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temperatures between 1,550 and 1,630° C. have proved impractical, however, for ceramic firing and in commercial practice there is no demand for cones deforming between these temperatures.
The process of manufacture of these standard pyrometric cones is as follows: The various ingredients are weighed in proper proportion and thoroughly ground to a homogeneous mixture. This mixture is made plastic by additions of a solution of dextrine and pressed in molds. After molding, the cones are baked at a low temperature to make them strong enough to permit handling. The shape and dimensions of the dried standard cone are shown in figure 14.
The material to be tested is ground to an impalpable powder and by a small admixture of dextrine, cornstarch, or similar solution is rendered sufficiently plastic to permit of its being pressed into cones of the same shape and dimensions as the standard cones. After drying these cones are ready for test.
METHOD OF TESTING.
A slab of fire clay is prepared and into this the cones to be tested are embedded about one-sixth their height. The cones should be spaced as closely as possible and not interfere with one another in deforming. Standard pyrometric cones should be placed upon the same slab or on separate slabs, and in as close proximity to the cones being tested as safety of operation will permit. The slab and cones, after being carefully dried, are so placed in an inclosed muffle furnace that no direct flame can strike the cones and the temperature is gradually raised until the desired deformation occurs (Pl. I, C).
A close watch must be kept over the process, as there are two stages to be observed the shrinkage stage and the deformation stage. The first or shrinkage stage is well in advance of the deformation stage in some cases, and a considerable rise in temperature may elapse before any evidence of deformation is noticeable. Specimens. that were removed from the furnace after reaching only the shrinkage stage have been found to be dense and slightly translucent, indicating that the process of fusion had begun in the individual particles but that nothing more than a cementing together of the particles had occurred. The shrinkage stage is characteristic of the standard pyrometric cones and hence these can not be used as standards by which the shrinkage may be gaged. However, as most standard cones do not begin to shrink perceptibly until heated within about. 40° C. of the temperature at which deformation begins, it is practical
FIGURE 14.-Standard cone for deformation test.
to use as a standard for gaging the shrinkage one of the standard pyrometric cones No. 12 to No. 14, which will certainly not begin to shrink until any feldspars that may be used in ceramic manufacture are well advanced in the process of deformation.
In placing the cones on the slab of fire clay it is highly desirable to slant them all slightly and uniformly in the direction in which it is desired that they should deform.
The process of deformation begins when the first evidence of bending is discerned, and is completed when the point of the cone touches the surface of the slab on a level with the base. Stages of deformation beyond the latter point are not easily determined owing to uncertain effect of the supporting action of the plate on two points of the cone. The first evidence of deformation of the feldspar should be carefully recorded with data regarding the degree of deformation of the standard pyrometric cones against which it is being standardized. As its deformation progresses frequent records should be made both of the feldspar cone and the standard cones. This data is valuable in direct proportion as the number of careful readings, since these indicate the rate of deformation.
Time is a factor which must be carefully considered in expressing the temperature of deformation of any feldspar or standard cone. A careful record should be made of the time which elapses between the application of heat to the feldspar and standard cones and the beginning and termination of the deformation process. Also a record should be made of the rate of increase of temperature in the inclosure containing the cones. This data may be obtained by providing a series of standard pyrometric cones which deform at stated intervals from red heat to the critical temperature of the feldspar being tested.
USE OF PYROMETERS.
A more satisfactory means of procuring this data is by employment of one of the numerous pyrometers designed for temperature determination, many of which are equipped with an automatic recording device by which the rise in temperature is plotted on a sheet suitably prepared for such record. Pyrometers other than pyrometric cones may be classified as follows: (1) Thermoelectric pyrometers, (2) heat-radiation pyrometers, and (3) optical pyrometers. A brief description of these thermometers which has been given in Bulletin 53 is included here:
a Watts, A. S., Mining and treatment of feldspar and kaolin in the southern Appalachian region: Bull. 53, Bureau of Mines, 1913, pp. 23-24.
The thermoelectric pyrometer is an instrument for ascertaining the temperature in an oven or kiln. It consists of a thermoelectric couple, made by fusing a platinum wire and a wire composed of 90 per cent platinum and 10 per cent rhodium, which is exposed to the temperature. The difference in temperature between the hot and the cold junctions of these two wires is proportional to the electric current generated, and this is recorded on a galvanometer. The deflection of the galvanometer varies with the current generated, and the dial of the galvanometer, being scaled in centigrade degrees, permits the operator to read directly the temperature of the furnace. Such an instrument is reliable to 3° C. under ideal laboratory conditions, and is reliable within 5 to 10 degrees under factory conditions. The electric pyrometer is highly satisfactory for use in testing feldspars, but as the deformation of a feldspar is a pyrochemical process, in which heat and time are factors, the time consumed in heating the sample to the deformation temperature should be considered in expressing the deformation temperature of any feldspar or feldspar mixture.
The thermoelectric pyrometer deteriorates rapidly at temperatures above 1,500° C., hence for testing kaolins and quartzes Seger cones or some form of heat radiation or optical pyrometer must be used.
In the heat-radiation pyrometer the heat radiated from an incandescent body, in the furnace or kiln, is focused on a thermocouple and the electromotive force generated is indicated by the deflection of an attached galvanometer, which is read on a dial scaled in centigrade degrees. The precautions that the operator must consider in using such a pyrometer are to have the incandescent object focused sharply upon the thermojunction and to have the image so focused of greater size than the junction.
Such a pyrometer is reliable only within 10° C. under the most favorable conditions; hence its use is little, if any, more satisfactory for temperature measurements than are the pyrometric cones.
The optical pyrometer of La Chatelier consists of a telescope that carries a small comparison lamp attached laterally. The image of the flame of this lamp is projected on a mirror at 45 degrees placed at the principal focus of the telescope. The images of the object viewed and of the comparison flame are side by side and are brought to equal intensity by suitable adjustment of the instrument. Under the most favorable conditions this instrument is subject to any error of vision of the operator and for high temperatures should hardly be expected to give results more accurate than 10° C.
As the deformation of feldspars may in some cases be completed within a temperature range of 5° to 8° C., the use of thermocouples or optical pyrometers does not furnish a graphic comparison. Seger cones, placed side by side with the sample to be tested, is by far the most satisfactory method of comparison, although it is always advisable to use a thermocouple or optical pyrometer to check the temperature of the deformation.
The cone of material of which the deformation point is to be determined is placed on a fire-clay slab to which it is made fast by means of a fusible slip or by packing clay about the base. If clay is packed about the base care must be used that the cone be set not more than one-fourth inch in the clay lest the deformation be retarded. If the deformation temperature is to be determined by means of cones, these should be placed about the cone to be tested and as near as possible without danger of contact when deformation begins. If the cones are not set exactly vertical, care must be taken that the same slant be given to all, otherwise the results will not be comparable.
As the rate of fusion is an important factor, the cones should be closely watched and the time at which each standard cone is exposed to the heat should be recorded, the time at which it begins to deform, and the time at which its point reaches the level of its base, the cone forming a semicircle. A similar record for the cone of the material being tested enables one to determine, by referring to the standard-cone record, the temperature of the beginning of deformation and also the temperature at which its point reaches the level of its base. The range of temperature between these two stages of deformation is very important as indicating the range of temperature within which the material under test will be valuable as a flux in pottery manufacture.
PROPERTIES OF FELDSPAR IN PORCELAIN MIXTURES.
The following description of methods of testing the pyrometric properties of feldspar is taken from Bulletin 53: a
TESTING BY MEANS OF STANDARD TRIALS.
The rôle of feldspar in the porcelain mixture is that of a cementing material or solvent, its activity depending on the temperature attained in the firing process. If the temperature only softens the feldspar the latter can do nothing more than bond the quartz and kaolin with which it is intimately mixed. If the feldspar is heated until it becomes fluid, it can take into solution part of the quartz and kaolin, and thus form a more or less homogeneous mass. Impurities in the feldspar may not seem very injurious in their action when the feldspar is tested alone, but may materially affect the speed of reaction and other properties of the feldspar when used in porcelain mixtures.
The action of a feldspar in porcelain mixtures is the only safe and proper basis for judging its industrial value. Experience has proved that it is not essential that the proportions of a porcelain mixture for testing be industrially correct but rather that those proportions be selected which will cause the ingredients to display most pronouncedly any faults that they possess. Thus an excess of feldspar will increase the tendency to warp and also to produce bad colors. For practical test the following proportions have been found most satisfactory: Feldspar, 20 per cent; kaolin, 50 per cent; and quartz, 30 per cent; mixed with 50 grams of water.
A standard for each of these materials should be selected from the best material on the market and should be thoroughly tested as to its physical and chemical properties. From these standard materials a standard trial mixture or blank should be prepared in the proportions given above and this standard trial should be tested in exactly the same manner as the mixture containing the material under test.
METHOD OF PREPARING TRIALS.
The standard plastic trials are produced by mixing the materials in the proportions given above. Especial attention should be given to the thorough mixing of these materials into a homogenous mass, as otherwise the trial mixture lacks uniformity and is unreliable. After being thoroughly mixed and kneaded the material is formed, by jiggering or by pressing into molds in such a manner that a product of varying thickness will be obtained.
For this trial a wedge-shaped rectangular block, which may be any desired length, is most satisfactory. If one edge is sharp and the other three-fourths of an inch thick the trial can be used for translucency and color tests; and by impressing the face with a metal die a record of linear shrinkage may also be obtained. Care should be observed that this mark for shrinkage or identification does not interfere with the translucency test.
a Watts, A. S., Mining and treatment of feldspar and kaolin in the southern Appalachian region, Bull. 53, 1913, pp. 32-36.
For testing the feldspar a mixture should be prepared in every way similar to the standard trial or blank, except that the feldspar to be tested should be substituted for the standard feldspar. In the molding of trials the process employed in making the standard trials must be carefully duplicated if comparable data are expected. All test pieces should be conspicuously marked to insure identification.
As soon as trials are removed from the molds they should be placed where they may dry without warping, and when thoroughly dry the drying shrinkage may be determined by measuring with calipers the impression made by the die.
In firing the trials the temperature attained should be that to which the feldspar will probably be subjected in commercial work. For convenience this temperature may be safely assumed to be about that at which, in the deformation of the trial, the point reaches the level of the base, as described under deformation-point determination.
Vitrification range is that range of temperature within which the feldspar being tested produces a vitreous body that does not warp. The vitrification range of feldspar is determined by means of a bar one-half by one-half by 6 inches composed of a mixture similar to the standard porcelain mixture, except that the standard feldspar is replaced by the feldspar being tested.
This bar, after thorough drying, is so placed in the kiln that it is supported 1 inch from each end, leaving the 4 inches in the middle unsupported. The temperature at which warping begins marks the highest temperature that is practical for this feldspar in this proportion and is considered the maximum temperature of the vitrification range. The minimum temperature of the vitrification range is determined by firing to various temperatures the trials containing the feldspar being tested; they are then carefully weighed dry, and after standing for 24 hours in pure water are removed, carefully dried on the surface only, and reweighed. The increase in weight indicates the absorbed water content. The minimum temperature which renders the trial nonabsorbent is the minimum temperature of the vitrification range.
In the absence of a standard system of measuring color, the method employed is by comparison, using the standard porcelain mixture made and fired under comparable conditions as a standard.
Translucency is determined by placing the wedge-shaped translucency trials over a 1-inch hole in a box that contains a 16-candlepower electric lamp of constant brilliancy. The maximum thickness of the trial, expressed in centimeters, through which a No. 20 wire can be detected on the face of the trial next the lamp with the lamp 3 inches distant, is taken as a measure of translucency.a
Shrinkage is determined by measuring the length of the die impression made in the wedge used in the translucency test. The total shrinkage is the difference between the original length and the length after firing.
a Weelaus, Charles, and Ashley, H. E., Report of the committee on the classification of white ware: Trans. Am. Cer. Soc., vol. 13, 1911, pp. 104-105.