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.

This is L4588 red burning porcelain. I got the idea after coffee break and was rolling and pressing specimens right after lunch. How?

The major reason is not being intimidated by mixing body and glaze tests. Another is having all the tools and materials close at hand (shown here). Third is a good record-keeping system where I make sure to enter results, notes, pictures and links to related tests at every stage of a project.

• 10.30 am: Got the idea to make this, created a record in my Insight-live account, code-numbered it and printed the recipe.
• 10.35 am: Started weighing it out.
• 10.40 am: Blender-mixed it really well to remove all agglomerates.
• 10.45 am: Poured it onto my plaster bat.
• 12.30 pm: It had stiffened to the perfect consistency to roll a couple of SHAB test bars and press and stamp some porcelain theme-rocks.
• Next day: In a cone 6 kiln load.
• Next day: Incredible red porcelain.
• Following weeks: Study the results and decide what recipe changes would improve it for round 2.

This body is 70:30 kaolin:feldspar (fired at cone 6 with glaze G2926B). The fired body has a vitreous porcelain-like surface. But, this dense craze pattern indicates a very serious fit problem. The COE (thermal expansion) of this body is much too low. There isn't a glaze-fix solution to this. Reformulating it enough to fit, while maintaining the firing characteristics, would be well beyond the expertise, material availability and budget of any typical pottery or manufacturer. All that work would just boil down to "compensating for an unusually low-expansion body". Adding quartz to the body to raise its COE is the appropriate conventional solution. An initial test would be to substitute 25 of the kaolin for 200 mesh silica. It is refractory, and while the finer particles will be dissolved by the feldspar, enough of the larger pure quartz ones will remain to impose their very high COE on the fired matrix.

The other issue is with the flat particle shape of kaolin. The throwing process has lined up the predominant kaolin particles concentric to the centre. During drying, and especially firing, more shrinkage occurs across them than along them. All ten of the cups, which I made and dried in my normal, careful fashion, cracked like this! The solution is adding a filler, one with non-platy particles. Silica is perfect; the grains act like aggregate in concrete, strengthening the matrix and separating the clay particles, forcing them to orient more randomly.

In order to add 0.5% of CMC gum to this glaze I need to calculate the water content to know how much powder is present. Other scenarios for this are adding Frit 3135 to cone6ify a cone 10 glaze, adding stain to color it, adding zircon to opacify it.

I put a teaspoon of the glaze (well stirred) on the tared tray. It weighed 5.04g (this 0.01g scale is accurate to 1/100th of a gram). WhenI dried the tray under a heat lamp, it weighed 2.46g. The water content of the slurry is thus (5.04 - 2.46) / 5.04 * 100 = 51.2% (the solids content is thus 48.8%). To blender mix 0.5% CMC gum into this I weighed the jar with glaze at 680g and the jar itself at 35g. Thus 680 - 35 = 645. 48.8% of 645 is 315g of powder. 0.5% of 315 is 315 * .005 = 1.6g of CMC gum.

The ceramic materials we depend on are no longer commodities we can assume will always be cheap and available. Lithium materials can suddenly multiply in price. Cobalt colorants can double. A familiar plaster can disappear. Zircon and stains can jump with global mineral markets. Products made from these (e.g. glazes, underglazes) follow suit. And freight can make even inexpensive clay expensive.

The answer is not to stockpile everything—it is to learn how to adapt. A potter who tests, documents and understands materials can reduce cobalt, replace lithium, qualify a new plaster, reformulate around a discontinued feldspar, make better use of a local clay, and even make his/her own glaze and underglaze. Accumulated knowledge from ongoing testing becomes insulation against both inflation and disruption. Think of your test records as a savings account. Every melt test, firing schedule, recipe adjustment, clay test and photograph is a small deposit. When supply problems or price increases arrive, years of organized observations suddenly become very valuable, giving your choices and options others don't have.

The reason for this rather strange-looking wrinkling, experienced by one potter, is not obvious. But I suspected the high clay percentage in the G1214Z matte base glaze recipe (which increases drying shrinkage, thus cracking, leading to crawling). I suspect these wrinkles are actually partially healed crawling cracks. The high-clay glaze shrinks and develops tiny cracks or locally loses its bond with the bisque during drying. During firing, the fluid melt begins to pull away at these fault lines, but then flows enough to heal most of them. What remains are these unusual wrinkles rather than the bare patches normally associated with crawling.

To reduce shrinkage, I supply the kaolin as a mix of raw and calcined. The original recipe called for 37 kaolin; I supplied that with 20 EPK and 15 calcined kaolin (15 is the LOI adjusted amount). For most people, the original 37 raw kaolin version is too high (unless a high solids glaze slurry is thinly applied to porous bisque). If you need to prove this, glaze identical test pieces with all-raw G1214Z and raw/calcined G1214Z1, at the same measured thickness and SG. Before firing, inspect them with a magnifier, not just when they first appear dry, but again 5-15 minutes later (high-clay coatings can continue shrinking after they look dry).

This pitcher was made by Elora Pottery. Like everyone using glazes that require lithium (from Spodumene or Lithium Carbonate), they are facing the unbelievable price increases these materials are seeing. Staci actually calculated the weight applied to each piece, determining the $$ dip cost! Notice her costs on the photo. During efforts to reduce the spodumene content, I noticed the expensive carbonate colorants and asked if these were causing blisters in the glaze (because of gasing associated with their LOI). When I saw this picture, it became clear that they are. But in a good way. Part of the variegation we are seeing is doubtless the mechanical disturbance they cause in the fluid-melt phase-separated glass. But the melt fluidity of the glaze appears to be sufficient to heal them and smooth out the glass during cooling.

What did we do about the spodumene? We calculated to use lithium carbonate to supply the Li2O instead - it took one fifth as much.

Spodumene is super expensive; one dunk in a dipping glaze can now cost dollars. But lithium carbonate is even more so. However, both are used as a source of Li2O. The latter is 6+ times more concentrated. And spodumene is more troublesome in glazes (issues with settling, dusting, slurry properties, consistency). And it can be hard to get. So using the pure lithium material might be a better choice for you.

This side-by-side calculation seems to indicate that 15 Lithium Carbonate, 55 Kaolin and 30 Silica can substitute for 100 parts by weight of Laguna Spodumene Substitute. However, that is not quite correct. Note that my substitute recipe calculates to an LOI of 16.3. If I assume the Laguna material has no LOI (for convenience, it actually has 0.2%), that means each of the amounts need to be divided by (100-16.3)/100=0.84. Thus, to substitute for 100 parts of spodumene, you need ~120 of this mix (18 lithium carbonate, 66 kaolin and 36 silica).

These plates are made at the HomeVSS factory in China. The plate is placed on a soft rubber-surfaced spinning wheelhead that is rotating fast enough that a ladle full of glaze poured into the center is expelled within a second and dries within four seconds. At the two-second mark, a sponge is used to catch drips and even coverage on the lip. The thixotropic character of the glaze enables the quick drying (since no CMC is needed). The plate is finished by a follow-up bottom-dip, using a suction cup holder. Finally, the lip is finished using a brush with the same glaze slurry. Watch this on Instagram using the link below.

The Zircon needed to opacify glazes has a side effect: Crawling. Even low percentages can induce this issue when other factors interplay (as is the case here). It often happens at sharp concaves (e.g. inside bottom corners on mugs). But here it is on surfaces disturbed during formation of the spout. This G2934 recipe has good melt fluidity, so that is not the issue. The issue appears related to adherence of the initial laydown. Here are some questions to consider:

• Did the glaze crack on drying? Using some calcined kaolin, as specified in the recipe, helps reduce drying shrinkage.
• Is it applied too thick? If mixed as a thixotropic slurry (1.44 specific gravity, and gelled) it should go on evenly but thinner. And stick better.
• These areas that were disturbed during spout forming may require special attention to make sure the glaze adheres well (this is a Plainsman body, soluble salts that migrate to the surface during drying are affected by handling in the plastic and leather-hard states). Mix up some as a brushing glaze and apply a thin layer onto the bisque in the areas likely to crawl (the gum will ensure better adherence). Then dip the whole piece in the dipping glaze version.
• An 80:20 blend of G2934 and G2926B glossy will be less likely to crawl. The mix can be adjusted to enable tuning matteness to a drop-and-hold firing schedule.
• Use tin oxide as the opacifier instead of zircopax or superpax.
• Mix the batch as a base coat dipping glaze, then it will adhere better to the bisque.
• Reduce the amount of calcined kaolin (in the recommended recipe) in favor of raw kaolin (perhaps 5%) - that may produce a slurry with better coverage and adherence.