This is a GBMF test, it compares the melt fluidity of the Gerstley Borate based cone 6 Perkins Studio clear recipe original (left, our code number G2926) and a reformulated version that sources the boron from Ferro Frit 3134 instead (right, our code number G2926A). The latter is less amber in color (indicating less iron). The good news was that it melted so much better that we were able to add significant Al2O3 and SiO2 to really drop the thermal expansion (improving glaze fit on common clay bodies), which produced our G2926B base recipe. Every time I use it I think of how unfortunate we would have been had we continued to use the Gerstley Borate original.

Gerstley Borate is a ceramic glaze flux, sourcing boron to melt far better than any other common raw material. It has been a foundation material in low and middle temperature pottery glaze recipes for many decades. Potters have a love/hate relationship with it: Enjoying its low melting point but enduring its problems (inconsistency, gelling of the slurry, crawling, micro-bubbles, boron-blue discoloration). Strangely most people have used it without knowing what it really was. And few realize how easy it is to replace. Yes, existing substitutes work sometimes - but it is better to adjust each glaze recipe to source boron from a frit (fixing other issues also). How? Visit gerstleyborate.com.

Luke Lindoe was a prolific glaze tester and developer, especially for cone 10R. We have finished taking and cataloging pictures of a treasure trove of glaze samples from Luke's testing from the 1970s to the 1990s. On one hand, Luke loved flashy copper reds and barium blues. On the other hand, he revelled in rustic surfaces of bare fired clay and matte earthtone glazes to complement them. Every one of the many hundreds of specimens was code numbered and we already know the recipes of some. We are confident that other cataloguing work will uncover more of his notes and provide clues and information to derive the recipes on many of the samples. The ultimate goal is to make all the pictures and documentation available on line for students and potters.

These mugs were fired at cone 10R. The body is L4168G5, I mixed it myself using 50% Plainsman Saint Rose Red, 40% Plainsman A2, 10% Custer feldspar. The Saint Rose clay contributes the color, the A2 the speckle and plasticity and the feldspar matures the body enough to avoid black coring. The heavy iron specking is being sourced by these very unique clays, both were ground at 42 mesh only. The left glaze is GR10-CW Ravenscrag Talc matte with added Zircopax. The right one has that same glaze on the inside and G2571A bamboo matte on the outside. The unglazed body is a beautiful deep red. These are certainly not porcelain strength but the glazes fit, the mugs are durable and serviceable for normal use. This type of ware is the domain of potters only, no industry would be able or even want to make them.

This body has high porosity, almost 25%. It is L4410P, a dolomite-based low-fire whiteware, Plainsman Clays makes this as a product named "Snow". But this high porosity has some advantages, one of them is that it soaks up silicone sealer very well (called "liquid quartz" by some). The slip-cast piece on the left was sealed (you can see the surface sheen) and it is impermeable to water penetration (the glaze is not crazed so water cannot penetrate there either). The piece on the right soaks up water readily (on the lower unglazed portion). Sealing this specific body is doubly important because the dolomite particles within can rehydrate over time, especially in damp climates, causing pieces to crack. Even the foot rings of functional pieces should be sealed, not just to prevent hydration but also waterlogging.

This glaze is G3933A, the body is Plainsman M390. The one on the right is more matte and has a richer brown color, definitely a nice surface. The left one was fired using the PLC6DS drop-and-hold firing schedule. But the one on the right used the C6DHSC firing schedule, that one adds a slow cool down to 1400F. That gave the iron in the body time to bleed up through the glaze. And it gave the MgO time to do what it does best: Create a pleasant matte surface.

This is GR6-L, is the standard GR6-A Ravenscrag Slip cone 6 base recipe + 10% chrome tin stain (the body is Midstone, the inside glaze is G2926B, the firing schedule is C6DHSC). Chrome tin stains are picky about their host glaze, if it does not have a compatible chemistry they fire grey. Obviously, there is a love affair going on here! But the mug on the left has an issue. The glaze on the left has gone on in varying thicknesses and these are producing crystallizations and runs and the incising is not being highlighted. The one on the right is under control. What is the difference? The rheology of the slurry for the bad mug was wrong - the specific gravity was too high (the water content was too low). Even on a quick dip it was building thickness unevenly and way too fast. And there were drips that were so big they had to be shaved off with a knife! After the addition of a lot of water, to take the specific gravity from 1.55 to 1.45 it was watery enough to accept some Epsom salts to make it thixotropic. The difference was amazing, it went on totally smooth without a single drip, producing the result on the right.

The glaze is G3933A, the clay body in Plainsman Coffe Clay. The front tile was fired using the C6PLST schedule that just goes to cone 6, holds for a short time and then free falls. The body is stained with raw umber, that material has a high LOI and gases right around cone 6. During the short hold at cone 6 the glaze percolated and foamed up with bubbles. The shut-off froze that in place. The mug was fired using the C6DHSC drop-and-hold and slow-cool-firing. The drop-and-hold shut off the gas expulsion the umber and gave the glaze a chance to shed the bubbles. The slow cool gave it lots more time to smooth out and heal every single pinhole.

Two mugs have dried. The local terra cotta native clay on the left shrinks 7.5% on drying, the porcelain one on the right only 6% (it is made using Kentucky ball clay). Yet few pieces of the terra cotta are ever lost due to drying cracks, even if it is uneven! For example, in a batch of a dozen mugs none of these will be lost whereas one or two of the white ones will always crack. Why? Dry strength. The clay on the left is very strong in the dry state, likely double or triple the white clay (the strength is a by product of its high plasticity and particle size distribution profile). That strength is enough to more than counter the extra shrinkage.

The body: M370. Glaze: G2934Y (with added green stain). Firing: Cone 6 drop-and-hold. Glazing method: dipping (using tongs). Thickness: The same. Surface: Clean on both. The difference: Wall thickness. The one on the right was cast much thinner so the glaze took a lot longer to dry. Common pottery glazes contain clays which need to shrink somewhat during drying. The bond with the bisque, although fragile, is normally enough to prevent cracking during drying. But drying needs to occur quickly. Quick drying is only possible when the body has enough porosity to absorb all the water quickly. Otherwise, cracks appear and these become crawls during firing. A complicating factor is that stain and/or zircon additions make an already-crawl-susceptible glaze even worse. One or a combination of the following can be done to minimize crawling on even very thin-walled pieces: -Apply a thinner glaze layer. -Heat the bisque before dipping. -Glaze the inside and outside separately (with drying between). -Deflocculate the glaze to reduce water content. -Brush or spray it on in multiple coats.