TERRA COTTA PROBLEMS SUGGESTED FOR DISCUSSION AND INVESTIGATION'

BY C. W. HILL

ABSTRACT An occasional review of the manufacturing problems of any industry regarding

which positive information is fragmentary or lacking is often of value. Such a review of some of our terra cotta problems may be productive of discussion which wjll bring to light unpublished data and may inspire our industrial ceramists to increased activity in fundamental research. It may also be of value to our University friends who are frequently in search of research problems of a practical nature. Naturally any summary prepared by an individual will not be complete but the omission of points which others regard as important will usually be noted and supplied by discussion.

The following subjects are proposed for discussion and investigation:

I. Plaster 11.

111. Glaze N. Process V. Patching

VI. Service problems.

Materials: (a) Body mix and materials, (b) barytes, (c) grog

I. Plaster of Paris Since plaster of Paris does not enter into the body or glaze and hence

does not directly influence the ware, it apparently has not received much attention from terra cotta ceramists. The tendency has been to require plaster manufacturers to supply a quality of plaster which will satisfy the workers and the foreman. The standards of the A. S. T. M. are not especially applicable to plaster for our use. A complete set of specifica- tions based on actual tests would be of value to the industry. It is, of course, necessary to bear in mind that local shop conditions influence the qualities desired in plaster. The setting time may differ between a shop having piece work systems and one paying by the hour, or between one in which a moulder works on one mould at a time and one in which he carries several moulds at once. The relation of physical properties, as shown by tests, and the strength, absorption, warping and chipping of moulds has apparently not been established. The selection of the best possible plaster, considering cost of plaster, volume of set plaster per pound, efficiency in use, cost of repairs, and life of moulds would doubtless effect a considerable saving to a factory during a year.

The possibility of adding agents to plaster so as to increase the rate or extent of absorption of moisture from the clay without undue decrease in mould strength is a most interesting subject for investigation.

The development of proper apparatus for uniform gauging of plaster and the supply of gauged plaster to the worker, while largely a mechanical

1 Read before the Terra Cotta Division, St. Louis, Mo., Feb. 28, 1922.

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problem, is one which requires a familiarity with ceramic problems. It would seem to be a matter quite promising of practical results.

A t present costs the question of recovery of used plaster is one of com- mercial value when several plants are located fairly close toge'ther. Theo- retically, there is no reason why the plaster can not be recovered. It should be possible to do this on a commercial basis provided enough waste plaster is available in a locality to warrant the cost of the installa- tion. It would apparently not be profitable for any individual plant. Some efforts to reclaim plaster have been described in the literature, the common result being a plaster of low strength due to the presence of a high percentage of anhydrous sulphate. A careful study of vapor pres- sure would indicate the requirements of a suitable process. These proc- esses may be outlined qualitatively as a means of heating a mass of plaster to the desired temperature for conversion without loss of moisture, re- moval of moisture under control and cooling under control. The fineness of the product would require the same attention as in the manufacture of new plaster. Deviations in properties of recovered plaster could prob- ably be compensated by mixing before use with a suitably selected fresh plaster. The commercial possibilities of the process are, of course, related to the price for new plaster, freight rates, cost of coal and availability of a sufficient quantity of waste plaster in the locality.

11. Body Mix and Materials The subject of body mixtures and materials has, of course, been studied

by each plant as dictated by its available clays and the plant equipment and process. The relation of body composition to strength, porosity, expansion, etc., is being made the subject of investigation by the Bureau of Standards with the cooperation of the National Society and need not be discussed here. There remain, however, certain points of general interest which merit investigation.

Barytes.-Many manufacturers of ceramic products have found it necessary to add barytes t o . fix the large amount of soluble salts in their clays. Others, who have but small amounts of soluble salts, add barytes to be on the safe side, without having determined the efficacy of the addition or the amount required. Some are knowingly adding barytes in excess since it is rather difficult to watch the amount of soluble sulphates and any great increase might not be detected until quite a ton- nage of ware was pressed and in the dryers.

Some manufacturers purchase barytes on specifications based on chemical analysis while others make only a qualitative test. Chemical tests of barytes. are not sufficient guides for purchase for the reason that the ac- tivity of the barytes is not entirely dependent on its purity. The fineness and method of preparation influence the extent to which it will react with

734 HILL-TERRA COTTA PROBLEMS SUGGESTED

soluble sulphates. The effectiveness of barytes is extremely doubtful in cases of low concentration of sulphates. The percentage of sulphates necessary to-give rise to trouble has not been established. Some of these points have been investigated in our laboratory and will be made the sub- ject of a larger paper when the work has been completed. The indica- tions are that a considerable amount of money is being spent for barytes unnecessarily.

M. E. Gates1 has discussed the use of barium hydroxide in place of barium carbonate. While the superiority of the hydroxide can hardly be doubted, the point was not established experimentally in Mr. Gates' paper which dealt primarily with a suitable apparatus for the use of hydroxide.

Grog.-The investigations of Clare and Albery2 have shown the impor- tance of further work on the nature of the grog and of the use of grog of controlled sizing. Additional investigation should, if possible, have re- sults expressed quantitatively in terms of physical properties of the body as well as qualitatively in factory terminology. Some assistance on the problem may be anticipated from the work now in progress a t the Bureau of Standards.

Wilson3 has suggested the further investigation of grog sizing particu- larly with reference to slip cracking.

Davis4 in an admirable paper has shown the effects of using grog of definite screen size and suggests the study of grists.

Flux.-The tightening of the body by additions of suitable fluxing agents is an interesting field for investigation. The subject will be dis- cussed later, since papers on this subject have been promised.

111. Glaze Investigations on terra cotta glazes have been extensive but the reports

as published unfortunately do not as a rule present sufficient data to per- mit of duplication or of practical use. This is due largely to the failure to give the chemical analysis of feldspar used or to express formulae in the accepted XRO, A1203 Y SiOz manner. In some cases the expression has been given in this form but the feldspar has been calculated to the theoreti- cal feldspar formula. It is common knowledge that even feldspar from the same mine varies sufficiently in content of free silica and in alkali content and ratio as to make the direct substitution of material from one carload for that from another shipment impossible. It is commended to

1 Gates, M. E., Jour. Amer. Ceram. Soc., 3,313 (1920). a Clare, R. L., and Albery, D. F., Ibid., 3,384 (1920).

4 Davis, H. E., Ibid., 3, 641 (1920). Wilson, Hewitt, Ibid., 3, 114 (1920).

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the attention of our writers and editors that papers on glaze should con- tain chemical analyses of feldspar and that glaze formulae should be expressed on a common basis. This will be a decided advantage although it will not tell us all that we should know, since the chemical analysis does not indicate the amount of free SiO2 as distinguished from that which is combined.

Tenacity.--The subject of glaze tenacity and of the degree of penetra- tion or union with the underslip or body are all of great importance. The microscopic method is the common method of attack and while it is pro- ductive of a great amount of information it is unsatisfactory in many respects. On account of the apparatus required this is a subject for university research along physical as well as chemical lines. Undoubtedly the use of underslip renders the problem more difficult and probably tends to lower glaze adherence.

IV. Processes The processes of mixing, ageing, forming and drying are well understood

in a practical way. While our information could be increased by further research it would seem that other manufacturing problems are more pressing.

The burning presents many problems of both theoretical and practical value. One of the most important is the determination of the critical cooling ranges of the burned ware. While these may vary'somewhat with the different bodies it is quite probable that bodies maturing a t the same temperature will have approximately the same characteristics in this respect, assuming no great difference in tightness of body. Doubtless our product may be cooled rapidly through certain ranges while in other zones the cooling must be slow or dunting will follow.

Hill' has discussed the results of Purdy and Boeck and has suggested 650"-500°C as the critical zone, more as a result of deduction than that of experimentation with a typical terra cotta body. With periodic kilns the point is not as important as with tunnel kilns in which full advantage may be obtained of rapid cooling. The use of tunnel kilns will doubtless bring out the critical ranges as a result of long experience and operation, but the ranges could be determined experimentally much quicker and a t lower cost. R. I,. Clare2 has done some work which leads him to favor a tempering of the ware.

On account of the relatively small amount of free silica present in the body it is not to be anticipated that changes due to the transition of silica from one form to the other plays an important r61e. Likewise the malogy which some investigators have attempted to draw between clay ware and

New Jersey Ceramist, 1, 73 (1921). 2 Private communication.

736 HILL-TERRA COTTA PROBLEMS SUGGESTED

glass is extremely doubtful since the similarity of the bond to glass (a homogeneous solid solution) is probably masked by other factors. How- ever, it is recognized in all plastic compositions that the characteristics of the bonding material frequently influence the properties of the mass far beyond the proportion which the bond bears to the mass of the inert constituent's. These speculations would lead to a study of the character- istics of the bond and inert material separately as well as those of two variables together.

Considerably more research and engineering is required on tunnel kilns before they may be acclaimed a success for terra cotta. It would seem that better and less expensive types than the muffle tunnel kiln are commercially possible. This may be seen by the following considerations.

The rBle of the muffle in our intermittent kilns is largely that of pro- tection against flame and ash. A muffle also offers a partial protection against the reducing gases. On account of the fact that all commercial refractories when hot are permeable by gases, there is a tendency for all flue gases to diffuse into the muffle. Usually there is sufficient access of air to the muffle to oxidize the CO and hydrocarbons and to prevent the reducing action of Son. SO2 is usually present in a muffle during the cer- tain parts of the burn and although it may issue from the parts in concen- tration sufficiently to be stifling, it does not do harm to the glaze as long as oxygen is present. With the usual design and operation of kilns the diffusion through the muffle free from leaks is not in excess of the amount which can be oxidized by air drawn into the muffle, except during the latter part of the burn when CO is frequently found in percentages as high as 0.2%. Ortmanl reports as high as 1.6% but this could hardly have been obtained under good conditions.

This leads naturally to the assumption that terra cotta can be burned without a muffle, provided it it protected from flame and ash and that the flue gases which come in contact with it are the products of complete combustion with a slight excess of air. These conditions are found in the surface combustion type of furnace. A wide variety of slips and glazes were fired in a surface combustion kiln, the fuel being city gas. It was found that the colors were exact duplicates of similar burns in a muffle kiln. A further test was made using unpurified producer gas from an- thracite coal, care being taken to insure an excess of oxygen. It was found that the SO2 under these conditions had no effect on the glazes or slips. Among the slips and glazes tested were several containing iron oxide which had been found to be somewhat sensitive to improper kiln conditions.

A kiln of the surface combustion type would possess the advantages of simple construction with comparative low cost, and should be more efficient since the ware receives the heat directly without the interpolation

1 Jour. Amer. Ceram. SOG., 3,487 (1920).

10R DISCUSSION AND INVESTIGATION 737

of the m d e . The absence of the m d e should result in a shorter kiln and less dirsculty from overlapping of temperature zones. It is to be expected that a kiln of this type should give less fire-cracking since the heat is transmitted to the ware by convection of the burned gases instead of radiation from the m d e and is taken away in cooling in a similar manner. The temperature gradient within the piece is therefore much less than when it is heated by radiation and, by proper design and control the gradient between the flue gases and the ware can be made quite long.

Theory and experiment, therefore, point to the possibility of burning terra cotta in a tunnel kiln without a m d e and with results not to be expected in a r n d e kiln. While other means of securing complete com- bustion without flame may be possible of development, that of surface combustion is the only one with which the writer is familiar. The adapta- bility of the system to large sized chambers has been proved in other lines of work as well as the absolute control and constancy of the kiln atmos- phere.

The preliminary results of trials with surface combustion kilns for terra cotta are sufficiently encouraging to warrant a most careful investi- gation by the individual manufacturers before going ahead with the muffle type of tunnel kiln.

The installation cost appears to be about 50% more than that for intermittent kiln6 of the same capacity and not more than half that of the m d e type of tunnel kiln. The labor cost of operation should be about the same as the m d e tunnel kiln, burning the same fuel, whereas the efficiency from a thermal-time standpoint should be much higher. On account of the lower temperatures and absence of flame the life of the simple refractory shapes should be very long indeed. The repair should be a comparatively simple and inexpensive matter.

Patching.-The repair of pieces scarred or chipped in shipping where mechanical strength of the piece is not impaired or where there is no danger from the infiltration of water when placed in the building, is a matter frequently of saving an expensive piece of ware. The develop- ment of a completely satisfactory cement would be of great value to the industry. The requirements are that the cement be capable of use by the average artisan, and that it have permanency of strength and color. Many cements change color on weathering, making an unsightly blemish on the piece. It may not be amiss to appeal to all terra cotta ceramists to publish their investigations on patching cements. It is quite probable that some manufacturers have developed a better cement than others and are inclined to regard the information as a trade or company secret, not realizing that the use of an inferior cement by any company injures the business of all companies. As a research matter the application of dental cements to this purpose offers a very attractive field. A complete

738 HILL

survey with comparative tests of all the cements given in the literature for ceramic ware or stone, and those now in the market would constitute a valuable contribution to our section.

V. Service Problems The effect of lime and other components, method of preparation and

application on the permeability, shrinkage, and strength of cements used for setting terra cotta is a subject of vital importance to the industry. While much can be done by proper construction and by the use of flashing for exposed work, the joints still afford a means for the introduction of water into a terra cotta wall and a large percentage of the failures attri- buted to terra cotta have been due to the admission of water through poor mortar. Although it might be said that research along this line is not within our field it is certain that investigation is needed and that we are all as much interested in knowing what is the best mortar obtainable as any one else can possibly be.

Along the same line it should be noted that many terra cotta manufac- turers are still showing construction details which cannot fail to give trouble from the introduction of moisture. The chemist can be of assis- tance to his company by emphasizing the need of protecting iron from corrosion and the danger of cracking of terra cotta due to the expansive force of rusting iron imbedded in terra cotta which may become moist.

ATLANTIC TBRRA COTTA CO. F'ERTH AMBOY, N. J.