Patent GB-1049888-A (GB)
Title: Improvements in methods for producing ceramic glasses
Inventor: HAGY HENRY EDWIN
IPC: C03C10/00
Filed: 1963-10-30
Abstract: <PICT:1049888/C1/1> In a method of making a semi-crystalline product from a glass containing crystallizable ingredients by a process of controlled heat treatment, the temperature of the glass is controlled to maintain, within the range of measurement error or instrument accuracy, the viscosity of the glass at a substantially constant value within a range suitable for crystallization.  A beam of glass 13 to be crystallized by the heat treatment is positioned in the chamber 12 of a furnace 10 heated by resistance elements 11 and supported at each end.  To maintain the viscosity of the beam 13 substantially constant, a weight 15 is hung by a hook 14 from the beam 13 and will move down at a constant rate for any given viscosity.  The weight 15 is attached to the core 18 of a linear variable differential transformer 17, the coils 16 of which are driven downwardly by linear drive motor 25 at the rate according to the desired viscosity; the core 18 being initially at a null position between coils 16.  Any variation of viscosity will vary the rate of descent of core 18 and transformer 17 will emit a signal 40 to error signal recorder-controller 19 which emits proportional signal 41 to comparator 20 which gives output signal 48 to variable speed motor 27 to drive the temperature control point in controller 30 which is equipped with current adjusting type power control and magnetic amplifier and which thereby varies the current in heating coil 11 to restore the viscosity value.  To prevent core 18 moving outside coils 16, controller 19 has limit switches and at predetermined movement of core 18 from the null position, signal 42 is sent to programmer 22, formed by relays, and a relay is closed to give signal 44, closing the circuit between controller 19 and null correction motor 23, the signal 51, which has a voltage proportional to error signal 40, being directed into motor 23 by signal 43 which throws motor 25 out of gear and itself operates to shift coils 16 back to a null position, when signal 40 is again zero.  To maintain the signal 48 correctly recording the error in viscosity, programmer 22 at signal 42, also sends signal 45 to motor step control 21 which emits signal 46 to comparator 20, the signal 48 being the sum of the signals 41 and 46, such signal being the same as if the renulling operation had not been necessary.  The temperature in chamber 12 is sensed by a thermocouple 35 and recorded in recorder-controller 30 as temperature variations against time and this record can be utilized in setting up temperature gradients in a conventional lehr so that the glass articles passing therethrough are crystallized with substantially constant viscosity.  When it is desired to remove samples of glass at different stages in the crystallizing operation without disturbing the control system of furnace 10, a second furnace is provided for the samples and this furnace is maintained at the same temperature by thermocouples in the two furnaces, with their positive elements connected in opposition and their negative elements connected to the input terminals of zero centre controller which amplifies the potential due to any difference of temperature in the furnaces and provides the power input for controlling the temperature of the second furnace.