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 we receive.

Super white translucent porcelains are expensive, approaching $200/box in some countries! Even so, variation in properties is common (certainly not good for a "tipping point body" that is difficult to make). The idea of making your own clay body is actually feasible here. Yet Kirsty Kash, maker of this mug, told me that this amount of DIY was something she had never really considered, thinking it would be too complicated without guidance. But ongoing issues with the commercial clay gave her the motivation to give it a try (using a recipe similar to L3778D). She weighed out the materials, slurried up the mix using a propeller mixer, finished by blender mixing and then dewatered on a super-clean plaster bat.

By the third batch, something good happened: The porcelain blistered (tiny bumps on the surface). Subsequent correspondence brought the realization that this type of body is "walking a recipe tightrope" that requires control of the percentage of the key ingredient: Feldspar. It determines the maturity of the fired product. Too much brings blisters, too little and translucency is lost. Simple testing is all that is needed to determine the needed amount. More good news: The change enabled increasing the kaolin percentage by the same amount. That, in turn, enabled reducing the percentage of veegum (reducing the cost).

Her comment a few days ago was inspiring: "I'm getting to know my material so intimately. I have been learning SO much." When I suggested she might end up buying commercial again she responded: "I just bought large bags of all the materials and plan to keep going. I like having the control and being less reliant on the boxed clay. You've converted me!".

Pyrax (Pyrophillite) is a mineral having a very low thermal expansion. It stands to reason that if we can maximize its percentage in a body and not fire the body to a point that changes the crystal structure, it will be resistant to thermal-shock cracking. To that end, I mixed it with only kaolin (ball clay would add some quartz that would increase thermal expansion). I made slip-cast pieces of similar thickness for testing. I fired them to cone 2 (after finding that by cone 4, shock-resistance begins to decline). As you can see from the video, the addition of grog actually harms the performance! The higher the Pyrax, the better. To get a real appreciation for how well this body endures differential heat stress, wait till the end to see how a glazed porcelain piece compares.

By itself, without copper, the G2926B recipe (right) produces a better and more durable glass (comparing the cups in the back). But a 2% copper addition, front, turns its surface to a mass of unhealed bubble-escapes. The G3808A recipe, on the left, develops much more melt fluidity, the extra mobility enables the bubbles, created by the decomposing copper, to coalesce, grow, break at the surface and heal before the melt stiffens too much.

This is one of the things Gerstley Borate commonly does (when its percentage is high enough). It is also highly thixotropic - this can be stirred vigorously to thin it, yet within seconds it turns back to jelly again. This is part of the reason it is often referred to as “ghastly borate”. Side effects of this include high water content, slow drying, excessive shrinkage on drying, cracking and crawling. To attempt a fix, I deflocculated it with Darvan. It was stable enough to dip bisque ware, with difficulty (but pieces dried very slowly). But overnight it has turned back into a gel. What can be done with a mess like this? Start over, with three options:
1. Identify the mechanism to isolate the base recipe and substitute another.
2. Replace the Gerstley Borate with an equivalent that does not do this (Gillespie Borate).
3. Do a little chemistry to source the B2O3 from a frit instead (e.g. in an account at insight-live.com).

In 2024-25 my wife and I have been on three cruises from Vancouver. Our driving speed of 2-3 days from here (there and back) slows to 4-5 days when we visit potters along the way. The trip this fall was awesome. I cannot believe how resourceful, determined and talented some of our long-time customers are. And I did not stop to think how long some of them have been studio potters (more than 50 or even 60 years). We try to spread our morning coffee out over these and others I have gotten recently (with a preference for those made from Candian clays, of course). It's a real joy to have served you all. We have more trips planned this and next year, so I hope to see many more of you.

Engobes can be incredibly opaque. This very thin layer of L3685Z2 completely covers these terra cottas (L210 and L215). It's color is whiter than paper! Using my G1916Q and G3879 clear overglazes, ware can appear as white as porcelain! But notice there are tiny cracks in the white on the edges of contours (most noticeable on the left sample). It appears to work well when applied to the bisque (because I added CMC gum), but during firing, it shrinks 2%, putting it under tension (the body had already shrunk during its bisque). If it were applied to the leather-hard ware that would not fix the problem. Why? Because the body shrinks 4%, that would put it under compression, looking for opportunities to flake off at edges (e.g. rims of mugs). How to optimize this? The engobe needs about 3% Ferro Frit 3110 to raise its firing shrinkage by 2%. And, to be applied to leather-hard ware.

The mug on the right is terra cotta slipware fired at cone 04 using underglazes and a leaded transparent over-glaze (lead glazes are still commonly used in many parts of the world and considered safe there). Mug on the left: This potter wants to use the same technique on cone 6 stoneware. This is a typical transparent glaze (fluxed using a frit or Gerstley Borate). The result is micro-bubble clouding, boron blue, washed-out colors and surface defects. Because it is a dipping glaze it went on too thick and didn't cover well over the colored brushstrokes. However, achieving better warm browns is possible. A more refractory underglaze (made with stains, not iron oxide) that does not bleed. A more fluid melt transparent glaze that is better able to shed bubbles. A drop-and-hold firing would reduce surface defects. Finally, careful control of the glaze thickness and quality of laydown. To achieve the latter, it might be worth preparing the transparent as a brushing glaze, at least for application on the outsides (enabling a dense and even laydown over the whole surface).