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Kiln Firing: Transformation Through Evaporation
Ceramics are some of history’s most enduring objects. Check out Groupon’s study of how a lump of clay becomes a piece of art to last a lifetime.
Though it was once clay, a ceramic pot will never revert back to a moldable state—not even when an archeologist digs it up in 2,000 years hoping it contains fossilized cookies. This permanent transformation is due to the pot’s adolescent incubator, the kiln, whose scalding belly fundamentally alters the molecular make-up of clay as it bakes inside. In its pliable state, clay is composed of silica and alumina molecules lightly bonded by molecules of water. For the transformation to occur, these water molecules have to go, and the heat of a kiln causes the moisture to evaporate. This process must happen slowly, however, lest the water turn to steam and cause the piece to explode.
Once the kiln reaches water’s boiling temperature (212 degrees Fahrenheit), much of the moisture has left—except for the molecules bonding the clay together, which don’t evaporate until around 572 degrees. At this point, no amount of water splashed onto the clay will wake it up or make it pliable again, but the sauna session isn’t over yet. Further milestones are reached at 1,063 degrees, when the silica oxide molecules rearrange themselves (a process called quartz inversion), and at 1,652 degrees, when the individual clay particles begin to fuse together (or sinter). At this point, the clay pot can adopt its new name: ceramic pot, Esq.
Here, a crossroads: if the process stops now, the piece could serve as a bisque, which can be painted or glazed and fired again later. But if the fire rages on, the ceramic will eventually reach a stage of vitrification, in which the particles bond together fully until it’s impervious to water. Shortly after this point, the artist must start cooling the kiln or risk melting the piece into an unusable blob suitable only for a last-minute Father’s Day gift.
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