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Article #136: Putting Patterns on Porcelain

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






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