New Ceramic Table Tops

MAY, 1937

INDUSTRIAL AND ENGINEERING CHEMISTRY

P. Hubanks for his part in the assay of the samples. C . H. ~~i~~~~of the Red Star yeastand products company gave many suggestions with a view to producing the yeast under conditions approaching those of commercial practice. The work was supported in part by a grant from the funds of the Graduate School, University of Wisconsin.

Literature Cited Balls, A. K., and Brown, 3. B., J . Biol. Chem., 62,789(1924). Claassen, H., 2.Ver.deut. Zucker.-lnd.,84,713(1934). Eijkman, C., van Hoogenhuijze, C. J. C., and Derks. T. J. G., J . Biol. Chem., 50, 311 (1922).

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(4) Elvehjem, C. A,, J . Asaoc. 0.Uicial Agr. Chem., 18,354 (1935). (5) Nelson, v. E.,Heller, v. G.,and Fulmer, E. I., IND.ENQ. CHEM., 17, 199 (1925). (6) Nielsen, N., Compt. Tend. trav. lab. CaThheTg, 20, No. 1 (1934). (7) Scheunert, A,, and Schieblich, M., Z . Vitaminforach., 4 , 294 (1935). (8) Schwartz, W., and Kautzmann, R., Arch. Mikrohiol., 2, 537 (1931). (9) Stiles, H. R . , Peterson, W. H., and Fred, E. B., J . Bact., 12, 427 (1926). (10) Taxner,'C., $.Inat. Brewing, 41, 27 (1935). (11) Williams, R. J., and Saunders, D. H., Biochem. J., 28, 1887 (1934). RECEIVED October 9, 1936.

NEW CERAMIC TABLE TOPS

I

N 1928 when plans for

The commonly used materials are lacking in one or more properties

furnishing the new buildthat the ideal laboratory table top should possess. A recapitulation ing of Mellon Institute of the required properties is presented. A program of research resultwere being made, the selection ing in the formulation and manufacture of a new ceramic material of material for the laboratory table tops was discussed with is described. I n addition to the properties usually possessed by ceramic manufacturers of laboratory bodies, it is important that this material should have a low coefficient furniture. Since no material of expansion which imparts to it high resistance to thermal shock. appeared to be completely The body is made impervious to liquids by impregnation with bitumisatisfactory, it was suggested nous materials and subsequent special heat treatment by means of that through research a new product might be devised which the original pore spaces are filled with carbon in the form of coke. khat would possess all the adThe properties of the new material are described and other applicavantages of the commonly tions are considered. used materials and none of their shortcomings. After careful c o n s i d e r a t i o n the study of the problem was STUART M. PHELPS AND EDWARD E. MARBAKER begun in the institute as a Mellon Institute of Industrial Research, Pittsburgh, Pa. fellowship project under the s u p e r v i s i o n of the senior author. Research and deordinary wood top, it is often chosen with the idea that, if velopmont carried out during the ensuing five years finally anything serious should happen, it can be easily and economiresulted in the small-scale production stage a t the institute. cally replaced. If such replacement is not made when needed, During the past two years developmental work has been the utility of the top is diminished and its appearance is continued and actual commercial production has recently rendered less attractive. been attained Wood tops of the highest grade, made from maple or birch, Discussion of Table-Top Materials finished to resist acids, alkalies, and solvents, and constructed in thickness from l b / s to l3/4 inches, are almost as expensive The most extensively used material for laboratory table as tops made from soapstone or specially manufactured tops is wood (8),usually maple or birch, qrotected by fillers, materials. varnishes, lacquers ( 6 ) , rubber composltions, or special It is not unusual to install a wood top of ordinary construcmastic coatings, or made acid-resistant by chemical treatment. tion and then to cover it with some material to provide The principal advantages of wood are its relatively low cost protection against conditions peculiar to its use. For and its ease of procurement and application. Manufacturers example, in laboratories where special work involving the use of laboratory furniture use improved methods of construction of concentrated sulfuric acid is carried on, sheet lead is used. to prevent warping, but these procedures add to the cost. Such a protected top is acidproof but is not completely Wood is not fireproof, and many a table top has been ruined fireproof because of the heat conductivity and low melting by charring when a flame has been burned too long under a point (327" C., or 620.6' F.) of the lead. Ordinarily 6- or hot plate placed upon it, or when a lighted burner has been 8-pound sheet lead is used, and the joints are burned so that overturned. Resistance to the absorption. of liquids and to the cost is relatively high. Moreover it is never perfectly the attack of acids and alkalies, in which respects wood itself smooth and so does not present an attractive appearance. leaves much to be desired, depends mostly upon the properNickel ( 3 ) and stainless steel are being satisfactorily used as ties of the protective coating. Wood tops require considercoverings for wood tops; although both metals add much to able care if they are to be kept in good condition, and the the appearance of a laboratory, they are expensive. finish must be replaced from time to time so that this mainWood tops are sometimes protected by asbestos in the tenance expense must be added to the cost, Because of the form of sheet or board to decrease the fire hazard, but this low initial investment involved in the installation of an


542

material absorbs liquids easily, is readily attacked by acids, and, because of its relatively rough surface, is difficult to keep clean. To overcome these defects, it is necessary to apply a protective coating such as sodium silicate or a heatand acid-resistant lacquer. Wence, although the original

JCRZEN /

'FILTER CLoTn

'MOLD

PAN

0"TLC.r FOR WATER

Fioom 1. DIAGEAM OF MOLDINGPAN

cost of wood is low, adequate protection and maintenance add so much to the expense that i t is questionable whether in the end its use is attended by any real economy. Asbestos is inherently fireproof, and therefore it bas been considered worth while to improve its strength, decrease its absorption, and make it more resistant to chemical attack, by ', incorporating in its fibers cement, bitumens (9), or resins (61 and to fabricate it under high pressure into monolithic sheets to 2 inches thick (8). Because of the high cost of heavier material i t is customary to install '/l-inch sheet on a wood base and to build up only the exposed edges to full thickness. Cement-asbestos materials ahsorb liquids and are attacked hy acids; their surfaces do not long r e tain a pleasing appearance and are difficult to keep clean. When heated or exposed to solvents, the pitch-impregnated type does not stand up well, although in other respects it is satisfactory. The resinous type is relatively new and its performance has not been definitely established. It is also more expensive than the other types. Soapstone (8) is a naturally occurring material that has a position second only to that of wood in table-top construction. The grade used for this purpose is a variety of serpentime marble that is much harder and more refractory than ordinary soapstone. IIowever, it is sufficiently soft to permit its fabrication into table tops by sawing, and is easily shaped and machined. But in service the surface may soon become scratched and pitted, with the low of the original smoothness which is so desirable from the point of vier of cleanliness. I t is therefore necessary to apply protective coatings from time to time, and occasional resurfacing is recommended. Such treatment adds to the cost of maintenance. The structure of soapstone is dense, and it is therefore practically impervious to liquids. It is not readily attacked by acids or alkalies in the concentrations generally used. I t is fireproof, but when subjected to severe localized heat it may crack. Its relatively high thermal conductivity causes what to some lahoratorians is a disagreeable coldness to the touch. Because, in the natural state i t often contains veins of quartz, calcite, or pyrite, which tend to decrease its strength and acid resistance, the care that must he exercised in selection adds to the cost of slabs satisfactory for table tops. Slate (8) is another mineral product that finds

VOL. 29, NO. 5

someuseas a table-topmaterial. It is a rock metamorphosed from shale under high pressure, with well-defined cleavage that aids in the quarrying of slabs. Slate is fine-grained,dense, and harder than soapstone. Its resistance to acid attack is good, more because of its fme texture than its composition; but when heated it spa& readily hecause of its characteristic cleavage. The cost of slate is high owing to the difficulty of obtaining it in slabs of sufficient thickness and free from acid-soluble mineral admixtures. Vitrified ceramic tile (8) is widely used in the fabrication of table tops. This material is hard, dense, and resistant to breakage by impact. It is impervious to water and acids do not attack it readily, hut when heated it may crack or spall. Its greatest disadvantage lies in the fact that i t is produced in relatively m a l l sizes, and the joints between the tiles are subject to the seepage of water, acid attack, and the accumulation of d i r h While progress has h e n made in overcoming this objection by the formulation of more adequate cements, it is difficult to lay tile with siifficient smoothness to produce a really satisfactory table top. Plate glass can he produced in large sheets and in a variety of colors so that it presents a beautiful appearance. It is impervious to liquids and unattacked by ordinary acids in any concentration, but i t is rather easily broken and will not withstand localized heat. Because of these disadvantages it has not heen recommended by laboratory designers except for special purposes. It is important to mention that a glass of greatly increased impact strength-tempered glass-is being used as a tabletop material. It is clear from this discussion of the characteristics of the materials most widely used in the construction of table tops

MAY, 1937


for general chemical laboratory service that none of them has in itself all the qualities that are regarded generally as requisite for satisfactory service.

Criteria of Excellence Experience has shown that laboratory table tops are subject to hard usage; because of the importance of the subject .to laboratory managers who are called upon to select wisely the materials for such service, the mecessary requirements have been given careful consideration. The preceding discussion of the properties of materials commonly used for the purpose has shown that, in addition to low initial and maintenance cost, a n ideal table top should possess the following important characteristics : PHYSICAL REQUIREMENTS: Appearance and Cleanliness. A table top should be smooth, unifprm in structure, and constructed with few joints. The reflection of light should be low to minimize glare. Transverse Strmgth. The number of sections should be small, to reduce the number of joints. Hence the material should have high transverse strength so that excessive weight may be avoided and few supporting members be required. Scratching and Abrasion. The working surface should not be easily scratched or marred by the abrasive action of sharp objects. It should be possible to maintain for a long time and at minimum expense the original degree of smoothness. Impact Strength. The table top should not crack or break when a heavy object is dropped on its surface, and it should not chip easily. Imperviousness to Liquids. Liquids spilled on the table top should not be absorbed. Effect of Nonaqueous Solvents. The surface should not be stained or roughened by the solvent action of organic liquids in contact with it. Heat and Thermal Shock. The table top should not crack nor spa11 when heated over a small area to relatively high temperatures and then cooled rapidly. CHEMICAL REQUIREMENTS: Noncombustibility. The table top should be completely fireproof. Effect of Acids and Alkalies. The table top should be chemically inert; it should not be attacked by acids and alkalies, thus preventing staining or corrosion. If chemical action takes place, the original smoothness is decreased and it may be de-

stroyed.

lbevelopment of an Improved Material The results of preliminary study indicated that the problem of producing a material to meet the foregoing criteria should beattacked from the point of view of developing a specific, scientifically compounded ceramic material that would be primarily resistant to chemical action and have the necessary strength, hardness, and uniformity of structure. Such a body should easily be formed not only into slabs for table tops but also in many other shapes to extend its usefulness to a wider field. It was recognized immediately that a ceramic body would necessarily be porous unless it was fired to vitrification, as is the case with chemical stoneware. The latter material was known to have practical limitations, such as the diffirulty of producing large slabs without warping, imperfcct surface, and relatively poor resistance to thermal shock. Upon the inception of research indicated by this study, ceramic bodies of the more usual composition were first investigated. Because of the nature of their ingredients and the process employable in their fabrication, they offered advantages such as drying and firing without warping and relative ease of control of physical properties. It was then thought that a porous body with the required mechanical properties could be made impervious by impregnation with suitable substances. The first successful experiments in impregnation involved

543

the use of sulfur and of suitable Bakelite solutions. Later, however, a more promising process was evolved, based on impregnation with bituminous substances followed by destructive distillation to form a residue of coke within the pores of the body. It was plain that a body, originally comparatively soft and therefore easily surfaced and otherwise machined, would be increased in hardness and strength after coking. The problem of improving resistance to local heating a t relatively high temperatures, followed by rapid cooling, was also studied at this stage. It was recognized that, in order to accomplish this improvement, the thermal expansion must be lowered and the heat conductivity increased. This conclusion led to a n investigation of low-expansion materials that could be used as grog and finally to the production and use of cordierite. From this work it seemed apparent that a highly inert ceramic body, comprising a grog of low thermal expansion and filled throughout its structure with carbon in suitable form, should, offer excellent resistance to chemical action and to rapid changes of temperature, and therefore be a highly desirable material for the construction of laboratory table tops. After the decision was reached that a material possessing the necessary impermeability could be made by the introduction of a suitable impregnant into a porous body of the required ceramic properties, the research was concentrated on developing such a product. This work embraced the systematic study of the following problems: (a) the production of a ceramic body bf the desired type; ( b ) increasing the porosity of this body; ( c ) impregnation; and (d) after deciding to use bituminous substances, coking by cracking and distillation.

Production of the Body The development of the body involved the preparation of many batches from various combinations of ceramic materials, the study of the working qualities, and the determination of the ceramic properties by laboratory tests. The water content on which workability depends was found to require adjustment according to the method used in the molding of the shape in which the body was finally to appear-in the first instance, as a table-top slab. Such pieces were formed by hand-pressing in wood or plaster molds, by casting as a slip in plaster molds, and, in the final process, by a modified filter-pressing step in which the water was expelled from the mix by air under pressure. I n the preparation of ware such as slabs for table tops that have a large area and relatively small thickness, the body must be formulated to provide for low drying and firing shrinkage whereby warping can be avoided. This provision is especially difficult when the product must be uniformly fine-grained. The body must also possess adequate strength to facilitate handling in the various stages of manufacture and to meet the constructional requirements of the service in which it ultimately finds its use, The effect of temperature and time of firing on the properties of the different bodies was studied carefully. A high degree of strength was not required of the body itself, as has been pointed out; because of this feature and the necessity for a n open structure, firing a t a high temperature was not desirable. This fact was advantageous from the standpoint of production cost. In the course of the work, bodies in which the desired strength and structure could be developed at temperatures from 9 0 0 9 t o 1250" C . (1650" to 2280" F.) were studied. Because 1000" C. (1830" F.) is easily attained in an electric furnace, that temperature was used until the semiplant production stage was reached. It was originally

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intended that a n electric furnace should be employed in fullto attain a higher degree of permeability for efficacious scale production, but later developments made necessary the impregnation, to study substances that could be incorporated use of gas- or oil-fired furnaces. in them and could eventually be burned out to provide more The properties of a ceramic body are profoundly influenced pore space. Such combustible materials as pitch, coke, by the nonplastic ingredient, grog, which ordinarily consists wood flour, and other finely divided cellulosic substances of high-fired clay bodies not naturally very pervious to were used experimentaily. Paper pulp produced $he best liquids. For the attainment of the properties required of results because it increased the strength of the body in the green and dry states, aided in keeping the shrinkage low, a table-top body, such grog was not appropriate and the and gave rise finally to characteristic capillary pore spaces solution of the problem of selecting and producing a suitable that aid& the penetration of impregnating liquids. grog involved much experimentation. Eventually a satisfactory grog was prepared by firing the plastic part of the Impregnation body to approximately the same temperature as the completed body. This procedure resulted in a uniform grog Because ceramic bodies are inherently resistant to acid material that had little or no effect on the shrinkage and was attack, the impregnant had to be selected with the idea of as permeable as the completed body. maintaining this resistance. As stated previously, the A good table top must be able to withstand not only first impregnant used was sulfur, either with or without relatively high temperatures but also rapid changes of teminhrofiers (1); later Bakelite and other resins in suitable perature. This property is associated with a low coefficient organic solvents were employed for a time. Bodies impregof linear expansion. The early bodies possessed coefficients of nated with these materials were found to possess properties expansion between 0.000005 and 0.000008 (from 20 O to 400 O C., of promise in the laboratory and other constructional fields. or 68" to 752" F.) which is the range of ordinary ceramic Coking bodies. This expansion is not sufficiently low to prevent cracking or spalling under conditions of rapid heating and Finally, however, carbon was decided upon as the ideal substance with which to render the body impervious. The cooling. Because a lowering of the coefficient of expansion development of this idea was necessary, a s t u d y was made of the results of required the study of the properties of a large numexperimental w o r k conducted a t the N a t i o n a l b e r of bituminous s u b stances. Low v i s c o s i t y Bureau of Standards on a s y n t h e t i c a l l y prepared was sought in order that penetration might be m i n e r a l called cordierite rapid and complete; and (Mg&SiS018) which has Fiber high carbon residue, rea n extraordinarily low comaining in the. form of efficient of expansion (4). Dtflocoulated I I coke after volatile matter It was thought that, if had been removed by disthis mineral could be protillation, was most desirduced commercially and able. Furthermore, it was used as grog, the coeffidesirable that contraction cient of expansion of the during coking should not resulting material would be sufficient to cause the be lower than that of any body to check or crack. known ceramic body and These considerations led therefore most desirable Tunnel Kiln to the rejection of many for the production of table materials, but finally pertops. Many experiments tain pitches and asphalts w e r e c a r r i e d out on its were chosen which production and use with afforded excellent results satisfactory results. The when used separately or bodies containing cordiertogether. ite as grog were shown to have coefficients of exManufacturing pansion of the order of Impregnat e d Process 0.000002 and to withstand As the development of h e a t i n g to redness and bodies possessing the reI Coked I immediate chilling withquired ceramic properties out cracking or spalling, proceeded, the mechanie s p e c i a l l y when a relaI Polished I cal methods of forming, tively small amount of impregnating, and coking s i l i c o n carbide was inDIAGRAM OF THE PRODUCTION OF KEMITE FIGURE 4. FLOW the slabs were investigated TABLETOPS corporated to increase the systematically. The first heat conductivity. Fortupieces were m a d e b y nately cordierite can be h a n d - p r e s s i n g in wood p r e p a r e d with an open or plaster molds, but subsequently it was found t h a t more structure and hence high permeability which, after impregnasatisfactory results could be secured by the use of sliption and coking, leads to finished ware of uniform structure. casting in plaster molds; this method was used in much Increment of Porosity of the earlier work. Later a new method was developed that was not only more convenient but also produced slabs Although ceramic bodies of the type studied are generally of more uniform structure. This procedure was based on considered to be quite porous, it seemed desirable, in order

MAY, 1937


t h e principle of the filter press, the water being forced by air pressure out of a slurry contained in a mold of special construction. As illustrated in Figure 1, the pressing apparatus was assembled by placing in the bottom of the mold (which was provided with an opening for the escape of water) a piece of wire cloth of relatively coarse mesh and the same size RS the mold. This fabric was covered completely with a piece of canvas duck to serve as a filter for the slurry. The slurry was then introduced to the required depth and leveled. A piece of r o b h e r s h e e t i n g w a s clamped to the mold just above the slurry to prevent the passage of pressure air through it. Air under pressure was applied directly to the rubber, which, in expanding, compressed the slurry, expelling the water through the filter a n d t h e openings in the hottom of the mold. Originally this operation required the assembly of the mold in a suitable press, but later the same result was obtained bv placing the mold, filled wit& slurry, in a drum which was then tightly closed. The air under pressure was admitted to the drum, and, as compression proceeded, the water was expelled through the opening in the bottom of the mold to the outside by means of an appropriate connection in the drum wall. The air pressure necessary to express the water depended upon the nature of the slurry; in general, 50 pounds per square inch was sufficient, and the operation was completed in approximately 6 hours. When removed from the mold, the slab thus produced was sufficiently dry and strong to he placed on edge and could be dried in this position without slumping. The structure of such a slab possessed exceptional uniformity because the pressure exerted during molding had been uniformly distributed over the entire surface, a condition that cannot be realized by the ordinary mechanical methods of molding. The development of this method took a great deal of experimental work not on!y in the mechanical design of the apparatus but also in determining the proper condition of the slurry. The fastening of the rubher sheet to the mold offered an interesting problem that was finally solved by the use of an arrangement whereby the air pressure holds the rubber sheet in place, thus obviating the use of clamps (Figure 1). An advantage of this apparatus is that the mold pan can be constructed of light materials because the pressure in the drum supports it completely and is effective on the slurry only because of the connection through which the water is expeUed to the atmosphere. Another advantageis obviousthe number and size of slabs tha.t can be produced in one operation depend only upon the number of mold pans available and the size of the drum.

545

FIGURE5 (LeJl). KEMITE TABLE TOP AKD X A R C ~ TS E lNK INSTALLED IS A >iARORATOFCY

The drying of the slabs offered no serious problems, principally because of their open texture. They were slowly dried on edge, first at room temperature and then at higher temperatures, until practically all the water was eliminated. The dried slabs mere fired either in the electric furnace already mentioned or in a gas-fired furnace. The latter was designed especially for this work, which required high operating efficiency and uniform heat distribution obtained by recirculation of the eomhustion gases. The construction of this kiln is shown in Figure 2. Standard stone-working machinery was used for cutting the fired slabs to specified dimensions, for rubbing down to attain smoothness of siiriace and proper thickness, and for machining operations such as drilling and grooving. The impregnation of the slabs was conducted in autoclaves of various sizes. First they were soaked for a suitable time in the impregnant held a t a temperature predetermined to render the viscosity as low as possible without excessive decomposition. The optimum temperature for the bituminous impregnants mas about 200" C . (390' F.). Alter soaking, usually for one hour, the cover of the autoclave was bolted down t,ight, and air under 45 pounds per square inch pressure was admitted. The pressure was usually maintained for one hour. The coking operation was carried out by placing the impregnated slab in a fairly tight container equipped with an exit pipe for the volatile matter, and this vessel was placed in the gas-fired kiln to which reference has already been made. The physical properties of the coke formed in the body can he varied by control of the heat treatment, and much cxperimental work was necessary to evolve a time-temperature schediile ior the production of a coke with an order of

546


reversible expansion similar to that of the ceramic body itself. Experimentally the time of heating from 200 " to 400-500 " C. (390" to 750-930" F.) varied from 40 to 100 hours, but the best results were obtained by coking from 200' to 400" C. over a time range of 60 to 80 hours. The developments were carried through the experimental stage to small-scale production a t Mellon Institute. Certain features of the body and also the production processes have been patented ( 7 ) . A number of finished table tops were made, and several of them were placed in actual laboratory service where the utility of the new material was amply demonstrated. This new material has been given the name Kernite.

Production of Kernite In order to continue the development of Kernite and to carry it to commercial production, a fellowship was established in Mellon Institute early in 1935, with the junior author as incumbent. The results of the work described were studied systematically for some months, and plans for making Kernite ware on a plant scale were formulated and applied to the study of production problems under actual industrial conditions. At the outset, the solution of the engineering problems involved in the preparation of slurry, the molding of slabs, and their handling through the drying and firing operations were of chief importance. A direct air-pressure mold, similar in construction to the apparatus previously described, was built, and slabs 60 X 54 X 2Ii2 inches were made. The upper surface of the slabs was improved by completely covering the slurry in the mold with a sheet of '/(-inch pressboard which prevented the formation of air pockets because of its absorptive nature. The slabs were dried a t 180" C. (356" F.) and fired to 1025' C. (1877" F.) in a periodic muffle kiln. Impregnation and coking were carried out in experimental equipment. The result of this preliminary work indicated that Kernite could be manufactured economically. At this time a change of composition was considered in order that the resulting body might be fired in the tunnel kiln under the temperature conditions regularly used for the burning of terra cotta and in that way reduce the production cost appreciably. Thus the maturing temperature of the body had to be increased from 1025' to 1190" C. (1877" to 2174" F.). Slabs were made from a batch prepared to meet this requirement and fired in the tunnel kiln with results sufficiently satisfactory to warrant continuation of the work. Further study of this problem was carried out jointly a t Mellon Institute and a t the plant. Several new bodies were made, and in each case the slabs possessed approximately the same properties as those produced from the original materials. At the same time the conditions necessary for the large-scale production of cordierite were worked out at the institute and plans were made for firing the raw mix in a periodic kiln. More slabs and a few sinks were produced from mixes designed for burning in the tunnel kiln, and this ware was coked successfully in a newly designed gas-fired oven. It was then decided that a regular schedule for the production of table tops should be followed, and this work was begun in May, 1936. It was necessary to improve the plant layout and to build a coking oven of improved design with a capacity of 10 to 12 thousand pounds of ware, as shown in Figure 3. This oven was heavily insulated and equipped with a gas burner of special construction and a system of flues and ducts arranged for the recirculation of the combustion gases to secure high efficiency and uniform distribution of heat. The sequence of manufacturing operations is shown in

VOL. 29, NO. 5

Figure 4; for the sake of clarity, the details of the operating procedure are briefly described. The slurry is prepared in a circular tank equipped with a mechanical stirrer. The necessary water and paper pulp are placed in the mixer, and the dry ingredients are added slowly with constant stirring. Towards the end of this operation the mix becomes quite stiff and the deflocculant is added. The slurry then becomes thin enough to flow readily although the water content is only 30 per cent. Mixing is continued for a n hour in order to remove as much entrapped air as possible. The slurry is transferred to molding pans assembled in the manner already described. These pans are placed on racks on cars that are run into a horizontal steel tank, 48 feet long and 6 feet in diameter. The outlets of the pans of each group are connected into the header leading to the outside of the tank, the door is tightly closed, and air pressure of 40 to 60 pounds per square inch is applied overnight. After removal from the tank, each slab is transferred from its pan to a pallet designed to permit air to come in contact with the surface of the slab resting upon it and thus to ensure the even drying that prevents warping. The pallet is set a t a n angle of about 60" in a space kept free from drafts, and the slab is dried partially a t room temperature over a period of 48 hours. The slab, still supported by the pallet, is then placed in a dryer and heated slowly to 180" C. (356" F.) in a current of air for 48 hours, when the slab is sufficiently dry to be placed in the kiln. The slab is now strong enough to be handled independently. It is lifted from the pallet and, with several other slabs, is placed vertically on a long edge upon a kiln car. It is carefully supported to ensure against falling and to aid in burning straight. A loaded car is placed in the charging end of the tunnel kiln every 2 hours; as it proceeds, the temperature of the ware is gradually increased until it reaches the hot zone which is maintained a t 1190" C. (2174" F.). Four hours are required for the passage of the car through this part of the kiln, during which time the ware is completely fired. As the car continues through the kiln, the temperature of the ware is decreased; at the end of 120 hours the car arrives a t the cooling chamber from which it is removed 2 hours later. Each slab is then ground to specified thickness on a horizontally rotating rubbing bed of heavy iron construction fed with a mixture of sand and water. It is cut to the required length and width by means of a silicon carbide wheel; any further machining, such as drilling or grooving, is done with appropriate tools. The slab is heated in the coking oven to 150' C . (302' F.) until all moisture has been eliminated, and then with several other slabs is placed in a rack and immersed in the impregnant contained in an autoclave, the temperature of which is held at 210" C. (410" F,). The cover of the autoclave is lowered and the slab allowed to soak for one hour. The cover is then bolted down, and air pressure (40 to 60 pounds) is applied for an additional hour. The impregnated slabs are then taken from the tank and drained, excess impregnant is removed, and the rack is placed on a car and run into the coking oven. When the coking oven has been fully charged, the door is bolted in place. At this time the temperature of the oven is about 150" C. (302' F.). The temperature is increased to 400" C. (752" F.) during a period of 60 to 72 hours, depending upon the weight of the charge. As the heating continues, the volatile matter from the impregnant is expelled, The vapor escapes through three openings in one side of the oven to a header and thence to a simple air-cooled condenser through which the distillate is conducted to collecting drums. Heating a t the maximum temperature is continued until the flow of distillate ceases. The gas is then turned off and the oven allowed to cool for 48 hours, the

.

MAY, 1937


final temperature being about 50” C. (122’ F.). The slabs are then removed and, after a final polishing either by hand or machine, are ready for shipment.

Properties of Kernite Kemite is a ceramic body of fine uniform texture; its pores are filled with carbon in the form of coke that imparts to it a black color. It is hard, dense, and virtually nonabsorbent. Internally it is free from laminations and other lines of weakness. Kernite is lighter in weight than the commonly used table-top materials. Table tops and other ware made from it can be polished, and the finished surfaces present a pleasing appearance and are easy to keep clean. The crushing and transverse strength meet fully exacting requirements for table-top construction. It will not crack or break when objects of ordinary weight are dropped upon it. It is resistant to abrasion and does not chip easily. The high resistance of Kernite to thermal shock is noteworthy. This property is due to a coefficient of linear expansion much lower than that of materials generally used in table tops. Because of the low expansion and the inherent heat conductivity, it will not crack or spa11 when heated to redness and cooled rapidly. Mineral acids, except hydrofluoric, have no appreciable effect on Kernite. Concentrated solutions of alkalies cause superficial etching, which does not affect the original smoothness of the surface. The foregoing facts indicate that Kernite conforms closely t o the requirements for laboratory table tops defined earlier in the paper. The physical properties of Kernite are as follows:

Uses KemiLe was developed primarily as an improved laboratory table-top material. As experience with it has increased, it has become evident that wider uses can be found for it because of its inherent properties and the fact that it can be fabricated in complicated shapes of large size without joints. For many purposes high resistance to thermal changes is not important. In such cases the cordierite grog can be replaced by suitable materials without affecting the other properties. This conclusion has led to the production of ware from both types of body, and it has been deemed advisable to distinguish the noncordierite ware by the name Karcite. (The properties of Karcite are similar to those of Kernite except the coefficient of expansion which is 0.0000060 to 0.0000075.) Either body can be impregnated with sulfur and many resinous materials to produce ware with properties fulfilling the requirements of specialized uses, and in white, brown, and other colors.

547

Kernite, as originally planned, has proved to be a satisfactory material for table tops (Figure 5 ) , hoods, and other forms of laboratory construction requiring low thermal expansion. For such applications Kernite has been given the additional specific designation Labstone. Karcite is being used in the laboratory field for shelves, sinks (some of which are of complex design, Figure 6), tanks, pipe, and pipe fittings. Either material can be employed for flooring to meet special requirements in the laboratory and in chemical industry. In the latter field Kemite should prove to be especially useful for vats, tanks, and other apparatus, because of its resistance to the action of chemicals and its low thermal expansion. In the electrical industry the basic body, properly impregnated, can be used for switchboard panels, specially molded parts, and complicated shapes, and as an insulating material. I n general building construction, Kemite should prove useful, particularly in ornamental forms which would deteriorate by the action of moisture and changes of temperature if made from other materials. Karcite can be employed in many forms, such as sanitary ware, partitions, roofing, flooring, stair treads,, wainscoting, and window sills. It should be applicable in external construction such as base courses, and for other purposes where the color would be suitable. Kemite can be used in applications of this kind which require higher resistance to thermal changes, such as fireplace construction. Karcite has been suggested for use as the base of pool and billiard tables because it can be produced in slabs of the required size, uniformity, and smoothness, and particularly because its weight is less than that of the materials commonly utilized for the purpose. The ceramic body that is the basis of Kernite is being subjected to trial for special purposes for which accurate molding and resistance to rapid changes of temperature are important, and doubtless other uses will be proposed as the product is more widely introduced commercially.

Acknowledgment The authors wish to acknowledge their appreciation of the helpful assistance rendered by G. R. Daniels and B. Purcell.

Literature Cited (1) Darrin, Marc, IND.ENG.CHEM.,20, 8 0 1 4 (1928). (2) Fisher, H.S.,U. S. Patent 1,924,601(1933). (3) Fraser, 0.B.J., IXD.ENQ.CHEM.,17, 604 (1925). (4) Geller, R.F.,and Insley, Herbert, Bur. Standards J. Research, 9, 3546 (1932). ( 5 ) Hoover, C. R., IND. EXQ.CHEM.,15,569-70 (1923). (6) Loetscher, E.C., U. S. Patent 1,863,799(1932). (7) Phelps, S.M., Ibid., 1,774,812(1930);1,832,913(1931);1,978,691 (1934). (8) Swan, J. N., “Rept. of Natl. Research Council Comm. on Construction and Equipment of Chemical Laboratories,” Sect. 7, pp. 105-8, New York, Chemical Foundation, 1930. RECEIVED February 1, 1937.

New Ceramic Table Tops

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