Cobalt glass

Smalt (also Schmalte, safflower or Zaffra ) is a cobalt (II ) oxide colored potassium silicate glass. The chemical formula for smalt is SiO2 K2O · · CoO.

History

Smalt is a precursor of cobalt blue the oldest known cobalt pigment. First applications of Smalt is ground into a powder pigment already existed in ancient Egypt. The medieval Venetian blue glass production is also based on the use of smalt.

In the 15th century smalt won in the European panel painting in importance. In the painting of the Baroque it was important for various sky. In the 17th and 18th centuries it was an important blue pigment. The most important production site for smalt was from the 16th century in Schneeberg in Saxony, so that the Electors of Saxony for a long time controlled a large part of the Central European Smalteproduktion.

In papermaking, smalt was formerly used as a whitener.

Production

For the production of smalt glass base is made ​​of quartz sand and potash, the potassium carbonate is mixed. For the blue color, the addition of about 5% to 7 % of cobalt (II ) oxide is required. These powders are mixed well. Then the mixture is melted at a temperature of 1150 ° C for 30 to 45 minutes. The resulting glass body is quenched in cold water, wherein it is divided into grains and blue can then be rubbed on the pigment powder. Smalt was mainly produced in specialized blue color works.

Properties

Smalt has good resistance to light, is acid and alkali resistant and compatible with all pigments. Milling is used as a blue color, which differ according to the fineness here the following ways:

• Scattered blue ( also grit ): coarse, angular powder
• Coleur: medium fine powder
• Eshel (also Aeschel ): finest powder

The intensity of color is dependent on the cobalt content. Rich cobalt smalt is referred to as royal blue, the darkest smalt as azure.

Use

The most common use of smalt made ​​in the glass and ceramics industry, where there is a color body for further processing. Smalt can be found in all binders and all techniques, it has been mainly used in oil and Kalktechniken. Occur occasional bleaching on the pigment surface by exchange reaction with oil-containing binders.

In Wertheim in Baden- Württemberg, the blue version of the framework in 1999 was made with smalt after original findings back to a half-timbered house (next to the County Museum ).