Large Crystals on Porcelain

The formation of large crystals involves processes that seem fairly similar, with a rapid heating to a peak of very high temperature followed by a controlled kiln cooling, with one or several soaking periods to allow the growth and shape of crystals.



Principle of a salt marsh: In summer, when the sea water evaporates from the eyelets of salt works, the concentration of salt increases. The work of paludier is to maintain a steady supply of brackish water in these eyelets, to increase the salt concentration as much as possible. When the concentrated brine reaches saturation, the slightest evaporation on the surface, under the effect of wind and sun, allows for the spontaneous formation of salt crystals known as "Fleur de Sel" (French Sea Salt)

But this is not all,
The saturation threshold increases with temperature, and warm water may contain more dissolved salts than cold water.
(Example: water at 0 ° C may contain 357g of dissolved sodium chloride per liter, while at 100 ° C it may contain 391g)
When a saturated brackish water is cooled, the saturation level drops and leads to the crystallization of the excess salt.


A salt saturated solution which cools produces crystalline precipitates, gradually, because of its saturation level which decreases.
It's this principle which is most similar to that giving rise to crystalline glazes.
A crystalline glaze is formulated to contain an excessive dose of certain compounds, such as zinc oxide. The glaze composition is intentionally unbalanced, in order to be beyond a critical concentration when its complete melting is achieved at a high temperature. The cooling phase will gradually lower the threshold of saturation of these elements in the molten glaze. The excess of the dissolved part, will give rise to crystalline solids precipitates, like Willemite crystallisations in the case of a zinc oxide excess
 Willemite crystallisation in a zinc glaze
A ceramic molten glaze is not as fluid as brackish water, its viscosity is much more higher.
The excess elements have to migrate from the molten glaze to its surface, and join the crystal lattice structures under construction.
They need to have some mobility and time needed for the crystal growth.
It's why soaking periods are necessary, to insure the crystal growth during the cooling phase, in temperature ranges where softened state of the molten glaze remains high with a sufficiently low viscosity.

Crystalline Glazes - breaking news
 Colouring experiment
Self-reducing glazes in oxidation firing
Glazes without Catchers (French)
Crystallizations observation (French)
Crystals coloration (French)
Crystals pictures (French)


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