A potter uses a 70% potash feldspar, 20% whiting, 10% kaolin glaze for a crackle cover over iron oxide brush strokes. While it fires crystal clear, it has a problem: it crawls. While it can be flocculated with Epsom salts, and the application and dry adherence are fairly good; this crawling problem is occurring. By mixing it as a brushing glaze we can fix three of its problems without changing the recipe: Its tendency to go on too thick, the entrained bubbles in the clear glass and the crawling. Don’t dismiss this idea immediately. This is a special-purpose surface and brushing can be done more quickly than you might think. Careful control of thickness is possible and the CMC gum in the glaze will make it adhere much better (eliminating the crawling). The consistently thinner coverage will also make it possible to produce crystal clarity with fewer micro-bubbles in the glass. A hybrid approach is also possible: Pour glazing the inside and brushing the outside.

USG specialty plaster is entering a final-inventory phase in the ceramic supply chain, and rationing has begun at some suppliers even though the Southard plant does not close until November.

These two cone 6 glazes are being compared in a melt-flow tester. Their calculated oxide chemistries are essentially the same, but G2934Y4 sources KNaO from Ferro Frit 3110 rather than feldspar, and much of its MgO from Ferro Frit 3249 rather than talc (talc is so refractory that it is a key ingredient in making kiln shelves!). The Y4 recipe even sources Al2O3 from calcined alumina (another kiln shelf material!). Yet the fritted glaze flows much farther.

How can the frit make such a difference in the melt, despite the burden of the Al2O3? The feldspar and talc require decomposition/reaction before the oxides become available to the melt, whereas the fritting process has already combined the oxides into a glassy, highly reactive homogeneous form.

Example of a whiteware clay bloating at cone 10 oxidation. While it appears stable at cone 8, it isn't. This is a variation on the 25x4 porcelain recipe. It employs 200 mesh silica, kaolin, ball clay and feldspar. Whiteware bodies like this often soften and melt without ever bloating, but not this one. These fired bars appear to show that this vitrifies at cone 8. However, ware fired at cone 8 exhibits the classic evidence of overfiring: It warps. Overfire it still more, to cone 10, and we enter bloating territory by a new development: The ball clay in the recipe contains soluble salts; these become a thin glaze that glosses and seals the surface, trapping LOI gases which create internal voids (top bar). This line graph shows it as a massive spike in absorption/porosity going into cone 10. It also shows that cone 6, the bottom bar, provides the lowest porosity and highest fired shrinkage, while not being on a volatile part of the curves.

This phenomenon is a reminder that the percentage of feldspar (or feldspar-containing clays) is best tuned such that the body is just reaching (or has just reached) its minimum achievable porosity (assuming it is for functional ware). This can be done by making SHAB test bars like this and firing them at a range of temperatures up to and beyond the target to get a broad view of how maturity progresses.

This reduction celadon is crazing badly. Why? Its chemistry is dominated by feldspar, which supplies lots of K2O and Na2O fluxes. Scores of recipes being traded online contain 50% or more feldspar; they benefit from these powerful fluxes but inherit their high thermal expansion (COE). Many work only because the body happens to have sufficiently high expansion - often aided by significant residual quartz.

Crazing is more than a cosmetic problem in functional pieces. It reduces ware strength and durability, can compromise food safety and turn into a customer complaint. Let's fix it, starting without chemistry:

• Use recipe logic: Transplant the color mechanism into a transparent base glaze that already fits the body. In this case, the celadon color is largely produced by an iron addition under reduction, so there is no need to preserve the high-feldspar base itself.
• Use material sense: A 2:1 silica:kaolin mix supplies an Si:Al ratio similar to typical glossy glaze chemistries. A 10% addition thus shouldn't change the degree of gloss but will likely lower the COE, reduce melt fluidity and improve durability.
• Use glaze chemistry. Substitute some of the high-expansion KNaO with much lower-expansion MgO. Both are fluxes, and even a 0.1-mole shift can make a significant difference. Couple that with an increase in SiO2 where the melt can tolerate it, and glaze fit can improve dramatically.

These pure material powders were applied to bisque as slurries, suspended by gelling with Epsom salts, and fired at cone 10R. On the left is Custer feldspar on a Grolleg porcelain (P700); on the right is Covia nepheline syenite on a whiteware body (P600).

Feldspars and nepheline syenite supply large amounts of K2O and Na2O, oxides having very high thermal-expansion contributions in glaze chemistry. When these materials are melted essentially by themselves, the resulting glasses have very high thermal expansion relative to typical ceramic bodies - thus the spectacular crazing seen here. Glaze recipes containing high percentages of them can likewise be prone to crazing unless enough lower-expansion oxides, especially SiO2, Al2O3 and MgO, are present.

Here is what Redart fired like in 2010 when evaluated for physical properties using the SHAB test and LDW test. These bars are fired from cone 5 down to 06 (top to bottom). The clay reached zero porosity between cone 1 and 2. Beyond that, it was stable to cone 4. Its drying shrinkage was only 5%, indicating low plasticity (compared to a typical pottery clay). It is thus an ideal candidate for a ball clay addition to create a plastic body for handbuilding or throwing.

However, its plasticity is just right for slip casting; it can be used as is. It has no soluble salts and a perfect score on the drying performance DFAC test. Cone 04 and 06 porosity and fired color are very typical of terracotta clays. The fired color of this material really is stunning. And the stability of that color across such a wide temperature range.

On this M390 red clay test tile, the titanium-variegated cone 6 calcia matte glaze G1214Z1 looks excellent (rutile has a similar effect). But that appearance depends on a combination of factors: glaze chemistry, 5% titanium dioxide, the titanium grade, application thickness, the red M390 body and the PLC6DS firing schedule. The relief of the tile especially flatters it - forcing the glaze thin over high points and allowing it to pool in recesses, displaying a whole range of effects in just a few centimetres.

The tile tells you what effects a glaze is capable of; it does not necessarily tell you what the glaze will look like on ware. These mugs demonstrate that. They were fired using the C6DHSC schedule and are made from buff-burning M340 (left) and M390. The mug on the right also has a heavier, less-even application. Surprisingly, some of the thickest runs become more transparent rather than more opaque. That is a reminder that the pale, variegated character of this glaze is not simply the result of titanium acting as an opacifier; it depends heavily on melt thickness and titanium-driven crystallization during cooling.

The objective is to discover the ratio of Redart to ball clay that reaches near-zero porosity by cone 10, with minimal bloating and rich color. And with high plasticity. This is stage 1 in creating a new body; nine more tests eventually achieved L3118J.

These test bars were fired at cone 10R and 11, 10, 9 and 8 oxidation (top to bottom). It reaches zero porosity by cone 8 and blistering is already happening there (which makes measuring fired shrinkage or porosity beyond that pointless). However, the cone 10R bar is not blistering. It clearly needs less fired maturity. A real bonus: Plasticity is fantastic yet drying shrinkage is well below 7%! In the final body I ended up using 20% Redart, it still imparted good color and near-zero porosity by cone 10.

This is L3118J, a chocolate brown burning super-plastic base clay (to which 25% coarse grog is added). The base matures at cone 6, yet the grog stabilizes the fired matrix enough that it stands up in the kiln at cone 10R, firing to a dense vitreous product (its porosity of 4%, comes from the grog). Only 20% Redart clay provides both the color and the K2O flux (no feldspar is needed). There is a second factor that makes this clay functional: An engobe to smooth the surface under the glaze. These two mugs employ L3954N (on the inside applied at the leather hard stage), they enable the smooth surface needed for the glaze. The glaze is brown and blue colored versions of G2571A.