Traditionally it has been considered that matter could occur in three forms: solid, liquid and gaseous. New means of investigation of its intimate structure—particularly during the century—have revealed other forms or states in which matter can present itself. For example, the mesomorphic state[16]​ (a liquid form with its smectic,[17]​ nematic[18]​ and cholesteric[19]​ phases), the plasma state "Plasma (state of matter)") (or plasmatic state, typical of ionized gases at very high temperatures) or the vitreous state, among others.

Bodies in the vitreous state are characterized by presenting a solid appearance with a certain hardness and rigidity and that, when faced with moderate external stresses, they deform in a generally elastic manner. However, like liquids, these bodies are optically isotropic, transparent to most of the electromagnetic spectrum of visible radiation. When its internal structure is studied through means such as X-ray diffraction, it gives rise to diffuse diffraction bands similar to those of liquids. If they are heated, their viscosity gradually decreases – like most liquids – until reaching values ​​that allow them to deform under the action of gravity, and for example take the shape of the container that contains them as true liquids. However, they do not present a clearly marked transition point between the solid and liquid state or "melting point".[20]​.

All these properties have led some researchers to define the glassy state not as a distinct state of matter, but simply as that of a subcooled liquid or liquid with such a high viscosity that it gives it the appearance of a solid without being one.[21]​[22]​ This hypothesis implies the consideration of the glassy state as a metastable state to which a sufficient activation energy of its particles should lead to its equilibrium state, that is, that of a crystalline solid.

In support of this hypothesis, the experimental fact is adduced that, when a body is heated in a glassy state until it obtains a clearly liquid behavior (at a temperature high enough for its viscosity to be less than 500 poise, for example), if it is cooled slowly and carefully, at the same time providing it with the activation energy necessary for the formation of the first solid corpuscles (seeding of microcrystals, presence of activating surfaces, nucleation catalysts, etc.), it usually occurs. solidify giving rise to the formation of sets of true solid crystals.

Everything seems to indicate that bodies in a glassy state do not present a specific internal order, as occurs with crystalline solids.[20]​ However, in many cases an ordered disorder is observed, that is, the presence of ordered groups that are distributed in space in a totally or partially random manner.

This has led different researchers to propose various theories about the internal structure of the glassy state, both geometric, based on both atomic and energy theories.

According to the geometric atomic theory, in solid crystallized silica the silicon atom is surrounded by four oxygen atoms located at the vertices of a tetrahedron, each of which unites it to neighboring silicon atoms. A plan view of this arrangement is schematized in Figure 1, in which the fourth oxygen would be above the plane of the page. When this silica passes into the glassy state, the tetrahedral arrangement is still maintained at the individual level of each silicon atom, although the bonds between oxygen and silicon atoms are made in an apparent disorder,[23]​ which nevertheless maintains an initial unitary organization (see figure 2).

However, none of these theories is sufficient to explain the complete behavior of the vitreous bodies, although they can serve to answer, in specific and well-determined cases, some of the questions that arise.

Substances capable of presenting a glassy state can be both inorganic and organic in nature, among others:

• - Chemical elements: Si, Se, Au-Si, Pt-Pd, Cu-Au.

• - Oxides: SiO, BO, PO, and some of their combinations.

• - Compounds: AsS, GeSe, PS, BeF, PbCl, AgI, Ca(NO).

• - Silicones (substances considered semi-organic).

• - Organic polymers: such as glycols, sugars, polyamides, polystyrenes or polyethylenes, etc.

Silica glasses

Silica is called "Silicon oxide (IV)") to a silicon oxide with the chemical formula SiO. It occurs in a crystalline solid state under different enanciotropic forms. The best known are quartz (the most common and stable at room temperature), cristobalite and tridymites. In addition to these forms, up to twenty-two different phases have been identified, each of them stable from a perfectly determined temperature.

When quartz is heated slowly, it goes through different enanciotropic forms until it reaches its melting point at 1723 °C. At this temperature, a colorless and very viscous liquid is obtained that, if cooled relatively quickly, becomes a glassy substance that is usually called quartz glass.

This quartz glass presents a set of properties that are very useful and applicable in multiple disciplines: in scientific and technological research, in domestic life and in general in all types of industry. The following stand out as most relevant:

Other properties, however, make its preparation and use difficult. In particular, the following:

Sodium silicate glasses

The most common sodium salts have melting points below 900 °C. When an intimate mixture of finely divided quartz is heated with a salt of these alkali metals, for example NaCO, to a temperature greater than 800 °C, a fusion of the alkali salt is initially obtained, the liquid of which surrounds the quartz grains, producing a series of reactions that can be included in the following result:

SiO + NaCO

NaSiO + CO H = -5.12 kcal/mol.

This slightly exothermic reaction releases gaseous carbon dioxide - which bubbles through the melting mass - and leads to a first sodium silicate, with a melting point of 1087 °C.

According to thermodynamics, the mixture of two substances with different melting points has a "liquidus point"[24]​ that is between those of the two substances in contact. In this way, the mixture of silica and sodium silicate formed gives rise to a product of SiO and silicates, already in a liquid state at temperatures that do not exceed 1200 °C, far from the more than 2000 °C necessary to prepare quartz glass.

The product thus obtained is commonly given the generic name sodium silicate, although this name identifies a set of products derived from the fusion of quartz with sodium salts (generally carbonates) in different proportions of one component and the other. Industrially, sodium silicates are prepared with molar proportions of each component between:

These sodium silicates have a glassy, ​​transparent and very brittle appearance. To achieve a viscosity of the order of 1000 poise (necessary for molding), temperatures are required that, depending on its composition, range between 1220 °C for the silicate richest in SiO, and 900 °C for the poorest. They are very soluble in water: between 35% and 50% by weight of silicate, depending on the SiO content. Their lack of mechanical rigidity and their solubility in water make them useless as substitutes for quartz glass in any of its applications.

They are rarely presented in the industry in solid form, but rather in the form of an aqueous solution. Its solution in water is used as a very effective ceramic glue or as a raw material for the production by hydrolysis of silica gel, a substance used as a moisture absorber (gas drying towers, etc.) or as a component of certain products such as vehicle tires and other applications in the chemical industry.

Its production is carried out in continuous balsa furnaces heated by the combustion of petroleum derivatives and frequently also with electrical energy, at temperatures as high as possible (within a certain profitability) in order to increase the productivity of the furnace. These temperatures are usually between 1400 °C and 1500 °C.

Sodium calcium silicate glasses

In order to obtain a product with properties similar to those of quartz glass at lower temperatures and therefore that are achievable by technically profitable means, a sodium silicate glass is produced to which other components are added that make it more mechanically resistant, and that stabilize the structure of the glass to prevent it from dissolving in water and therefore being more inert to chemical agents at room temperature, and that maintain its transparency to light, at least in the visible spectrum.

These components are alkaline earth metals, particularly magnesium, calcium or barium, as well as aluminum and other elements in smaller quantities, some of which appear contributed as impurities by the raw materials (in the case of iron, sulfur or others). The raw materials used to make glass of this type are chosen from those that have a lower price:

• - For quartz:

• Feldspathic sands, with SiO purity greater than 95% and with the lowest possible content of ferrous components (between 0.15% and 0.01% in terms of FeO)

• Ground quartzites.

• - For sodium:

• Natural sodium carbonates (deposits in the United States and Africa).

• Synthetic sodium carbonate, the most used in Europe.

• Synthetic sodium sulfate, byproduct of the chemical industry.

• Natural sodium nitrate (Chile nitrate).

• Sodium chloride or common salt.

• These last three, used in small proportions, due to the release of polluting gases during glass production: SO, NO, Cl.

• - For calcium:

• Natural limestone.

• - For magnesium:

• Natural dolomites.

• - For barium:

• Natural barium sulfate (barite).

• - For aluminum:

• Natural feldspars (kaolins).

The industrial production of this type of glass is carried out, as in the case of sodium silicates, in glass furnaces, generally made of raft, heated by the combustion of petroleum derivatives with the support, in many cases, of electrical energy at temperatures ranging between 1450 °C and 1600 °C. A slightly moistened powder mixture (5% water) and previously dosed of the raw materials already mentioned is introduced into these ovens. This mixture of mineral materials reacts (at appreciable speeds and, obviously, the higher the better) to form the set of silicates that, combined and mixed, will give rise to that substance called .

The properties of common glass are a function of both nature, raw materials, and the chemical composition of the product obtained. This chemical composition is usually represented in the form of percentages by weight of the most stable oxides at room temperature of each of the chemical elements that form it. The compositions of the most used sodium silicate glasses are within the limits established in the attached table.

Many studies—particularly in the first half of the century—have attempted to establish correlations between what was called the internal structure of glass—generally based on atomic theories—and the properties observed in glasses. The product of these studies was a set of relationships, of an absolutely empirical nature, that represent many of these properties in a surprisingly precise manner through linear relationships between the content of the chemical elements that form a given glass (expressed in the form of the percentage content by weight of its most stable oxides) and the magnitude representing said property. Interestingly, correlations with compositions expressed in molar or atomic form are much less reliable.

The MgO, FeO and SO contents are a consequence of the impurities of limestone, sand and sodium sulfate, respectively*.*.

The light absorption (or transparency) of sodium silicate glasses in the visible spectrum area (0.40 μ to 0.70 μ) depends on their content of transition elements (Ni and Fe in the example). However, in both ultraviolet and infrared, glass behaves practically like an almost opaque object, independently of any of these elements.

• - (1) The density is somewhat higher than in fused quartz (2.5 versus 2.2 g/cm³).

• - (2) The coefficient of linear thermal expansion at room temperature is noticeably higher than that of fused silica (about 20 times higher), so sodium silicate glass objects are less resistant to "thermal shock".

• - (3) Its refractive index is slightly higher than that of quartz glass and can be increased by using additives.

• - (4) The tensile strength in any type of glass is a magnitude that depends extraordinarily on the state of the surface of the object in question, so its quantification is complex and unreliable.

• - (5) The resistance to chemical or physical attack (dissolution) of common glasses is fundamentally a function of their chemical composition. However, in all of them this resistance is high. It is usually measured through a series of internationally standardized tests. Among the most used:

• DIN 12116

• DIN 52322

• DIN 12111

• The attackability of glasses is also modified by surface treatments with SO, Sn, Ti, and others.

• - (6) To mold glass, a viscosity between 1000 poise and 5000 poise is necessary. In the case of silica, temperatures of more than 2600 °C are necessary, while for common glasses approximately 1 200 °C is sufficient.

• - (7) The absorption of light is influenced by the intimate structure of these transparent materials. In the case of a Si-O structure, photon absorption is low, even for small wavelengths (transparency to UVA rays). This is not the case when other elements (Na, Mg, Ca, etc.) are added to this simple structure, which decisively affect the absorption at small wavelengths (less than 200 nm) and in the infrared (greater than 700 nm). On the other hand, the presence in the glass network of transition elements (see Periodic Table of Elements) produces selective absorptions of visible radiation, which allows, among other things, to color the glasses with a wide range of shades.

Las principales características del vidrio (su transparencia y su dureza), a pesar de las restricciones impuestas por su principal limitación (su fragilidad), lo convierten en un elemento imprescindible en numerosísimas aplicaciones, formando por sí mismo un grupo de materiales de una enorme importancia económica.

Building and architecture

Since the middle of the century, glass facades have become an almost essential hallmark of large buildings in the main cities of the world.[26] These facades are usually made using pieces of flat glass with a very wide range of colors, which facilitates the creative work of architects. These glasses are normally subjected to certain processes that improve their thermal and acoustic insulation properties; and its ability to attenuate external light.

In conventional facades, glass continues to maintain its predominant role in the windows, integrated into different types of carpentry (from traditional wooden ones, through steel ones, aluminum ones, and even PVC ones), with single panes or double panes separated by a confined layer of air.

Nowadays, glass has become an essential element in home decoration. Thanks to its elegance, transmission of external light and transparency, glass makes spaces spacious and clean. For this, choosing the right glass is very important, especially for architects and designers who are the ones who use this material to create their projects.

Furthermore, as it has different colors and textures, glass can be used in numerous ways in countless elements, such as:

Glass wool is used as thermal and acoustic insulation in buildings, placed between the exterior and interior walls of many buildings.

There are some pioneering projects that have used fiberglass treated with resins for use in small bridges[27]​ and walkways, taking advantage of the advantages of its lightness. Likewise, the use of fiberglass bars has been proposed for reinforcing concrete, thus avoiding the effect of corrosion on metal reinforcements in especially aggressive environments.

Windshield

Since the first carriages designed to transport passengers, all companies manufacturing means of transport (railways, shipbuilding, the automobile industry and the aerospace industry) have been linked since their origins to the production of glass elements used in windows and windshields as well as in the interior and exterior lighting systems of all types of vehicles. Likewise, another element linked to the automobile industry is the manufacture of rear-view mirrors.

A clear example is the evolution of automobile design, which went from using flat glass exclusively to integrating sophisticated curved glass elements in windshields and windows. Both the aerospace and automobile industries have benefited and in turn have made notable contributions to the development of increasingly lighter and stronger glasses, such as Gorilla Glass,[28]​ later widely used in the manufacture of cell phones.

Packaging

Glass (despite competition from cheaper containers such as aluminum or steel cans; waxed or aluminum-coated cardboard bricks; and plastic bottles) is still one of the containers preferably used for the marketing of most alcoholic beverages (among which wine and beer can be massively included, despite the progression of other types of containers in these two cases), a multitude of preserves (especially jams and vegetables, which benefit from visibility of the product through glass), soft drinks of all kinds and perfumery products such as colognes or certain beauty products (to which glass containers[29]​ with original designs provide undeniable added value).

From the first half of the century, when food companies were in charge of collecting containers for cleaning and new use (a common practice at that time in dairy, brewing and soft drink industries), until the 1980s, in which the use of non-returnable containers[30]​ intended to be recycled in the manufacture of new bottles became widespread, glass has been shown to be one of the least polluting and most easily recycled materials.

Likewise, the pharmaceutical industry frequently uses glass containers for many of its liquid preparations such as syrups or injectables.

energy production

Energy production systems such as photovoltaic panels[31]​ and solar thermal power plants use glass elements massively to capture solar energy. In the case of photovoltaic panels, they protect the silicon cells (and finally concentrate the light), and in the case of solar thermal power plants they are the key element of the collector mirrors (and in some systems, also of the collectors through which the fluids with which the sun's heat accumulates circulate).

Improving the properties of these glasses (cost, transparency, thermal and chemical stability, resistance to dirt and environmental agents...) is key to the profitability of the costly investments necessary for the commissioning of these facilities.

Optics

It constitutes one of the main specific applications of glass since the Renaissance, when quality lenses began to be produced with increasingly perfected procedures. Some of the scientific foundations of optics had already been laid before (since the year 1000, Arab mathematicians like Alhacen had studied the geometry of mirrors). However, it was not until Galileo Galilei appeared with his lens telescope, Anton van Leeuwenhoek with his primitive microscope, and Isaac Newton himself with the development of the mirror telescope, when the bases of the importance of optical instruments were definitively established, until reaching the theoretical limits of resolution at the beginning of the century, with the achievements of Carl Zeiss based on the theoretical discoveries of Ernst Abbe, based on the use of different types of glass.

The applications of glass optical technology focus mainly on instruments for image processing and capture; in scientific devices for the study of light; in digital communications; and in the ophthalmological correction of human vision defects through lenses:.

Laboratory material

A large part of the equipment in chemical and pharmaceutical laboratories "Glassware (chemistry)" (test tubes, beakers, flasks, pipettes, condensers "Cooling tube (chemistry)"), plates for microscopic preparations...) is made of glass. Sometimes special glasses are used, prepared to withstand high temperatures or certain chemical attacks.

Appliances

Televisions systematically use glass screens to protect the different systems of luminous pixels through which they form the images.

Conventional ovens, microwave ovens and ceramic cooktops include heat-resistant glass elements in their design.

Likewise, washing machines usually incorporate a circular glass door, and many refrigerators use glass shelves to improve the feeling of space and interior light.

Lightning

Since the invention of gas or oil flame lamps, glass bell jars have been used to prevent both the extinguishing of the flame and its accidental spread. With the invention of the electric incandescent bulb, the characteristic glass bulb that protects the filament has become an irreplaceable element, which has been progressively adapted to the greater thermal requirements demanded by sodium vapor lamps, halogen lamps (with pure silica glass) or xenon lamps (which use special glass). Even in fluorescent tubes, whose operating temperatures are low, the glass that contains the neon gas is an essential element. Only the development of LED light systems (due to their low heat emission) can allow the replacement of glass with translucent plastic materials, which are cheaper, lighter and easier to manufacture.

Many models of lamps in which light points are mounted use glass elements to disperse and give a certain decorative appearance to the light they project. In this sense, we can mention the enormous chandeliers "Spider (lamp)") made up of numerous pieces of interlocked glass, characteristic of the large halls of public and private buildings from the Victorian Era to the First World War.

Cell phones and touch devices

The use of luminous screens (increasingly larger) on cell phones and touch devices, made with especially resistant glass, such as Gorilla Glass, has become widespread.

Watchmaking

Traditionally, watch faces have been protected with domed glass, with flat profiles later adopted. In the case of wristwatches, it is an essential requirement both when they mount pointer devices (to prevent them from being damaged) and when it comes to digital devices (the glass allows the screen to be shown to the outside). High-end watches usually have sapphire crystals, whose extraordinary hardness prevents them from being easily scratched.

Kitchen and kitchenware

Many kitchen utensils can be made of glass (such as platters or bowls). The appearance of borosilicate glass, capable of withstanding very high temperatures, extraordinarily expanded the use of glass in the kitchen, until it became a material widely used in dishes for preparing roasts in the oven.

At the table, both glasses and all types of cups, as well as jugs and containers for most liquids, are usually made of glass, and there are also tableware in which the plates are also made of this material, replacing ceramics.

Decoration and jewelry

Special quality colored glass is frequently used in jewelry, replacing much higher priced natural gems. An example is Swarovski glass, which is used to produce a wide range of decorative products, as well as fashion accessories linked to jewelry.

In other types of products, the sale of colored glass beads that are used in the making of handmade necklaces and bracelets is common, both for marketing and for pure entertainment.

According to data published by the CIAdvanced website,[32]​ the main glass manufacturing industries in the world (the vast majority share this activity with the manufacture of ceramic materials), are concentrated in Japan, Europe and the United States.

The economic potential of these companies is considerable. The largest of them, the French company Saint-Gobain, had a turnover of 45 billion dollars in 2014 and employed nearly 60,000 workers, with about 3,700 dedicated to research and development tasks. Its main clients are the automotive industry, aerospace, energy plants and ophthalmic optics. The size of Japan's companies is also notable, accounting for four of the top ten.

The smallest of the ten, the Austrian RHI AG, in 2014 had a turnover of 1.9 billion dollars, and employed 8,000 workers.

On the other hand, the American company Owens-Illinois is the world's largest manufacturer of glass containers. It has a turnover of 7.4 billion dollars annually and employs 24,000 workers.

(See: Glass manufacturing).

• - Mineraloid.

Types of glass:

• - Wiktionary has definitions and other information about glass.

Traditionally it has been considered that matter could occur in three forms: solid, liquid and gaseous. New means of investigation of its intimate structure—particularly during the century—have revealed other forms or states in which matter can present itself. For example, the mesomorphic state[16]​ (a liquid form with its smectic,[17]​ nematic[18]​ and cholesteric[19]​ phases), the plasma state "Plasma (state of matter)") (or plasmatic state, typical of ionized gases at very high temperatures) or the vitreous state, among others.

Bodies in the vitreous state are characterized by presenting a solid appearance with a certain hardness and rigidity and that, when faced with moderate external stresses, they deform in a generally elastic manner. However, like liquids, these bodies are optically isotropic, transparent to most of the electromagnetic spectrum of visible radiation. When its internal structure is studied through means such as X-ray diffraction, it gives rise to diffuse diffraction bands similar to those of liquids. If they are heated, their viscosity gradually decreases – like most liquids – until reaching values ​​that allow them to deform under the action of gravity, and for example take the shape of the container that contains them as true liquids. However, they do not present a clearly marked transition point between the solid and liquid state or "melting point".[20]​.

All these properties have led some researchers to define the glassy state not as a distinct state of matter, but simply as that of a subcooled liquid or liquid with such a high viscosity that it gives it the appearance of a solid without being one.[21]​[22]​ This hypothesis implies the consideration of the glassy state as a metastable state to which a sufficient activation energy of its particles should lead to its equilibrium state, that is, that of a crystalline solid.

In support of this hypothesis, the experimental fact is adduced that, when a body is heated in a glassy state until it obtains a clearly liquid behavior (at a temperature high enough for its viscosity to be less than 500 poise, for example), if it is cooled slowly and carefully, at the same time providing it with the activation energy necessary for the formation of the first solid corpuscles (seeding of microcrystals, presence of activating surfaces, nucleation catalysts, etc.), it usually occurs. solidify giving rise to the formation of sets of true solid crystals.

Everything seems to indicate that bodies in a glassy state do not present a specific internal order, as occurs with crystalline solids.[20]​ However, in many cases an ordered disorder is observed, that is, the presence of ordered groups that are distributed in space in a totally or partially random manner.

This has led different researchers to propose various theories about the internal structure of the glassy state, both geometric, based on both atomic and energy theories.

According to the geometric atomic theory, in solid crystallized silica the silicon atom is surrounded by four oxygen atoms located at the vertices of a tetrahedron, each of which unites it to neighboring silicon atoms. A plan view of this arrangement is schematized in Figure 1, in which the fourth oxygen would be above the plane of the page. When this silica passes into the glassy state, the tetrahedral arrangement is still maintained at the individual level of each silicon atom, although the bonds between oxygen and silicon atoms are made in an apparent disorder,[23]​ which nevertheless maintains an initial unitary organization (see figure 2).

However, none of these theories is sufficient to explain the complete behavior of the vitreous bodies, although they can serve to answer, in specific and well-determined cases, some of the questions that arise.

Substances capable of presenting a glassy state can be both inorganic and organic in nature, among others:

• - Chemical elements: Si, Se, Au-Si, Pt-Pd, Cu-Au.

• - Oxides: SiO, BO, PO, and some of their combinations.

• - Compounds: AsS, GeSe, PS, BeF, PbCl, AgI, Ca(NO).

• - Silicones (substances considered semi-organic).

• - Organic polymers: such as glycols, sugars, polyamides, polystyrenes or polyethylenes, etc.

Silica glasses

Silica is called "Silicon oxide (IV)") to a silicon oxide with the chemical formula SiO. It occurs in a crystalline solid state under different enanciotropic forms. The best known are quartz (the most common and stable at room temperature), cristobalite and tridymites. In addition to these forms, up to twenty-two different phases have been identified, each of them stable from a perfectly determined temperature.

When quartz is heated slowly, it goes through different enanciotropic forms until it reaches its melting point at 1723 °C. At this temperature, a colorless and very viscous liquid is obtained that, if cooled relatively quickly, becomes a glassy substance that is usually called quartz glass.

This quartz glass presents a set of properties that are very useful and applicable in multiple disciplines: in scientific and technological research, in domestic life and in general in all types of industry. The following stand out as most relevant:

Other properties, however, make its preparation and use difficult. In particular, the following:

Sodium silicate glasses

The most common sodium salts have melting points below 900 °C. When an intimate mixture of finely divided quartz is heated with a salt of these alkali metals, for example NaCO, to a temperature greater than 800 °C, a fusion of the alkali salt is initially obtained, the liquid of which surrounds the quartz grains, producing a series of reactions that can be included in the following result:

SiO + NaCO

NaSiO + CO H = -5.12 kcal/mol.

This slightly exothermic reaction releases gaseous carbon dioxide - which bubbles through the melting mass - and leads to a first sodium silicate, with a melting point of 1087 °C.

According to thermodynamics, the mixture of two substances with different melting points has a "liquidus point"[24]​ that is between those of the two substances in contact. In this way, the mixture of silica and sodium silicate formed gives rise to a product of SiO and silicates, already in a liquid state at temperatures that do not exceed 1200 °C, far from the more than 2000 °C necessary to prepare quartz glass.

The product thus obtained is commonly given the generic name sodium silicate, although this name identifies a set of products derived from the fusion of quartz with sodium salts (generally carbonates) in different proportions of one component and the other. Industrially, sodium silicates are prepared with molar proportions of each component between:

These sodium silicates have a glassy, ​​transparent and very brittle appearance. To achieve a viscosity of the order of 1000 poise (necessary for molding), temperatures are required that, depending on its composition, range between 1220 °C for the silicate richest in SiO, and 900 °C for the poorest. They are very soluble in water: between 35% and 50% by weight of silicate, depending on the SiO content. Their lack of mechanical rigidity and their solubility in water make them useless as substitutes for quartz glass in any of its applications.

They are rarely presented in the industry in solid form, but rather in the form of an aqueous solution. Its solution in water is used as a very effective ceramic glue or as a raw material for the production by hydrolysis of silica gel, a substance used as a moisture absorber (gas drying towers, etc.) or as a component of certain products such as vehicle tires and other applications in the chemical industry.

Its production is carried out in continuous balsa furnaces heated by the combustion of petroleum derivatives and frequently also with electrical energy, at temperatures as high as possible (within a certain profitability) in order to increase the productivity of the furnace. These temperatures are usually between 1400 °C and 1500 °C.

Sodium calcium silicate glasses

In order to obtain a product with properties similar to those of quartz glass at lower temperatures and therefore that are achievable by technically profitable means, a sodium silicate glass is produced to which other components are added that make it more mechanically resistant, and that stabilize the structure of the glass to prevent it from dissolving in water and therefore being more inert to chemical agents at room temperature, and that maintain its transparency to light, at least in the visible spectrum.

These components are alkaline earth metals, particularly magnesium, calcium or barium, as well as aluminum and other elements in smaller quantities, some of which appear contributed as impurities by the raw materials (in the case of iron, sulfur or others). The raw materials used to make glass of this type are chosen from those that have a lower price:

• - For quartz:

• Feldspathic sands, with SiO purity greater than 95% and with the lowest possible content of ferrous components (between 0.15% and 0.01% in terms of FeO)

• Ground quartzites.

• - For sodium:

• Natural sodium carbonates (deposits in the United States and Africa).

• Synthetic sodium carbonate, the most used in Europe.

• Synthetic sodium sulfate, byproduct of the chemical industry.

• Natural sodium nitrate (Chile nitrate).

• Sodium chloride or common salt.

• These last three, used in small proportions, due to the release of polluting gases during glass production: SO, NO, Cl.

• - For calcium:

• Natural limestone.

• - For magnesium:

• Natural dolomites.

• - For barium:

• Natural barium sulfate (barite).

• - For aluminum:

• Natural feldspars (kaolins).

The industrial production of this type of glass is carried out, as in the case of sodium silicates, in glass furnaces, generally made of raft, heated by the combustion of petroleum derivatives with the support, in many cases, of electrical energy at temperatures ranging between 1450 °C and 1600 °C. A slightly moistened powder mixture (5% water) and previously dosed of the raw materials already mentioned is introduced into these ovens. This mixture of mineral materials reacts (at appreciable speeds and, obviously, the higher the better) to form the set of silicates that, combined and mixed, will give rise to that substance called .

The properties of common glass are a function of both nature, raw materials, and the chemical composition of the product obtained. This chemical composition is usually represented in the form of percentages by weight of the most stable oxides at room temperature of each of the chemical elements that form it. The compositions of the most used sodium silicate glasses are within the limits established in the attached table.

Many studies—particularly in the first half of the century—have attempted to establish correlations between what was called the internal structure of glass—generally based on atomic theories—and the properties observed in glasses. The product of these studies was a set of relationships, of an absolutely empirical nature, that represent many of these properties in a surprisingly precise manner through linear relationships between the content of the chemical elements that form a given glass (expressed in the form of the percentage content by weight of its most stable oxides) and the magnitude representing said property. Interestingly, correlations with compositions expressed in molar or atomic form are much less reliable.

The MgO, FeO and SO contents are a consequence of the impurities of limestone, sand and sodium sulfate, respectively*.*.

The light absorption (or transparency) of sodium silicate glasses in the visible spectrum area (0.40 μ to 0.70 μ) depends on their content of transition elements (Ni and Fe in the example). However, in both ultraviolet and infrared, glass behaves practically like an almost opaque object, independently of any of these elements.

• - (1) The density is somewhat higher than in fused quartz (2.5 versus 2.2 g/cm³).

• - (2) The coefficient of linear thermal expansion at room temperature is noticeably higher than that of fused silica (about 20 times higher), so sodium silicate glass objects are less resistant to "thermal shock".

• - (3) Its refractive index is slightly higher than that of quartz glass and can be increased by using additives.

• - (4) The tensile strength in any type of glass is a magnitude that depends extraordinarily on the state of the surface of the object in question, so its quantification is complex and unreliable.

• - (5) The resistance to chemical or physical attack (dissolution) of common glasses is fundamentally a function of their chemical composition. However, in all of them this resistance is high. It is usually measured through a series of internationally standardized tests. Among the most used:

• DIN 12116

• DIN 52322

• DIN 12111

• The attackability of glasses is also modified by surface treatments with SO, Sn, Ti, and others.

• - (6) To mold glass, a viscosity between 1000 poise and 5000 poise is necessary. In the case of silica, temperatures of more than 2600 °C are necessary, while for common glasses approximately 1 200 °C is sufficient.

• - (7) The absorption of light is influenced by the intimate structure of these transparent materials. In the case of a Si-O structure, photon absorption is low, even for small wavelengths (transparency to UVA rays). This is not the case when other elements (Na, Mg, Ca, etc.) are added to this simple structure, which decisively affect the absorption at small wavelengths (less than 200 nm) and in the infrared (greater than 700 nm). On the other hand, the presence in the glass network of transition elements (see Periodic Table of Elements) produces selective absorptions of visible radiation, which allows, among other things, to color the glasses with a wide range of shades.

Las principales características del vidrio (su transparencia y su dureza), a pesar de las restricciones impuestas por su principal limitación (su fragilidad), lo convierten en un elemento imprescindible en numerosísimas aplicaciones, formando por sí mismo un grupo de materiales de una enorme importancia económica.

Building and architecture

Since the middle of the century, glass facades have become an almost essential hallmark of large buildings in the main cities of the world.[26] These facades are usually made using pieces of flat glass with a very wide range of colors, which facilitates the creative work of architects. These glasses are normally subjected to certain processes that improve their thermal and acoustic insulation properties; and its ability to attenuate external light.

In conventional facades, glass continues to maintain its predominant role in the windows, integrated into different types of carpentry (from traditional wooden ones, through steel ones, aluminum ones, and even PVC ones), with single panes or double panes separated by a confined layer of air.

Nowadays, glass has become an essential element in home decoration. Thanks to its elegance, transmission of external light and transparency, glass makes spaces spacious and clean. For this, choosing the right glass is very important, especially for architects and designers who are the ones who use this material to create their projects.

Furthermore, as it has different colors and textures, glass can be used in numerous ways in countless elements, such as:

Glass wool is used as thermal and acoustic insulation in buildings, placed between the exterior and interior walls of many buildings.

There are some pioneering projects that have used fiberglass treated with resins for use in small bridges[27]​ and walkways, taking advantage of the advantages of its lightness. Likewise, the use of fiberglass bars has been proposed for reinforcing concrete, thus avoiding the effect of corrosion on metal reinforcements in especially aggressive environments.

Windshield

Since the first carriages designed to transport passengers, all companies manufacturing means of transport (railways, shipbuilding, the automobile industry and the aerospace industry) have been linked since their origins to the production of glass elements used in windows and windshields as well as in the interior and exterior lighting systems of all types of vehicles. Likewise, another element linked to the automobile industry is the manufacture of rear-view mirrors.

A clear example is the evolution of automobile design, which went from using flat glass exclusively to integrating sophisticated curved glass elements in windshields and windows. Both the aerospace and automobile industries have benefited and in turn have made notable contributions to the development of increasingly lighter and stronger glasses, such as Gorilla Glass,[28]​ later widely used in the manufacture of cell phones.

Packaging

Glass (despite competition from cheaper containers such as aluminum or steel cans; waxed or aluminum-coated cardboard bricks; and plastic bottles) is still one of the containers preferably used for the marketing of most alcoholic beverages (among which wine and beer can be massively included, despite the progression of other types of containers in these two cases), a multitude of preserves (especially jams and vegetables, which benefit from visibility of the product through glass), soft drinks of all kinds and perfumery products such as colognes or certain beauty products (to which glass containers[29]​ with original designs provide undeniable added value).

From the first half of the century, when food companies were in charge of collecting containers for cleaning and new use (a common practice at that time in dairy, brewing and soft drink industries), until the 1980s, in which the use of non-returnable containers[30]​ intended to be recycled in the manufacture of new bottles became widespread, glass has been shown to be one of the least polluting and most easily recycled materials.

Likewise, the pharmaceutical industry frequently uses glass containers for many of its liquid preparations such as syrups or injectables.

energy production

Energy production systems such as photovoltaic panels[31]​ and solar thermal power plants use glass elements massively to capture solar energy. In the case of photovoltaic panels, they protect the silicon cells (and finally concentrate the light), and in the case of solar thermal power plants they are the key element of the collector mirrors (and in some systems, also of the collectors through which the fluids with which the sun's heat accumulates circulate).

Improving the properties of these glasses (cost, transparency, thermal and chemical stability, resistance to dirt and environmental agents...) is key to the profitability of the costly investments necessary for the commissioning of these facilities.

Optics

It constitutes one of the main specific applications of glass since the Renaissance, when quality lenses began to be produced with increasingly perfected procedures. Some of the scientific foundations of optics had already been laid before (since the year 1000, Arab mathematicians like Alhacen had studied the geometry of mirrors). However, it was not until Galileo Galilei appeared with his lens telescope, Anton van Leeuwenhoek with his primitive microscope, and Isaac Newton himself with the development of the mirror telescope, when the bases of the importance of optical instruments were definitively established, until reaching the theoretical limits of resolution at the beginning of the century, with the achievements of Carl Zeiss based on the theoretical discoveries of Ernst Abbe, based on the use of different types of glass.

The applications of glass optical technology focus mainly on instruments for image processing and capture; in scientific devices for the study of light; in digital communications; and in the ophthalmological correction of human vision defects through lenses:.

Laboratory material

A large part of the equipment in chemical and pharmaceutical laboratories "Glassware (chemistry)" (test tubes, beakers, flasks, pipettes, condensers "Cooling tube (chemistry)"), plates for microscopic preparations...) is made of glass. Sometimes special glasses are used, prepared to withstand high temperatures or certain chemical attacks.

Appliances

Televisions systematically use glass screens to protect the different systems of luminous pixels through which they form the images.

Conventional ovens, microwave ovens and ceramic cooktops include heat-resistant glass elements in their design.

Likewise, washing machines usually incorporate a circular glass door, and many refrigerators use glass shelves to improve the feeling of space and interior light.

Lightning

Since the invention of gas or oil flame lamps, glass bell jars have been used to prevent both the extinguishing of the flame and its accidental spread. With the invention of the electric incandescent bulb, the characteristic glass bulb that protects the filament has become an irreplaceable element, which has been progressively adapted to the greater thermal requirements demanded by sodium vapor lamps, halogen lamps (with pure silica glass) or xenon lamps (which use special glass). Even in fluorescent tubes, whose operating temperatures are low, the glass that contains the neon gas is an essential element. Only the development of LED light systems (due to their low heat emission) can allow the replacement of glass with translucent plastic materials, which are cheaper, lighter and easier to manufacture.

Many models of lamps in which light points are mounted use glass elements to disperse and give a certain decorative appearance to the light they project. In this sense, we can mention the enormous chandeliers "Spider (lamp)") made up of numerous pieces of interlocked glass, characteristic of the large halls of public and private buildings from the Victorian Era to the First World War.

Cell phones and touch devices

The use of luminous screens (increasingly larger) on cell phones and touch devices, made with especially resistant glass, such as Gorilla Glass, has become widespread.

Watchmaking

Traditionally, watch faces have been protected with domed glass, with flat profiles later adopted. In the case of wristwatches, it is an essential requirement both when they mount pointer devices (to prevent them from being damaged) and when it comes to digital devices (the glass allows the screen to be shown to the outside). High-end watches usually have sapphire crystals, whose extraordinary hardness prevents them from being easily scratched.

Kitchen and kitchenware

Many kitchen utensils can be made of glass (such as platters or bowls). The appearance of borosilicate glass, capable of withstanding very high temperatures, extraordinarily expanded the use of glass in the kitchen, until it became a material widely used in dishes for preparing roasts in the oven.

At the table, both glasses and all types of cups, as well as jugs and containers for most liquids, are usually made of glass, and there are also tableware in which the plates are also made of this material, replacing ceramics.

Decoration and jewelry

Special quality colored glass is frequently used in jewelry, replacing much higher priced natural gems. An example is Swarovski glass, which is used to produce a wide range of decorative products, as well as fashion accessories linked to jewelry.

In other types of products, the sale of colored glass beads that are used in the making of handmade necklaces and bracelets is common, both for marketing and for pure entertainment.

According to data published by the CIAdvanced website,[32]​ the main glass manufacturing industries in the world (the vast majority share this activity with the manufacture of ceramic materials), are concentrated in Japan, Europe and the United States.

The economic potential of these companies is considerable. The largest of them, the French company Saint-Gobain, had a turnover of 45 billion dollars in 2014 and employed nearly 60,000 workers, with about 3,700 dedicated to research and development tasks. Its main clients are the automotive industry, aerospace, energy plants and ophthalmic optics. The size of Japan's companies is also notable, accounting for four of the top ten.

The smallest of the ten, the Austrian RHI AG, in 2014 had a turnover of 1.9 billion dollars, and employed 8,000 workers.

On the other hand, the American company Owens-Illinois is the world's largest manufacturer of glass containers. It has a turnover of 7.4 billion dollars annually and employs 24,000 workers.

(See: Glass manufacturing).

• - Mineraloid.

Types of glass:

• - Wiktionary has definitions and other information about glass.