This is a fritted stoneware fired to cone 03. There is plenty of glass development to form a strong interface with glazes and underglazes. But this green commercial underglaze is not bonding well with the body. Repeated blows to the surface by a hammer (insufficient to break the piece but enough to test the glaze) are chipping off chunks of glaze/underglaze at the latter's interface with the body. This does happen with most other underglazes tested. The green underglaze is obviously more refractory and should be reformulated so that its stain medium develops enough fired maturity to counteract the refractory stain.

Right: My attempts to apply a layer of watery white underglaze to leather-hard clay. Not surprising that I measured the water content of this at 68%. Yikes!

Left: After adding about a gram of Epsom Salts (and blender-mixing it in to dissolve), the slurry became thixotropic and gelled enough to enable applying a thicker layer with no problem. Note how it stays on the spoon. However, there is still an issue: Multple layers are needed, with lots of drying time between them. The low solids loading of this product also makes it tricky to use on bisque ware (more time is needed between the thinner coats it creates). Allowing it to settle and pouring off water, or evaporating some of the water to raise the specific gravity, would make it even better.

Amaco V-303 Terra Cotta underglaze was applied in one coat at the leather-hard stage (on these Plainsman Snow mugs). It appeared to be thick enough on application and after bisque (left), but the clear overglaze on the right has diffused into it to melt it enough to lose its opacity. A higher specific gravity (lower water content) would make it possible to apply it thickly enough in one coat; this is especially important because each layer rewets the piece and adds a lengthy drying period. An appropriate specific gravity is thus the difference beween practical and impractical. Acheiving a better terra cotta texture and color is a prime candidate for DIY underglaze.

Since white burning slips are made from refractory materials they are not nearly as vitreous as terracotta bodies, they need help to mature and a frit is the natural answer. With the right amount, fired shrinkage can be matched to the body while the slip remains opaque.

L3685A slip on the left: An adjustment to the popular "Fish Sauce" recipe (which is basically a super plastic white body). 8% Frit 3110 replaces 8% Pyrax to make it glass-bond to the body better while remaining completely opaque.

Right L3685C: It has 15% frit, moving it into glaze territory, with translucency replacing opacity. Although expensive zircon could be added to restore opacity, that doesn’t fix the excessive firing shrinkage it now has, mismatching it with the body (which becomes more eventful the thicker it is applied).

This was an effort to do SHAB test bars, at various temperatures to form a fired maturity profile of a blue commercial underglaze. It did not go as planned. It was very difficult to make these bars because the product has such a high content of gum. This dramatically impedes drying and it is impossible to form and dry them (they split and crack into many pieces). So I crushed and ball milled the dried scraps, roasted the powder to 1000F to destroy the plasticity, then added enough bentonite to be able to make them plastic enough to form the test bars.

Notice that at cone 06, the absorption is 15.4% with 3.7% fired shrinkage. By cone 04, the bottom bar, that is down to 1.2% porosity and up to 11.% shrinkage (basically a colored porcelain). The top bar is fired to cone 6, and melts (like others at previous cones).

Both of these are within its claimed firing range. So what does "within that range" mean? They mean "true-to-color", it is the same color throughout the range. But that bottom bar, cone 04, is at the edge of what an underglaze should be ceramically (if applied in multiple layers that build up a thickness).

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.

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.