clay substance is higher than that of any admixture of common clay substance with silica, although it is commonly known that the other ingredients generally present in mined clay, such as pyrites, mica, felspar, and the like, are very active in reducing the melting point. The more finely divided the silica and other impurities, the more intense the action in bringing about the softening of the clay substance at high temperatures, for the fine state of division brings about a great area of active contact surface. It is this fine material that cannot be removed by settling, and which the Osmosis process successfully eliminates.

The sintering or vitrifying temperature of a clay may be taken to be the temperature at which the rapid contraction of the clay in firing ceases; above this point little contraction occurs. Sintering takes place in osmosed clays at lower temperatures than in raw clays; consequently, goods made with osmosed clay can be finished at lower temperatures, resulting in a saving of fuel. The reduction of the sintering temperature, together with the higher melting-point, enables articles to be made that show little further contraction after burning. The amount of the reduction of the sintering temperature varies for different clays; it is as much as 300° C. for low-grade clays, and much less for high-grade clays. Bricks made from some clays, after osmosing, are better buint at 1100° C. than from the raw clay at 1300° C.

Many alluvial red-burning clays cannot be fired to vitrification because, before vitrification temperature is reached, the clay commences to blow. Many such clays, after the osmosis treatment, have a margin of from 100° to 200° C. between the vitrifying temperature and the temperature at which decomposition starts. This enables vitrified bricks and vitrified roofing tiles to be made from a product which, without treatment, could not be used for such purpose.

In the manufacture of porcelain and earthenware, osmosed clays yield whiter bodies or bodies freer from specks and stains. Osmosed fireclays are entirely free from pyrites, and goods made therefrom are not subject to green stains when glazed.

By the use of fine osmosed materials, chemical porcelain has been produced of the highest quality, the body being made of pure kaolin only, which, owing to the fineness of the particles, completely vitrifies; the locking of the glaze to the body by the fine sillimanite crystals, which form on firing and penetrate the glaze, was shown in a photo-micrograph.

The form given to the apparatus for the commercial purification of clay consists of a tank of suitable form, containing at the lower part two paddles, which serve to keep the suspension in agitation, and which direct it in a stream through the numerous small spaces in the cathode fixed immediately above, surrounding the lower half of the anode. The anode consists of a metal cylinder, revolving at a speed of about one revolution in three minutes, at a distance of about three-quarters of an inch from the cathode. A scraper removes the clay from the anode, whence it falls down a shute clear of the machine. fresh clay suspension is fed into the lower part of the container, and the water effluent returned to be mixed with fresh clay. A machine with a cylinder, two feet diameter and five feet long,

The

produces about 1000 tons of pure clay per annum. The manner in which the machine acts towards the clay slip is as follows:

The clay in suspension, in passing through the laminated or perforated cathode, becomes negatively charged and is immediately attracted to the anode cylinder, the water being driven towards the cathode. There is thus obtained a dry layer of clay on the anode cylinder and a watery zone of clay suspension round the cathode. Fresh clay entering the machine encounters the watery zone on its passage to the anode, in which zone the electro-osmotically indifferent particles such as pyrites, mica, and quartz, become freed from the clay, and are washed away with the effluent from the machine. The effluent with these particles also contains some clay. It is carried to a settling tank where the impurities quickly settle out, and thence to a blunger, or other mixing machine, where it takes up fresh clay and returns again to the machine through one or more settling tanks. The clay leaves the machine in the form of a blanket from one-quarter to one-half of an inch in thickness, from which all the water has been driven except about 25 per cent, and in this form admits of ready drying when required. The process is extraordinarily flexible and, therefore, lends itself to the treatment of many varieties of clay from which many different products are obtained. From some marls and fireclays, considerable quantities of pyrites are obtained, and from others silica in a very finely divided form is separated. From china-clay deposits, exceedingly fine mica and sand are obtained. Again, the clay particles themselves vary greatly in fineness, and the very fine particles can be separated from the coarser. This exceedingly fine clay is useful for many purposes, and doubtless many new applications of its use will arise. The finest particles of ball clay are almost jelly-like in fineness.

The cost of working varies widely according to the class of clay treated. The electricity used varies from as low as 20 up to 70 units per ton of machine product.

Another application of electro-osmosis is the electro-osmotic filter press, which has been developed for the de-watering and purifying of many substances in a finely divided state. The press in its simplest form consists of a series of chambers into which the suspension is fed under a head of, say, 1oft., sufficient to ensure a rapid filling of the chambers. The chambers are closed on both sides by filter cloths in the ordinary way, but the cloths are held in position by perforated or grooved metal or carbon or other conducting plates, one of these plates forming a cathode and one an anode. An electrical pressure of 20 to 100 volts, depending on the substance to be filtered, is established between the plates, and the water is forced towards the cathode. In this press, very fine materials can be de-watered, materials fine enough to choke the ordinary press. This press can be used for filtering clays and many other materials in a colloidal state which are difficult to filter in the ordinary pressure press.

The application of the principles of electrical osmosis are by no means confined to the purification of clay. The applications are, in fact, so many that it is possible in the time at our disposal to refer only to a few examples, such as the tanning of leather, the treatment of metallic

slimes produced in various stages of metal purification. The removal of ash from gelatin and the separation of glue and gelatin into several products has been accomplished.

(The paper was illustrated by a large number of lantern slides and experiments.).

DISCUSSION.

The CHAIRMAN (Mr. A. A. Campbell Swinton), in introducing Mr. Highfield, who read the paper, said the subject of electrical osmosis was one that was partly chemical, partly electrical, and partly physical, and in its applications came into the region of engineering, so that it was perhaps reasonable that three authors should have taken part in the paper-Mr. Highfield, who was member of the Council of the Society and well known in the electrical world, Dr. Ormandy, who was well known in the electrical world, and had previously lectured before the Society, and Mr. Northall-Laurie, who was also a distinguished chemist.

a

Sir HERBERT JACKSON, K.B.E., F.R.S., said he had been always interested in electrical osmosis, and at the time during the war when there was some anxiety as to the supply of glass, the hope of getting something better in the way of pots for optical glass was of importance, and it was realised that if the lines pointed out in the paper could be followed it would be of great benefit. The facts were very simple and clear with regard to the advantages of the clay. In his own case, in using an ordinary and well-known form of clay crucible or muffle for experiments, he was very lucky if he could ensure getting anything like 24 hours' work out of it, but it was quite easy to obtain something in the order of ten or fifteen times as much work out of a muffle that was produced from the osmosed clay. He exhibited two specimens of crucibles which had been given to him by the authors, one of which had been baked at a temperature between 800° and 1000°, and the other had been used as an experiment to see how far it would contract at a higher temperature, and inside it and fused into a mass that was quite soft, was some of the clay of a crucible that he at one time used, so that the ordinary clay crucible had fused inside the authors' crucible. When the crucible was being taken out of the furnace, which had a pure silica lining, it had to be lifted out with considerable force, and the silica was drawn out in fine rods, but it was not possible to deform the clay crucible even in taking it out with considerable force. When material was in a fine state of division, so that each particle could exercise its best attraction on the neighbouring particles, a condition of equilibrium and great stability was reached, and when there was stability under heat there was also stability under chemical action for most things. There was great hope that many things which had been difficult to prepare with ordinary clay could be prepared in a state of greater purity and more readily when the process of the osmosed clay was understood thoroughly and became regularly used. He had had the pleasure of meeting Dr. Ormandy and Mr. Highfield some time ago, and took the whole matter up with Dr. Ormandy, and they started out and achieved the brilliant success they had shown that evening, and he felt confident it would be as

brilliant a success when carried out on a large scale.

Mr. WALTER C. HANCOCK said those who had had any experience in the preparation of a lecture for such an audience would be very much impressed with the magnificent show the authors had given. The photographs, taken under very high magnification, were really quite triumphs of art. He did not think he would be giving away any secrets if he said that he came to the meeting that afternoon from a place where they had been more or less discussing the problem of the production of glass, and he was happy to say that the production of high-grade optical glass of all descriptions was now being wrested by this country from many former competitors, and success would depend entirely upon the utilisation of some high-grade refractory material. Up to the present, manufacturers had been content to take the raw material as it was mined and submit it to the most primitive methods of purification. The osmosis process would supersede any method employed hitherto for the production of high-grade china-clay and other forms of clay, and a material would be obtained which was capable of resisting the effects of high temperature on the one hand and of chemical action upon the other.

Mr. W. MURRAY MORRISON said he had seen a large number of samples of osmosed clay, and he believed there was a very large field of application for that clay in this country; in fact, it would seem that the field was almost unlimited. The Society was very much indebted to the authors for having brought the subject forward.

Mr. W. H. PATCHELL said the slides shown on the screen were very beautiful, but the specimens were more fascinating still when seen under the microscope. With regard to the question of colloids, he thought the first strike on record was due to colloid chemistry, when the Egyptians cut off the supply of straw from the Israelites for making bricks. In 1910 he saw the work on osmosis being carried out by Count Schwerin in Germany, and he sent some samples of clay from Cornwall and received most beautiful results from the Count, but he did not appreciate at that time what those samples really meant. They showed a finer clay than could be produced by the ordinary method adopted in Cornwall, but there was no demand in this country for such beautiful products; it was said not to be commercial and so was turned down. About three years later he had one or two interesting interviews with Dr. Ormandy, and after that Sir Herbert Jackson took the matter up. The Cornishman, though generally a Radical, was a most conservative man in his methods. The ordinary Cornish clay washing had gone on with practically no change whatever, and when the offer was made to show the Cornishman better methods it was very difficult to get him to move, and as he could sell more clay than he had been able to produce he really could not be blamed. The difficulty now was not in producing the clay but in getting it out of Cornwall. Where very pure products were required the new process must be of very great benefit.

Captain C. J. GOODWIN said he noticed in a paper by Dr. Ormandy he referred very extensively to the use made of the osmosis process in Germany. It was no secret that Dr. Ormandy and probably some of his colleagues had again visited

the Continent, and it would be of great interest to know to what extent the process had been used during and since the war in Germany and other foreign countries. Apparently the industry was in its infancy in this country. He was anxious to know whether anything had been done to apply the process to the drying of peat. It was a very important question at the present time, because of the great shortage of fuel. In some Continental countries, notably Italy, lignite and peat deposits could be very well utilised if they were amenable to that sort of treatment. In the chemical industry of nearly every country there were certain byproducts and factory wastes which were to some extent of the colloidal nature, and those products very often contained material which, if it could be sufficiently recovered, would yield a very handsome profit. One of the commonest instances was perhaps sewage, in connection with which considerable use had been made of centrifugal machines and filters. It would be interesting to know how the osmosis process would compare with other methods that were used, and to have some idea of what the cost of treating the clays was, in order to have a comparative basis in considering the problems. There was also the question of the conditions under which various bodies were capable of undergoing osmosis treatment. The authors had dealt almost entirely with suspensions in water, and he would like to know whether the process was at all applicable to materials suspended in other liquids, such as ferric hydrate in a caustic soda solution arsenical suspensions in sulphuric acid. He noticed that the clay separated out very rapidly, and he would like to know what the weight of separation was, and whether it varied to any considerable extent with different materials. At the present time considerable interest was being taken in the question of suspension of coal dust and other combustible material in oil, to which the generic name of "colloidal fuel" had been given and it occurred to him that possibly investigation into the time required for the separation might | be a useful method of determining the efficiency of those suspensions in oil.

or

Dr. W. R. ORMANDY, in replying to the discussion, said a very fine piece of work was involved in the coloured photographs which necessitated Mr. Northall-Laurie working very late hours, as even the vibration of passing traffic would ruin the photographs altogether. With reference to the remarks of Sir Herbert Jackson, an osmosed clay muffle, made from a certain clay, lasted thirteen journeys, heated up to 1500° C., as shown by the pyrometer, whereas muffles made from the same clay used by the muffle maker without treatment would never last three journeys. Taking the finest English china-clay which had been treated with 97 tons of water in order to wash 3 tons of clay, it was still possible to separate from that clay 7 per cent by weight of a product | which consisted almost entirely of silica and mica and large rouleaux of china clay, which were not broken up or dispersed by the alkali. Mr. Hancock had rather emphasised the point raised by Sir Herbert Jackson that in the manufacture of optical glass there was a great demand for higher grade refractories than were available previously. That was not only due to the mechanical properties of the clay, but for optical purposes it was essential

that the clay substance itself should not enter into the mixture in a pot to any appreciable extent. As most fireclays contained a good deal of iron, iron, which was a colouring matter, was introduced. Therefore, for the optical glass maker, it was essential that he should have a crucible which would stand the high temperature and would not be eaten away by the corrosive action of the products which were being melted. In the manufacture of ordinary commercial glass there was a very great opening for the use of clay treated by the osmosis process, because at the present moment the great glass industries of this country, which were being developed to a degree unknown before the war, were severely handicapped by the kind of clay they had to employ for their tank blocks and for making crucibles. There was no question that an osmosed clay, owing to its refractory properties, would resist the corrosive action of the fluid glass in the tank. In the past manufacturers on the Continent had made great progress in the manufacture of glass, and if this country was to gain the industry to supply not only our own requirements but the world's markets, it would be necessary to recognise that the methods that were used by the Egyptians were still largely in use to-day, and were methods that had to be scrapped. It was no use stating that science was a great thing unless the manufacturer was going to support science and apply it. With regard to Mr. Patchell's remarks, many people had the idea that the straw used in Egypt was mixed into the bricks in the form of a binder, but that was not the case. The Egyptians used to put the straw into tanks and allow it to ferment in a hot climate until it went into a colloidal rotting mass, and that was used to mix with the sandy Nile-clay in order to make it more plastic. The lack of demand in this country for purified osmosed china-clay in the early days merely showed that the users were not sufficiently educated to realise that it paid to use a scientifically purified product. The manufacturers had to be educated, and to a very large extent the directors of some of the big companies needed education. With regard to Captain Goodwin's remarks, the osmosis process was derived primarily from the Continent, where there had been a greater time to develop it. In Austria there were already china-clay works turning out 60 or 70 tons a day, and there were works at Klingerberg being worked by the process, and very large works with seven or eight machines near Coblenz, and plant was already being erected in Spain. With reference to the drying of peat, that was a subject that had occupied his and his colleague's attention very considerably. The osmose filter press was, in his opinion, the only method which had yet been offered that showed a possible outlet for the treatment on a commercial scale of colloidal peats, which could neither be centrifuged nor pressed. Unfortunately the bulk of peat in the world was of a colloidal type. Such peat could be treated by the osmosis process, and he thought no long time would elapse before Mr. Highfield would be in a position to deal with the matter in another paper. Sewage experiments had also been carried out. One of the troubles was that, whereas clay would travel from one pole definitely to the other, in sewage there was a heterogeneous mass of material, some of which went to one pole and

some to another, and some had no electrical property whatever and would not move. As far as his own experiments went, there did not appear to be any immediate prospect of the electrical treatment of sewage proving any solution of the difficulty. As to the conditions of use, it was quite obvious that a particle could not move in an electric field if there was a good deal of soluble salts present, and that ruled out the possibility of separating colloidal ferric hydrate from a caustic soda solution, as the soda would convey the current and a very large amount of electricity would have to be used. The only way in which colloids suspended in an electrolyte could be dealt with would be to use a modification of the process. First of all, the electrolytic salts should be removed from the solution, leaving the colloid in a watery solution, and afterwards it could be dealt with in the same way as clay. With regard to the rate of output, that was conditioned to a certain extent by the electric pressure used, but there was an economical rate which it did not pay to exceed. The figure for all clays seemed to be pretty much the same. The purer the clay the larger the output. Coal dust suspended in oil behaved as a pseudo-colloid and was subject to the same laws as clay suspended in water, but that was a branch of the subject which had only just come into prominence. It would have to come into very much greater prominence, because oil was getting in greater demand every day, and the supply was not growing at more than one-third of the rate of the demand, so that the question of mixing coal dust with oil was a problem of the very greatest importance.

Mr. HIGHFIELD said it was quite possible to dry peat economically; with a moderate consumption of electricity the water could be forced out of the peat, and when that was done the peat was in the form of little curled up bits of material like cocoanut shavings. The difficulty was to know what to do with the peat in that form. It could be made into a briquette, but that involved a further consumption of energy, and unless it was made into a briquette it was difficult to carry, because it was bulky, and the cost of freight was high. If a peat was mined on a really large scale, tens of thousands of tons a month, the difficulty would be that, as the peat lies in moderately shallow deposit, it would be always running further away from the plant, as the plant could not be placed upon the yielding beds, so that the cost of bringing the peat to the plant was continually increasing and there were many other difficulties to be overcome apart from drying. He had to thank his colleagues for the enormous amount of work they had done on the paper, and Dr. Ormandy for having answered the questions raised in the discussion. He also wished to thank his assistants, who had been of great help in preparing the experiments.

The CHAIRMAN, in proposing a hearty vote of thanks to the authors, was sure the audience would agree that the experiments had been beautifully shown, and that the whole demonstration had been a model of its kind.

The motion was carried and the meeting then terminated.

Pre

GRINDING is one of the most ancient of arts. historic man shaped his instruments of stone and later of metal by rubbing them on rocks which possessed abrasive qualities. It is not a matter of record when the idea of cutting out a circular block of stone, mounting it on a spindle, and revolving it by hand was first thought of.

Sandstones were originally used in the industries where grinding operations were performed, although the applicability of emery was generally recognised by the Greeks of the early ages, who found it on the island of Naxos. The extensive use of emery in competition with the sandstone was limited until around the year 1870, when a method was invented for binding the grains together with a suitable medium and forming the wheels into necessary shapes for use in grinding.

Improvements in the methods of manufacture of grinding wheels naturally included a betterment of the abrasive material, with the result that the artificial abrasive was developed to overcome the imperfections of the natural emery, and to make available an abrasive in sufficient quantity to meet the ever increasing needs of industry.

Abrasives. In general, there are two types of abrasive-aluminous and silicon carbide, the former consisting essentially of aluminum oxide, and the latter of a chemical union of the elements carbon and silicon.

Aluminous abrasives occur in nature as minerals in the form of emery and corundum. Aluminous abrasives are also manufactured by electric furnace methods and sold under the trade names "alundum," "aloxite," "borolon," &c.

Silicon carbide abrasives do not occur in nature but are manufactured in the electric furnace and sold under such trade names as "crystolon," "carborundum," "carbolon."

The Norton brand of aluminous abrasive, alundum, is made from the natural mineral, bauxite, containing as high a percentage of aluminum oxide as it is possible to obtain. The ore is carefully analysed and a mixture so made that the product of the furnace operation is fully controlled. The mixture is fused in an electric furnace of the arc type, and during fusion the material is purified and changed from soft bauxite into hard crystals of aluminum oxide.

The Norton brand of silicon carbide abrasive, crystolon, is manufactured by heating pure silica sand and coke together in a special resistance type of electric furnace. The material is not fused, but a chemical reaction results from the high temperature employed, with resulting crystals of abrasive.

Sizing the Abrasive.-The alundum and crystolon abrasives are received at the Norton grinding wheel plant in irregular pieces about six inches in diameter. This material is passed through a series of jaw crushers, rolls, washers, &c., and is *From Proceedings of the Engineers' Society of Western Pennsylvania, April, 1920.

finally sized by being passed through standard| mesh screens. The standard grain sizes begin at 8 mesh and continue through 200. The flour which is left is designated as 200-F, and is further refined and classified by hydraulic means into such sizes as F, 2F, 3F, XF, &c. The number giving the size of the grain indicates approximately the number of holes to the linear inch in the screen through which the grain will just pass. For instance, a 30 grain will just pass through a screen having 30 small holes to the linear inch or 900 small holes to the square inch.

After passing over the sizing screen, the abrasive grain is stored in tanks, ready to be sent out as polishing material or to be used in the manufacture of grinding wheels.

Manufacturing Processes.-Grinding wheels are manufactured by four processes-vitrified, silicate, elastic and rubber.

By far the larger proportion of grinding wheels is manufactured by the vitrified process. In this process, the abrasive grain is mixed with the proper amounts of clay and water until the mixture has the consistency of a mud, which can be easily poured into moulds. After pouring into moulds, the wheels are taken to drying rooms and left until they are thoroughly dry. They are then shaved on special shaving machines to the approximate shapes and dimensions required, and placed in dry storage, ready to go into the kilns for vitrification. Vitrification in the kilns takes place at about the melting point of steel, and the length of time required for heating, the length of time held at high heat, and the cooling period are very important. Kilns are of the type used in the pottery industry, and are fired by a series of hardcoal fires uniformly spaced around the base of the kilns. In the larger types of kilns, it is approximately three weeks from the time the kiln is charged, until it is drawn. This time is absolutely necessary, and regardless of the emergency nature of any order, every wheel burned in the large kilns must remain there for the full time. After the wheels have been burned, they are sent to machines where they are shaped to exact size by means of hard metal cutters.

Silicate wheels, as the name indicates, are made by using a bonding material composed of silicate of soda. These wheels are made by tamping into iron moulds, and they are baked at a comparatively low temperature. By this process, all wheels 30 or more inches in diameter are manufactured, 60 inches in diameter being the maximum size that can be made.

Elastic wheels are made by using a bond having quite a degree of elasticity. The bond is of an organic nature and is composed mostly of shellac. These wheels are also tamped into iron moulds and are baked at a comparatively low temperature.

Rubber wheels are made by mixing abrasive with rubber, and later, vulcanising the resultant product.

Silicate wheels are used largely in the cutlery industry in general to replace sandstone; and for all wheels over 30 inches in diameter, which cannot be manufactured commercially by the vitrified process. Elastic wheels are used where very thin wheels are required for cutting off stock; also for finish grinding of chilled iron rolls, and for other work where a fine finish is desired. Rubber wheels

are used on the same type of work as elastic wheels, except that they have a somewhat harder action and are, therefore, used only where grades harder than those made by the electric process are required.

Abrasive Action.-The abrasive action of an aluminous abrasive is dependent upon the amount of crystalline aluminous oxide present and also upon the temper or brittleness of the abrasive.

The best grade of emery comes from Turkey. It contains up to about 65 per cent of corundum, which is the form in which the cutting element occurs in emery. Emery mined in America has as low as 10 per cent corundum. The chief impurity of emery is magnetic iron oxide. Alundum abrasives contain more than 92 per cent of this cutting element, and a special alundum known as No. 38 contains more than 98 per cent aluminum oxide. Practically no magnetic iron oxide is present in these abrasives.

Artificial aluminous abrasives are more efficient than emery, because: (1) they contain a much higher percentage of cutting element; (2) being free from impurities, they are capable of variations in toughness to suit the work to be done; (3) they are made to a definite standard of composition and temper.

The grinding wheel to meet present day requirements must be a scientifically developed cutting tool. Its action when at work is similar to that of the steel milling-cutter. On the face of the wheel are millions of cutting teeth at work every minute, and although these teeth are not as long or as strong as the teeth of the steel cutter, and cannot cut as deep, they are capable of working at a much greater speed. Each little cutting tool, which in substance is a grain of abrasive material, cuts off a chip at each revolution. The chips resemble, in shape and character, the chips cut off by the milling-cutter.

The

Uses in Industry.-The uses of grinding wheels in industry to-day are many and varied. great refinement attained in the case of the gasoline motor used for automobiles could not have been reached without the use of the grinding wheel and artificial abrasives. Likewise, the grinding wheel plays an important part in the manufacture of tractors, motor toucks, gas-engines used for farm purposes, &c.

The ball- and roller-bearing industry, which has grown up alongside the automobile industry, is likewise absolutely dependent upon artificial abrasives for the refinement and accuracy of its product. Being composed of hardened alloy steels, the only way that these could be brought within the required limits of accuracy and finish is by means of such aluminous abrasives as alundum. Likewise, the requirements of this industry are so great that existing supplies of natural abrasives, even though they were of proper standards of purity, would not begin to meet the demand.

The phonograph, the typewriter, the adding machine, the cash register, and other apparatus of similar nature could not be made as economically to-day, if it were not for the grinding wheel industry and artificial abrasives. The agricultural implement industry uses grinding wheels and abrasive grain in large quantities for the manufacture of harvesting and threshing machinery, ploughs, planters, &c. Even such activities as the