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the dike material or until the grade of the product is proven too low for commercial use. The most convenient small shaft is a 3 by 6 foot rectangular shaft, with the larger dimension across the dike. In sinking such a shaft, frequent and detailed record should be made of the appearance of the rock, and at intervals not exceeding 5 feet a 50-pound sample should be taken, and each of these samples crushed and quartered to a 5-pound sample after the manner described above. If more than one band of the dike is exposed in the shaft, each band should be sampled separately.

use.

TESTING FELDSPAR.

If a feldspar as marketed was the powder of a pure mineral, the chemical analysis would doubtless afford a reliable means of checking the various shipments and thus insure uniform results from its As the feldspar of commerce necessarily contains other minerals which are associated with it and as the term "feldspar" is used to cover a broad range of chemical composition, the safest course for the user who desires uniformity of results is to determine the chemical composition and also the physical properties of the feldspar. For the methods of chemical analysis of feldspars, the reader is referred to United States Geological Survey Bulletin 422. The most important physical properties of feldspars are as follows: Temperature of deformation or fusion; rate of deformation; color when deformed and after complete fusion; shrinkage which its introduction imparts to a standard porcelain body.

TEMPERATURE OF FUSION.

Feldspars fuse at widely varying temperatures, owing to their great variation in chemical composition. When to this cause of variation is added that of an uncertain proportion of mineral other than feldspar, only a very general prediction can be made as to the physical and pyrometric properties of the mixture.

Fusion is a progressive process and covers the entire pyrophysical change from the solid to the fluid state. The only means of measuring the degree to which the process has attained is to determine the change in viscosity of the mass.

Measurement of the viscosity of fusion by means of physical apparatus is a difficult operation, and the apparatus at present available is not suitable for the use of any but highly trained experimentors. Fortunately for the practical man, as well as for the more highly trained investigator, there is a simple and accurate method of measuring viscosity change in the early stages of fusion which affords a reliable means of expressing the relative temperature and rate of fusion.

a Hillebrand, W. F., The analysis of silicate and carbonate rocks: U. S. Geol. Survey Bull. 422, 1910, 239 pp.

TESTING WITH STANDARD CONES.

The method referred to is standardization against standard pyrometric cones of known deformation temperature and rate of deformation. Deformation may be defined as the first visual evidence of fusion and indicates the reduction of viscosity to the point where the mass is no longer able to support its own weight. The rate of deformation evidences only the rate of fusion in its early stages, but by the method employed the possibility of error is reduced to the minimum.

The standard pyrometric cones are made from a series of mixtures of carefully tested and standardized materials similar to those employed in porcelain manufacture. They were first produced by Dr. Herman Seger under the patronage of the German Government. The temperature at which the cone deforms is regulated by its composition. Seger made a series of standard cones to fuse at intervals of 20° C. between 1,150° C. and 1,650° C., and numbered them 1 to 26. Later a series of standard cones for temperatures from 950° C. to 1,130° C. were devised by E. Cramer, an associate of Seger. The members of this series also fuse at temperature intervals of 20° C., and are numbered 010 to 01. A series of standard cones were later produced for temperature above 1,650° C., and these are known as Nos. 27 to 42, the latter being the fusing point of pure alumina. The temperature at which these pyrometric cones fuse is affected to a greater or less degree by the heat treatment which they receive, but with firing conditions constant they are reliable to within a few degrees.

The standard cones of the Seger series, which are the only ones in which the feldspar investigator is interested, are as follows:

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Cones Nos. 21 to 25, inclusive, have proved so erratic in their behavior under commercial firing conditions that their manufacture and use have been abandoned both in Europe and America. The

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. drying these cones are ready for test.

METHOD OF TESTING.

After

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 pyro

metric 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

FIGURE 14.-Standard cone for deformation test.

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

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.

THERMOELECTRIC PYROMETER.

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.

HEAT-RADIATION PYROMETER.

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.

OPTICAL PYROMETER.

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.

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