5. Nonrefractory clays, fusing below cone 20.

Determination of fusibility.-The temperature at which a clay fuses is determined either by means of test pieces of known composition, or by some form of apparatus or mechanical pyrometer, the principle of which depends on the expansion of gases or solids, thermoelectricity, spectrophotometry, etc.

Seger cones. These test pieces consist of a series of mixtures of clay with fluxes, so graded that they represent a series of fusion points, each being but a few degrees higher than the one next to it. They are so called because originally introduced by H. Seger, a German ceramist. The materials which he used in making them were such as would have a constant composition, and consisted of washed Zettlitz kaolin, Rörstrand feldspar, Norwegian quartz, Carrara marble, and pure ferric oxide. Cone No. I melts at the same temperature as an alloy composed of one part of platinum and nine parts of gold, or at 1100° C. (2012° F.). Cone No. 20 melts at the highest temperature obtained in a porcelain furnace, or at 1530° C. (2786° F.). The difference between any two successive numbers is 20° C. (36° F.), and the upper member of the series is cone 36, which is composed of a very refractory clay slate, while cone 35 is composed of kaolin from Zettlitz, Bohemia. A lower series of numbers was produced by Cramer, of Berlin, who mixed boracic acid with the materials already mentioned. Hecht obtained still more fusible mixtures by adding both boracic acid and lead in proper proportions to the cones. The result is that there is now a series of 58 numbers, the fusion point of the lowest being 590° C. (1094° F.), and that of the highest 1850° C. (3362° F.).

As the temperature rises the cone begins to soften, and when its fusion point is reached it begins to bend over until its tip touches the base, Plate XIV.

For practical purposes these cones are very successful, though their use has been somewhat unreasonably discouraged by some. They have been much used by foreign manufacturers of clay products and their use in the United States is increasing. The full series can be obtained from Messrs. Seger and Cramer, of Berlin, for $0.01 each (or about two and one-half cents apiece, including duty and expressage), or numbers .010 to 35 can be

obtained for $0.01 each from Prof. E. Orton, Jr., of Ohio State University, Columbus, O. The table of fusing points of these cones and their composition is given below.

If the heat is raised too rapidly the cones which contain much iron swell and blister and do not bend over, so that the best results are obtained by the slow softening of the cone under a gradually rising temperature. In actual use they are placed in the kiln at a point where they can be watched through a peep hole, but at the same time will not receive the direct touch of the flame from the fuel. It is always well to put two or more cones of different numbers in the kiln, so that warning can be had, not only of the end point of firing, but also of the rapidity with which the temperature is rising.

In determining the proper cone to use in burning any kind of ware, several cones are put in the kiln, as for example, numbers .08, I and 5. If .08 and I are bent over in burning and 5 is not affected, the temperature of the kiln is between 1 and 5. The next time numbers 2, 3 and 4 are put in, and 2 and 3 may be fused, but 4 remains unaffected, indicating that the temperature reached the fusing point of 3.