked. This liner glaze, G2926B, is special at cone 6. It is a good example of how I found a recipe, recognized its potential and tuned and adjusted it to be better. It has proven itself as a base to host all manner of colorants, opacifiers and variegators. One of the reasons it is so widely used is that it has a story, it is well documented, with a code number that Google indexes. Drinking from a mug having a quality functional surface instills pride as its maker. And it minimizes complaints from customers.

After watching a youtube video (link below) about a Karelian potter, who uses this technique to make cookware, I could not wait to try it. He unloads the ware from his kiln (which appears to be a standard electric top loader used by potters in the west), and while still hot he immerses pieces in a bucket of milk for a few seconds. When he withdraws them they are steaming. I mixed some 2% milk and cream (to get closer to the whole milk he was using) and cold-dipped an 1850F bisque-fired jar and tile (of Plainsman L210) for about a minute (to enable it to soak in as much as possible). The potter claims to fire his ware to 300-350 degrees. I fired 500F/hr to 612F (350C), then held for 10 minutes and shut off to free fall. And it worked beautifully, high enough to get lots of carbon (which is only on the surface), not high enough to burn it away. The surface is smooth and pleasant-to-touch, it is odor-free. The potter claims it retains this surface over many years despite repeated oven use. This clay body, L210, is well suited since it is very fine-grained and fires to such a smooth unglazed surface. And the carbon makes it much better. Indigenous cultures throughout history have learned how to prepare, cook and store food in terra cotta clays like this, they withstand thermal shock better than vitrified stonewares and porcelains. Of course, more testing is needed, I will report as I proceed.