Crystal glass

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Crystal glass , also hard glass , mirror glass or crown glass , is a colorless glass that often contains metal oxides or ions as additives. In the case of lead oxide, one speaks of flint glass . However, it is not crystalline in the physical sense , but amorphous like all types of glass. The term “crystal glass” refers to the increased refractive index and the increased dispersion compared to soda-lime glass , which cause refraction and color effects that are reminiscent of crystals such as quartz .

history

Already in the 15th century in was Venice crystal clear glass, and touted as "cristallo" in which a mixture of sand and potash in the pot furnace was heated to about 1400 ° C. Later this name was used for all glossy, colorless glasses with high light refraction, which could be produced in an optimal thickness for glass cuts without losing transparency .

It is true that there were glasses mixed with lead oxide as early as the Middle Ages ; however, the invention of the lead crystal in the Baroque era is attributed to the English. It was a crystal-clear type of glass that is ideally suited for cut glass.

composition

Lead-free crystal glasses were developed in Bohemia by adding calcium carbonate in the form of chalk to the glass mass - hence the name chalk glass or Bohemian crystal glass . Today, simple crystal glass is made by partially replacing the sodium oxide in the glass with potassium oxide. This gives the glass a more beautiful brilliance .

The Crystal Glass Labeling Act specifies which ingredients the glass must contain so that it can be designated as crystal glass. For the purposes of this law, crystal glass is a glass that either

  • Contains lead oxide (PbO), barium oxide (BaO), potassium oxide (K 2 O) or zinc oxide (ZnO) alone or together in an amount of at least ten percent, has a density of at least 2.45 kg · dm −3 and that is based on sodium -D-ray related refractive index is at least 1.52 or
  • Contains lead oxide (PbO), barium oxide (BaO) or potassium oxide (K 2 O) alone or together in an amount of at least ten percent, has a density of at least 2.40 kg · dm −3 and a surface hardness according to Vickers of 550 ± 20.

Around 1970, cut vases and bowls, sometimes two-colored in two layers, were marked with labels such as “ lead crystal (glass) 24% PbO content” as a quality feature. Since the toxicity of lead became public and that it got into drinking water through old lead water pipes and was emitted as lead oxide dust by tetraethyl lead in gasoline from car engines, the lead content of crystal glass has no longer been emphasized by producers and dealers. Lead is bound to be sparingly soluble in crystal glass, but it can be dissolved out by vinegar (acid) and should therefore not come into contact with food. Even with high-temperature processes such as waste incineration, lead can escape as an oxide in the form of dust.

See also

Individual evidence

  1. ↑ Entry on crystal glass. In: Römpp Online . Georg Thieme Verlag, accessed on October 19, 2015.
  2. ↑ Crystal Glass Labeling Act of June 25, 1971 (BGBl. I, p. 857) - legal text (PDF; 42 kB)