Putting Patterns on Porcelain
Today there are many options for putting colorful patterns onto porcelain china. Some, like decoupage, waterslide decals and air-dry paints like Delta Air-Dry PermEnamel are within the reach of any home crafter. Others, like dye sublimation printing, transfer printing and hand-glazing high-fired pottery require substantial investment in equipment and are best suited to well-capitalized businesses and artists’ cooperatives. The two classic ways of putting patterns onto porcelain, hand-painting and transfer printing, still exist today. In addition, there is a high-tech version of waterslide decals used commercially which consists of screen-printing decals with glazes and applying the decals to the porcelain. In each case, the pottery is high-fired before decorating to at least cone 6.
[Cone is a measure of heat absorption resulting from heat applied over time. Cone 6 translates to between 2165 and 2269 degrees F (depending on how fast the kiln heats – or ramps – up).] Such high-firing produces the hard almost-translucent quality of genuine porcelain. Then the piece is decorated and lightly fired repeatedly to melt and fuse the glazes to the porcelain. Incidentally, the term “porcelain” has been applied more and more broadly as new techniques developed.
Ask any potter to define “porcelain” and he will likely give you the classic definition. To a potter, genuine porcelain is high-fired (cone 6 or higher) white clay that is at least somewhat translucent. It has a large proportion of kaolin clay, with the remainder being primarily feldspar and silica. This clay composition accounts for the pure white gleam of porcelain. Artisans who paint porcelain (rather than actually make it) refer to three grades of porcelain: hard-paste, soft-paste, and bone china. They all contain kaolin but only hard-paste has feldspar and silica and is high-fired. The high temperatures cause the body and the glaze to fuse. When hard-paste porcelain is broken, it is impossible to distinguish the body from the glaze. Soft-paste porcelain adds ground glass or frit (material for glass that is not yet fused and vitrified) and is fired to between cone 01 and 1 (1999 to 2109 degrees F). Because soft-paste porcelain is fired at lower temperatures, it does not completely vitrify and remains slightly porous.
When soft-paste porcelain is broken, you can distinguish a grainy body covered with a glassy layer of glaze. Bone china has bone ash added to the kaolin and vitrifies (becomes glass-like) somewhere between cone 2 and cone 5 (2034 to 2205 F). Though not as hard as true porcelain, bone china is more durable than soft-paste porcelain. The bone ash greatly increases the translucence of the porcelain. Finally, go to a tile store and look at their “porcelain” tiles. If you turn them over, you will see base clay ranging from white to brown to gray. The definition of “porcelain” in the tile industry has nothing to do with the clay content or level of firing. Rather, tile manufacturers define as “porcelain” any tile fired to the point where it absorbs less than 3% moisture.
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