Pure feldspar applied as a glaze: Possible because of the magic of thixotropy.

These are pure Custer feldspar and Nepheline Syenite. The coverage is perfectly even on both. No drips. Yet no clay is present. The secret? Epsom salts. I slurried the two powders in water until the flow was like heavy cream. I added more water to thin and then started adding the Epsom salts (powdered). After only a pinch or two, they both gelled. Then I added more water and more Epsom salts until they thickened again and gelled even better. The result is a thixotropic slurry. They both applied beautifully to these porcelains. The gelled consistency prevented them from settling in seconds to a hard layer on the bucket bottom. Could you do this with pure silica? Yes! The lesson: If these will suspend by gelling with Epsom salts then any glaze will. You never need to tolerate settling or uneven coverage for single-layer dip-glazing again!

Related Pictures

Suspending pure feldspar and applying it as a glaze

Most people think that would be impossible. But it is not. This slurry will stay in suspension for days. How? It is flocculated using a tiny bit of powdered epsom salts. Without the epsom salts it is watery and will settle in seconds. How does the slurry apply to this porcelain? Since it contains no clay it has complete permeability. Against the immersed bisque a layer builds very rapidly, pieces must be dipped and removed immediately. Does it dry hard enough to handle? Yes.

Craze city: Feldspar and Nepheline Syenite on cone 10R porcelain bodies

These were applied to the bisque as a slurry (suspended by gelling with powdered or dissolved Epsom salts). On the left is Custer feldspar, the right is Covia Nepheline Syenite. Notice the crazing (feldspars, and nepheline syenite, always craze because they are high in K₂O and Na₂O, these oxides have by far the highest thermal expansions).

See the magic of thixotropy as I mix a 20kg batch of G2926B glaze

In this video, I mix 20kg of glaze powder into 20kg of water using our powerful propeller mixer. The resulting slurry is like water, absolutely unusable. Yet on measuring the specific gravity (using a hydrometer because it floats freely) I find that it is too high, I actually have to add more water! How is that even possible? Instead, I add Epsom salts and mix again and the slurry gels and hangs on in a perfectly even layer when I dip the spatula. This is a thixotropic gel, it will apply evenly to bisque ware yet not go on too thickly. We normally recommend a specific gravity of 1.44 for this glaze, but in this case, it seemed watery enough at 1.46 (on use, it will become clear if 1.46 is OK e.g. if it goes onto the ware too thick). If that happens I'll just add water to 1.44 (and more Epson salts if needed). At the time of writing, based on online pricing at this time, coverage is 650-5000% cheaper than buying jars of transparent brushing glaze (I am considering both the total powder weight and the specific gravity difference between this and commercial glazes we use).

Links

Troubles	Powdering, Cracking and Settling Glazes Powdering and dusting glazes are difficult and a dust hazard. Shrinking and cracking glazes fall off and crawl. The cause is the wrong amount or type of clay.
Materials	Epsom Salts
Glossary	Thixotropy Thixotropy is a property of ceramic slurries of high water content. Thixotropic suspensions flow when moving but gel after sitting (for a few moments more depending on application). This phenomenon is helpful in getting even, drip-free glaze coverage.

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