[Trade Journal]
Publication: Journal of the American Institute of Electrical Engineers
New York, NY, United States
vol. 42, no. 7, p. 743-747, col. 1-2
The Production of Porcelain for Electrical Insulation —IV
BY FRANK H. RIDDLE
Champion Porcelain Company
Jeffery-Dewitt Insulator Company
Review of the Subject.—The three previous articles have been devoted to giving a brief description of the theoretical points involved in the development of porcelain. The following articles will briefly describe the actual manufacturing methods and the application of the theoretical points.
Recent developments in plant management and control have been applied to porcelain manufacture in the same manner as they have to other industries, and the old rule of thumb and trade secret methods are fast disappearing.
Ample storage room for raw materials is necessary for a uniformly high grade product can only be made when there is ample time to test the raw materials prior to using them, and to permit of the rejection of inferior materials when necessary without causing any delays in production.
Accurate methods of weighing the various raw materials to form the final raw body are necessary. Care must be used in determining the per cent of moisture in each raw material so as to compound the body on a dry weight basis.
The old method of preparing the body by mixing or blunging, with the idea of merely suspending the raw materials in water without regard for grain size is to be avoided. Many potters are now grinding the materials in pebble mills and working to definite grain sizes.
The ground body is passed over a magnetic separator to remove magnetic iron, then passed through a fine mesh (120 meshes to the lineal inch) lawn and filter pressed to eliminate excess water, and thus put the ingredients in a workable condition (approximately 22 per cent water). The materials for porcelain can also be stored easily while in this condition. By storage the plasticity is not only increased, but time is available in which to run actual tests on the materials before releasing them for production.
The moisture content in filter press cakes is not uniform throughout the cross-section of the cakes. A greater degree of homogeneity is produced by pugging the body after filter pressing. A pug mill develops uniformity to a high degree when properly handled. Several methods of shaping the body into final form require different methods of preparation. Each of these will be described in later issues.
CONTENTS
Review of the Subject. (360 w.)
Process of Manufacture. (225 w.)
Raw Material Storage. (150 w.)
Preparation of the Body. (1500 w.)
Pugging. (300 w.)
PROCESS OF MANUFACTURE
MODERN efficiency methods and ceramic research have brought about changes in plant design in the manufacture of porcelain just as similar studies in other industries have made marked improvements in their methods. Where it has been impossible to make changes in older plants on account of such conditions as lack of space or other unavoidable difficulties it has still been possible to install modern equipment. As a result many plants have been able to improve their methods in one or more respects.
In the early days potters were very secretive and their processes and particularly their formulas were guarded with great care, the knowledge being handed down from father to son. This condition still exists to more or less extent in some branches of the potting industry. Those who have been slow to give information to others however have been slow to learn from others and have not been prone to adopt methods which are more modern and based on sound engineering principles. Continued practise along the old lines can, of course, result in only one end.
In describing the methods of manufacture it will be necessary to repeat, in some instances, part of the work previously described, particularly when discussing raw materials, body mixtures and the behavior of the bodies and glazes during burning. This will only be done in such cases as will make it possible to describe the processes more thoroughly.
RAW MATERIAL STORAGE
The supply of raw materials an insulator plant should carry in stock should be governed not only by production and the time necessary to receive new shipments but also by the time it requires to test new materials upon receipt, and before they are certified for use in production. The stock should also be large enough, so that if any cars of new materials are rejected on account of quality there is still sufficient stock to permit production to go on undisturbed. These conditions being considered, it is necessary that the testing methods be such that new materials can be checked up without delay. It is obvious that great care should be used in selecting the sources from which the raw materials are purchased, taking into consideration not only the integrity of the dealer, but also the quality uniformity and size of the deposit from which the material is secured.
Most potters secure their materials from several sources and proportion the different ones, in compounding their bodies and glazes, so that some material is used from each source, thus eliminating somewhat a chance for serious trouble in case one of the sources supplies an inferior grade of material. Ball clays hold moisture very tenaciously and hence are usually received in a moist state. It is well to keep them in this state, in fact some potters keep clays of this type well dampened not only to prevent loss of plasticity but also to make it possible for the material which is received in rather large lumps to be blunged up or put in suspension more easily in the preparation of the body.
The weighing of the bodies and glazes is an operation where considerable care should be used and some method arranged for checking. The most satisfactory method is by use of charging scales in which the weights are set and locked by the foreman thus obviating the necessity of the operator's knowing the quantities used and also the possibility of errors caused by incorrect weighing.
In weighing out a batch it is always necessary to determine the per cent of moisture in each raw material and allow for same. The moisture content in flint and feldspar is usually under two or three per cent, however china clay may run as high a content as ten or twelve per cent and ball clay even higher. With a body mixture containing 50 per cent of clay with a moisture content of 15 per cent it would mean that there would be a shortage of 7 per cent of clay in the body if the moisture content was not allowed for. This is assuming that the remaining 50 per cent of the body batch was dry. This is likely to be so as the remainder of the batch would be flint or feldspar, both of which have a low per cent of moisture. These exact conditions are not unusual in some general ware factories today.
PREPARATION OF THE BODY (THE MIXED INGREDIENTS)
The usual method of preparing or washing the body, as the raw material mixture is called, is to blunge it. A blunger (Fig. 15) is a large tank equipped with revolving arms. The batch, together with a proper amount of water, is placed in the blunger and churned until the raw materials are disintegrated and suspended in the water. This method of preparation, although quite commonly used, is inferior in many respects to grinding or even mixing in pebble mills. The blunging process simply disintegrates the lumps of raw materials and suspends the particles in water. This is not so serious in the case of the clays, particularly the ball clays as they are naturally very fine, however the flint and feldspar are ground rock and the manufacturer who blunges is entirely dependent for grain size upon the miller. If there is any variation in grain size in the miller's product this same variation will be in the blunged body. The chance for variation here depends not only upon the care the miller uses but also upon how carefully the manufacturer checks the raw materials received. It is common practise for the miller to supply 120 or 140 mesh product and it has been found and will be shown later that 140 mesh product does not produce so good a porcelain insulator product as finer material does. By 140 mesh product is meant a product which is sufficiently fine so that all of it will pass through a lawn which has 140 openings per lineal inch, a great proportion of the material being much finer than 140. It is also important to know not only the size of the coarsest particles but also the percentage of various fines contained. This should always be considered as some millers, in using one type of grinder will get various percentages of fines where another method will produce an entirely different ratio of the various sizes.
In the grinding process pebble mills (Fig. 16) are usually employed. As the illustration shows they are cylinders supported on trunnions so as to revolve around the horizontal axis. They are lined with quartz or porcelain tile and filled about half full of small porcelain or quartz pebbles. They can be used for either dry or wet grinding, however wet grinding is usually employed in a modern pottery. If a pebble mill is not operated properly it will give a wider range of grain size and be less dependable than a blunger but where used properly it is beyond comparison. It is essential to standardize on not only the quality of the grinding parts as pebbles and lining, but also on the revolutions per minute, total time of grinding, weight, bulk and size of pebbles, tonnage of the grinding charge and particularly upon the quantity of water. If the water content is too low the sticky crass will cement the pebbles together and nothing will be accomplished, while if too much water is used the slip will be so thin that the pebbles will wash around in the mass and accomplish very little grinding. With just sufficient water to form a thick slip each pebble will be coated with a layer of slip which will adhere to the pebble as it rises up out of the slip, and rolls back down over other pebbles into the slip, as the ball mill revolves.
Where a body is made up of coarse grained non-plastics such as flint and feldspar, and fine grained clays such as English ball and China clays it may be necessary to grind the non-plastics and put the clays into the mill only near the completion of the operation and then just long enough for a thorough mixing. Where this is done additional water has to be added when the clay is added. It is also necessary, where the non plastics or grinding charge is rather coarse, to float it with a small amount of plastic material to prevent its settling and caking. The amount of water to add to a charge depends not only upon the character of the raw material but also upon the amount of water it already contains as natural moisture. After the slip is made it is passed over a magnetic separator and through a lawn. The fineness of the meshes of the lawn depends upon the quality of slip the potter desires. Usually a silk bolting cloth of 120 mesh is used. Obviously the lawn is only used to eliminate coarser pa- tides of foreign material. This is extremely important in insulator porcelain.
Recently the writer's attention was called to a manufacturer who was importing English clays in drums in order to prevent contamination. This would unquestionably facilitate handling, but it is doubtful if the expense was warranted if the proper equipment was employed in the body preparation. Punctures can be caused in porcelain if hairs or threads from sacks are in the body when it is formed as these burn out later and leave thread-like holes. If the proper lawn is used in screening the slip and it is properly inspected this trouble should not occur if the foreign material is incorporated before lawning, however, if the materials get into the body after lawning, troubles can occur. This is one reason why care should be used in the selection of proper quality filter bags.
The filtering off of part of the water to reduce the water content in the slip to bring it to proper stiffness for plastic forming is almost universally done in a leaf type periodic filter press (Fig. 18). These presses usually produce seventy-two leaves of body 24 by 24 by 1-1/4 in. After filter pressing the body will contain about 22 per cent of water. The slip is forced through a pipe which enters the press at the end and passes on filling the first narrow chamber then through into the next chamber, etc. When all the chambers are full the hydraulic pressure starts to rise and the water is forced from the particles of body through the canvas linings of the chambers and out through small ports at the bottom of the grooved steel partitions between the chambers. The pressure used, sizes of meshes of the weave of the canvas, speed of pumping and several other variables all effect the quality of the finished insulator. If the slip forced into the filter press is too thin it will cause a separation which will result in a lack of uniformity in the cross-section of the cake.
The usual method of applying pressure to the slip to force it into the presses is to use a plunger pump. Obviously the plunger has to be lubricated and as a result some oil enters the filter press with the slip. A more refined pump is the diaphragm pump which prevents grease from contaminating the slip. Both these methods produce a pulsation which tends to some extent to fill up the meshes of the filter cloths and retard the flow of the water. A third method is the use of compressed air. The air is pumped into a tank containing slip and forces the slip into the press. This produces a uniform pressure and keeps the slip free from contamination. It not only makes it possible to filter more rapidly but at a lower pressure and results in a cake of greater uniformity.
Some plastic process plants form the body immediately after filter pressing it, pugging it or mixing it. However, it is considered good practise to store or age the body for a period before using it. This aging is usually done in a close damp room or cellar where the body cannot dry out. (Fig. 18). There are several reasons for aging; the principal one being that an aged body will not only form better but yield a higher percentage of first quality ware. Greater plasticity is developed. This is particularly useful in high grade wares where it is desirable to use just as white burning materials as possible. White burning clays are usually short or non-plastic while the ball-clays although plastic do not burn white and their content must be kept as low as possible. Several theories have been advanced regarding the causes of the development of plasticity during aging and its effect upon the final product. Additions of very small amounts of acids have been known to accelerate the effect of aging. Bacterial growth is also supposed to assist. The writer had experience during the war at the Bureau of Standards with an extremely short kaolin body from which special porcelain propellors were made for stirring optical glass experimentally. Two lots of bodies were used, one being six months old and the other newly made. The aged body worked very well while the new body could not be shaped at all.
One big advantage of storing body at this stage is that it gives time to experiment with preliminary lots through the several processes to see if the body is satisfactory. This is not essential in all grades of ware but in some high grade wares and where the body is extremely sensitive it is invaluable.
PUGGING
Filter press cakes whether taken directly from the press or aged, require a thorough mixing in order to produce better homogeneity. This is done in a pug mill, Figs. 19 and 20, which not only thoroughly mixes the mass but also tends to develop plasticity as well as assisting in eliminating any enclosed air. Proper pugging necessitates careful study as each body works differently and requires special attention.
As shown in the illustration, a pug mill is a barrel shaped machine which has a series of knives or propellors revolving in it around a central shaft, the knives having a pitch to them which will keep forcing the material down through the barrel from the top where it is fed in. This forcing down and packing as mentioned above, improves the quality of the body.
At the bottom of the barrel is a horizontal shaft also equipped with knives which forces the body sideways out through a port or die in the side of the pug mill. Sometimes pieces are shaped by the die and simply cut to length. If this is done it is necessary to have the conditions as near right as possible. Two sources of trouble arise in a pug mill known as auger lamination and die lamination. The first is caused by the knives or propellors polishing the clay so that it does not properly cohere when forced through the die. This gives an effect similar in a way to the spirals on a stick of candy. The second trouble is die friction. A column of clay like other liquid or semi-liquid masses flow fastest where there is the least friction. This will sometimes cause laminations or slips in the mass which will not heal or adhere and the result is that these slippage cracks will remain in the mass clear through the process resulting in a damaged porcelain, and the defect in most cases being so hidden that it is not noticeable until severely tested either in final inspection or in service.
