Watch the G1214Z video to see me convert the G1214M cone 6 clear base into G1214Z cone 6 calcium matte using simple glaze chemistry and recipe logic. This first appeared in the Digitalfire desktop Insight instruction manual 30 years ago. It is an understatement to say that this process is interesting if you want to know more about glazes, their chemistry and recipe logic. Watch this video and see me adjust the recipe of my high-calcium transparent cone 6 glaze to convert it into a calcium matte. In an Insight-live.com account, the process is easy enough for anyone. We'll cut the Si:Al ratio, increase the CaO, maintain the thermal expansion for glaze fit and make the recipe shrinkage-adjustable using a mix of calcined kaolin and raw kaolin. We will even compare it with the High Calcium Semimatte from Mastering Glazes.

Ceramic glazes, like this GA6-B, are actually just glass. But they are not like bottle glass. The latter is formulated to work well in forming machines (harden quickly), melt and stiffen quickly, have low melt viscosity and resist milkiness and crystallization on solidification. The chemistries to accomplish this have adequate resistance to leaching and adequate durability for a few uses. A stoneware glaze melt needs to be much more viscous (to stay put on vertical surfaces). And, it must have a lower thermal expansion (to match common clay bodies). And, it must resist crystallization much more (since it cools slowly). Fortunately, meeting these needs brings along big benefits: Greater durability, hardness and resistance to leaching. Stoneware glazes and bottle glass share a common trait: They have about the same amount of SiO2. But the similarity ends there, stoneware glazes have:

-High Al2O3. Three to five times more! It is the key oxide for durable glass. And it stiffens the melt (that disqualifies high levels from bottle glass).
-The same fluxes (CaO, MgO, K2O, Na2O). But they distribute very differently (half the CaO, half to one third the KNaO, much more MgO). Other fluxes like SrO, Li2O are also common.
-Low KNaO (which they call R2O). In glazes, it produces crazing, 5% is a typical maximum. But bottle glass can have double or triple that (the high thermal expansion is not an issue, and its cheap source materials supply lots of melting power).
-B2O3 melter. It is expensive but can be justified because the glaze is just a thin layer. Glazes at the low end of the stoneware range have 5% or more boron.

Far right: A glass bottle. Left: Small test bottles made from dark and light burning stonewares. Third: A production ceramic bottle. Notice how much the dark body darkens the GA6-B glaze.

I have seven open side-by-side. There are hundreds of them, and all are well-documented with test results and photos. There are glazes, engobes, bodies, materials and special-purpose recipes. All of them are ones that have been shared over the past decade from our Insight-live.com account. These are great to open beside recipes you are evaluating or testing, it can be a real eye opener to see the chemistries and recipes compared.

These videos from Eastfork Pottery demonstrate their use of thixotropic glaze slurries. Watch them to see how effective a highly gelled glaze is. It enables a quick dip, stays fluid while draining, gives even coverage and dries in seconds. These don't hard-pan or settle out in the bucket either. They work on porous or dense bisque. Almost any glaze can be thixotropic if you take the time to learn how to do it. The fast drying enables the use of twin running (or twin belt) foot wiper machines (best shown on these Instagram and Facebook videos).

If your pottery glaze is doing on drying then it will crawl during firing. Wash it off, dry the ware. Then check the water content. If the glaze has worked fine in the past then it is likely going on too thick because the specific gravity is too high - just repeat cycles of adding a little water and dip testing (make it thixotropic if needed). But that was not the issue here. Glazes need clay to suspend and harden them, but too much clay means trouble. This was Ravenscrag Slip, a clay, being used pure as a cone 10R glaze. The glaze appeared to go in perfectly and it dried to the touch in ~20 seconds. But shrinkage continues after that, revealing after a couple of minutes. Fixing the issue was a matter of adding some roasted Ravencrag Slip to the bucket. That reduced the shrinkage and therefore the cracking. Any glaze containing excessive kaolin can be fixed the same way (trade some of the raw kaolin for calcined kaolin). Some glazes that contain plenty of clay also have bentonite - a simple fix for these is to simply remove the bentonite.

Although Nepheline Syenite and Custer Feldspar are used as effective body maturing agents and fluxes in glazes past cone 6, curiously, neither of them melt well by themselves. Thus, both of these come 6 melt fluidity tests add 20% Ferro Frit 3134 to get them flowing. This is a 2021 shipment of the feldspar and a 2022 shipment of the nepheline.

Most low-fire bodies contain talc. It is added for the express purpose of increasing thermal expansion. The natural quartz particles present do the same. These are good for glaze fit but bad for ware like this. There are also sudden volume changes associated with cristobalite, but it forms (from quartz) at stoneware temperatures so should not be a concern in terra cotta or a white low fire body. You could fiddle with the clay recipe or change bodies, but better to change the firing schedule. The quartz in stonewares goes through a sudden volume change between 950-1150F on the way down. Quartz particles in low fire bodies will do the same. A simple fix is to slow down the entire cooling cycle like this potter did. Or, learn to program your kiln to approach this range more slowly, then ease down through it. No electronic controller? Learn a switch-setting-schedule to approximate this down-ramp (buy a pyrometer if needed).

Potters love plastic clay. On the wheel it enables pulling larger, more overhang, thinner walled pieces. For beginners it can make the difference between success or a collapsed lump of mud. The downside is high drying shrinkage and danger of cracking. But potters know how to exercise care in drying to get success anyway.

This industrial jiggering machine has the opposite priority: Ability to hold shape immediately after forming and to dry crack-free quickly. The secret is low plasticity stiff clay (notice how it splits around the edges when flattened). Notice, in the video, how much water is used yet it does not stick to the heated metal mold. Note also how the machine avoids tearing it by applying pressure slowly right to the end. Even then, the vertical splitting on the outer belly and the crumbly way it cuts verify its poor plasticity.

These have just been thrown on the wheel. I find it to be a foolproof method of comparing the plasticity of two clays. They were slurried up and dewatered to about the same moisture content and the same amount was thrown to compare the size achievable. While the Grolleg is stickier and dewaters a little slower, it is not nearly as plastic as EPK (which itself is not that plastic compared to others). Curiously, New Zealand kaolin (halloysite) is quite a bit less plastic than the Grolleg but it responds to plasticity augmentation (in porcelain recipes) just as well as Grolleg (similar amounts of bentonite producing similar plasticities). And, bodies containing EPK also need about the same amount of bentonite to produce plasticity suitable for throwing large forms. So, the plasticity that a kaolin appears to have by itself is not completely indicative of what it will contribute to a body (if augmented with bentonite). The EPK used here is the darker and more plastic of the two varieties Plainsman receives.

A restoration project faced a tile-matching challenge. At installation in a bathroom 90 years ago, the tiles were not crazed. But between then and now it happened (shown inset upper right). Now, a restoration specialist is tasked with duplicating the aged effect (one unsuccessful attempt is shown here). The shade, opacity, degree of matteness, bubble-free matrix and surface character of the original are all real challenges. Duplicating the crazing is even more difficult. Why? Matching "time-crazing" with a crackle glaze pattern will be temporary (it will craze much more after installation).

The reason why functional mattes seldom craze can be seen in the chemistry. This chart compares the thermal expansions of the oxides that combine to form the fired glaze matrix. ~80%+ of the makeup of almost all common base glazes (without colorants, opacifiers) is SiO2 and Al2O3 (orange bars). Mattes almost always need a low Si:Al ratio (e.g. below 6:1). The rest is fluxing oxides to melt them (the blue bars + B2O3). Here is the problem with making a crazing matte: Almost all crazing is caused by high levels of K2O and/or Na2O (the top two bars on the graph). But they produce high gloss (as can be seen in this test tile). The main matting fluxes and agents are MgO, CaO, SrO, BaO; they have a low COE (and don't craze glazes). Further, both zircon and tin oxide, the opacifiers needed, also have low thermal expansions!

Other possibilities of making crazed matte:
-A matte glaze can have a high SiO2:Al2O3 ratio and craze if it is very melt fluid (containing lots of KNaO) and cooled slowly so that micro-crystals cover the surface. The downside is unpleasantness to the touch.
-Glossy glazes can be matted by the addition of micron-fine alumina (e.g. 800 mesh, this is done in the tile industry).
-A low expansion body with no ball clay or silica (e.g. just kaolin and feldspar with enough bentonite to get the needed plasticity) will craze most glazes. Adding pyrophyllite will further lower its COE.
-Print the lines on the tile (using ceramic transfers) and use a translucent matte glaze (like G2934).