Watch the G1214Z video to see me convert the G1214M 20x5 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.

To get a better, more brilliant transparency than G1947U, at cone 10R, I tried sourcing Na2O from a frit. The inside surface on this mug is the result (v2), it is so glossy it looks like a low-fire leaded glaze. This screenshot of side-by-side panels shows the frit-enhanced version(s), G3910 and G3910A. Frit 3110 has a much higher concentration of KNaO than the feldspar, a key to making a brilliant glass. While this increases the COE, the fact that it comes from this frit brings multiple benefits:

-A brings a little B2O3, a better melter than zinc (which volatilizes anyway at cone 10).
-It dissolves more low-expansion glass-forming SiO2 (in v2 I switched to 400 mesh silica to enhance that even more).
-The glaze can then tolerate some low-expansion MgO (introduced by Frit 3249 or its equivalent, Fusion Frit F-69).

More details about this project can be found by clicking/tapping the Insight-live recipe code numbers G1947U, G3910 or G3910A. The red outside glaze is G2571D1.

These tiles were fired at cone 6. The melt-flow comparison behind them shows how much more freely the G3808A clear base flows than G2926B. G2926B produces a crystal-clear glass by itself and works well with some colorants. But with 2% added copper carbonate, its melt does not clear the bubbles and heal the surface completely. G3808A has enough melt mobility to produce a smooth surface.

That extra fluidity comes at a price: Notice the glaze running down the right-hand tile. To get this clear, glossy copper color while controlling movement, strategies include thinner application, keeping the glaze farther above the foot, restricting it to more horizontal surfaces or using a tested catcher glaze below it.

To melt a frit batch, the first challenge is the crucible and release from it. These disposable crucibles were cast from L4404A refractory slip. The front one has an alumina-based liner L3693E. Without the liner, the ingot in the back is inseparable from the crucible wall. But in the other, the alumina has remained as a powder and can be cleaned off the ingot, leaving almost no residue. This test was done at cone 04, so only a tiny amount of Al2O3 is taken into solution in the melt.

Why would a potter consider doing this? Problems with commercial frit availability and consistency are motivating some to investigate making their own. But there are more challenges. Commercial frit manufacturers control melting conditions to produce a homogeneous glass, then crash-cool it to limit crystallization and facilitate grinding. Their furnaces ensure heat distribution throughout the batch (a real challenge in a crucible in a pottery kiln). But some frit compositions are considerably more resistant to grinding; a studio ball mill might not be up to the task. Producers also melt frits at cone 13-20 (a potter frit-firing to cone 10, what a producer smelts to cone 15, risks producing a partially reacted mixture of glass and undissolved solids, rather than the homogeneous frit intended by the formulation). And their furnaces are in no danger of cracking during firing. And they have expertise to formulate and test for low water solubility (yes, glass having an unstable chemistry can dissolve in water). They also have tight control over the particle size of the finished powder. They also have equipment to control dust during crushing and grinding (remember - that the dust is tiny angular glass shards!).

She is likely getting it from nearby. She knows what works: A mix of sand, clay and organic matter. She isn't even the best mud-nester in the bird world. Some species can stick their nests on to walls and cliffs.

If a bird can find and use a native clay, should a potter or even hobbyist be intimidated by the prospect of doing the same? She doesn't overthink it; she likely just samples different spots and uses the one that works the best. The clay appears to be a typical terracotta; if her nest were fired, it could well come out like these overlaid SHAB test bars (the two types, a sandy and smooth, were fired at cone 4 and 04).

Finding a native clay for pottery can begin just as simply: collect small samples from different spots, see how each one works, and watch how it dries. Then take the step the bird has no reason to take—fire test bars. These show how I compare drying shrinkage, firing shrinkage and fired absorption. A nest proves that mud can be built with. Test bars help determine what that mud can become in a kiln.

If you manufacture clay bodies, bentonite deserves more incoming-material testing than its small percentage in recipes might suggest. Because of its highly concentrated nature, bodies are disproportionately affected by variations in its plasticity, soluble salts, particulate contamination, fired color (and another property we will see in a moment). Consider why it is wise to accumulate test data to establish a reliable baseline against which to compare every shipment.

The top two bars are a Wyoming bentonite fired to cone 8 and 2 oxidation. But they are not made from the raw bentonite alone. The test mix is 15% raw bentonite and 85% of the same material after calcining. The calcined portion acts as a non-plastic filler, reducing the extreme plasticity enough that SHAB test bars can be formed and dried reliably. They provide a revealing picture of the fired properties as a pure material.

The bottom two bars use a different approach: A standardized dilution of 15% bentonite mixed with 85% silica. These make evident its effect on the fired characteristics of a pure white, refractory and non-plastic material. The loss of whiteness is obvious, and the lack of fired specks notable. But another thing still reinforces viewing this bentonite as a “controlled impurity” addition: These bars should be much more refractory, but they are porcelain-like, albeit with high porosity, the bentonite itself is acting as a body flux.

This recipe is pretty far out on the edge for a glaze rescue. But the potter loves the appearance, so it is still worth preserving for decorative or non-food-contact use. Of course, one must go in with eyes open about its application, fit and durability issues. Consider the obvious red flags:

• Almost 30% barium carbonate, combined with SiO2, this raises serious durability and barium-release concerns.
• Only 5.7% kaolin means poor slurry suspension and troublesome application.
• There is no added silica, and the calculated SiO2 is very low for a high-fire glaze.
• Custer feldspar is no longer available to many users.
• More than 43% nepheline syenite supplies a lot of KNaO, and the very high calculated thermal expansion strongly suggests crazing risk on many bodies.

But this glaze has one thing the potter values: it appearance (see image). So rather than discard it, we reworked the material sourcing while keeping the oxide chemistry very similar. I reduced the nepheline syenite substantially and introduced Ferro Frit 3110 to supply much of the KNaO without bringing much Al2O3 with it. That enabled the kaolin percentage to almost triple, from 5.7% to 16.2%, while calculated Al2O3 stayed at 0.37. The result should be a much more practical slurry while giving the glaze a reasonable chance of retaining the character that made it worth rescuing in the first place.

However, the new recipe still has 29% barium carbonate and a calculated expansion of 9.4, so this is not a “fixed glaze.” It is a more usable version of an intentionally problematic glaze.

A potter uses a grogged body that contains a mix of three clays (with small amounts of feldspar, bentonite and silica, likely for tuning fired maturity, plasticity and COE). It fires to a dense light tan at cone 10 (as shown). Although his pugmill does not have a vacuum chamber, until now the body has worked well. For whatever reason, it now has become insufficiently plastic, so some "recipe engineering" seems like the best option.

The obvious response is adding more bentonite. But bentonite works best when the mixing system can thoroughly disperse it, and this pugmill likely cannot do that reliably, even with the 3.5% already in the recipe. Fortunately, physical testing I did years ago on Hawthorn Fireclay, Goldart and OM#4 ball clay provides data that enables comparing their plasticity and fired maturity. Armed with that, we can change the proportions of these clays in the recipe (see the full report by clicking/tapping their code numbers: L2950, L3320, L3254).

The three fire to a similar color, which simplifies things. So here is my test #1: Increase the ball clay by 20% at the expense of Hawthorn, the least plastic of the three. Drop the bentonite. The body will fire more vitreous, so compensate by trading 5% of the feldspar for 5% more ball clay. An SHAB test comparing the original and the new mix will give direction to test #2. Since ball clay is so high in quartz, that test could trade all of the silica for Hawthorn fireclay (glazes will likely fit without it, and the body will be even more plastic).

This mug blends the stages of ceramic production into one and builds appreciation for how magical the ceramic process is. That can help set realistic expectations for production. Consider:

• How can a minority of microscopic clay particles, through their surface charges and enormous collective surface area, bind a mass of other nonplastic mineral particles into something that can be shaped? Using just water as the glue?
• How is it even possible that a handle can become rigid during drying while the mug body continues to shrink and dry, yet no crack occurs?
• How can a clay-containing glaze powder, which must shrink on drying, stick to a smooth bisque surface and not crack off?
• Temperature gradients are inevitable, meaning different parts of the mug are not passing through the same stages of quartz inversion, yet it does not dunt.
• How can a clay body soften as if it vitrifies in the kiln yet still stand up and hold its shape?
• How can two ceramic powders, neither of which melt, melt if they are mixed together?
• How can a glaze melt enough to produce a brilliant gloss and yet not run down off the ware?
• How can micro-bubbles in a glaze on a vertical surface find their way to the surface, break and yet the surface heals behind them?

These things seem impossible or improbable, yet ceramics does them routinely. That should encourage us to understand the mechanisms, measure the variables that matter and build enough margin into our processes that normal variation does not push them over the edge into failure.

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 DIY 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.