Figure 1. Atomic structure of glass


Dirk Schrijvers

Glass seems to have an eternal live. It is an amorphous solid material which is often transparent and is based on the chemical compound silica (silicon dioxide), primary constituent of sand, to which other components such as sodium oxide (Na2O), sodium carbonate (Na2CO3), calcium oxide, and several other agents are added.
The atomic structure (Figure 1) of a glass lacks a crystalline form although the chemical bounding between oxygen and silicon results in a high degree of short-range order with respect to local atomic structure. The amorphous arrangement of glass molecules is a result of the energy that is trapped during the cooling process after the production of glass by heating up the components.
Although the cooling process is a gradual one to avoid the build-up of tension, it is still too fast to assure the formation of a crystalline structure. This makes that, unlike the crystalline structure of other materials, randomly arranged glass molecules are more vulnerable to external forces.

Posted 23 February 2015

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The strength of a particular type of glass depends on the bonds between the molecules in the glass. These bond strengths can vary with different materials of the glass and manufacturing techniques (1).
Glass can be destroyed by different mechanism. Physical forces like mechanical energy resulting from acceleration and subsequent stopping (e.g. a falling drinking glass) or an external (e.g. stone against a window, glass cutter) or internal force (e.g. tension during the cooling process); vibration; or temperature changes can destroy glass, as can chemical reactions that change the structure of glass. The causes of glass destructions are discussed in this article. 

Figure 2. Destruction of glass by cracks

Physical causes
Mechanical force
A glass object can break when it falls to the ground or when a stone is thrown through a window. This is the result of the conversion of mechanical energy into surface energy. Different theories are proposed to explain the propagation of the first fracture to the whole object. A first theory states that glass fractures develop through the stretching and breaking of individual inter-atomic bonds one after another (Wiederhorn), enlarging the crack. Another theory proposes that glass fractures develop through submicroscopic cavities that form ahead of the crack tip.
The force to break a glass can be decreased when the surface of the glass is scratched (e.g. glass cutter) to hamper its integrity to external forces (Figure 2)

Any physical object has frequencies at which they naturally vibrate (= resonance frequencies ). Glass can be broken by vibration caused by sound. For a vibration to break glass several conditions have to be fulfilled. The glass have to be one that resonates at a particular harmonic frequency such as a crystal object. The sound has to set up sympathetic vibration in the glass by having the same resonance frequency as this of the glass and having a high enough amplitude of waves (loudness), which causes the displacement of the molecules exceeding the strength of the glass to resist the vibrations. When the sound gets too loud for the glass to vibrate, it shatters the glass.
Temperature changes
Glass is made by melting different materials at high temperature (500-1600°C). It reaches a liquid stage where it can be manipulated into different forms. Cooling of the material from a liquid into a high viscous/solid state changed the nature of particle diffusion in the glass.
The rotational and directional movement of the glass particles through space is coupled at high temperature but changes during the transition period when the glass reaches a viscous state. At that moment, the rotation and directional movements become decoupled and the rotation starts slowing down more. This results in the built-up of tension in the glass when the cooling process is too fast, causing glass to crack.
Another mechanism by which temperature changes can cause is due to different expansion in different regions. Glass expands as it heats up and shrinks as it cools whereby it has a moderately low thermal conductivity. This is the cause that glass can crack when it is suddenly cooled the surface will cool almost instantly on contact, but the middle has to wait for heat to conduct away before it can cool. This creates an uneven thermal profile, where the surface is cold but the inside is hot, resulting in breakage.
Chemical causes
The surface of the glass is not completely smooth but has 'lattice' or 'honeycomb' patterns. Organic and inorganic contaminants fill the gaps in the surface and react chemically with the glass, firmly bonding to its surface. This causes the glass to become stained and discolored, difficult to see through and difficult to clean and keep clean. Different chemical reactions can occur, that cause destruction of the glass object.
Although glass is considered to be resistant to corrosion, it will corrode under certain conditions. It can be attacked by chemicals like hydrofluoric acid, concentrated phosphoric acid, hot concentrated alkali solutions and superheated water.
-Hydrofluoric acid attacks any type of silicate glass.
-An alkali solution attacks a glass surface and dissolves it. This process will continue as long as there sufficient supply of alkali.
-An acid solution corrodes glass by dissolving the alkali in the glass resulting in a porous surface that consists of the silica network with holes where the alkali has been removed.
-Corrosion by water is similar to acid corrosion in that alkali is removed from the glass surface. Water corrosion acts at a much slower rate but at high temperatures, water corrosion can become significant.
Aqueous corrosion
Glass is hydrophilic and can attract and hold moisture. This results in a molecular layer of moisture on the surface of the glass. When this layer increases, it can obscure visibility and create a risk to comfort or safety while it will also cause the destruction of the surface of the glass.
This process can be divided in 2 stages:
-The first stage is aqueous corrosion and is the result of the moisture which causes an ion exchange or alkali extraction (leaching). An ion exchange occurs between sodium ions from the glass and hydrogen ions from the solution. The remaining components of the glass are not altered, but the effective surface area in contact with the solution is increased. The increase in surface area leads to further extraction or leaching of the alkali ions from the glass, leaving a silica-rich layer on the surface. As the silica (SiO2) concentration in the glass goes down, surface area increases through dissolution of the glass surface.
There are two types of aqueous corrosion:
-Static aqueous corrosion, which is caused by an entrapment of moisture on the surface of the glass.
-Dynamic aqueous corrosion in which the corrosion solution is replenished due to condensation run-off.
-The second stage of corrosion is a process of destruction of the leached surface layers of glass. Glass is resistant to most acids but is highly susceptible to attack by alkaline materials, especially a concentration of OH- ions giving a pH greater than 9.0. The result is an attack of the network forming silica-oxygen (Si-O) bonds, leading to dissolution of the glass surface. 

Glass weeping and crizzling
In rare instances, "weeping glass" can occur in which droplets of moisture are formed on the surface of a glass object. These droplets of moisture can actually leach out unstable components of the glass producing an alkaline solution (Figure 3). If these alkaline droplets remain on the surface of the glass for a long period of time, the surface will develop a fine network of cracks. This phenomena is referred to as "crizzling". When the cracks become deeper and more pronounced, the glass begins to flake and eventually the object disintegrates. Both crizzling and weeping are believed to be the result of improper formulation during glass manufacture.

Figure 3. Glass weeping

Iridescent surface
Glass can be permanently damaged by lengthy exposure to acidic or alkaline conditions. Glass that is buried in the soil for long periods of time develops a matte, scaly and "iridescent surface".
Glass seems to be an eternal material, but can be destroyed by different causes. The most known are physical forces that destroy the glass by causing cracks that are propagated over the surface and destroy the object. Also chemical decay may be a reason that glass loses its integrity. Everybody involved in the handling of glass should be aware of these causes of glass destruction.


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