Open AccessEffects of thermal treatment on glass as shown by precise viscometry
Author(s) -
Vaughan H. Stott,
D. T. Turner,
H. A. Sloman
Publication year - 1926
Publication title -
proceedings of the royal society of london. series a, containing papers of a mathematical and physical character
Language(s) - English
Resource type - Journals
eISSN - 2053-9150
pISSN - 0950-1207
DOI - 10.1098/rspa.1926.0126
Subject(s) - platinum , viscometer , crucible (geodemography) , materials science , glass tube , tube (container) , metallurgy , viscosity , composite material , chemistry , biochemistry , computational chemistry , catalysis
Viscometers which have previously been described by various authors for use with molten glass have suffered from a common defect, namely, the impossibility of following any changes which might occur in the viscosity due to prolonged thermal treatment of various kinds. This defect can only be overcome by the use of apparatus which is practically insoluble in molten glass, and which permits the glass to be cooled to room temperatures and reheated. In the present state of our knowledge such apparatus must be constructed of a platinum alloy, the high cost of which tends to restrict the weight of the portions of the apparatus in contact with glass. In the present apparatus viscosity is measured by a determination of the thickness of the film of glass which adheres to a thin wire of 10 per cent. iridio-platinum which is withdrawn at a known velocity from the molten glass contained in a small crucible of the same material. The glass is heated in a vertical cylindrical platinum resistance furnace having two concentric windings of platinum foil 1 inch wide by 0·002 inch thick. The inner tube of the furnace, which is of alundum, has an internal diameter of 1½ inches, and is 24 inches long. The glass is contained in an iridio-platinum crucible of 1¼ inches diameter at the top, and 2 inches high, which is supported in the centre of the furnace. The furnace stands on three adjustable legs carried by a table, which can be swung to one side of its normal position so as to allow of convenient access to the various parts of the apparatus. Such a furnace is capable of withstanding high temperatures tor prolonged periods. During viscosity determinations, the furnace was run continuously at working temperatures when not otherwise mentioned in the text. The weighed iridio-platinum wire, which is to be lowered into the glass and thence raised, is tied to platinum wires fixed in slots in the cut-away portion of an alundum tube, as shown in fig. 1. The upper end of the alundum tube is fixed to the lower end of a counterpoised steel tube which can be raised or lowered in guides by means of an electric motor. Fig. 2 shows the general arrangement with the tube at the bottom of its strobe. The useful speeds of the motor can be varied in the ratio of twenty-five to one, and two gears are available in the ratio of four to one, the available linear velocities of the tube varying from 0·01 to 1 cm./sec. The steel tube A is actuated by means of a rack and pinion through the agency of a toothed clutch operated by the experimenter. The motor carries a flywheel of such inertia that the drop in speed of the motor on letting in the clutch is inappreciable. The speed of withdrawal of the wire is measured by an electric chronograph which makes contact with successive teeth of the rack. The chronograph is accurate to about 1/100 sec. The temperatures are measured with a specially designed disappearing filament pyrometer capable of a precision of one or two degrees between 800° and 1600°. The pyrometer is sighted on the glass through a totally reflecting prism B mounted above the steel tube. When the tube rises in the course of an experiment, the prism swings away from it on pivots.