Don't listen to people that say you can just replace frit 3134 with 3124 in glaze recipes. That is wrong. Frit 3124 has five times the amount of Al2O3 (the second most important oxide in glazes) and half the amount of boron (the main melter). The glaze chemistry approach is much better, and easier than you think. To be able to do it you need two other Ferro frits, 3110 and 3195. As it turns out, Frit 3195 is more important than is 3124! A key goal in the way it is done is to end up with at least 15% kaolin (to suspend the slurry). I have chosen three types of recipes to demonstrate, dealing with each requires a unique approach. Two of the calculations produce improved slurry properties and one a recipe of significantly lower cost. Stay tuned for a video on how to do this in your insight-live.com account. If you have a recipe that needs this, get an account, enter it there and I can help you do the calculation.

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.

Perhaps you are shocked that a material this dark and dirty (the bars are fired from cone 1 to 7 oxidation, bottom to top), would be used in porcelains. Why? Bentonites are very difficult to process. This is just raw bentonite (HPM-20), dry ground to -325 mesh (to guarantee no fired specks). That grinding does not reduce the soluble salts (that melt by cone 4) or the iron (which accounts for the dark-burning color). These undesirable properties must be tolerated (as whiteness loss) to get the plasticity supercharge 3-5% of this can impart. Why not use super-white bentonites or smectites instead? They can cost ten or even twenty times as much!

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.

This reduction celadon is crazing. Why? High feldspar. Feldspar supplies the oxides K2O and Na2O, they contribute to brilliant gloss and great color (at all temperatures) but the price is very high thermal expansion. Any glaze having 40% or more feldspar should turn on a red light! Thousands of recipes being traded online are high-feldspar, some more than 50%! There are ways to tolerate the high expansion of KNaO, but the vast majority are crazing on all but high quartz bodies. Crazing is a plague for potters. Ware strength suffers dramatically, pieces leak, the glaze harbours bacteria, crazing invites customers to return pieces. The simplest fix is to transplant the color and opacity mechanism into a better transparent, one that fits your ware (in this glaze, for example, the mechanism is simply an iron addition). Fixing the recipe may also be practical. A 2:1 mix of silica:kaolin has the same Si:Al ratio as most glossy glazes, this glaze could likely tolerate a 20% addition of that quite easily. That would reduce running, improve fit and increase durability. If the crazing does not stop the next step is to substitute some of the high-expansion KNaO, the flux, for the low-expansion MgO, that requires doing some chemistry in your insight-live.com account.

This is a fluid melt cone 6 glaze with colorant added and partially opacified. It runs into contours during firing, thickening there (notice the darkening around the logo), this is a desired visual effect. However, notice that drips and runs coming down from the rim, they are producing darker streaks. This is an application issue. Glazes that fasten-in-place too slowly will drain unevenly on extraction from the bucket (after dipping). This can be solved by making a thixotropic slurry. If bisque ware is too dense, glazes have a more difficult time fixing-in-place in an even layer, especially if they have no thixotropy. If glazes lack clay (e.g. less than 15% kaolin) they do not gel as easily. Slurries containing too much gum dry slowly and drips are almost unavoidable. If the problem is too much melt fluidity, choose a more stable base glaze can really help. Just because melt fluidity is less does not mean that it will be less glossy.

Six layers of any normal dipping glaze would be impossible, flaking usually starts with the second layer. Yet this slurry is 85% plastic clay, it shrinks so much that it would be like a "dried up lake bed" on the first layer. By the second layer it would all just fall off! How was it possible to dip six layers here? A 1% CMC gum addition (via a gum solution). Gums are often added to low-clay-content glazes to dry-harden them. But with all the clay in this one, no help is needed for hardening. This is an incredible demonstration of the power of a gum as an adhesive and hardener: It has sufficient power to actually counteract drying shrinkage! Of course, there is a down side: A drying period is needed between each layer, the length depends on the porosity and wall thickness of the ware and the amount of gum. This also demonstrates the difference between the function of Veegum (and similar materials) with CMC. The former, if added to this recipe, would would gel the slurry, require more water and drastically increase the shrinkage, making the cracking even worse. Of course, one could simply use a mix of calcine:raw Alberta Slip to control drying shrinkage and gum would not be needed.

I found six secrets. 1. A lot of Zircopax, in this case 20%. 2. The whitest burning materials. New Zealand Halloysite as the kaolin, Veegum as the plasticizer and Nepheline syenite as the feldspar. 3. A terra cotta body the engobe fits (has the same fired shrinkage). I found a mixture of these materials that fits the terra cotta bodies I am using at cone 04: L4170B (20% nepheline syenite, 55% New Zealand kaolin and 25% silica), L215 and L210. 4. 55% kaolin will certainly produce drying cracks, to stop it the secret is 3% Veegum (to gel the slurry) and 1% CMC gum (to glue it and slow drying). These make it paintable and work for dipping on bisque. It works so well that pour-glazing the insides of these two slip-cast pieces produced an absolutely even, yet very thin paper-white layer. This is an excellent base for clear-glazing and brushwork decorating. 5. A clear glaze and 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, preserving the color of the engobe. 6. The right firing curve: The 04DSDH drop-and-hold schedule.

Quartz particles have a high melting point, they must enter the glaze melt by being dissolved by it (usually the last particles to do so). Obviously the silica should be as fine as possible to increase its surface area to be more readily dissolved. The more that dissolves the closer the physical properties of the fired glaze will be its theoretical (e.g. degree of melting, thermal expansion, transparency, durability). This brand of silica, #90 (likely 45 microns) classifies as 200 mesh even though 2.8% remains on the 200 mesh screen. Not surprisingly, their #45 grade retains 1.9% on the 325 mesh screen. However, the most significant aspect is how much of the #90 is on the 325 and 270 mesh screens: 26%. The #45 grade only has 2.6! This is a huge difference and shows the value of using the finer material. It would take a typical ball mill hours to make this difference.