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).

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 right now 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.

Fit? It has to stick well. And stay stuck during drying (and shrinking). And the bond has to survive shrinkage that happens during firing. This potter is doing thick applications of each slip (actually that makes them engobes). She uses stains, that's wise, metal oxides bring baggage when used to color slips (e.g. their decomposition can affect the bond, they can gel the slurry, flux the fired product thereby increasing the firing shrinkage of the slip). Stains are better because they affect slurry and fired properties less. But there are still enough issues that each colored slip deserves testing. This potter first slaked B-mix as a slip (it is highly plastic), using it at a runny yogurt consistency. But it bubbled when fired hotter than a cool cone 6. A switch to porcelain slip (which is non-plastic) is shown here. It flaked off as it dried (even in a damp box for 24 hours), also after bisquing to cone 08, and sometimes even after firing to cone 6. This signalled a drying mismatch between body and slip, the bond that managed to survive drying was weakened enough to fail on firing.

The solution was an engobe recipe that is super plastic and sticky. The popular Fish Sauce recipe is an example, it contains 10% bentonite and is unbelievably sticky. We formulated L3954B with this in mind. It adheres well to leather-hard clay and doesn't flake off (misfit is instead evident by surface cracking if its shrinks more than the body). And it is not highly vitreous, keeping its fired shrinkage low enough to match stoneware bodies. Mixing your own recipe also enables compensating the amount of feldspar if the stain affects the slip's degree of vitrification, and therefore fired shrinkage (e.g. blues, oranges, yellows).