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).
Page detailing the manufacturing process of hard porcelain, its glazing, and its physical and electrical properties.
| Identifier | ExFiles\Box 24\2\ Scan104 | |
| Date | 25th January 1921 guessed | |
| - 17 - On the other hand too little feldspar prevents homogenous formation and reduces the dielectric strength. The surface of porcelain insulators is generally provided with a glaze. The process of manufacturing hard porcelain is somewhat as follows. After forming the sludge, caoline feldspar and quartz are mixed damp in the manner desired, the water is mostly removed by filter presses and the mass is stored for considerable periods in damp, cool cellars in which it becomes ready for treatment. The substance when it has become plastic is energetically mixed in mixing machines and is then formed into the desired shapes by pressing, stamping, turning, pouring and working in plaster moulds. It is left to dry slowly in the air, until most of the moisture has evaporated and is then placed in the oven and raised to a temperature of about 900°. In this process the substance undergoes the change that it no longer disintegrates in water and in the subsequent process of glazing on account of its great porosity it greedily absorbs the glazing fluid which consists of a thin paste of china clay and silicic compounds. If the insulator are to be colored, suitable oxides are added to the glass flux which form silicates with the silicic acid. For instance to obtain black urane, green chromium, blue cobalt and brown manganese are used. When the porcelain is drawn through the glass this clings to the surface, the water penetrates the pores and a layer of about 0.5mm. thickness is formed. The most difficult part of the manufacture now follows, namely burning on the glaze, which calls for great experience. The process takes place at a temperature of about 1500° and lasts, according to the type of oven, 25 to 35 hours. The cooling in the oven takes an equal length of time, so that big ovens can only be operated once or twice a week. The temperature is controlled by so-called seger-cones, or by removing test cups. In order to protect articles made of porcelain against soot and products of combustion, they are enclosed in fire proof covers and pieces that are very often of very complicated forms must be provided with supports at weak points in order that they may not sink. The pieces that are to be burned are placed in the furnace on polished plates of clay or porcelain. Great attention should be given to the fact that porcelain shrinks considerably during the burning, to the extent of 17 to 20 per cent in linear dimensions. Hence it is very difficult to maintain the shapes quite exactly and the size of flat plates that can be made in limited. From an electrical point of view, porcelain has great advantages. It is cheap, its dielectric strength and insulating properties are very high; it is fireproof and withstands heat, is not hygroscopic, conducts heat away quickly and is not attacked chemically by the atmosphere nor by the formation of oil or ozone, that is to say, it does not age. At the same time it has great power under compression. contd. | ||