Morgan Harris Pottery
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.
How Visible Crystals Form
The macro-crystals found in crystalline glazes form around a nucleus of a tiny 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. The same crystalline structure occurs in nature as willemite.
The Firing Schedule
In order for Crystals to form and grow in a glaze not only must the composition and application of the glaze be correct, but the firing schedule is critical. I fire my crystalline quickly up to a top temperature of approximately 2300°F and hold it there for a soaking of roughly 20 minutes. As the glaze contains literally millions of potential crystal nucleation “seeds” the top temperature and hold time allows most of them to burn out or run off of the pot. Without this runoff the crystals would completely cover the pot and grow out of the glaze surface creating a very rough texture. Then I allow the temperature to fall rapidly to the crystal growing stage and hold it there for several hours before turning off the kiln. Generally speaking, crystals begin to form as needle-like shapes at about 2084°F. If the temperature is held at about 2012°F, a double-axe head shape will usually form. Holding the temperature between 1994°-1850°F will encourage the shape to round out. Fully rounded crystals give a distinctly flower-like effect. To sum up the firing; Top temperature and hold time determines the amount of crystals while the following lower temperature and hold time determine the shape and size of the crystals. Depending on the desired effect each firing takes around 13 to 18 hours with an equivalent cool down before viewing the results.
The Chemical Glaze Composition
In general, crystalline glazes require relatively high percentages of zinc. Crystalline glazes are far lower than normal in their alumina (a glaze stabilizer used in most glazes) content, generally none at all. Because of these requirements, crystalline glazes tend to be quite runny. The liquidity of the molten glaze allows the zinc and silica molecules to migrate through the glaze and bond with one another to form the crystalline structure. Additionally, the glaze must be applied quite thick compared to most other glazes; often as thick as 3/16” at the top, thinner towards the bottom. I fire all my crystalline pots with a pedestal basin glued to the bottom to catch the runoff glaze. After firing the basin must be removed (without breaking the pot) and the pot's bottom must to be ground and polished.
Glaze and Crystal Coloration
Because of the crystal's molecular structure, only certain colorants can migrate into and color the crystal. These are typically cobalt, nickel, 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 remain in the glaze matrix, making it yellow, grey, or brown depending on the concentration and firing schedule. If cobalt is not present, nickel takes next precedence in migrating into the crystal, then manganese, then copper, then iron. All of the coloring oxides by themselves with color the crystal and the background the same color , with shade variations, except for nickel. Nickel as the only coloring oxide present will always produce blue crystals with an amber/gold/beige background and sometimes the background will have tiny secondary clear crystals.