Crystalline Glaze for Cone 6
Crystalline glazes are at once fascinating and frustrating. Successful crystalline glazes are the result of using a clay body with no grog or sand (ideally porcelain), manufacture of forms with proper support for the glaze firing, accurate glaze formulation and application and matching a firing schedule to the glazes (perhaps the most critical factor).
Crystalline glazes are divided into two basic categories: micro crystalline and macro crystalline. Mat glazes are types of micro crystalline glazes. The surface is created by numerous crystals (too small to be seen by the naked eye) suspended in the glaze matrix. The masses of crystals refract light in such a way as to create a non-shiny surface. Other micro crystalline glazes are referred to as aventurine. The crystals in these glazes are very small, suspended in the glaze, reflecting light like little flitters. Iron and/or titanium are used most often to create this effect.
Macro crystalline glazes can be large and spectacular. Crystal shapes such as fans, circles, rods and stars may appear in various sizes seeming to float in a smooth background. The crystals are formed from a combination of zinc and silica, known as zinc orthosilicate, similar to the naturally occurring mineral willemite. Titanium is also used to form crystals to a lesser extent.
Normally the ratio of silica (glassformer), flux (melter) and alumina (stabilizer) is established to form a glaze that will remain stable (non-flowing) on a vertical surface at a given temperature. Crystalline glazes differ in the ratio of silica and flux to the amount of alumina. The amount of alumina is small, often, none at all, resulting in a glaze which is very fluid when fired to its maturing temperature. A fluid glaze is necessary to form crystals. The glaze is saturated with silica (positive ion) and zinc (negative ion). During the melting process the positive and negative ions are attracted and bond to each other. As the glaze continues to flow molecules are attracted and bond to form the zinc orthosilicate crystals.
There are numerous variables that might affect the formation of crystals in a glaze. Contributing factors, singularly or in combination, may include: thickness of glaze application, firing time, shape of the pot, maximum firing temperature, soaking temperature and soaking time.
Factors in Crystal Development
- Glaze must be fluid to form crystals.
- Glaze coating needs to be thicker than usual.
- Glaze is best sprayed or brushed on, but can be dipped/poured.
- Glaze which is fresh, well mixed and screened works best.
- A fast firing and long soaking (1/2 to 4 hours) time produces more crystal growth.
- Fritting of compounds or use of frits, rather than raw materials, is recommended. Most frequently used frits are Ferro #3110 & #3134.
- High soda content is better than potash or calcium.
- Manganese or iron, under 2%, promote larger crystals.
- Zinc should range 10 – 35%.
- Titanium contributes to smaller, but evenly distributed crystals. No more than 10%.
- Alumina must be less than 10%.
Materials most commonly used in crystalline glazes
- Silica - 325 mesh: up to 25%
- Zinc oxide – calcined: up to 35%
- Frits – sodium, Ferro frit 3110 or GF 134: 50 – 55%
- Alumina - alumina oxide or hydrate, calcined kaolin - .75 - 2 parts
- Titanium dioxide/ Rutile – 1 - 6%
- Tungsten - crystal enhancer, .5 -.75 parts
- Vanadium Pentoxide - crystal enhancer, .1 -.5 parts
- Molybdic oxide - crystal enhancer
Coloring Oxides in Crystal Glazes
Cobalt: Almost always blue on a lighter blue background. Use .2 – 3%. Addition of zirconium and/or tin will lighten and opacify. Works well with copper and manganese.
Copper: Various greens on green background. Green on cream background and turquoise blue also occur. Use 1-3%. Combinations with cobalt, nickel and manganese.
Nickel: May produce silver on brown, blue on amber or blue-green on chartreuse. Use 1-3%. Use with copper and cobalt. Do not use with titanium.
Manganese: Results may yield lavender/pink on tan, silver on tan, orange, brown or purple/brown. Use .5-4%. Works with cobalt and copper.
Iron: Gold on brown, gold/brown on orange and various tan/brown combinations. Use 2-5%.
Rutile: Golden brown on brown to tan background. Is an impure form of titanium; do not use too much. Works as a modifier of other colors.
Illmenite: May create blues on tan/brown. Use .5-2%.
Tungsten: Can produce a luster surface. Reacts similar to titanium as a crystal enhancer. Do not use very much. Use .5-2%
Combinations of oxides will produce any number of results. Factors include: specific oxides, percentage of oxides and the base glaze formula. One should expect to see changes in color of the crystals, the color of the background and the shape and size of the crystals. Colorants may also affect the glaze melt. Most colorants act as fluxes, except nickel, which acts as a refractory.