Opaque glazes

All sand for opaque glazes should be fritted with soda... 

Opaque glazes contain opacifiers either in frit (zircon, fluorides) or as a secondary component (Sn02). They are used wherever the body colour is to be covered or to attain pastel colour shades, in particular in building ceramics. Lead-free glazes are likewise given preference in industrial production. 

When the same ceramic paste that is used for making terracotta is fired at higher temperatures (above 950°C), the material obtained is known as earthenware (see Fig. 55). Earthenware is more vitrified and therefore less porous and stronger than terracotta, although it is also opaque. Its porosity generally varies within the range 5-10%. Earthenware is often glazed. 

Bismuth subcarbonate [(BiOl CO ] is used to make other bismuth compounds, cosmetics, enamel, and ceramic glazes. Its major use is as an opaque substance placed in the digestive tract to show up on X-rays. The bismuth blocks X-rays, and thus the physician can see patterns inside the stomach and intestines. 

Glazes are thin, transparent coatings (colored or colorless) fused on ceramic materials. Vitreous enamels are thin, normally opaque or semiopaque, colored coatings fused on metals, glasses, or ceramic materials. Both are special glasses but can contain little silica. They have typically low melting points and are often not easily mixed in with more traditional glasses. 

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