This glaze has just been applied to a bisque tile. It contains wollastonite, a curious mineral because it has a shelf life (it can agglomerate in storage as a powder). Surprisingly, it can also form them in slurries. Other materials can also agglomerate; these include barium carbonate, iron oxide, cobalt oxide, clays, tin oxide, zircon and titanium dioxide.

This glaze was propeller-mixed, but that was not enough to break down the white lumps. Is sieving the answer? Maybe not. While screening can reduce the size of the lumps, it does not prevent their regrowth in the slurry (because smaller agglomerates that survive sieving can become starting points for larger clusters). A better option is blender-mixing. When that is done and re-agglomeration occurs, then the problem is more complicated.

Yes. The two glaze cones are both the same Grolleg-based porcelains. Both of them have the same base glaze: 1947U transparent. But the one on the left uses EP kaolin, and the one on the right uses Grolleg kaolin. The Grolleg glaze has a dramatically cleaner appearance. The Grolleg does not suspend the slurry quite as well as EPK; however, it responds to gelling better (using vinegar or Epsom Salts, for example). This ability to facilitate an easy-to-use thixotropic slurry more than compensates.

USG specialty plaster is entering a final-inventory phase in the ceramic supply chain, and rationing has begun at some suppliers even though the Southard plant does not close until November.

These have just been thrown on the wheel. I find it to be a foolproof method of comparing the plasticity of two clays. They were slurried up and dewatered to about the same moisture content and the same amount was thrown to compare the size achievable. While the Grolleg is stickier and dewaters a little slower, it is not as plastic as dark EPK (which itself is not that plastic compared to others). Curiously, New Zealand kaolin (halloysite) is quite a bit less plastic than the Grolleg but it responds to plasticity augmentation (in porcelain recipes) just as well as Grolleg (similar amounts of bentonite producing similar plasticities). And, bodies containing EPK also need about the same amount of bentonite to produce plasticity suitable for throwing large forms. So, the plasticity that a kaolin appears to have by itself is not completely indicative of what it will contribute to a body (if augmented with bentonite). The EPK used here is the darker and more plastic of the two varieties Plainsman receives.

Left: OptiKast, a coarse-particle kaolin intended for slip-casting. Right: Plainsman A2 ball clay. These DFAC test (drying performance) disks were dried with their centers covered, deliberately creating uneven drying. The exposed perimeter dries and stiffens first; the covered center dries and shrinks later. The resulting cracks reveal how each clay responds to that stress. OptiKast feels smoother, yet its individual clay particles are much coarser than those typical of ball clay. This kaolin has low drying shrinkage, so its disk remained intact. The ball clay shrinks more. As its center dried and tried to contract against the already stiff perimeter, stresses developed, splitting the disk and opening numerous cracks around the edge.

So why use ball clay? This particular kaolin has so little plasticity that even making this disk was difficult. And the dried disk is fragile. Yet it is very useful in casting bodies, where plasticity impedes permeability. Ball clays are highly plastic; high drying shrinkage comes with that. Adding some of this to poor-plasticity casting or forming bodies that can tolerate a darker fired color can transform them into ones that both form well and dry harder and stronger.

This two-inch stack of clay body and glaze mix tickets represents only about half of my testing over the past year. Along the way, I also added thousands of photos to my Insight-live.com account. Looking at this pile, I realized why I am testing more: Organized records make it easier to keep multiple projects moving. I can see what I have tried, compare results and pick up where I left off. Each test builds on earlier work, and that visible progress motivates me to keep going. If you want to do DIY glazes and bodies, this is the secret is good record-keeping.

The underglaze is the 20x5 cone 6 base transparent glaze (G1214M with 5% Mason 6666 black stain). Overglaze left: GR6-H Ravenscrag Oatmeal. Overglaze right: GA6-F Alberta Slip oatmeal. Both produce a pleasant silky matte texture. Double layering using typical dipping glazes is prone to crawling (because of shrinking and cracking during drying). By keeping layers fairly thin, it is possible to achieve success, but with glazes high in clay content (as these are), more is needed. A mix of raw and roasted Alberta and Ravenscrag Slips is the answer here. Start with a 50:50 ratio and adjust to establish a balance between drying hardness and drying shrinkage. For more assurance in production situations, mix the underlayer black as a base-coat dipping glaze.

There are enough variables that a record-keeping system is needed. Start now, and you won't regret it.

Watch the G1214Z video to see me convert the G1214M 20x5 cone 6 clear base into G1214Z cone 6 calcia matte using simple glaze chemistry and recipe logic. This first appeared in the Digitalfire desktop Insight instruction manual 30 years ago. It is an understatement to say that this process is interesting if you want to know more about glazes, their chemistry and recipe logic. Watch this video and see me adjust the recipe of my high-calcium transparent cone 6 glaze to convert it into a calcia matte. In an Insight-live.com account, the process is easy enough for anyone. We'll cut the Si:Al ratio, increase the CaO, maintain the thermal expansion for glaze fit and make the recipe shrinkage-adjustable using a mix of calcined kaolin and raw kaolin. We will even compare it with the High Calcium Semimatte from Mastering Glazes.

While some sectors of modern ceramic manufacturing are highly automated, crockery and holloware production relies heavily on skilled workers who execute the same fundamentals as studio potters. And they use equipment a studio potter can make. But the speed and cooperation at which they work is astonishing. Consider:

-Foam and sponge belts: To finish unfired surfaces and wipe glaze from feet.
-Spinning sponge tables for rim and edge dry finishing.
-Rheology Control: Glaze slurry rheology (specific gravity, thixotropy, fluidity) is adapted to each situation. Workers dip, spin, and extract pieces in seconds without runs, drips, or extended drying pauses.
-Custom Tooling: Instead of standard shop tongs or studio brushes, workers use single-point holding tools, custom wire dipping tongs, and painting and stamping sponges.
-Traditional Jiggering: Workers turn large masses of soft clay into cups and bowls in seconds, or they feed stiff clay into semi-automatic machines.
-Handle Preparation: Workers use custom plaster holders and specialized blade cutters to shear slip-cast handles to match vessel contours.
-Fountain & Dip Glazing: Workers glaze interiors and rims simultaneously.
-Direct Screen Printing: Workers drape a silk screen directly around pieces and scrape ink through via a hand rubber scraper.

Tap/click the typecode link below for the videos.

To get a better, more brilliant transparency than G1947U, at cone 10R, I tried sourcing Na2O from a frit. The inside surface on this mug is the result (v2), it is so glossy it looks like a low-fire leaded glaze. This screenshot of side-by-side panels shows the frit-enhanced version(s), G3910 and G3910A. Frit 3110 has a much higher concentration of KNaO than the feldspar, a key to making a brilliant glass. While this increases the COE, the fact that it comes from this frit brings multiple benefits:

-A brings a little B2O3, a better melter than zinc (which volatilizes anyway at cone 10).
-It dissolves more low-expansion glass-forming SiO2 (in v2 I switched to 400 mesh silica to enhance that even more).
-The glaze can then tolerate some low-expansion MgO (introduced by Frit 3249 or its equivalent, Fusion Frit F-69).

More details about this project can be found by clicking/tapping the Insight-live recipe code numbers G1947U, G3910 or G3910A. The red outside glaze is G2571D1.