Crystals in Glazes
Invisible crystals inhabit many if not most glazes. Many matte glaze textures and opaque glazes are the result of multitudes of micro-crystals, or crystals that are so small that they are invisible to the naked eye. The macro-crystalline glazes, or more commonly known simply as crystalline glazes, have crystals that grow large enough to see.
The glaze on a fired pot is generally an amorphous super-cooled liquid. As the glaze is melted and cooled in the kiln, glass molecules bond together in random strings. 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 image is courtesy of Mara Cammi Orsi.
How Visible Crystals Form
The macro-crystals found in crystalline glazes form around an nucleus of a 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 downfiring 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.
In general, crystalline glazes are also high-fire glazes and require relatively high percentages of zinc, titanium, or lithium. Lithium is able to encourage crystal growth even in lower temperature glazes.
Crystalline glazes are lower than normal in their alumina content. In addition, the amount for free silica in both the glaze and the clay body must be kept at a minimum. Otherwise, cristobalite may form, making the pot much more brittle and susceptible to thermal shock.
Because of these requirements, crystalline glazes tend to be quite runny. Pots should be fired on a bisque pedestal-saucer (see illustration) to catch all drips. The pot's bottom may need to be ground and polished after removal from the kiln.
Because of the crystal's molecular structure, only certain colorants can migrate into and color the crystal. These are cobalt, nickle, copper, iron, and manganese. However, due to molecular characteristics these colorants do not all act the same way.
Cobalt is the strongest; it will override the attraction of the other colorants and move into the crystal structure alone. For example, if cobalt and manganese are both present, the cobalt will migrate into the crystals making them blue, and the manganese will remian in the glaze matrix, making it yellow. If cobalt is not present, nickle takes next precedence in migrating into the crystal, then manganese, then copper. Copper, if by itself, will color glaze and crystal fairly evenly.