Consider the hazards and hassles before choosing a black matte or gloss recipe that has high individual or combined percentages of manganese dioxide, cobalt or nickel.

Gloss blacks: These are super popular as the base for layering of reactive glazes. DIY dipping versions thus make a lot of sense. They make even more sense when they don’t turn to jelly in the bucket because of the high percentage of red iron oxide in all blacks made using metal oxide colorants. And when the total percentage of pigment is as high, or higher than 15%. And when the pigments cause crystallization (especially when overloaded).

Matte blacks: The human eye can detect even slight differences in the degree of matteness (which is very difficult to keep consistent). Raw metal oxides affect the matteness, especially when overloaded with pigment. They are prone to cutlery marking if too matte. By using stains, manufacturers and even potters have learned to tune recipes (lower left) and firing schedules to achieve consistency and functionality (even tourist souvenirs (lower right) feature them now). With stains, only one material is producing the color, its percentage (which can be as low as 4%) can be tuned.

This is G2826A3, a transparent amber glaze at cone 6 on white (Plainsman M370), black (Plainsman 3B + 6% Mason 6666 black stain) and red (Plainsman M390) stoneware bodies. When the glaze is thinly applied, it is transparent. But at a tipping-point-thickness, it generates boron-blue that transforms it into a milky white. Glazes that are very glassy but on the edge of structural instability do this. So they are not good for functional ware.

This is an adjustment to the 50:30:20 Gerstley Borate base recipe (historically used for reactive glazes, often on functional surfaces! This cuts B2O3 and adds significant SiO2. But it still has double the boron of a typical functional glaze. While the chemistry of the original was within the territory of boron blue development (relatively low Al2O3), this one is better because of the increased SiO2 (the high MgO:CaO ratio is likely also helping). Boron blues like the lower Fe2O3 content or Gillespie Borate. One more factor: I am using 325 mesh silica here, it dissolves in the melt better.

Would you like to be able to use your own found-clays, ones native to your area or even your property, in your production? Follow me as we evaluate a mystery clay sample provided by a potter who wants to do exactly this. I will use ordinary tools that any potter either already has or can buy at low cost. We will describe this clay in terms of plastic clay bodies and common ceramic materials that most potters already use. The potter who submitted it has worked enough with the material to suspect it has potential and he wants to know how to best utilize it (e.g. at what temperature, with what glazes, mixed with what, processed in what way). In technical terms what we are doing is called "characterization".

This artisan, Dennis Cuku, is the king of DIY tile, making "actually HANDMADE" product using a red-burding terra-cotta-like middle temperature clay body. He also makes glazes in-house and fires using 36 shapes. He mixes 129 glazes and produces about 50,000 ft.² of tile per year. Tile making presents many unique challenges, not the least of which is the need for consistency and predictability of surface character and color. This endeavour is made possible with data, a lot of it. Not just glaze recipes, but many forming, glazing and firing procedures and techniques that must be documented.

Watch this 30-second video to see. Gelled (thixotropic) slurries for dipping are so much better to work with; you'll never go back once you have mastered this DIY technique. While some glazes and engobes gel naturally, especially those with high clay content, these almost always work best when the water content is within a certain range, so fine-tuning like this is still needed. Although not shown here, if over-gelling happens, a drip or two of deflocculant (e.g. Darvan) brings back the fluidity, this is more likely to happen with engobes since they need more gel (for dipping and even more for painting). A side benefit of this: No settling in the bucket.

This reduction stoneware glaze is producing white streaks on some pieces (left center). The body is a coarse iron stoneware. A magnification is needed to better explain this.

It is 2025, many smartphones now have dedicated macro lenses and can be held as close as a 1 centimeter. They automatically sense placement and switch to using the macro lens. Of course, the phone must be held rock steady and good lighting is essential. If you are a doubter of what they can produce, look at the two magnifications on the right. On the top one, the white streak is clearly visible, floating in a sea of phase-separated glass patterned by earlier-escaping bubbles. The extreme magnification on the bottom right appears to implicate tiny crystals growing in an area where late bubbles have escaped, changing the pattern of phase separation. This doesn’t yet explain the cause, but it is valuable information courtesy of a macro lens.

This is G3948A (similar to the popular Amaco Ancient Copper product). To get this stunning result, it needs to be applied thickly. Therefore, it runs a lot. But the catcher glaze around the bottom of these mugs has stopped the flow. The catcher is a glossy black, G3914A (but Amaco Obsidian would also likely work). I have learned to put it on with the right height (about 2cm) and right thickness, and then apply wax emulsion to prevent the iron red glaze from sticking during dipping. The inside glaze, G2926B, is one I have tested and developed to fit Plainsman clay bodies as a liner.

Watch the G1214Z video to see me convert the G1214M cone 6 clear base into G1214Z cone 6 calcia 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 calcia 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.

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.

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).