|Monthly Tech-Tip |
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 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. 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! Read the page "Thixotropy", it will change your life as a potter.
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.
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).
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.
Thixotropy is a property of ceramic slurries. Thixotropic suspensions flow when you want them to and then gel after sitting for a few moments. This phenomenon is helpful in getting even, drip free glaze coverage.