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Blog

A seismic event in ceramics almost happened:

The demise of our plaster supply

Mini beer bottle mold

The ceramic industry depends on some surprisingly specialized materials made by very few companies. When one of those companies exits the market, we need to notice. Consider how much of ceramics depends on plaster: slip casting of pottery and sanitaryware, jiggering, RAM pressing, mold making and countless studio processes. Yet plaster is so inexpensive and familiar that we tend to forget how technologically specialized—and indispensable—it actually is. USG #1 Pottery Plaster has been an industry standard for generations.

The Bad News An example: USG is closing its historic Southard, Oklahoma gypsum operation, apparently ending production of #1 Pottery Plaster, Hydro-Stone, Ultracal and other specialty gypsum products.
The Good News Fortunately, there is another major North American producer: Georgia-Pacific. Its K-59 Pottery Plaster is a remarkably close counterpart to USG #1, having essentially the same normal water/plaster ratio and density, with similar strength and expansion. Without products like these from Georgia-Pacific, the USG withdrawal could have been a genuine supply-chain earthquake for ceramics. Instead, it should be a wake-up call.
Cautious Optimism Georgia-Pacific has a long-established Industrial Plasters division with dedicated sales managers covering both the U.S. and Canada. GP already manufacture pottery and tooling plasters, ceramics is one of the industries they explicitly target. If they decide to embrace the opportunity, they would become the primary North American supplier of specialty ceramic plasters. The biggest uncertainty is whether they can produce enough to replace what USG has historically supplied.

Context: Plaster, Beer Bottle Master Mold..

Thursday 30th July 2026

Color variation in wall tile:

Why are these edges darker?

Look carefully along the edges of these wall tiles. A band of darker blue appears around the perimeter. At first, it might seem that the cause is pooling at the edges during glaze application. But tile factories know all about even application (by bell, waterfall, disc, or spray). Instead, this is probably happening during firing - this is a fluid melt and likely flows slightly, with surface tension causing it to accumulate a little thicker at the edges. While this cobalt blue glaze is somewhat transparent, the thicker layer absorbs more light and appears darker. While potters often value color variation like this, ceramic tile manufacturers work hard to control “edge darkening” by adjusting glaze viscosity, application thickness, and firing conditions to achieve a more uniform surface. If they fail, here is what can happen: When thousands of tiles are laid side-by-side, those darker edges line up and visually create a grid pattern across the entire installation. And they can also make the grout lines appear stronger and the surface less uniform.

Could some glaze chemistry help? Maybe. Al2O3 is a classic melt stiffener; in many glazes, higher levels of it can be tolerated without losing any gloss. However, tile glazes must heal during very fast firing and high Al2O3 can introduce defects (tile people prioritize perfect surfaces over small color variations). It can also make color less intense and reduce thermal expansion (a possible issue with fit and engobe compatibility). In tile plants, engineers usually prefer process adjustments over chemistry changes because they disturb the system less.

Context: In pigmented glossy transparent.., Ceramic Tile

Sunday 26th July 2026

Can bone-dry ware be glazed too thickly?

Yes! When the specific gravity is 1.7.

In testing to determine the best specific gravity (SG) for this Darvan-deflocculated glaze slurry (G2926B), I arrived at what I thought would be great: 1.7. For 2.5kg of glaze powder only 1100g (1100ml) of water is needed, half the normal amount. And the slurry flows well. There appears to be enough water to form a good bond with the body. But a one-second dip of a bone dry piece produces twice the desired thickness, it dries too slowly and it is running. So, what should the SG be? I'll keep increasing the water until I can induce some thixotropy to hold it in place (using Epsom Salts), and it goes on thinner in a quick dip. If thixotropy is difficult, I’ll add 1% bentonite to increase particle surface area. And, I'll document the results at each step for future reference (in my Insight-Live account). Of course, these are going to need help drying, either by preheating the ware or putting the pieces in a drier.

Context: Drip glazing and bare.., Specific gravity, Once-Fire

Saturday 25th July 2026

This surprised me about calcia matte glazes:

Most are highly melt fluid.

Unfortunately, many matte glazes are such simply because they are not melted enough. That is not the case with cone 6 G1214Z tested here. When applied as a thin layer on pottery, it seems normal. But this GLFL test demonstrates that it fires highly melt fluid, much more than one might expect from a matte glaze. A quick look at its oxide chemistry reveals the reason: It is a calcia matte. It has high CaO, so as the melt cools, it combines with SiO2 and even Al2O3 to form calcium silicate crystals (instead of staying in the glass).

What is the G2928C? That is Ravenscrag cone 6 Tri-Matte; it employs three matteness mechanisms: High CaO, MgO and Al2O3. It contains significant Tin Oxide (thus the white color).

Context: G1214Z1, G2928C, Calcia vs Magnesia matte..

Saturday 25th July 2026

Secret #4 of crystalline glazes: They contain little clay.

So without help, they don't suspend or harden on drying.

Brush application of crystal glaze

Potters used to working with dipping glazes that contain plenty of clay are shocked when they see what crystal glazers have to work with: Slurries that contain very little clay because the Al2O3 contributed impedes crystal growth. They end up with dipping glazes conditioned with gum and bentonite that are bummers to work with. But here is a way to mix as a gelled brushing glaze that works surprisingly well. This requires two gums. Let's mix 500ml.

Step 1: Blender-mix 340g powder into 440g water. We now have a watery slurry that settles in seconds.
Step 2: Thoroughly blender-mix 5g of CMC powder into it. We now have a slow-drying but thin slurry.
Step 3: Blender-mix in, on high speed, 6g of VeeGum. Slowly add it. We now have a gelled slurry that will stay in suspension. And paint well with a fan brush.

Full gel might not kick in until aging it overnight. Of course, if it overgels for you, then use less VeeGum the next time. Why not add the gums together or the VeeGum first? Agglomerates will form, and even the blender can't remove them.

Context: Fan Brushes - The.., Secret 3 of crystalline.., Glaze large bowls inside-and-out.., Crystalline glazes, Glaze laydown

Monday 20th July 2026

Trap a crystalline using a catcher glaze.

Stop the crazing using a custom body.

This small vase was quick-cooled so crystals did not grow. But it was fired to the full temperature and held to give the glaze opportunity to run as much as possible. But no "glaze catcher" was needed because I used a "catcher glaze" instead. The upper has a typical high-zinc and high-sodium fritted crystalline glaze. The lower half is just a functional melt-stable cone 6 transparent, G2926B, (having the same amount of cobalt). Notice how it arrests the flow of the runny one.

There is more to this vase than meets the eye. It was slip cast from my L3802F DIY casting porcelain. That body has much more silica than is typical, which raises the thermal expansion of the body enough to stop the crazing (crystalline glazes have high Na2O so they are guaranteed to craze on normal porcelains).

Context: Secret 5 of crystalline.., Crystalline glazes

Monday 20th July 2026

A magnesia speckle matte at cone 6 oxidation:

The road toward a cone 10R look-a-like is clearer

I am getting closer to reduction speckle in oxidation. I make my own speckle by mixing the body and a glossy glaze 50:50 and adding 10% black stain. Then I slurry it, dry it, fire it in a crucible I make from alumina, crush it by hand and screen it. I am using G2934 cone 6 magnesia matte as the glaze on this mug on the left. To it I added 0.5% minus 20 mesh speck. Right is a cone 10R dolomite matte mug. Next I am going to screen out the smallest specks, switch to a matte glaze when making the specks (they are too shiny here), switch to dark brown stain. Later we will see if the specks need to bleed a little more. The next step is to tune the degree of matteness in the glaze and add a tiny amount of blue stain. I am now pretty well certain I am going to be able to duplicate the reduction look in my oxidation kiln. Methodical testing with good records are the key to fine tuning the color, speckle density and size distribution and glaze surface character.

Context: Making your own crucibles.., Reduction speckle a product.., Oxidation fired speckled glaze.., Blue stain in a.., Reduction Speckle, Magnesia Matte

Friday 17th July 2026

Permeability of Texas and Montana talcs:

Larger round particles vs tiny platelet particles

Texas talc (left) quickly absorbs all the water poured on it. Montana talc (right) resists whetting of the particles much more, the water is just sitting on top and has not penetrated at all.

The Montana material is more pure, and it is a platy talc. Talc platelets expose broad basal water repellent surfaces that are difficult for water to wet. When these fine plates settle parallel to the surface, they can form a tightly packed, water-repellent layer with very small pore entrances. Surface tension and trapped air then prevent water from entering, so it remains pooled on top.

The Texas is chemically and physically different. It contains appreciably more calcium-bearing material and other non-talc components. It also has rounder, fluffier and substantially coarser particles. These characteristics provide larger, better-connected pore passages and more readily wetted mineral surfaces. Once water enters those pores, capillary suction quickly pulls it through the powder.

Context: Pioneer 2661 Talc, Talc, Natural Talc C-98, Permeability

Wednesday 15th July 2026

Does bisque ware need washing for glazing?

These pieces demonstrate why not.

A light coating of ordinary ceramic dust is made of the same minerals as the ware and glaze itself. During dipping, the incoming water instantly penetrates and incorporates surface dust. Glaze are dust mixed with water! However, washing trades a tiny and often harmless surface contaminant for a more serious problem: partially saturated pores. Bisqueware is a porous ceramic sponge. Its job during dip glazing is to suck water out of the slurry quickly and evenly, leaving behind a stable layer of particles on the surface. If the bisque is damp from washing, the suction is weakened. Of course, there are common-sense exceptions, but compressed air, a soft brush, or a barely damp sponge used sparingly are better cleaning solutions to preserve the essential dryness.

These thin-walled mugs demonstrate: Just pour-glazing the inside has waterlogged them, they must be dried before applying the outside glaze. Imagine if they had been washed also!

Context: Should I glaze the.., Bisque, Crawling

Sunday 12th July 2026

Fitting a brushing engobe to a porcelain:

Three things are very important.

Black porcelain engobe

First, the black engobe must be tuned to have the same degree of vitrification as the porcelain (using EBCT test). Second, to brush like a paint onto leather-hard ware, it needs CMC gum. Third, it needs a low water content so one coat covers. Here's how I made a pint:

-Put 280 g water into a blender.
-Add 500 g of L3954F powder.
-Add 75 g of Laguna CMC Gum Solution.
-Blender mix thoroughly to get all the lumps out.

I was so excited about being able to have a black porcelain surface on P300 that I made my own label (showing its code number from my Insight Live account). Subsequently, the piece was bisque-fired, black-glazed, and fired at cone 6. The band painted on the base, which I did as a fix-up for a few tiny white bare spots, demonstrates something unexpected: The gum enables it to dry on the vitrified surface without cracking, and this thin layer even fired on without peeling.

Context: Here s how I.., The best way to.., Is porcelain engobe good.., Engobe

Thursday 9th July 2026



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