From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Technical description of the properties and manufacturing of glass and porcelain for insulation purposes.
| Identifier | ExFiles\Box 24\2\ Scan103 | |
| Date | 25th January 1921 guessed | |
| - 16 - chemically. Only a part of these components has changed. Silicate of potash and silicate of soda, called water-glass, are substances that dissolve easily in water; on the other hand silicate of lime and the silicates of the other alkaline earths are insoluble and difficult to fuse. Glass consists of a mixture of both kinds. The approximate method of a commercial manufacture of glasses is as follows - Quartz (Si O2) i.e. sodium and lime are fused in suitable muffles and soda, i.e. sodium flux is produced. It is also possible to take sulphates of the alkaline compounds mixed with carbon and then the sulphur salts are reduced to metallic sulphides by the carbon. These in turn are transformed into silicates of the metals by combination at high temperatures. According to the components that are used one must distinguish between potassium and sodium glass. The first called Bohemian glass, which is difficult to melt and harder than the sodium glass, is used for ordinary purposes such as for windows, bottles etc. The dielectric strength of glass is extremely high. It reaches 20,000 volts for 1 mm. thickness, and on this account it is used in America for insulators on high tension lines. Its greatest weakness is its sensitiveness to variations of temperature and glass insulators are even liable to crack in the transition from hot days to cold nights. It cannot be machined and is very brittle. The surface of glass is attacked by rain and water, by the production of free alkali which enters into chemical action with the silicon dioxide. This action becomes stronger with rise of temperature to different degrees from different kinds of glass, and the insulating qualities of the surface are greatly reduced on account of the roughness of surface produced. Although its composition is purely organic it cannot be regarded as fire resisting according to the regulations of the Institution for heavy-current installations, as although it does not burn or soften in contact with a flame yet it cracks. It is still put to various uses in high tension installations. Porcelain is a very important insulating material that is used in the most general manner in low and high tension installations. It is composed of a mechanical mixture of aluminium silicates. For electrical purposes only hard porcelain need be considered that is sintered from a flux consisting of caoline and feldspar. Caoline is an earth containing water produced by the weathering of ancient stones. It is a material that is extremely difficult to melt at the highest temperatures. On the other hand feldspar is a double compound of silicate of alumina and silicate of potash, that melts when it burns and sinters with caoline, generally with addition of silicon dioxide to form a doughy mass. Too great an addition of quartz is detrimental to the formation while an excess of feldspar in the burning produces too strong liquification. contd. | ||