is manufactured in several colour intensities and with varying grey proportion (smoked glass). Colour glass is used separately or combined with crystal or other colours. When combining glass of different colours it is essential to use glass of equal expansion coefficients to avoid a subsequent rupture of the product.
Colour glass usually has the same composition as crystal glass, the refining, however, may differ in order to achieve a certain redox balance of colouring oxides, present in the glass in higher valance. Colour glass containing 24 % PbO is made only exceptionally – such glass is suitable for cutting and subsequent chemical polishing.
Due to the large variety of colours and relatively moderate volume of orders, pot furnaces are used for fusing such glass. Exceptionally, also small tank furnaces are used – profiting from electric heat source. In such case, an optimum exchange of colour tones must be secured and as well as the application of transition colours (i.e. progressive addition of oxides – starting from clear crystal and passing to yellow, green and blue up to black; ruby bed is mixed separately).
The term crystal glass stands for clear (untarnished), perfectly discoloured glass with high brilliance, i.e. high glance, light refraction, translucency, glistering etc.
Historically, both potash-lime glass made in Bohemia (Bohemian crystal) and British and French lead glass were called crystal. In many countries, efforts striving for a more precise delimitation and definition of "crystal glass" resulted in the issuance of regulations and standards classifying glass types and defining crystal glass based on the composition and certain physical and chemical properties. EU-countries (France, Germany, Italy, Benelux countries, UK, Ireland, Denmark) introduced a Glass – Crystal Directive.