Art 151 Basic Glaze Technology

GLAZE

Glaze is a liquid suspension of finely ground minerals, that after being applied to the bisque ware and heated to the proper temperature, melt to form a glassy coating on the clay surface.

GLAZE CLASSIFICATION

Glazes are classified according to the following criteria:

            1. Composition- i.e. Lead, Feldspathic, Saturated Iron, etc.

            2. Surface texture- i.e. glossy, mat, crackle

            3. Interior- i.e. transparent, translucent, opaque

            4. Temperature- i.e. high, low, mid-range

            5. Body- i.e. porcelain, stoneware, earthenware

GLAZE COMPONENTS

All standard ceramic glazes contain the following components:

           1.            Glass former- silica (SiO2), melting point 3119 degrees Fahrenheit   
                           (RO2)  

           2.            Flux- lowers the melting point of silica; provides the great                             variety of surfaces in ceramic glazes. The common fluxes are:                          sodium, potassium, calcium, barium, magnesium, zinc, lithium,                           and lead. (RO)

           3.            Stabilizer- alumina, keeps the glaze on a vertical surface;                           stiffens the melt. (R2O3) 

TWO MATERIALS

These two materials play a key role in ceramics:

                       Kaolin- Al2O3 :2SiO2:2H2O, melting point 3164-3245 deg. F.

                       Feldspar- K2O :Al2O3:6SiO2, melting point 2228 degrees F.

A TYPICAL STONEWARE GLAZE

            HT Celadon, Cone 10

           Silica                                   20 (glass former)

           Potassium Feldspar            30 (glass former, flux, stabilizer)

           Kaolin                                  20 (glass former, stabilizer)

           Whiting                                30 (flux)

                                                         100

            + 2% Red Iron Oxide(color)

COLOR IN GLAZES

Color in glazes is most commonly attained by the addition of small amounts (10 % or less) of metallic oxides. An oxide is a chemical combination of any element with oxygen.

COMMON COLORING OXIDES (colors listed are for Cone 10 reduction firing)

Iron Oxide (FeO)-                                    greens and browns

Cobalt Oxide (CoO)-                               blues and violets

Copper Oxide (CuO)-                             greens and reds

Chrome Oxide (Cr2O3)-                         greens

Manganese Dioxide (MnO2)-                   browns and purples

Nickel Oxide (NiO)-                                 gray, green and brown tints

Rutile (TiO2 & FeO)-                               tan, brown and blue/gold

SOME COMMON GLAZE FLAWS

Crazing-     fine network of cracks in the glaze surface. Called crackle if desired.     Caused by the presence of high expansion oxides in the glaze and the resulting excessive shrinkage of the fired glaze (glaze is too small for the surface it is covering).

Shivering-  opposite of crazing. Small slivers of glaze pop off the fired form. Caused  by the lack of shrinkage of the fired glaze (the glaze is too big for the surface it is covering).

Crawling-   the glaze pulls itself apart, leaving bare spots of clay on the fired surface. Can be caused by dirty bisque or excessive shrinkage of the raw glaze.

Pitting/Pinholing-  small pits and/or pinholes in the fired glaze surface. Canbe caused by the surface of the clay, the composition of the glaze, the firing cycle or a combination of the three.