Crystalline Glazes: Study of Colouring Oxides/Carbonates

The focus of this thesis paper will be on macrocrystalline glazes and their colouring oxides/carbonates which will be applied to porcelain ceramic forms. A ceramic glaze is a special sort of glass, differing from window-glass and glassware in its lower thermal expansion and higher alumina content, which increase its viscosity and helps it to adhere to the clay body.1 Crystalline glazes are considered special effect glazes. They are the most challenging and elusive of all the potters\u27 glazes. Their production hinges on the perfect synthesis of numerous variables and technical skills which will be detailed in the following pages Crystalline glazes essentially fall into two categories: macrocrystalline and microcrystalline. Macrocrystalline glazes grow sunburst, flowers, or snowflake like forms floating in a sea of color. Often there are very tiny (1/10 cm to 1/5 cm) secondary crystals usually of a different formation and displaying a different color. Ghost crystals sometimes appear. They are like round blobs with no definite formation, floating on the very surface of the glaze. They are always very pale and sometimes can only be seen when the pot is tipped to catch the light. Often the color of the crystal differs from the background or matrix. Microcrystalline glazes have microscopic, needle like crystals present in great quantities. When light strikes the surface of the glaze, it is broken up by crystals and reflected out in all directions. This often causes the surface to sparkle in the light and produces the satin matte surface that is pleasant to touch.2 There are many challenges working with crystalline glazes as opposed to other high fire glazes. The process begins with the porcelain clay body and continues with the form, the glaze formula, the application of the glaze, the firing, the cleaning of the bottoms of the vessels and all the little but important things in-between. The final process for my pottery is the glaze which gives the piece color, shine, and durability. It is the glaze that produces the special effects responsible for the piece\u27s character. In my research and experimentation of many pure and simple glazes there are none so difficult to create as crystalline glazes. It is a unique process where science meets art in an extraordinary cooperation of energy and time