The Components of Glass
What is glass made of ?
The most common types of glass used today (for construction, containers, etc.) are soda-lime glasses, i.e. mainly composed of silica (SiO2), sodium (Na) and calcium (Ca). Silica is the main component of glass: nearly 70% of the mass. Quartz sands contain mainly silica. Silica is the vitrifying element (oxide forming the vitreous network) of glass.
The melting point of silica is 1700°C. In order to lower this temperature (to less than 1400°C), sodium carbonate (Na2CO3) is added as a flux, which represents about 14% of the mass. In the past, and elsewhere in the world, other flowing agents have been used.
Finally, limestone (CaCO3), magnesia (MgO) or alumina (Al2O3) are added to stabilise these raw materials at a rate of approximately 10% by mass.
“Cullet” or “frit” (recycled, cleaned and ground glass) is also used to lower the melting temperature. It is added in varying proportions to these raw materials.
Additives and colorants make up 5 to 6% of the mass.
The dyes are metal oxides. Introduced into the structure of the glass as ions, they absorb one or more wavelengths of white light. It is therefore the selective interception of radiance that produces a given colour. Cobalt oxide, for example, absorbs the colour orange, therefore producing blue glass.
All these elements, brought together in very precise proportions, are heated to approximately 1500°C in a furnace in order to transform them into liquid glass.
Where do the raw materials come from ?
Silica is one of the main constituents of the earth's crust (more than 60%). In nature, silica is very common in quartz form: as quartz, as chalcedony, or in amorphous form, as diatomaceous earth (consisting of the remains of unicellular aquatic microalgae with a silicon shell).
Limestone (CaCO3), a rock mainly made up of calcium carbonate, is the main stabilizer of glass. The limestone used for antique glass came from carbonated shells in the sand. Today, glass manufacture benefits from extremely pure natural limestone.
Over the years, various fluxes have been used. Most have alkaline elements in common, such as sodium (Na), potassium (K) and even calcium (Ca). The fluxes in the first glasses were ashes from halophytic plants (plants adapted to salty environments, such as glasswort or mangrove). These have long been used as flowing agents to produce Venetian glasses.
In Roman times, natron was used, a mineral soda exploited in the emerged part of salt lakes or lagoons in Lower Egypt (the north of present-day Egypt). The term “natron” comes from the Ouadi Natroun, or Wadi el Natrun (“natron valley”, an arid region in Egypt's western desert, west of Cairo). The Romans manufactured natron (sodium) type glass on an industrial scale along the eastern coast of the Mediterranean (Israel, Syria, Palestine). This glass was transported by sea to Europe where workshops re-melted it and transformed it into a finished product.
In medieval Europe, wood ash was used. This produced potassic glasses, which were less stable than older glasses. Local raw glass production and the manufacture of finished objects were located under one roof (“Hüttenglasproduktion”). Subsequently, lead-based glasses (Bohemian glass, English crystal) were produced.
Currently, other fluxes such as borax are used for heat-resistant glass. Industrial soda ash is also produced.
Here are some examples of metallic colouring agents and the colours they produce :
Copper (I) oxide or cuprous oxide (Cu2O) green
Copper (II) oxide ou cupric oxide (CuO) blue
Chromium (Cr) green or green-blue
Iron (II) oxide or ferrous oxide (FeO) brown
Mangangese (II) oxide (MnO2) violet
Colour changes can be observed depending on the oxidation stage of the ion.
Antique glass produced from natural raw materials had a green or bluish tint. It was therefore necessary to bleach them in order to make them look good. The Romans used antimony or manganese to obtain pure clear glass.
Venetian glass or crystal was made from extremely pure quartz sand and plant ashes, which were leached to extract the salt. It was Angelo Barovier, a descendant of a long family tradition of glassmakers, who around 1450 came up with the idea of purifying plant ashes.
The famous ruby red glasses are coloured by nanoparticles of metallic copper and gold.
Coloured with copper, red glass becomes opaque after a certain thickness. To preserve its transparency, this glass is almost always made up of thin layers of red deposited on transparent glass: the layers of colours are superimposed by dipping the gob (the mass of molten glass) into one or more layers of red molten glass.
U. Bergmann, R. Hasler, Y. Jolidon, A. Kaiser, B. Kurmann-Schwarz, S. Trümpler, Raconte-moi le vitrail, Musée Suisse du Vitrail, Romont, 2001
M.-H. Chopinet, Les couleurs du verre (industriel), 7e Entretiens Physique-Industrie, 2003
J.-M. Haussonne, Céramiques et verres : principes et techniques d’élaboration, Presses Polytechniques et universitaires romandes, 2005
J.-P. Houdaer, La coloration des verres, in Cours sur la couleur au Collège de France, 2005
Jean Phalippou, Verres - Aspects théoriques, Techniques de l'ingénieur, 2001
Jerzy Zarzycki, Les verres et l'état vitreux, Masson, 1982
Glaströsch, Le verre et ses applications, Glas Trösch Holding, Bützberg, 2013