For the production of glass are required the following raw material:
- Quartz sand: is the most important raw material for the production of glass and represents the largest part of the mixture.
- Lime:strengthens the resistance compared to chemical and physical influences.
- Flux: are i.a. Soda and Potash
Soda = sodium carbonate
Potash = potassium carbonate
- Stabilisers:are oxides of multivalued metals like for example lead oxide, barium oxide, dolomite and magnesite. They give the glass physical and chemical qualities for example resistance, solidity and hardness.
For the colouring of glass are used pure chemicals. One reaches a great colour palette by addition of iron, copper, nickel and other metal compounds as well as silver and gold. With admixture of fluorine containing materials depose finest crystalline particles in the glass mass which make it cloudy and opaquely
The single raw materials are prepared in certain amounts for the respective glass species. This combination or recipe is also called "glass composition". If the "glass composition" is prepared, mixed and prepared for the melt one speaks of mixture.
The Melting of glass take place in different phases:
The first step is the rough melt. She includes the melting of the mixture and his homogenisation. The second step is the refining. Hereby are expelled the gases. The third and last step is the killing melt of the glass, that means the glass is cooled down for the further moulding.
There are two types at the glass melting in the furnace: the pot furnace and the tank furnace.
In the pot furnace takes the melting place in separate periods, so-called harbours. The harbour is a heat-resistant container in which the glass melts. At first the furnace is heated until the required temperature is reached. In the next step the mixture is carried in the single harbours and is heated until it is liquid. After the heating the mixture cools down on the right working temperature and can further processed.
The tank furnace can be melted continuous. So he is suitable for the automatically processing of glass. The progression of the steps is here areal separated. First of all the mixture is heated in a rectangular tank and must correspond to the withdrawal of glass. The heating of the melting tank happens slowly. In the refining area remaining bubbles are expelled of the glass melt. Because of the high viscosity of the melt this happens only slowly, thus there are also high temperatures necessary like in the melting range. The refining area follows the working tank. For the moulding are lower temperatures necessary, that's why the glass has to cool down before. A channel, also called dog-hole, connects the melting tank with the working tank. The glass runs then from the working tank to the point of the withdrawal.
3. Processing and rather moulding of glass
Glass is formed differently. It depends on the product which is desired. One differs between the techniques of the hot and cold moulding.
The hot moulding includes: casting, pressing, blowing, drawing and rolling. This procedures are depending on temperature and that's why you have only a liitle time for the moulding of the glass.
The cold moulding procedure includes: grinding, sawing, drilling and polishing.
4. Procedures of the glass production
Bar drawing technique
The bar drawing technique exists since 100 years and proves itself at small production volumes. At this procedure is directed a burner at the bottom of a glass bar, which melts the end of the bar. Thereby form a drop, which draw a thin glass thread during he is falling down. Afterwards he is wound onto a spool.
Diffuser drawing technique
The diffuser drawing technique is similar to the bar drawing technique, but here granulate materials are melted. The granulates are melted in a furnace with holes at the bottom side, so that the drops can fall down through the holes and the entrained glass bar can wound onto a spool.
Nozzles blowing technique
The basis of this procedure is the diffuser drawing technique. At the nozzles blowing technique runs through nozzles, which are in a tank, the liquid glass. By addition of stream are drawn fine bars and cut afterwards in small pieces. Thereafter these are dried and directed on a burner. A so called sieve drum collects the glass threads to a suction zone from which they are taken as an infinite fibre glass tape.
With this technique a hot glass stream runs on a turning disc. Because of the centrifugal forces spread the glass. Small drops are flung away by the rotation and draw fibres. These fibres are focused by airstreams on the acceptance. Another centrifugal technique takes place in a centrifuge. In the head of the centrifuge are holes through which the drops are flung and small fibres originate.
In every glass object originate mechanical tensions by moulding and processing. They can be measured with voltage testers.
The tension sensitivity depends on the expansion coefficient of the glass and has to be regulated thermally. One reduces the tensions by slow cooling down in the cooling area.
How long a glass object pass through the cooling area depends on the glass species on the bypass temperature and the strength of the object. The temperature decrease can be carried out with different furnaces.
There are periodical leers and continuous leers. The Leers are only suited for custom-made products and little amounts, because the furnaces has to be brought after every withdrawal of the work pieces on temperature again. In the industry are used leers. Hereby the products are slowly transported by graded heated furnace segments on steel rod mats or roles.