Glass

Glass is a state of matter, characterizing amorphous solids. The structure of glass neither exactly resembles that of a crystalline solid nor that of a liquid.

The properties of glass vary depending on its composition (over 400.000 types of glasses have been manufactuered) and thus it has big range of uses.

The most commonly used component of glass is silica, an element that is abundant in the Earth's crust (30% by mass). During the formation of our planet, rocks containing a high percentage of silica were melted and cooled rapidly under the hostile conditions of the environment (volcanic eruptions, lighting strikes), thus natural glass was made such as obsidian. Man-made glass was first produced about 4000 years ago and it was used mainly for jewellery. Then, glass couldn't be obtained in a pure/homogenous form and lacked transparency.

Glass continued to spread around the world and new techniques were invented but still, transparent glass could not be achieved until the middle ages.

This important step took place in the island of Murano. Cristallo was the first clear, colourless, transparent glass and was made by the Venetian glass blower Cesaro Barovier in the 15th century.

Another marking point in the glassmaking industry was the invention of crown glass, which was a simpler process that prevailed in the middle ages. The glass that was manufactured by this method is sometimes thicker in the bottom than at the top. That is not attributed to the viscosity of the glass (a common misconception) but rather to the glass making process itself. It was slowly replaced in the 19th century by cylinder blown sheet glass, another hand-blown technique.

It took many years for man to study the relation of glass composition and properties. The industry only saw very steep scientific progress in the 19th century.

After we became aware of the underlying science of the craft, the glass making process was automated and improved to accomondate all our needs. The mass production of glass objects is done with the help of machines that create the desired shapes of the products using moulds.

A common misconsception suggests that glass is a liquid of high viscosity. This not the case. Glass is its own distinct state of matter that doesn't coincide with any other classical one. Every liquid (except Helium) can be turned into glass, if a sufficiently rapid cooling takes place. This process is called vitrification. When a liquid is cooled (water for example) it normally goes through the process of crystallization. We say that the water is frozen as ice forms which has a very specific structure that is characterized by its stability (crystalline solid). If the cooling happens quickly enough, the water molecules don't have the opportunity to occupy the lowest energy sites and this is how the amorphous solids forms.

Annealing illustrates this phenomenon. When glass is cooled down in order to solidify, this process must be done during a specific time interval in order for the molecules to have enough time to position themselves in a more stable manner. If during the glass making process, the produced glass has been solidified too rapidly, then the stress present in the solid makes it too fragile to the point where it can rupture/shatter even during handling. By reheating the glass and slowly droping the temperature again, we ensure that the glass object has gained a much more stable structure.

As crystalline solids have a melting point, amorphous ones have a glass transition temperature which surprisingly depends on thermal history (how rapidly was the former liquid made into glass?). Around this temperature point, the viscosity of the glass increases rapidly and can be classified as a solid (under classical terms). It should be noted that the viscosity as well as other properties of the substance made into glass, present a continuous change as the temperature changes. This is not the case with "ordinary" freezing as liquid water turns spontaneously into a solid in a discontinuous manner (during the freezing process the temperature stays constant. Immediately below $0^{o}C$ all the water has turned into ice).

The glass state is metastable and its transition to a crystalline solid is thermodynamically favoured although kinetically inert.

The works of Zachariasen and Warren (Network Theory and its verification by X-ray diffraction) resulted in big advancements in the study of glass structure. This theory can predict a large number properties of conventional glasses. His theory was later extended by Dietzel.

Zachariasen presented three types of cations that form a glass:

In order for glass to form, the network-former should (although there are exceptions) form polyhedral groups in its simplest form. (For example silica, $Si{O}_2$ forms $Si{O}_4^{4-}$ tetrahedrals). Each polyhedron should only connect with its neighbouring once via bonding bridges formed by the anions (in this example an Si-O-Si).

The network-former of silicate glass is $Si{O}_2$ which is

This type of glass accounts for 90% of commercially used glass. Its uses range from windows to even jars. It is typically made of (w/w) 70% silica ($Si{O}_2$), 10% lime ($CaO$) and 15% soda ($N{a}_{2}O$).

Silica is the network-former of the glass. Silica, is found in nature as quartz and its pure form is the ideal glass. Nevertheless, soda is added in order to reduce the temperature at which the glass is softened, thus making its production cheaper and better to work with. Since sodium cations are very soluble in water, lime is added in order to decrease the solubility of the glass.

Silica is obtained by sand and mixed with soda ash ($N{a}_{2}C{O}_3$) and limestone ($CaC{O}_3$). These materials are mixed together to get a powder called batch which is later combined with cullet (recycled glass pieces) in order for the glass to once again soften at even lower temperatures. The mix is heated into an oven at temperatures around $1000^{o}C$. Impurities that arise are removed and the melted mass is put into moulds to take its final shape. Annealing is also taking place at the end to ensure glass durability, which is the process of reheated the bottle and then slowly cooling it down in order to further stabilize the structure of the glass.

Soda lime glass is recyclable.

The Sun's rays are not monochromatic and represent photons of various wavelength that may or may not be part of the visible light. In glass, some ions absorb particular wavelengths in the visible light and emit only the remaining wavelength. This is what contitutes the colour of an object. If an ion absorbs photons of red light, it appears bluish. These ions are normally transition metals as they can absorb photons of wavelengths that we can see.

The colour that we perceive by the glass is not only related to the ions contained in the glass but it also depends on the network-former/modifiers present. This is because the ions interact with them, which influences each ion's orbital energy levels and consequently the light that they can absorb. Lastly the colour also depends on the concentration of the ion, its nature (valency) and the heterogeneity of the glass. A portion of the light that arrives in the surface of the glass gets reflected and doesn't interact further.

A list summarizing some generally perceived colours and their associated ions:

Glasses can also be coloured by metal colloids or to coloured particles.

Glass Chemistry by Werner Vogel (1994)