Pottery and China An Introduction
Pottery • One of the oldest (evidence exists back to 10,000 B.C.) forms of fabricating utilitarian objects from natural resources (clay). • Cooking and eating objects, containers, decoration. • Mechanical aspects: crush, grind, and mix clay; add water; shape object, fire pottery with or without glaze.
Industrial Revolution • Chemistry is involved in setting the clay mixture – type and amounts, making the glazes, and selecting the correct clay + glaze combination. • Like textiles, the pottery industry became mechanized and grew rapidly for many of the same reasons that the textile industry did. • New inventions, advances in power, new markets and sources of raw materials, improvements in transportation.
Earthenware, Stoneware, Porcelain • These contain naturally occurring clays or minerals resulting from the chemical weathering of rocks (e.g. + acid). • Identity of the minerals (mostly silicates = Si + O) and proportions vary. Some examples • (A) Antigorite: Mg3Si2O5(OH)4 • (K) Kaolinite: Al2Si2O5(OH)4 • (Q) Quartz: SiO2 • (F) Potassium Feldspar: KAlSi3O8
A Look at the Minerals • Kaolinite: http://mineral.galleries.com/minerals/silicate/kaolinit/kaolinit.htm • Quartz: http://www.galleries.com/minerals/silicate/quartz/quartz.htm • Feldspar: http://mineral.galleries.com/minerals/silicate/microcli/microcli.htm
From Minerals to Pottery, China • Clay + water is plastic and can be shaped into objects. Heating or firing the resulting pot or dish at high temperatures expels the water and produces physical and chemical changes. • As the firing temperature increases, the clays become more vitreous (like glass, amorphous), less porous and harder, more translucent.
How Pottery Types Differ • Kiln or Firing Temperature • Earthenware : 1000-1100 oC • Stoneware: 1200-1300 oC • Porcelain: 1200-1400 oC • Composition • Earthenware: A:K:Q:F = 25%:28%:32%:15% • Porcelain has mostly K • Earthenware: most porous, opaque, coarse
Glaze • Glazes serve several functions: make an object less porous, make it stronger and for decoration. • Glazes are vitreous or glassy materials that have various colors, consistencies. They are applied to the kiln-fired object and then fired again to make the glaze adhere to the object.
Composition of Glazes • Silica is the main component of glaze: SiO2 • Flux to help melt the glaze: e.g. BaO, CaO, Na2O, PbO • Stabilizer to extend glaze MP and stiffen glaze: alumina, Al2O3 • Opacifier to make glaze opaque: e.g. SnO2, TiO2 • Colorant: metal oxides
Glazes Impart Color • Metal oxides, inorganic compounds • Metal atoms absorb light and emit visible light; recall the discussion about textile dyes. • CuO, CuCO3, CoO, CoCO3, MnO2, Fe2O3, TiO2, V2O5, NiO
References • http://www.thepotteries.org/types/index.htm