COLLE DI VAL D'ELSA (Siena) - Italy

La Grotta del Cristallo © was founded in 1998 and is nowadays an enterprise where the processing of crystal is still deeply based on handicraft.

Our laboratory still working from 1966.

Analysing the tradition of Colle Val d'Elsa, centre of brightness, and applying daily technologies in the processing of crystal, La Grotta del Cristallo  has succeeded to produce both classical and modern crystals that are unique pieces of work and results of the work, the talent and the imagination of its master of crystal.

Definition, structure, types of glass and crystal

Glass is an homogeneous material with a random, non crystalline (liquid-like) molecular structure. The manufacturing process requires that the raw materials (batch) be heated to a temperature sufficient to produce a completely fused melt, which when cooled rapidly, becomes rigid without crystallizing. Within the three classical states of matter, there is no place for wood, rubber, plastics, dough, living cells, glass, or crystal. Glass is the fourth state of matter that combines the rigidity of crystals with the random molecular structure of liquids. It is often described as vitreous or glassy state. The atom in glass, although arranged at random, are frozen in position. Thus, glass combines some of the aspects of a crystalline solid and liquid; it is rigid like a solid but its random atomic structure is characteristic of a liquid. The American Society for Testing and Materials define glass as ”.....an inorganic product of fusion which has cooled to a rigid condition without crystallizing”.

Composition

Thousands of different chemical compositions can be made into glass. Different formulas affect the chemical, electrical, optical and thermal properties of the glasses that are produced. There is no single chemical composition which characterizes all glass. Glass can do most anything. From bottles to spacecraft windows, glass products include three types of materials: Formers are the basic ingredients. Any chemical compound that can be melted and cooled into a glass is a formers.  Fluxes help formers to melt at lower, more practice alto-achieve temperatures.

Stabilizers

Combine with formers and fluxes to keep finished glass from dissolving, crumbling, or falling apart. Chemical composition determines what a glass can do. There are already tens of thousands of workable glass compositions and a new one are being developed every day.

Types of glass and crystals

Nearly all commercial glasses fall into one of six basic categories are based on chemical compositions.

1. Soda-lime glass is the most common (90% of glass made) and least expensive form of glass. It usually contains 60-75% silica ,12-18% soda and 5-12% lime. Resistance to high temperature are not good and resistance to corrosive chemicals is only fair.

2. Lead glass (Crystal) has a high percentage of lead oxide (at least 20%of the batch). It is relatively soft and its refractive index gives brilliance that may be exploited by cutting. La Grotta del Cristallo is specialized in this type of work. It is somewhat more expansive than soda-lime glass and is favoured for electrical applications because its excellent electrical insulating properties. Thermometer tubing and art glass are also made from lead-alkali glass commonly called lead glass. This glass will not withstand high temperatures or sudden changes in temperature.

3. Borosilicate glass is any silicate glass having at least 5% of boric oxide in its composition. It has high resistance to temperature change and chemical corrosion. Not quite as convenient to fabricate as either lime or lead glass, and not as low in cost as lime, borosilicate’s cost is moderate when measured against its usefulness Pipelines, light bulbs, phototronic glasses, sealed-beam, head-lights, laboratory ware and bake ware are example of borosilicate products.

4. Aluminosilicate glass has aluminium oxide in its composition. It is similar to borosilicate but is greater chemical durability and can withstand higher operating temperatures. Compared to aluminosilicates are more difficult to fabricate. When coated with anelectrically conductive film, aluminosilicate glass is used as resistors for electronic circuitry.

5. Ninety-six percent silicaglass, melted and formed by conventional means, then processed to remove almost all the non silicate elements from the piece. By reheating to 1200°C the resulting pores are consolidated. This glass is resistent to heat shock up 900°C.

6. Fused silica glass is pure silicon dioxide in the non crystalline state. It is very difficult to fabricate, so it is the most expansive of all glasses. It can sustain operating temperatures up of 1200°C for short periods.

Difference between glass and crystal

The main difference between Glass and Crystal lies in the presence of leads oxide. This oxide gives glass certain Characteristics that make it particularly interesting. Pbo, in fact, increases the density of glass and consequently greatly increases its refraction index. This kind of glass thus approaches the optical properties of diamonds: the light that touches it is refracted much more than normal glass thus giving it that particular brilliance typical of lead crystal. The greater the content of lead oxide, the greater is his effect. Only glass that contains at least 24% lead oxide can be legally called CRYSTAL or Crystal glass. A higher content of lead oxide, however, would give a yellowish colour to the glass. Only with the close collaboration between specialised chemist and master glassblowers can the composition turn into a clear, transparent, brilliant and stable material, worthy of being called CRYSTAL.

Normativity	Characteristics			Label
Category	Oxide metal in %	Density gr/cm³	Refraction Index	Symbol
Crystal over 30%	Lead >=30%	>=3,00	Nd>=1,5450	Round label O gold colour
Lead Crystal	Lead >=24%	>=2,90	Nd>=1,5450	Round label O gold colour
Sound glass best quality	Zinc, barium, lead, potassic >=10%	>=2,45	Nd>=1,5200	Squared label □ silver colour
Sound glass	Zinc, barium, lead, potassic >=10%	>=2,40	Nd>=1,5200	Triangular label Ñ  silver colour