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).

3D print four of these and glue them together to make a large cookie cutter for producing slab-built mugs. 3D print the cup, fill it with plaster and remove the PLA using a heat gun. Roll out a thin slab of clay, press the cutter into it using a round wooden batt, make sure it is not sticking to the board and flip it over onto the plaster form. Handles can even be attached while it is on the form. If clay is plastic it can be used quite stiff. Experiment, adjust sizing and dimensions and reprint to fine-tune.

This has been drawn "parametrically" with OnShape. I only had to draw half of one of the quarters (I mirrored that, extruded and then did a circular pattern of 4). The downloads page (link above) has a free link to view this drawing at OnShape.com. To 3D print, right-click the part (from the list on the lower left, "Quarter 1" and "Cup"). Choose the Export option and select 3MF as the file type (it should go to your downloads folder). Open it with your slicer and print (turn the quarter over and make four). You can even export in formats that other CAD software can open. Better yet, import it into your OnShape account to see the design history and change multiple aspects of the geometry in the variables panel (the drawing will adjust automatically). Other dimensions (e.g. cut depth) can be edited manually. Parametric design is revolutionary and it is now accessible even to hobbyists; it fits my try-it, adjust-it and try-it-again way of working.

A search for "ceramic stain toxicity" shows 19 digitalfire hits out of 32. Pretty well all of these pages refer to the increased toxicity of metal oxides over stains. No alarm. However, a search for "manganese dioxide toxicity" (lower right) is alarming (with disturbing words like neurotoxicity, movement disorders, Parkinson’s, distonia, liver disease, iron depletion, etc.). So then, why are so many potters still using recipes for black that contain high percentages of manganese dioxide? Although many black glazes use a combination or iron oxide and cobalt (e.g. 10%/2%), that does not work as well so manganese is still commonly employed (along with other metal oxides). There is a better way: Black stain in a proven base recipe (like GA6-B, G2934, G2926B), as little as 4% is possible. The decreased bioavailability of stains and and the much lower percentage needed make them a no-brainer for coloring glazes more safely.