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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.
| Materials |
Wollastonite
A source of CaO in glazes and a porcelain body material. |
| Glossary |
Agglomeration
The fine mineral, oxide and clay particles used in ceramics often aglommerate during storage or even in the latter stages of production. These must be broken down later. |
| Glossary |
Sieve
Sieves are important in ceramics for removing particulates and agglomerates from glaze, engobe and body slurries. |
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