Primary Materials

Silica (SiO2, silicone dioxide) is the most common moulder of the vitreous lattice and is therefore the most important primary material for glass production. About half of the earth's crust is made up of silica (silicates and quartz), the main component in rock and sand. However, natural silica does not, generally, possess the necessary characteristics for glass production, because it forms complex minerals with other oxides (like, for example, in clays and feldspars with alumina, Al2O3), and because it contains some elements like iron which, even in small quantities, gives glass an undesired colour. Only silica that contains less than 0.1% of iron oxide (Fe2O3) can be used for sheet production. To produce table-glass or artistic glass, however, this percentage drops to 0.01% and only small quartz deposits guarantee this amount. For glass used in eyewear the acceptable level is even lower, less than 0.001%. It is a very small amount, the equivalent of 10 milligrams per kilogram of sand! The content of other minerals is even lower, like chrome, cobalt and copper etc. oxides which have the power to colour which is higher than the iron one. No natural sand can meet the needs of eyewear glass; for this reason, even sand from the best deposits must be purified further with special treatments.

To lower the quartz fusion temperature (about 1700° C) a flux is added, generally sodium oxide. In current production, it is added in the form of carbonate (soda) or nitrate. Whatever its origin is, either natural or artificial, soda, at about 800°, breaks down into carbon dioxide (gas) and sodium oxide. Sodium oxide has the ability to react, in the solid state, with the silica thereby transforming the quartz in sodium silicates which melt at a lower temperature.
Potassium or potassium carbonate (K2CO3) behave in the same way, and are also produced industrially now. Soda (or potash), in addition to making the silica more meltable, has the property of lengthening the range of temperatures within which the glass solidifies (production range), and makes - in the jargon - the glass longer.

In Roman and late-Medieval times, the flux was natron, natural sodium carbonate which is found in salt lakes in the Middle East. Glass, melted in Syria, Egypt or Lebanon with local calcareous-siliceous sand, was exported in the form of rough blocks of glass to then be remelted and worked in glass centres located in all the Mediterranean basin area and northern Europe.

In the Middle-Ages, natron was substituted by plant ash. Depending on the geographical location of the glass works, the plants which were burned were of marine or land origin. In the marine types, which were used prevalently in the Mediterranean area, soda was extracted; from the ashes of continental plants (oak, beech, ferns...), used mainly in northern Europe, potash was obtained.
As it was still impossible to carry out chemical analyses to determine the level of alkaline carbonates in the ash (generally very low and variable), the glass works judged it by the quality of the colour, the smell and with the aid of taste.
When one wanted to produce pure and uncoloured glass it was necessary to extract sodium (or potassium) carbonate from the ashes, through leaching, melting the ashes in boiling water and filtering the insoluble residue. After leaching, about 40% of alkaline salts (carbonates, sulphates and chlorides) could be obtained from the best ashes. This was the Venetian glazier's main secret which led to the invention of crystal, a glass which is so clear and uncoloured that it can be compared to rock crystal (quartz).
It was only at the end of the 1700s, in France, that soda began to be produced artificially, using sodium chloride as the primary material (marine salt or rock salt). In 1791 Nicolas Leblanc set up a process for the production of artificial soda, which is much richer in sodium carbonate than natural fluxes, but still contains many impurities. With this method, a product was obtained containing 70-75% of sodium carbonate. One of the drawbacks of the Leblanc process was the high production costs.
In 1865 in Belgium, a new process was devised to extract sodium from marine waters, via treatment with ammonia, to transform it then into sodium carbonate. This Solvay process produces soda which is much better and much cheaper and which, appropriately perfected, is still used.

Sodium-silicone or potasic-silicon glass is not stable; all you need is atmospheric humidity to ruin the surface, forming whitish and corrosive layers. In water, this glass is perfectly soluble and is used today in dishwashing detergents. To obtain stable glass, part of the soda is substituted with other compounds which strengthen the vitreous lattice, thereby improving its chemical properties. This effect can come from the bivalent calcium oxides (CaO), magnesium (MgO), barium (BaO), lead (PbO) and zinc (ZnO), which for this reason are called stabilizers. Further improvement can be obtained by the introduction in the glass of other oxides like alumina (Al2O3) and boric acid (B2O3).
Calcium carbonate is found in nature in the form of marble or limestone. It breaks down, at about 1000°C into carbon dioxide and carbon oxide, which then becomes part of glass. Dolomite, a carbonate mixture of calcium and magnesium, is used to substitute, partially or completely, the calcium carbonate. Alumina is added, generally, in the form of alkaline feldspats (compounds made up of silica, alumina, and sodium oxides or potassium oxides), minerals which are abundant in the earth's crust and are easily meltable. It is used to improve the chemical resistance of the glass and to control the melting viscosity. Lead is added in the form of oxide produced industrially (minimum, Pb3O4 or litharge, PbO).
A high percentage of lead lowers the fusion temperature, reduces the glass hardness and increases its shininess. Glass is a totally reversible material. It can be remelted and moulded an infinite number of times without losing or changing its properties. For this reason, scrap glass has become, for certain productions, one of the most important primary materials.
In blast furnaces for producing coloured glass, over 60% (in some cases nearly 100%) of the vitrifiable mixture is composed of recycled scrap, that is, glass from public rubbish collections (recycling or external scrap). All vitrifiable mixtures must contain a bit of scrap, in that it accelerates the fusion of the vitrifiable mixture and saves energy and primary materials. Every glass work saves and reuses its own production scraps (internal scrap) in the mixture.

The vitrifiable mixture is still not complete. The molten is a viscous fluid in which there are numerous gassy bubbles formed by the decomposition of the carbonates or other origins. To get rid of them, what are called thinner compounds are added, like arsenic oxides (As2O5) and antimony oxides (Sb2O3) associated with nitrates. Up until the industrial era, only manganese dioxide (MnO2) was used. In continuous modern furnaces, the principal thinner are the sulphates added to small quantities of reducing agent compounds (carbon, blast furnace slag etc.).
These compounds break down at a high temperature (over 1200° C) releasing oxygen bubbles that, going back up the molten, absorb the bubbles they meet up until reaching the surface. Crossing the glass layers at different densities, the bubbles also play a role in the homogenization of the molten.

Glass, obtained in this manner, is still not transparent and uncoloured, or coloured glass for artistic glass works. It is not enough to use synthesis primary materials or choose the purer ones; some elements, like iron and chrome, are always present even if in small amounts, but in any case enough to slightly colour the glass. Another component must be added to the mixture: a decolourant. These are certain elements which in small quantities correct the colour tone on the basis of a physical principle (overlapping of a complementary colour which cancels the iron one, for example) or a chemical one (oxidisation or reduction of the colouring element; iron, for example, concentrations being equal, colours much more intensely if it is found in the reduced state compared to the oxidised one).
The most well known colourant, which acts in both ways, is manganese dioxide which, for this specific property, was called glazier's soap. However, manganese, placed in the glass, still has the ability to capture solar light energy and then to oxidise itself, giving the glass a yellow-violet colour. An example are the lamps which illuminate piazza San Marco in Venice. Initially uncoloured, because of the manganese they have become violet, thereby giving off a soft light which has become a feature of the piazza at night. Because of this instability of manganese, today the element is substituted by a mixture of elements like selenium, cobalt and rare earth which, measured out singularly, give a more complete and stable result.

For the production of coloured glass, recourse is taken to the use of appropriate substances in the mixture. The intensity of the colour depends on the quantity of the colouring agent used in the glass composition, on the presence or lack of the presence of oxidising substances or reducing agents in the furnace atmosphere, on the thermal management of the fusion and the type of colour (ionic or colloidal).
The following table lists come of the principal colouring elements and compounds with the relative effects on the basis of the oxidising or reducing operation conditions.