|Monthly Tech-Tip |
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).
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.
In ceramics, feldspars are used in glazes and clay bodies. They vitrify stonewares and porcelains. They supply KNaO flux to glazes to help them melt.
Crazed ceramic glazes have a network of cracks. Understanding the causes is the most practical way to solve it. 95% of the time the solution is to adjust the thermal expansion of the glaze.