This is a cone 6 transparent fritted glaze (converted from a Gerstley Borate one). Its B2O3 content is high, sourced by Ferro Frit 3134. Bubbles in transparent glazes often plague potters; many just keep trying new glazes until one works, or give up on never finding one. Why not try to fix this problem?

The calculated thermal expansion of this glaze is 8.8, very high because of its high KNaO and low SiO2. These plates are unglazed on the underside. The upper one has a well-fitting glaze and its base remains flat. On the lower one, the high-expansion glaze on the inside contracts much more on cooling and, because it is bonded to the body, ends up in tension. The resulting stress is enough to pull the plate out of shape, bowing the base upward. This issue is common with ceramic tile, where an otherwise flat tile bows as the two layers contract differently on cooling.

A glaze this far into tension will also be highly prone to crazing. And its combination of high alkali and low silica raises another red flag: chemical durability. The solution is to reformulate, reduce KNaO in favour of lower-expansion fluxes (among which MgO is most effective), and increase SiO2 as much as the desired melting behaviour and surface will permit.

The chemistry for Custer Feldspar has been updated in the materials at Digitalfire and Insight-live. Until now, the Pacer-published values have been used, but at Ron Roy's request, this has been changed (see the link to his article below). The average of assays done from 2000 to 2012 has been used for the chemistry. The material name has been changed to "Custer Feldspar (post-2000)". However, Insight-live will still find it in recipes when needing its chemistry, since "Custer Feldspar" has been included as an alternate name. The old material's name has been changed to "Custer Feldspar (pre-2000)".

In this image, you can see the material chemistries on the right and each of them being used in a one-material recipe on the left. To verify they are calculating correctly, the calculation of each has been set to formula plus analysis.

If you have defined your own material for this (typically done to set a cost), everything should still work correctly (using your chemistry if you provided it).

The 80:20 GA6-A Alberta Slip base becomes oatmeal-like when oversaturated with rutile and/or titanium (left: 6% rutile + 3% titanium dioxide; right: 4% rutile + 2% titanium dioxide). The rough surface results from excessive crystallization of titanium-bearing phases from the melt during cooling. It is unpleasant to touch, readily shows cutlery marking and is less desirable for functional surfaces. In glazes of this type, rutile additions above about 4% can become increasingly difficult to keep smooth and consistent. Crystallization, rather than phase separation, increasingly dominates the variegation mechanism.

One way of avoiding problems is staying below the tipping point, keeping the percentage as low as possible while still getting the desired variegation (of course, that will vary depending on the melt fluidity of the glaze and kiln cooling). Surprisingly, making the glaze more fluid does not necessarily solve the problem. Greater melt fluidity increases ion mobility and can actually promote crystal growth during cooling, producing an even rougher surface.

On the right is L213, a body made using Texas talc and Kentucky ball clay. On the left is L3778H, a translucent cone 6 porcelain made using Grolleg kaolin, Nepheline syenite and silica. These test tiles were in the same cone 04 firing. Since both of these bodies have similar porosities and fired strengths at cone 04, does that mean one could use the porcelain? It appears the answer is yes. Why is that again? Because low-temperature artware clay doesn’t need vitrification; all the bodies are porous.

What could make this work? This crystal clear glaze, G1916QL1, fits the L213 but crazes after a few days on the porcelain. Increasing the firing temperature to cone 03 extends that time to 3 weeks. Cone 02 would likely extend it further. Adjustments to the glaze recipe (e.g. 400 mesh silica and increasing its percentage) would lower the thermal expansion and improve fit even more. This whole scenario is doable for DIY potters who mix their own bodies and glazes, especially for casting. The materials to make the porcelain are kaolin, silica and feldspar, even inexpensive kaolins would produce a body almost as white as this one at low fire.

Industrial spodumene is found in massive quantities across major mining regions like Australia, Africa, and North America. Yet if this 1.5kg rock was pure spodumene, it could be ground and sold for $45 (prices at ceramic suppliers in UK, US and Australia are about $31-35/kg in June 2026). The actual mining commodity price is US$2,100/tonne, pricing this rock at $3. That means finely ground ceramic spodumene is currently selling for 10+ times the mine/concentrate price. The battery industry is responsible; it has effectively repriced a ceramic feldspathic mineral as "lithium ore"!

What about lithium carbonate? Albemarle reported an average industrial price of US$19.53/kg in Q2 2026. So potters currently pay only up to 8 times that! Yet, the processing is formidable: The ore must be crushed and beneficiated, then subjected to a high-temperature calcination that converts alpha spodumene to the more reactive beta form, followed by acid roasting, leaching, neutralization, impurity removal and further conversion.

Fortunately, our kilns don’t care where the Li2O comes from: both spodumene and lithium carbonate have advantages, so understanding how to substitute between them is becoming increasingly important. As evidence that lithium is still important, one potter just purchased a 50 pound bag of spodumene for $1200 so she wouldn’t have to worry about it for the rest of her career!

This social media post is a little shocking: tin oxide at 1/5th the cost! I assume it is cheaper there because South American consumers are much closer to the tin-production and industrial-chemical supply chain. In North America, a potter is often buying a specialty imported ceramic material that has passed from the mine to the smelter/refinery, then the chemical manufacturer, then the importer, then the ceramic-material wholesaler, then the pottery supplier who repackages it into small sizes, documents it with an SDS and carries it on slow-moving inventory. Those services are legitimate costs, but they can dwarf the actual processing cost of converting tin into tin oxide. That being said, other materials share similar costs, why are they so much less expensive?

On a clay test tile, this titanium-opacified/variegated cone 6 oxidation glaze, G1214Z1, looks great. But it is important to recognize that the variegated fired appearance on the tile is a combination of multiple factors: The chemistry of the glaze, the titanium brand and grade and its high percentage (5%), a good quality laydown, the PLC6DS firing schedule, the red M390 clay body below and variations in the thickness. However, most of these factors change with the mug on the right! It is made from buff-burning M340! The C6DHSC firing schedule. Notice how it is actually going transparent where very thick. Notice the glazing drips, the uneven laydown that caused the dripping is the calcined kaolin; it contributes alumina and silica without providing the colloidal clay particles needed for suspension and thixotropy.

Look at the red clay tile again: The glaze naturally breaks thin over high points and pools thick in recesses, so one small tile displays a whole range of effects, almost guaranteeing attractive variation in a small area; smooth functional ware does not do that.

I am looking to match the reduction-fired dolomite matte mug on the right, but in cone 6 oxidation. I must duplicate the glaze color and surface character. But more difficult is the speckle size, color, density and edge bleeding. The speckle is the biggest challenge, but I met that by making my own by mixing the body and G2926B glossy glaze 50:50 and adding 10% black stain. Then I slurry it, dry it, fire it in a crucible (which I make from L4404A with an L3693E liner), crush it by hand using my shusher and screen it. I am using G2934 cone 6 magnesia matte as the glaze on this mug on the left. I add 0.5% minus 20 mesh speck to achieve this result. The next step in this project is to sieve out the smallest specks, switch to the G2934 matte glaze in the speckle recipe (the specks are too shiny here), and switch to dark brown stain instead of black. I will also need to control the degree to which the specks bleed at their edges (by adjusting the glaze:body ratio) and tune the degree of matteness (by adjusting cooling speed of the kiln). A small amount of green or blue stain is also needed to match the glaze color. There is no question that an authentic reproduction is possible; methodical testing with good records is the key to accomplishing that.

Typical pottery porcelains are variations on the traditional triaxial body and commonly contain around 50% clay, either kaolin, ball clay or a mixture. Since the clay fraction is normally the largest source of iron and titanium impurities, its purity has a major influence on fired whiteness—especially in reduction.

These mugs were fired in the same kiln load at cone 10R and have the same clear glaze, G1947U. The mug on the left is a Grolleg kaolin porcelain (50% Grolleg kaolin, 25% silica and 20% Mahavir feldspar, 5% bentonite). The one on the right is 15% M23 ball clay, 40% #6 Tile kaolin, 15% Nepheline, 25% silica, 3% bentonite. The grey is the cumulative effect of the impurities in these clays.

Grolleg is considerably more expensive, but this is one place where the extra purity is plainly visible. Want to push whiteness even further? Use an ultra-low-iron New Zealand kaolin and replace ordinary bentonite with Veegum or another clean-burning plasticizer.