Borosilicate glass
There are many different types of glass, each having different characteristics for different uses.The glass most frequently used for laboratory and technical ware and equipment is the borosilicate glass as the brand names “Pyrex”(Corning 7740) and “Duran”(Schott 8830).
These glasses are widely used because of their high resistance to chemical attack and to thermal shock, both needed is glassware for scientific laboratories and in chemical and physical research.
Glass in an organic mixture fused at high temperature which solidifies on cooling but does not crystallize.
Its basic components, network formers and modifiers are present in the form of oxides in ordinary glasses.Typical glass formers(network formers) are silica (SiO2), boric acid(B2O3),phosphoric acid (P2O5) and under certain circumstances, also aluminium oxide(Al2O3).
These substances are capable of absorbing(dissolving) certain amounts of metal oxides without losing their glassy character. This means that the incorporated oxides do not participate as glass formers but modify certain physical properties of the glass structure as network modifiers.
Glass structure
A large number of chemical substances solidify from the liquid state of the melt in the form of a glass.The glass formation depends on the cooling rate and requires mixed types of bonding(covalent bond and ionic bond) between the atoms or atom groups.As a result, glass-forming products have a strong tendency while still in the melt for three-dimensional crosslinking by polymerisation in a largely random manner.Crystals are formed when the individual atoms form a regular three-dimensional arrangement in what is known as a crystal lattice, as soon as the particular substance change from the liquid tot the solid state.
Glass, however, on cooling from the liquid state, forms a largely spatially random network.The main components which participate in the glass formation are therefore called network formers.Ions can be incorporated in this network of glass forming molecules, as a result of which they tear up the network in certain places and modify the network structure and thus glass properties.That is why they are called network modifiers.
Borosilicate glass has the following typical composition: % by weight
| Silica (SiO2) | 80,60 % |
| Boric oxide (B2O3) | 12,60% |
| Sodium oxide (Na2O) | 4,20 % |
| Alumina (Al2O3) | 2.20 % |
| Iron oxide (Fe2O3) | 0,04 % |
| Calcium oxide (CaO) | 0,10 % |
| Magnesium oxide (MgO) | 0,05 % |
| Chlorine (Cl) | 0,10 % |
Basic shapes are produced by automatic, semi-automatic and manual processes involving blowing, pressing and tube drawing.From these basic shapes a wide variety of scientific apparatus can be produced by subsequent modern engineering techniques and the high skill of the professional glass technician.
Annealing
The annealing of glass is the process by which it is heated and held for a defined period to relieve internal stresses.Careful cooling under controlled conditions is eesential to ensure that no stresses are re-introduced by chilling.
Temperature limits in service
In general it is recommended the strain point 515oC be regarded as the maximum safe operating temperature of borosilicate glassware.
For some shapes and for a relativily short time this limit can be exceeded, but at 600oC there is danger of deformation.
When heated above 515oC the glass may acquire permanent stress on cooling and this may result in subsequent breakage.
Chemical properties
The chemical resistance of glass is more extensive than that of all other known materials.
Borosilicate glass is highly resistant to water,acids, salt solutions, organic substances and also halogens such as chlorine and bromine.It also has relatively good resistance to alkaline solutions.
Only hydrofluoric acid, concentrated phosphoric acid and strong alkaline solutions cause appreciable surface removal of the glass at higher temperatures.
Safe use and handling glass components and instruments
When heating
- Use tongs or heat resistant gloves to remove glassware from sources of heat.
- Ensure that electric heating equipment is in good working condition.
- Use a soft flame with a bunsun-use a gauze or diffuser to prevent localised heating.
- Heat the whole lower hemisphere of your flask to ensure good efficiency and no localised hot spots.
- Take care when cooling and avoid sudden draughts to prevent thermal shock and possible failure.
Vacuum use
- Use a safety screen
- Do not use glassware beyond the recommended safety limit.
- Do not subject the glass to sudden pressure changes.
- Do not use scratched or abraded glassware, its strength will be seriously impaired.
Sintered ware
- Ensure that the lower pressure is applied to the concave side of the larger domed disc.
- Do not use a differential pressure exceeding 100 KN/m2 (15 psi).
- Do not subject sintered ware to excessive temperature changes or direct flames.
- Do not subject sintered ware, particularly of porosity grade 4, when cold and damp, to sudden excessive temperature changes.The evolution of steam may set up sufficient pressure within the filter to cause failure.
Retrun to Technical glass.
