[Trade Journal]
Publication: American Institute Of Electrical Engineers
New York, NY, United States
p. 343-346, col. 1-2
Porcelain for Electrical Insulation—I
BY FRANK H. RIDDLE
Associate of A. I. E. E.
Director of Research. Champion Porcelain Company and Jeffery-Dewitt Insulator Company
President of the American Ceramic Society
HISTORY
PORCELAIN is known to have been made by the Chinese about 1000 years ago. They developed the art to a high plane of artistic excellence. It was this porcelain, which, when brought to Europe, was mused many potters in a number of countries to attempt its reproduction. For centuries these attempts proved unsuccessful and the best that could be done by the Italian, French, and Dutch schools of artisans and artists was to create a type of faience pottery in which the coarse color of the clay body was hidden by an opaque, white enamel containing tin oxide. It was not until 1709 that Boettcher discovered the use of kaolin and first made a yellowish but vitrified material which could be considered a type of porcelain. In England Josiah Wedgwood in 1759 produced a type of semi-vitreous pottery, known as Queens Ware. In France at a much later date a frit porcelain was made in which kaolin and ground quartz were cemented together in firing by means of a fusible glass or frit. The knowledge of the art of porcelain making soon spread over Europe and finally resulted in two types: the so-called hard porcelain of continental Europe, composed of kaolin or clay, ground quartz and feldspar, a mixture which is vitrified at about 1400 deg. cent. (2552 deg. fahr. a bright, white heat); and the bone china of England, consisting of white burning clay, feldspar, introduced through the so-called Cornish stone, and calcined or burned bone ash. The latter porcelain known as bone china is caused to vitrify at about 1280 deg. cent. 2336 (leg. Fahr.). For electrical purposes only the hard porcelain is used in Europe; however, the American product although made at slightly lower temperatures than 1400 deg. cent. is considered superior.
In the United States porcelain for electrical insulation was developed about 1895 from the so-called semi-vitreous tableware, composed approximately of 50 per cent of kaolin and ball clay, 14 per cent of feldspar and 36 per cent of ground quartz. By reducing the content of quartz and increasing that of feldspar, a vitrified body was produced which matured at about 1330 deg. cent. (2426 deg. fahr.). In such a composition the amounts of feldspar and quartz are approximately 30 and 20 per cent respectively.
DEFINITION OF PORCELAIN
The general composition of porcelain has already been indicated as being essentially a mixture of clay substance, principally kaolin, feldspar and quartz. These minerals, intimately blended, are fired to a temperature at which the feldspar fuses to a glass and in doing so cements together the two more refractory constituents. Porcelain may thus be said to be an agglomerate of clay and quartz held together in a matrix of molten feldspar at least as far as the lower firing temperatures of the usual tableware porcelains are concerned. Such a mass is non-absorbent, i. e., when immersed in water, even under diminished pressure, practically none is taken up. Again, thin sections of the material transmit some light thus showing that there is present a continuous translucent glass phase.
These two qualities then are peculiar to porcelain. The color of the material is generally but not necessarily white. Examining a thin petrographic section(1) of porcelain under the microscope, especially with polarized light,(2) we are able to discern the fragments of fine quartz surrounded by glass and clay substance which may or may not be dissociated into a new crystalline mineral constituent. It will also be noted that the quartz grains are subject to a solution process inasmuch as many of them have lost their sharp edges and have become rounded. Indeed, very fine grains may disappear altogether. It is evident therefore that the feldspar glass behaves as a powerful solvent, tending to dissolve the quartz. This solution may actually be carried to completion in high fire porcelains.
At the temperatures to which transmission porcelain is burned 1350 deg. cent. to 1390 deg. cent. (2462 deg. fahr. to 2502 deg. fahr.) the solution of the quartz has become quite noticeable and the crystalline development shows more distinctly particularly at the higher temperature mentioned.
Parallel with the dissolving of the quartz goes the dissociation of the clay substance into a new and frequently crystalline compound, sillimanite,(3) which may be plainly recognized under the microscope.
But both the solution of quartz and the formation of sillimanite are not necessary qualifications of a porcelain, nor is it necessary that feldspar should be present or even quartz. It is quite possible to produce the incipient fusion required by means of other fluxes and it is equally true that quartz may be replaced by other mineral constituents of a more or less inert natural Clay, on the other hand, is a necessary component since its presence is indispensable in the initial molding of the ware. We might define porcelain then to be a ceramic product usually light colored, which is completely vitrified and is translucent in thin sections.
It is evident from what has already been said that porcelains may have a wide range of composition and that hence there must necessarily be a variety of types which possess certain characteristics. As the composition varies so must also the firing temperature varies since it is evident that a material carrying a high percentage of feldspar can be matured, or properly fired to a vitreous porcelain, at a lower temperature than one with less flux. Accordingly we may have porcelain high or low in clay substance, high or low in fluxes and high or low in quartz. The properties of one porcelain shade into those of another with gradual changes composition.
CERAMIC RAW MATERIALS
The principal raw materials of porcelain are clay, feldspar and quartz but there are to be considered all other constituents, such as calcium carbonate, magnesium carbonate, alumina, zirconium oxide, conium silicate and sillimanite, which are coming in use for special purposes in the modern development of the art.
CLAYS
There are to be considered principally two types of the less plastic, very white burning materials known as kaolins or china clays, and the very plastic, cream or yellow burning, ball clays.
The kaolins may again be divided into two classes those of primary, geological origin and those laid down as secondary deposits. The former type of kaolin found in contact with the parent, igneous rock, from which it has been formed through natural decomposition
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than the kaolins or china clays but much inferior in this respect to the so-called ball clays. The shrinkage of the secondary kaolins upon drying is quite large. amounting to about 30 per cent of the original volume. As a result they are inclined to crack or check under the strain of this large contraction unless the drying operation is carefully controlled.
The plastic bond or ball clays constitute the third group of clays used in the manufacture of porcelain and they represent by far the most powerful cementing materials for the bonding together of the non-plastic components. These clays are not only secondary clays. geologically speaking, but have been laid down either simultaneously with or in contact with material high in carbon of vegetable origin. In addition to the great inherent fineness of these clay grains the presence of this carbonaceous matter has resulted in building up not only well-developed plasticity but great bonding power as well. These materials, owing to the transporting processes they have undergone and their mode of sedimentation in swampy regions, have accumulated considerable amounts of iron and some other impurities and hence fire to a creamish or yellowish color. For the same reason, they fire to a dense structure at a comparatively low temperature unlike the kaolins which are refractory in nature and remain porous even at high furnace temperatures. It is the chief function of these clays to bond together the non-plastic components of the porcelain and to impart to the mixture sufficient strength to withstand handling in the dry state. The mechanical strength of the ball clays may be illustrated by the fact that bars made from a one to one mixture of such a clay and a non-plastic may show a transverse strength of as much as 600 pounds per square inch. Owing to creamish or grayish color which they impart to the porcelain the amount of ball clay used is limited to the smallest amount possible. The best types of these plastic bonding materials are the ball clays from North and South Devonshire, and Dorset, England. There are also ball clays found in Tennessee and Kentucky.
Owing to the dense structure the water of plasticity escapes from the ball clays at a much lower rate than is the case with the more open grained kaolins and the drying process must hence be conducted with still greater care.
The fluxes present in these clays cause them to vitrify at a relatively low temperature and for this reason they promote also the vitrification of the porcelain mixture to which they have been added.
The following are typical analyses of various clays:
1. A thin transparent section of the specimen (about 0.001 inch thick) fur use under the microscope for studying the structure and mineralogical constituents of same.
2. "Mineralogy" by Kraus and Hunt, p. 107 state in part as follows:
"Nature of Polarized Light—According to the undulatory theory, light is assumed to be a form of energy transmitted in waves in the ether, which pervades all things and space. The propagation of light takes place according to the laws of wave motion, the ether particles vibrating at right angles to the direction of propagation. The velocity of propagation has been determined to be about 186,000 miles per second.
"In the case of ordinary light., the vibration of the ether particles takes place in a plane at right angles to the direction in which the light is propagated, but the vibration direction in this plane is constantly changing.
"In plane polarized light, the vibrations take place in a definite direction within the plane and at right angles to the direction of propagation. Plane polarized light may be produced in three ways.”
Certain substances are transparent to vibrations in one plane, and opaque to, those in the plane at right angles to this, so that, in transmitting the light those vibrations are selected to which this plane corresponds. The plane of polarization is altered or rotated by the passage of polarized light through a quartz crystal built into the microscope. Owing to interferences, crystals show remarkable colors when polarized light is passed through them and it is hence valuable in the investigation of rock structure.
3. Sillimanite will he described in detail later. It is of interest at this time to note briefly that clay contains one atom of alumina and two of silica and that, at the proper tiring temperature this breaks up or dissociates into the sillimanite crystal made up of one atom of alumina and one align of silica, the other atom of silica going into solution in the glassy feldspar.
M2O-2Si0 - heat = Al2O2 - SiO2 Sillimanite + SuO2
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