The Definition and Composition of Crystal
The European Union definition of crystal, which is high quality glass, is that must have an index of refraction
of at least 1.520 and have a density of more than 2.45kg/l. The EU define Lead crystal as having a lead oxide
content of at least ten percent. It does permit contain that the glass for stemware may contain oxides
of potassium, zinc or barium.
More specifically the European Standards say that glass containing less than 4% lead oxide (PbO)
must be referred to as 'glass' whilst adding PbO to the composition above this percentage is referred to as
'crystal'. It should be noted that American percentages are different where pure glass is defined as
having up to 1% PbO and concentrations above this are defined as crystal.
The highest grade is where the PbO percentage is higher than 30% and this is commonly known as 'high lead crystal'.
Lead glass as used in lead crystal is potassium silicate glass that has been impregnated with 10-28% of lead oxide by weight during its manufacture. Lead plays two key roles in the case of stemware and decorative glass . Firstly to increase the refractive index of the glass and as a consequence the lustre or sparkle and secondly, to soften the glass making it easier to cut for decorative purposes. The reason why lead crystal glass has a much greater sparkle than conventional glass is because the lead oxide added to the molten glass increase index of refraction for light.
The composition of common glass is about 70% silicon dioxide, known as silica, which is an identical chemical compound to that found in quartz and its corresponding polycrystalline form which is sand.
Crystal and Quartz
Quartz, in its pure state is the base element of glass. Historically the quartz mined in the north of Bohemia was ground into sand and combines with deposits of iron oxide to produce normal glass for everyday usage. This everyday glass has a greenish tint which can be seen by looking at the side of machine cut sheet glass panels. To produce crystal the iron and other impurities need refining and additives of lead oxide (PbO) are added to the glass in its raw mineral form. This difference in composition is illustrated the fact that crystal resembles a true and colourless form of glass heavily contrasting with the visible attributes of window panes and other industrial glass products. Crystal is characterised by absolute clarity and brilliance with exceptional quality and hardness, produced by complex material chemistry independent of the fact that the composition may contains lead or not. It is evident that the addition of PbO to glass will increase clarity, brilliance and light refracting abilities and the corresponding colour spectrum emitted from a finished product. This is evident when viewing a cut crystal pendant hanging from a chandelier or a crystal vase viewed under a direct light source. In addition, lead oxide produces material softening of crystal which makes it easier to manipulate but lengthens the cooling time needed once the molten glass is taken from the kiln before for the glassmaker can begin work. In fact, compositions containing proportions of additional elements for instance calcium, potassium, barium and lead oxide all will affect a products durability and resistance to abrasion. Accordingly, the material composition is a key factor to be considered at the outset of manufacture so the material characteristics are right for the glassware artist to engrave the product and produce an item of beauty.
Differentiating Crystal and Glass
With respect to the characteristic differences between crystal and glass, the term crystal is often used to refer to the clarity of an object. In fact the word crystal derives from krystallos which is the Greek word meaning 'clear ice'. Romantics may say that crystal brings illusions of pure water in perfect stillness and is synonymous with absolute clarity. As far back as the 13th and 14th centuries workers in Bohemia began blowing glass in monastery workshops. This led to an increase in popularity in Europe during the late 13th century and the construction of 20 glassworks in Bohemia and Moravia. Today this number has grown and there are more than 40 production factories. Products around this era were predominately functional ones for commoners and exotic glass tableware was strictly the reserve of members of high nobility. The nobility were so fascinated by glassware they built glassmaking furnaces on their estates. The Bohemian glassmakers of the 17th century made progressive contributions to the advancement of glass and crystal production through the modifications of the chemical composition of molten glass. Their resultant secret recipes would further be known worldwide as Bohemian Glass. The glass composition they created consisted of potash-lime glass and calcium carbonate (or chalk) which produces a glass which is more suited for the intricacy of complex engravings and hand cutting methods. Through this it was now possible to produce crystal with exceptional hardness but still had good material properties for cutting and engraving. Adding a higher percentage of PbO results in a slight increase in mass and the enhancement of its brilliance, sparkle and transparency. This specialist knowledge is still the reserve of Bohemian glass and crystal makers who produce glassware today which is considered to be consistant with the Bohemian style artistry. Clear glass is now complemented with ruby glass, high enamel painted glass, and gold sandwich glass which are now popular examples of how the techniques have advanced to create the products in todays market.
Czech glassmakers represent the pinnacle in artistic design and flair. With crystal as a medium it has been the freedom and unburdened artistic exposure to glass which has evolved the trade. For centuries the glassware evolution is connected with time proven craft quality and represents the cornerstones of excellence. This has been the inspiration for the new generation of glass designers and artistic glassmakers that today produce fabulous products advancing the classic tradition of the Czech glassmaker.