To distinguish a glass object from a crystal one, a careful eye might be needed. In fact, crystal is very similar to glass , but it has particular characteristics of brightness, transparency, elasticity and sonority.
Characteristics that make it much more valuable than glass, despite their similar appearance. But the differences lie deeper: the presence of lead in the chemical structure of crystal, in fact, plays a fundamental role.
Tuscany is one of the largest producers and exporters of crystal, thanks to the perpetuation of the ancient and difficult glass art that has become a profitable economic activity today, one of the few sectors that also offers work to young people. Suffice it to say that in Colle Val d'Elsa, which is not by chance called the 'Italian Bohemia', 14 percent of the world's crystal and 95 percent of the Italian one are produced.
In this article we will see what differences characterize these two materials, we will understand how they are worked and how they are produced.
Definition and characteristics of glass?
The process of creating glass is obtained by melting a mixture of silicates at high temperatures , and then letting it cool. Among these we find quartz and other chemical substances, including boric anhydride and phosphoric anhydride, called "vitrifying substances" .
The material thus obtained is used to make windows, glasses, vases, chandeliers and many other objects.
Glass is a homogeneous material with a random and non-crystalline molecular structure ( similar to liquids ).
The manufacturing process requires that the raw materials be heated to a temperature sufficient to produce a completely molten mass of material. This, if cooled rapidly, becomes rigid without crystallizing .
If we consider the three classical states of matter (solid, liquid and gas), glass is like a fourth state of matter that combines the rigidity of the solid with the molecular structure of liquids. It is often described as the Glassy state .
Glass can be melted down again and reused an infinite number of times , which is why we push for separate waste collection.
Many countries use the “returnable” system to recover used bottles and reintroduce them into the market at minimal cost.
Definition and characteristics of crystal?
Crystal is a glass that has certain characteristics of brightness, transparency, elasticity and sonority. These qualities are provided by the presence of Lead Oxide .
Thanks to the lead, in fact, the crystal is brighter , because it has a greater refraction and to the touch it is more sonorous than glass.
This also increases the resistance of the material, allowing the production of objects that are much thinner than glass, but also more delicate. In particular, a glass becomes crystal only if it contains at least 24% lead.
The name “crystal” comes from the fact that it has a reticular structure , called “crystal lattice”, formed by atoms and molecules arranged in an orderly and regular manner.
Contrary to what intuition suggests, it is a relatively soft material. Furthermore, its characteristic brilliance , due to the refractive index, can be accentuated with cutting and engraving techniques.
It is slightly more expensive than soda lime glass and is preferred for use in the electrical field due to its excellent insulating properties.
The thermometer tubes and the artistic one are made of Lead Alkali Glass, commonly called Lead Glass.
This type is not resistant to high temperatures or sudden changes in temperature.
How is crystal worked?
As already mentioned, the base is transparent crystal glass in which the calcium components of common glass are replaced with lead oxide (PbO).
Crystal typically contains 18-35% lead (PbO). Lead oxide adds to the glass a higher density , lower heat conductivity and a higher refractive index (and therefore greater brilliance). In addition to this, it also has greater resistance (products containing lead oxide weigh more), although at the same time they are more delicate.
To obtain a colored crystal, different additives ( metal oxides ) are used . Thanks to this addition, crystals can be obtained that vary from ruby red, to violet blue, even to amber.
In this process, the finished product will be obtained, which is created by joining the transparent crystal layer with a colored one.
At this point the glassmaker blows the material at a temperature of about 800 °C. The final shape will then be obtained by pouring the still liquid crystal into a wet beech wood mold into which the master glassmaker will blow for the last time.
Differences between glass and crystal
It seems useless to say at this point, the substantial difference between glass and Crystal is given by the presence of lead oxide . This gives glass some characteristics that make it particularly interesting.
Lead, in fact, causes the density of the glass and therefore the refractive index to increase significantly. In this way the glass approaches the optical properties of diamond: the light that hits it is refracted more decisively than with normal glass, thus giving that effect of great brilliance typical of lead glass .
The effect is greater the higher the lead oxide content; only glasses containing at least 24% lead oxide by weight are legally allowed to be called crystal or “Crystal Glass”.
Such a high addition of lead oxide, or even more, would give a yellowish hue to the Glass. It is only the wise correction in the composition of the Glass made by specialized chemists in concert with master glassmakers that make the Lead Glass clear, transparent, brilliant, stable, worthy of being called Crystal.
To make it easy to recognise, all European countries have introduced an important EEC regulation (69/493) which characterises different types of glass with their own labelling based on their composition , refractive index and specific weight.
For centuries, the production of glass and crystal has represented a resource for the territory of Tuscany. We at Amarzo, with our production of 100% recycled glass design products, also participate in this extraordinary tradition. Come and discover the products on our Website .
Article by Renato Sarlo
