Fig. 67.—Seger Pyramids or “Cones.”
Seger Pyramids or “Cones.”—Seger, of Berlin, published in 1886 an investigation dealing with the production of silicates of progressive melting points. By varying the composition, he was able to produce a series of materials with melting points ranging from 1890° C. to 590° C., the interval between successive compositions being 20° between 1890° and 950°, and 30° from the latter temperature to 590°. The highest member of the series has the composition Al2O3, SiO2; and the lowest member 2SiO2, B2O3. For convenience in use the materials are made in the form of triangular pyramids, 5 cms. in height, and each side of the base 1·5 cms. long. Each pyramid is stamped with a distinguishing number, and altogether 60 are made to cover the range 1890° to 590°. When conducting a test, several pyramids are selected with melting points known to be near the temperature of the furnace, as discovered by previous trials. These are inserted in the furnace standing on a slab of refractory material, as in [fig. 67], and may be watched through a sight-hole or withdrawn from the furnace for examination after attaining the existing temperature. If the right pyramids have been chosen, the appearance presented will be as in [fig. 67], in which D is seen to have collapsed completely, C has bent over, B has been rounded at the top, whilst A is intact. The temperature of the furnace is then taken to correspond to the melting point of C, which is found by reference to a table in which the melting points corresponding to the different distinguishing numbers are given. The pyramids are extremely cheap, and only those with melting points near to the working temperature need be purchased. In cases where it is desired to increase the heat to a specified point, and then to allow the furnace to cool, these pyramids fulfil all requirements; an examination through a sight-hole closed with darkened glass enabling the furnace attendant to discover when the requisite temperature has been attained. The procedure is more difficult when it is desired to maintain a steady temperature, as this involves frequent renewal of pyramids already melted. These appliances are sold under the name of Seger “cones,” the latter word being evidently a misnomer.
Watkin’s Heat Recorder.—This arrangement consists of a small block of fireclay, having a number of cylindrical holes in its upper face. Pellets of materials of progressive melting points are placed in the holes, in which they fit loosely. The block is placed in the furnace, and afterwards withdrawn and examined, when those which have completely melted will be seen to have sunk, and to possess a concave surface; others which have been superficially fused, will show rounded edges, whilst others will be intact. The melting point of the highest member of the series which is observed to have rounded edges is taken as the temperature of the furnace. The materials used in the manufacture of the pellets are approximately the same as those employed by Seger, being the same in number (60), and differing progressively by similar intervals. It is not evident that the method of observation is superior to the use of pyramids, although some workers may prefer it, and the arrangement is merely an alternative plan of using the Seger compositions. Watkin has also introduced a modification in which straight bars of clay compositions are supported at the edges, the temperature being deduced by observing which numbers melt, droop, or remain intact.
“Sentinel” Pyrometers.—Under this name, Brearley, of Sheffield, has introduced a number of compositions, chiefly of salts, which possess definite melting points. These are made in the shape of cylinders, about 1 inch long and ¾ inch in diameter, which collapse completely when the melting point is attained. Compositions have been found which melt at certain temperatures known to give the best results in the treatment of different kinds of steel, and a cylinder of correct melting point, placed in the furnace on a small dish near to the steel, furnishes a simple and correct clue to the attainment of the desired temperature. The existing condition of a furnace may be discovered by taking a number of cylinders, having progressive melting points, and making observations after the manner described under the heading of Seger pyramids. A few “Sentinel” cylinders are frequently of use in the workshop or laboratory for other purposes, such as a rapid check of a given temperature in confirmation of the reading of an indicating pyrometer, or in discovering whether a certain temperature has been exceeded in a given case. “Sentinel” cylinders have been used in such a manner as to give audible warning of the attainment of a given temperature by means of a metal rod, which is made to rest on the cylinder, and which, when the cylinder melts, falls and completes the circuit of an electric bell. The upper range attainable by the use of ordinary metallic salts is not so great as in the case of silicates, but up to 1100° C. metallic sulphates, chlorides, etc., or mixtures of these, give results quite as good as those obtained with Seger pyramids.
Stone’s Pyrometer.—This instrument is intended to indicate the correct temperature at which a metal or alloy should be poured, and consists of a silica tube at the bottom of which is placed an alloy melting at the temperature at which the material operated on should be poured. A silica rod rests on this alloy, and is connected at its upper end to an iron extension, the extremity of which engages a pointer moving over a scale. When the alloy in the silica tube melts, the rod falls through the molten mass and moves the pointer over the scale, thus giving a certain indication that the desired temperature has been attained. Arrangements exist for adjusting the pointer to zero at the commencement of a test.
Fusible Metals.—Instead of clays or salts, a number of metals and alloys are sometimes used. These are placed in the form of short rods in numbered holes in a piece of firebrick and inserted in the furnace, and on withdrawal those which have undergone fusion will be seen to have taken the form of the holes in which they were placed. The temperature of the furnace is considered to lie between the melting points of the last of the series to undergo fusion and the first which remains unchanged. A series of metals of this description is more costly than clays or salts, but is more rapid in action, owing to the superior conductivity of metals.
Fusible Pastes.—These consist of salts incorporated with vaseline or other suitable fat, and are used to detect the attainment of a specified temperature by a piece of metal. If, for example, it were desired to heat a piece of steel to 800° C. for a given purpose, a paste containing common salt might be smeared on its surface before placing in the furnace. On heating, the vaseline burns away, leaving a white mark due to the salt, and this white mark will be visible till the salt fuses. The disappearance of the white mark therefore indicates that the required temperature has been reached; and the method is simple and useful in cases where a number of articles are to be worked at a uniform temperature.