How Is Porcelain Made? White, Translucent and Hard to Copy
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Hold a porcelain cup to a lamp and light comes through the wall. Tap it and it rings. Neither happens with earthenware or stoneware, and both come from the same cause: firing a particular mixture hot enough that part of it turns to glass and fills every pore, leaving a body with essentially no porosity at all. Reproducing that took European potters roughly six hundred years.
The materials and what each does
True porcelain is made from three ingredients whose roles are distinct and complementary:
- •Kaolin, a white clay chemically pure enough to stay white through firing, which supplies plasticity for shaping and the refractory backbone that holds the shape at high temperature
- •Feldspar, which melts during firing and acts as the flux, forming the glassy phase that flows into the pores and fuses everything together
- •Quartz, which acts as a filler controlling shrinkage and stiffening the body against slumping
- •Firing to roughly 1,300 to 1,400 degrees Celsius, far above earthenware, so that vitrification occurs and needle-like mullite crystals grow through the glass, giving strength
- •The result is a body with almost no open porosity, which is why porcelain holds water without a glaze while earthenware does not, and why it rings rather than thuds
- •Translucency comes from the glassy phase and the small difference in optical properties between it and the crystals, which lets light pass rather than scattering it away
The long Chinese head start
Porcelain developed in China over centuries rather than being invented at a moment, with high-fired white wares appearing by the Tang period and the material recognisably mature by the Song. The technical foundation was a combination not available elsewhere: abundant deposits of suitable kaolin and porcelain stone, and kiln designs including the dragon kiln built up a hillside that could reach and hold the necessary temperatures. Jingdezhen became the dominant production centre and operated on something close to an industrial scale centuries before European industry existed, with the work divided among dozens of specialists, each performing one operation on each piece. Blue and white ware, decorated with cobalt beneath the glaze and fired once, became a global commodity, exported across Asia, the Islamic world and eventually Europe in quantities that made it one of the great trade goods, carried as ballast in ships whose principal cargo was tea and silk.
The European pursuit
European potters could see porcelain and could not make it, because the ingredients were unidentified and the necessary temperatures were beyond ordinary kilns. The gap produced centuries of substitutes: tin-glazed earthenware, which coats a coloured body with an opaque white glaze and is called maiolica, faience or delftware depending on where it was made, and soft-paste porcelain, which mixed clay with glassy frit to imitate the appearance at lower temperatures while remaining porous and fragile. The problem was solved at Meissen around 1708, where Johann Friedrich Bottger, working under the Elector of Saxony as effectively a prisoner after failing to produce gold, and the scientist Ehrenfried Walther von Tschirnhaus identified kaolin and achieved the firing temperature. The formula was treated as a state secret guarded like a military one, and it escaped anyway through defecting workmen within a couple of decades, which is how Vienna, Sevres and the rest of Europe acquired it. England took a different route, adding calcined animal bone to produce bone china, which is whiter and more translucent than hard paste, fires lower and is more forgiving to make.
Shaping, glazing and firing
Porcelain is difficult to work because the qualities that make it beautiful make it unforgiving. The body is less plastic than ordinary clay, so it collapses easily on the wheel, and it shrinks by something like twelve to fifteen percent from wet to fired, which means every mould and every joint must anticipate the shrinkage precisely. Complex shapes are slip cast, pouring liquid clay into a plaster mould that absorbs water and leaves a shell of even thickness, which is how handles, spouts and figures are produced and assembled. Most porcelain is fired twice: a lower bisque firing to make the piece durable enough to handle and glaze, then a glaze firing at full temperature in which body and glaze vitrify together and fuse into one another, which is why the glaze on porcelain does not chip off in flakes the way it can on earthenware. Underglaze decoration, painted before glazing, must survive the full firing, which limits the palette to a few oxides including cobalt blue; overglaze enamels are applied afterwards and fired again at low temperature, which allows a full range of colours at the cost of a surface that wears.
The takeaway
Porcelain combines kaolin for whiteness and shape, feldspar to melt and fill the pores, and quartz to control shrinkage, fired above 1,300 degrees until it vitrifies. The resulting body has almost no porosity, which gives it strength, a ring when struck and translucency. China developed it centuries ahead of anyone else on the strength of its clay deposits and kiln design, and Europe reached it at Meissen around 1708 after long detours through tin-glazed earthenware and soft-paste imitations. It shrinks enough during firing that every mould must allow for it.