Crystalline glazes are specialty glazes that show visible and distinct crystal growth in the matrix of the fired glaze. Although most crystals are not this large, some can grow up to four or five inches across within the glaze matrix.
The macro-crystalline glazes, or more commonly known simply as crystalline glazes, have crystals that grow large enough to see.
Crystals occur if the glaze is fluid enough to allow molecules to move more and hot enough long enough to allow the glaze molecules to arrange themselves in structured strings, or crystals.
The macro-crystals found in crystalline glazes form around a nucleus of tiny titanium oxide or zinc oxide crystal. In the right circumstances, zinc and silica oxide molecules will begin attaching themselves to the nucleus crystal. These molecular bonds are in very specific arrangements, which we see as crystals.
For this to happen, there must be an extended time at higher temperatures to allow time for crystal growth, and the glaze must have the right type of chemical composition. These are the first two of three factors that potters deal with when working with crystalline glazes.
Crystals take a long time to grow. In order for this to happen, the glaze must remain molten for an extended period of time. Firing schedules for crystalline glazes usually require a soaking period at the end of the temperature gain, plus a down-firing ramp.
Generally speaking, crystals begin to form as needle-like shapes at about 2084 F/1140 C. If the temperature is held at about 2012 F/1100 C, a double-axehead shape will usually form. Holding the temperature between 1994-1850 F/1090-1010 C will encourage the shape to round out. Fully rounded crystals give a distinctly flower-like effect.
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