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 role 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.

FIRING.

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.

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.

COLOR.

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.

Translucency is determined by placing the wedge-shaped translucency trials over a l-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.

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.

TEST UNDER GLAZE.

An important property of all feldspars is their effect on the color of the body, both unglazed and glazed. Many porcelain bodies are of faultless color when unglazed, but when covered with a clear glaze the defects of the ingredients show, and the body as viewed through the glaze is so badly off-color that the commercial value of the ware is greatly reduced. Tests under glaze are therefore of much importance, and as the glazes used are of two classes-namely, raw-lead and fritted—it is advisable whenever possible to apply to the fired trials a thin coat of glaze. For convenience it is desirable to place a small quantity of each glaze on the same trial or test piece, in order that the effects of the different glazes may be most clearly compared. A small part of the trial should be left unglazed, in order that the effect that glazing has on the color may be determined by comparison.

For tests of materials in porcelain mixtures under glazes, those glazes should be chosen that are most likely to be used with the materials in commercial work.

For this test the following glazes have been found satisfactory and are recommenced for use at temperatures between cone 02 and cone 2—that is, approximately, 1,110° to 1,190° C:

This material is ground with water enough to produce a thin paste and is applied as a coat of uniform thickness to all the test pieces to be compared.

This frit is melted to a glass, cooled, and ground to pass a 150-mesh sieve. The ground melted frit is added to the glaze in the proportion given above, and the mixture is thoroughly ground before being applied to the trials. The fritted glaze is generally applied in a thinner coat than that necessary for the raw-lead glaze.

BLANK OR STANDARD TRIAL FOR FELDSPAR.

The blank or standard trial for feldspar consists of the standard porcelain mixture, and contains the standard feldspar, kaolin, and quartz in the proportions specified. The chemical composition of the standard feldspar is as follows:

The standard feldspar has a deformation temperature range of 1,300° to 1,320° C. When fused it becomes a milky glass without any yellow tint.

In the standard porcelain mixture this feldspar produces a vitreous mass at 1,310° C. and at 1,350° C. shows no indication of warping. The color is a vitreous white without any distinguishable cream or blue tint. At 1,350° C. the feldspar produces a translucency of 0.65, and a total shrinkage of 15.6 per cent, 3 per cent of which is drying shrinkage and 12.6 per cent firing shrinkage.

QUARRYING AND QUARRYING METHODS.

The methods of quarrying dike materials vary greatly in different localities, owing to the wide difference in structure of different deposits and the varying amount of worthless material which must be handled.

Most pegmatite deposits outcrop along a part of their strike, and the common error of opening a quarry where the dike material is most easily and cheaply accessible, without due consideration for

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future operations, has caused many a venture to fail which with a little more care might have been a success.

Dikes rarely have vertical walls, and if a dike is opened from the foot-wall side the removal of the material below the hanging wall may necessitate the handling of a large amount of hanging-wall refuse. In many cases the deposit may be as conveniently opened on the hanging wall, and the foot-wall material need not be disturbed, so that the expense of handling this material may be avoided.

Careful planning of the method of attack is especially necessary in dealing with the isolated lenses of foreign or wall material which are to be found in all dikes and sills, the removal of which often absorbs all the profit from the marketable material obtained.

The dikes often contain impure bands throughout their entire extent. If these impure parts are along the walls, the quarrying may be so planned that only at the entrance and in parts of the hanging wall need the impure material be disturbed. If the capping of the dike is worthless because of impurity, it may be removed first and the great annoyance of handling this refuse on the quarry floor with the marketable material be avoided.

Where there is a definite impure band in the heart of the dike it is often advisable to remove the dike material only on one side the impure band and leave the other part until the band can be separately removed without serious difficulty.

The reason for working a quarry in such an irregular manner is evident if the expense of sorting the quarried products by hand is thereby avoided. Where biotite or tourmaline is scattered throughout the deposit and hand sorting of all marketed material is necessary, it would hardly be advisable to quarry the two sides and the impure band separately.

THE OPENING OF A QUARRY.

Careful consideration should be given to the opening of a quarry in order that the material may be handled as economically as possible. This consideration may involve opening the deposit at a point widely removed from the easiest point of attack.

Where the strike of the deposit follows the face of a hill and the deposit has been proved over a considerable area, the opening cut should be made across the entire width of the dike. From this crosscut entrance the deposit may be worked across the entire face. Only by this method can the production be maintained uniform for any great period of time.

If the deposit strikes across the hill it is necessary to confine operations to one face only, except in extraordinary cases where both ends of the deposit are so situated that quarrying may be carried on equally well from either end. In most cases one face presents