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.

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.

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).

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.

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).

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.

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.

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!

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.

Left: The base recipe of GA6-C (80% Alberta Slip, 20% Frit 3134, 4% rutile). That recipe is stable; it does not run. Right: A 2% addition of copper carbonate transforms it into a runny, melt fluid glaze. It also produces a pleasant green even on this red-burning body (a good green is not easy in oxidation ceramics). Using a catch glaze (e.g. a frit-reduced version), this could be made safe on vertical surfaces. However, there are red flags here. Having only 2% copper would not destabilize a typical glaze (making it leachable). But phase separation is occurring here, likely because the base contains significant rutile. That means the phases are copper-bearing and possibly copper-concentrating. And, as noted, the copper has a greater-than-expected impact on the melt. Third, commercial variegated copper glazes can even leach in acids (as happened with the mug inset after a night with lemon juice). While the GA6-C recipe does have a 0.3:0.7 R2O:RO ratio, other indicators make it a course of wisdom to consider the GLLE test before use on functional surfaces.