Glasses are amorphous (non-crystallized) materials, with high mechanical strength and hardness, with a low coefficient of expansion. At higher temperatures they behave like superchilled liquids with high viscosity. They have no defined melting point. By heating it gradually softens, until it liquefies, which allows the glass to be processed by blowing, pressing, casting, rolling.

The bottles are generally obtained by melting in special furnaces a mixture consisting of quartz sand, limestone, sodium (or potassium) carbonate and auxiliary materials. The physical properties of bottles are determined by their composition.

Regular glass (sodium or potassium glass)
Sodium glass has the approximate composition 6SiO2•CaO•Na2O. It is used in the manufacture of glass and glass packaging. Potassium glass has the composition 6SiO2•CaO•K2O and is resistant to temperature variations. It is used in the manufacture of laboratory vessels.

Crystal (lead glass)
It is a glass in which sodium and calcium have been replaced by potassium and lead (6SiO2•PbO•K2O) and is characterized by good refractive properties and high density. Flint and strass contain a higher percentage of lead than crystal. Flint is used for prisms and optical lenses.

Jena, Pyrex or Duran glass
By adding small amounts of Al2O3 or B2O3, bottles resistant to sudden temperature changes are obtained, which are used in the manufacture of laboratory vessels. They have a high chemical resistance and a low coefficient of expansion.

Colored glass
They are obtained if some metal oxides (Fe, Co, Cr, Cu, etc.) are added to the melt, which form colored silicates. In the glass industry, a very large number of substances are used as colorants, which usually fall into three categories: ionic colorants, molecular colorants, and colloidal colorants.

Ionic dyes are generally metal oxides. For example, red glass also contains copper oxide, yellow glass cadmium sulfate, blue glass cobalt oxide, green glass chromium oxide, purple glass manganese oxide. Uranium trioxide gives a yellow-green color accompanied by a beautiful green fluorescence.

The molecular dyes are represented by selenium, which gives a pink color, by sulfur, which gives a yellow or yellow-brown color, and especially by sulfides and selenides of different elements. Very used is the mixture CdS + CdSe which gives a ruby-red color whose shade depends on the ratio between the two components.

Colloidal dyes are actually the metals that, through appropriate heat treatments, are dispersed in the form of a colloidal solution, printing colors on the glass that depend on the size of the colloidal particles. Thus, the fine gold dispersed in the glass gives a very beautiful ruby-red color. Silver gives shades from yellow to brown.

Colored bottles are melted in crucibles with capacities of hundreds of liters or in small furnaces where the temperature, and especially the character of the medium, can be rigorously controlled.

Colored bottles are used outside household items, in many important fields.

The big consumers of colored glass are air transport, ships, land. Light signaling in transport is of particular importance, the colors usually used are red, green, blue and yellow. Colored bottles are also used as filters for certain radiations. To protect the eyes of welders or those who look into incandescent furnaces, the so-called cobalt glasses are used, as well as other glasses that can retain caloric or ultraviolet radicals. Colored filters are part of optical or analysis devices, used in physics, chemistry or photographic laboratories.