[Trade Journal]
Publication: Journal of the American Institute of Electrical Engineers
New York, NY, United States
vol. 42, no. 4, 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
Review of the Subject.—This series of articles is presented with a view of acquainting the electrical engineer with the composition of porcelain, the raw materials, and processes employed in its manufacture, and with the structure and the physical properties of the finished product. The principles underlying the art will be discussed as well as the reasons for using certain materials or certain processes.
Recent developments in porcelain for automotive purposes have clearly demonstrated that the transmission porcelain of the future will be an improvement over that of the porcelain of today, provided however, that the consumer of transmission porcelain will be trilling to pay the increased cost, or provided the manufacturer will be able to lower his cost on the improved product when sufficiently developed. The fact that the porcelain used in transmission work today would make only inferior spark plugs makes it safe to predict this future improvement in transmission porcelain.
The present article deals briefly with the history of porcelain and its definition.
Generally porcelain is made of a mixture of ground feldspar, a mineral which melts to glass and makes the porcelain vitreous when fired; clay which makes it possible to form the unfired porcelain and have it retain its shape; and fine silica sand or quartz which acts as a filler or non-plastic material. These three materials are mentioned several times before they are described in detail but this brief description is sufficient for the present.
The beginning of a description of ceramic raw materials is given.
The writer wishes to ex press his thanks to Mr. A. V. Bleininger for his cooperation and suggestions in preparing these articles. Also to Prof. Albert Peck for his assistance in preparing and discussing the photomicrographs.
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 processes. It is generally accepted that these clays are formed from the feldspar minerals which may be represented by the mineral orthoclase or microcline, K20 A1202 6Si02. The feldspar, through leaching processes, is finally converted to the hydrous mineral, A1203 2SiO2 2H20, known as kaolin, corresponding to the percentage composition of 47 silica, 39 alumina and 14 of chemically combined water. The decomposition process extends usually to the surface depths of the original granite or other type of igneous rock but in some localities as in Cornwall, England, the gaseous agencies at work arose from deep seated sources. In this locality therefore the deposits are deeper than those found in the United States, in North Carolina, Maryland and Pennsylvania.
The primary kaolins are composed of crystalline kaolin fragments but to a still greater extent of particles which are so small that they can be barely recognized under the high-power microscope and which cling together to form aggregates or clusters. The plasticity or the property which enables the kaolin to be molded is sufficient for the purpose but the bonding power of the material, that is, its power to cement together inert, non-plastic particles is quite feeble. This is to be ascribed to the fact that the component particles, while very small, still are larger than those making up the other types of clay and are of the order of 0.005 mm. (0.0002 inch) in diameter. We are dealing hence with a coarsely dispersed system of clay grains, and the mass requires about 30 per cent of water, expressed in terms of the dry weight of the clay in assuming the plastic state. A clay is said to be in the plastic state when it is wet enough to be modeled into any desired shape. These kaolins yield up their water readily upon drying and contract in volume, proportionately to the amount of water evaporated, down to the point at which the clay particles come in actual contact. The water which is driven off during the contraction is called shrinkage water and that remaining, pore water, as it fills the pores between the particles of clay after they have come in contact. The rate of evaporation of most of the water held by clay is practically equal to that of free water.
The kaolins laid down as secondary deposits are primary materials removed from the original place of deposition by the action of water currents. During this transportation process the clays necessarily come in contact with iron carrying minerals and hence become more or less contaminated by them. In addition the attrition and grinding action involved has brought about a decided reduction in the fineness of the grains, causing the clays to become more plastic and workable. Such secondary kaolin deposits are those of Georgia and Florida. The clay particles in these are not only exceedingly fine but also of remarkable uniformity in size which, however, is not a desirable condition, since it does not contribute towards maximum bonding power. As a result we find that these clays are stronger 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
