Patent GB-888882-A (GB)
Title: Process and apparatus for the production of glass sheets having a plane surface of polished appearance
IPC: C03B17/06
Filed: 1958-04-24
Abstract: 888,882. Glass sheets. CAMPAGNIE DE SAINT-GOBAIN. April 24, 1958 [April 24, 1957], No. 13094/58. Class 56. A glass sheet having a plane surface of polished appearance is produced by cooling a plane sheet of glass of uniform thickness from a temperature at which all parts thereof are at a viscosity no greater than 10<SP>5</SP> poises to a temperature at which no parts thereof are at a viscosity of less than 10<SP>10</SP> poises, whilst maintaining the viscosity ratio, defined as being the numerical value obtained by dividing the viscosity in poises of the glass at one face of the sheet by the viscosity in poises of the glass at the other face of the sheet, at a value not exceeding 100, and subsequently cooling the sheet to the ambient temperature by any appropriate method. The viscosity ratio at the beginning of the cooling treatment does not exceed 100 and the cooling is effected so that the viscosity ratio decreases until it is less than 10, the rate of cooling being so controlled that the viscosity is at most doubled in value per minute. The glass sheet may be polished on one face, may preferably be an opaline sheet, a glass sheet flashed by deposition of a layer of glass, for example, coloured or opal, on a first sheet of glass, or a glass sheet of varied appearance by superposition of deposits of glasses of different natures or colours. The glass sheet is deposited on a thermally insulating horizontally movable support in a chamber, the glass, support and chamber being maintained at substantially the same temperature, which support carries the sheet through a heat insulated tunnel, the interior of which reduces progressively in temperature from that corresponding to a viscosity of 10<SP>5</SP> poises to that corresponding to a viscosity of 10<SP>10</SP> poises. The plane glass sheet of uniform thickness may be produced from a sheet having any surface state and any viscosity by raising the temperature of the sheet to that required to enable it to flow by gravity, so that the upper face of the sheet becomes plane. As shown in Figs. 1 and 3, glass flows from furnace 1 through platinum tube 2 into tank 3 which feeds the glass through slot 4 of length substantially equal to the width of the glass sheet to be delivered and disposed close to the surface of movable support 7, which comprises a conveyer member 11 comprising a metal belt lined with a fabric 12 impermeable to granular material and resistant to heat, supporting a layer 9 of granular refractory material, e.g. sand, from hopper 20, of size such that they can be readily compacted and levelled, by roller 21, and the layer is of sufficient thickness to insulate the conveyer 11. Support 7 is heated to substantially the temperature of the glass in compartment 8 prior to entering isothermal chamber 6. Plane glass sheet 25 of polished appearance leaves chamber 6 through insulated tunnel 26 in which the sheet progressively solidifies. A layer 10 of a finer pulverous product, e.g. crushed chamette, may be spread on layer 9 from hopper 22. On leaving support 7, the glass sheet passes over brush 28 and dust extractor 29 which removes any adherent finer pulverous product. The upper layer 15 of hot granular refractory material, freed from finer pulverous product by extractor 18 is fed to hopper 20, and the lower layer 16 of granular material is fed to hopper 17, for re-use. A flashed glass sheet consisting of subjacent layer 30 of clear glass and coating 31 of opaline is produced (Fig. 4) by depositing one layer on the other, on support 7 in isothermal chamber 6, from overflow tanks 3 and 3<SP>1</SP> having an overflow discharge disposed very close to the support and of a length substantially equal to the width of the glass sheet to be delivered, the tank being provided with heating means for imparting the desired temperature to the glass sheet. The subjacent layer 32 is obtained by rolling glass between rollers 33, 34 (Fig. 5), which may be disposed within preheat compartment 8 or isothermal chamber 6, and subsequently receives opaline layer 36 from tank 3. Regularity and rate of the feed of glass from the furnace may be improved by the use of two or more feed tubes 2. A plurality of feed tubes fed with glasses of different natures and colours may be employed to obtain a glass sheet of varied appearance. Specification 774,673 is referred to.