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Bohemian crystal became famous for its excellent cut and engraving.
They became skilled teachers of glass-making in neighbouring and distant countries.
The word "crystal" means, in most of the western world, the presence of lead.
In the European Union, labeling of "crystal" products is regulated by Council Directive 69/493/EEC, which defines four categories, depending on the chemical composition and properties of the material.
Several of these towns have their own glass museums with many items dating since around 1600.
Jablonec nad Nisou in particular is famous for the local tradition of manufacturing glass costume jewellery.
Bohemian glass-workers discovered potash combined with chalk created a clear colourless glass that was more stable than glass from Italy.Lead increases the weight of the glass and causes the glass to diffract light.Glass can contain up to 40% lead, if maximum hardness is desired.In the 16th century the term Bohemian crystal emerged for the first time in history to distinguish its qualities from the glass coming from other places.This glass contained no lead as is commonly suspected. In addition, resources such as wood for firing the kilns and for burning down to ashes were used to create potash.