Abstract: A method for monitoring a molding process carried out in cycles includes determining at least two nearest neighbors in the form of cycle data from at least two past cycles, such that the cycle data of the at least two nearest neighbors lie nearer to the current cycle data than the cycle data which do not pertain to the at least two nearest neighbors. Only those past cycles for which quality data are contained in the data collection are used for the determination of the at least two nearest neighbors. A predictability criterion is checked to determine whether a quality variation of the quality data of the cycles of the at least two nearest neighbors is smaller than a maximum variation and/or larger than a minimum variation. If the predictability criterion is not met, a first notification that a quality and/or a quality datum of the molded part is not reliably predictable is issued.

Abstract: Provided is a glass for a magnetic recording medium substrate, which is an amorphous glass, in which an SiO2 content is in a range of 54 mol % or more and 62 mol % or less, an MgO content is in a range of 15 mol % or more and 28 mol % or less, an Li2O content is in a range of 0.2 mol % or more, and an Na2O content is in a range of 5 mol % or less.

Abstract: Methods and systems for de-stabilizing foam produced in submerged combustion melters. A molten mass of glass and bubbles is flowed into an apparatus downstream of a submerged combustion melter. The downstream apparatus includes a floor, a roof and a wall connecting the floor and roof, but is devoid of submerged combustion burners and other components that would increase turbulence of the molten mass. The molten mass has foam on at least a portion of a top surface of the molten mass. One method includes directly impinging an impinging composition onto at least a portion of the foam in the downstream apparatus. Systems for carrying out the methods are described.

Abstract: Ion-exchangeable glasses having high hardness and high elastic modulus. The base cover glass formulation includes Na2O, Y2O3, Al2O3, and SiO2. The glasses may further include P2O5, B2O3, and any of the alkali, alkaline earth, and rare earth oxides, as well as other divalent metal oxides. The ion-exchangeable glasses offer higher hardness, which provides more resistance to micro-ductile scratching damage. Ion-exchange of these glasses increases their resistance to cracking caused by frictive damage and increases retained strength following formation of surface damage.

Abstract: A method is provided for reshaping a glass body rotating about its longitudinal axis. The method includes using a means for establishing a temperature profile, a radial forming tool and an axial forming tool. In the method, a first temperature profile is established and the glass body is reshaped by engaging the radial forming tool. Then a second temperature profile is established and the glass body is reshaped by engaging the axial forming tool.

Abstract: A fusion down-draw method for making a glass sheet using a forming body comprising an upper part (101) made of a first material and a lower part (103) made of a second material, where the first material has lower creep rate than the second material at the normal operating temperature of the forming body, and an fusion down-draw forming apparatus comprising such forming body. The invention is advantageous in that it solves the issues of internal stress and long-term creep issue of a unitary forming body made of a single material such as zircon.

Abstract: Methods and apparatuses are provided for casting silicon for photovoltaic cells and other applications. With such methods and apparatuses, a cast body of geometrically ordered multi-crystalline silicon may be formed that is free or substantially free of radially-distributed impurities and defects and having at least two dimensions that are each at least about 10 cm is provided.

Abstract: A glass melting furnace is operated under a set of control parameters. A glass batch is fed into the glass melting furnace and melted into molten glass. During the melting, a surface layer comprising a portion of the glass batch and foam forms over the molten glass. A plurality of thermograms of the interior of the glass melting furnace is obtained. The thermograms are analyzed to determine whether there is instability in the thermodynamics of the surface layer. The set of control parameters are then adjusted to reduce a determined instability in the thermodynamics of the surface layer.

Abstract: A thickness of at least one preselected portion of a substrate, such as glass substrate for example, is controlled. A laser beam is directed to the at least one preselected portion of the substrate in a viscous state, thereby increasing a temperature and reducing a viscosity of the at least one preselected portion of the substrate in a viscous state sufficiently to cause the at least one preselected portion of the glass substrate to attain a desired thickness. The laser beam after it is generated can be directed to a reflecting surface from which the laser beam is reflected to the at least one preselected portion of the substrate in the viscous state. The substrate can comprise a glass ribbon produced in a downdraw glass forming process for example, and the laser beam can be directed onto a plurality of preselected portions of the glass ribbon.

Abstract: A glass fusion draw apparatus for molten glass stream thermal profile control, including: a first enclosure; and a first isopipe situated within the first enclosure, the first enclosure can include at least one first heating element assembly integral with the wall of the first enclosure, and the at least one first heating element is in proximity to a portion of molten glass stream over-flowing the first isopipe within the enclosure. The apparatus can also include a proximity or temperature sensing system associated with the first enclosure that senses and controls the thermal gradient properties of the molten glass stream or streams in the first enclosure. Also disclosed are methods of making and using the fusion apparatus.

Abstract: Processes of controlling submerged combustion melters, and systems for carrying out the methods. One process includes feeding vitrifiable material into a melter vessel, the melter vessel including a fluid-cooled refractory panel in its floor, ceiling, and/or sidewall, and heating the vitrifiable material with a burner directing combustion products into the melting zone under a level of the molten material in the zone. Burners impart turbulence to the molten material in the melting zone. The fluid-cooled refractory panel is cooled, forming a modified panel having a frozen or highly viscous material layer on a surface of the panel facing the molten material, and a sensor senses temperature of the modified panel using a protected thermocouple positioned in the modified panel shielded from direct contact with turbulent molten material. Processes include controlling the melter using the temperature of the modified panel. Other processes and systems are presented.

Abstract: Systems and methods are provided for controlling temperature in a glass forehearth. In one implementation, a system includes at least one burner disposed in said forehearth, a manifold coupled to said burner, a combustion fuel supply coupled to said burner, a combustion air blower for delivering ambient air under pressure to said manifold, and a controller coupled to said burner for controlling operation of said burner. The system may include a temperature sensor operatively coupled downstream of the blower for providing to the controller a temperature signal indicative of temperature of air delivered to the manifold by the blower. The controller may be responsive to the temperature signal for controlling operation of the burner as a function of current temperature of air fed to the manifold. Operation of the burner may also be controlled as a function of an average air temperature over a preceding time duration.

Abstract: Apparatus for producing glass ribbon includes a control device configured at least periodic thermal stress compensation of the glass ribbon by independently adjusting operation of a plurality of temperature adjustment elements based on stress characteristic information at least periodically obtained from a stress sensor apparatus. In further examples, methods of producing glass include at least periodically sensing a stress characteristic of a glass ribbon and at least periodically changing a transverse temperature profile of the glass ribbon based stress characteristic information. In further examples, methods include the step of changing a transverse temperature profile of the glass ribbon only if a measured parameter is within an operating range associated with the parameter.

Abstract: A method and apparatus for forming a glass sheet using a fusion down-draw process, wherein the heating powers of the heating elements are managed such that in case of a failure of one heating element, the heating power of the adjacent heating element is immediately increased. The method decreases the thermal stress the forming body is exposed to due to the failure of the heating element.

Abstract: The present invention significantly modifies “The Overflow Process”. It includes a method and apparatus for measuring glass flow rate and maintaining a constant glass flow rate. It also embodies design features and methods that support and stress the forming apparatus in a manner such that the deformation that results from thermal creep is corrected, thus minimizing the effect of the thermal creep on the thickness variation of the glass sheet. The present invention also embodies design features that change the process from a single step (combined flow distribution and cooling) to a two step process; step one being flow distribution and step two being cooling.

Abstract: A method for melting inorganic materials, preferably glasses and glass-ceramics, in a melting unit with cooled walls is provided. The method includes selecting the temperature of at least one region of the melt is selected in such a way as to be in a range from Teff?20% to Teff+20%, where the temperature Teff is given by the temperature at which the energy consumption per unit weight of the material to be melted is at a minimum, with the throughput having been selected in such a way as to be suitably adapted to the required residence time.

Abstract: Apparatus for producing glass ribbon includes a control device configured at least periodic thermal stress compensation of the glass ribbon by independently adjusting operation of a plurality of temperature adjustment elements based on stress characteristic information at least periodically obtained from a stress sensor apparatus. In further examples, methods of producing glass include at least periodically sensing a stress characteristic of a glass ribbon and at least periodically changing a transverse temperature profile of the glass ribbon based stress characteristic information. In further examples, methods include the step of changing a transverse temperature profile of the glass ribbon only if a measured parameter is within an operating range associated with the parameter.

Abstract: A glass melting furnace is operated under a set of control parameters. A glass batch is fed into the glass melting furnace and melted into molten glass. During the melting, a surface layer comprising a portion of the glass batch and foam forms over the molten glass. A plurality of thermograms of the interior of the glass melting furnace is obtained. The thermograms are analyzed to determine whether there is instability in the thermodynamics of the surface layer. The set of control parameters are then adjusted to reduce a determined instability in the thermodynamics of the surface layer.

Abstract: Systems and methods are provided for controlling temperature in a glass forehearth. In one implementation, a system includes at least one burner disposed in said forehearth, a manifold coupled to said burner, a combustion fuel supply coupled to said burner, a combustion air blower for delivering ambient air under pressure to said manifold, and a controller coupled to said burner for controlling operation of said burner. The system may include a temperature sensor operatively coupled downstream of the blower for providing to the controller a temperature signal indicative of temperature of air delivered to the manifold by the blower. The controller may be responsive to the temperature signal for controlling operation of the burner as a function of current temperature of air fed to the manifold. Operation of the burner may also be controlled as a function of an average air temperature over a preceding time duration.

Abstract: A method and apparatus for forming a sheet wafer melts feedstock material in a crucible that is part of a crystal growth furnace, passes a plurality of filaments through the crucible to form a sheet wafer, and cuts a portion of the sheet wafer to form a smaller sheet wafer. The method and apparatus then determine the weight of the smaller sheet wafer, and control the temperature of the melted feedstock material (e.g., by controlling crucible temperature or by interfacing with another temperature control system) as a function of the determined weight.

Abstract: Reducing the adjustment complexity during the forming of glass products, such as the forming of glass tubes to obtain syringe bodies. The glass of a glass pre-product may be heated to be formed, a laser may be used that emits light having a wavelength for which the glass of the glass pre-product is at most partially transparent so that the light is at least partially absorbed in the glass.

Abstract: The present invention significantly modifies “The Overflow Process”. It includes a method and apparatus for measuring glass flow rate and maintaining a constant glass flow rate. It also embodies design features and methods that support and stress the forming apparatus in a manner such that the deformation that results from thermal creep is corrected, thus minimizing the effect of the thermal creep on the thickness variation of the glass sheet. The present invention also embodies design features that change the process from a single step (combined flow distribution and cooling) to a two step process; step one being flow distribution and step two being cooling.

Abstract: A method to produce a clear glass or a clear drawn glass includes the steps of melting starting materials to obtain a glass batch melt, refining the obtained glass batch melt, homogenizing the obtained glass batch melt, and producing a glass product in the drawing process. A sulfate refining agent, selected from an alkali-, alkaline earth- or zinc sulfate or mixtures thereof is utilized in a predetermined amount and a predefined refining temperature during refining of the glass batch melt which is 0° C. to 100° C. higher than that in a refining process using a refining system which contains antimony oxide on its own or in combination with one or several other refining agents.

Abstract: A method and apparatus for forming a glass sheet using a fusion down-draw process, wherein the heating powers of the heating elements are managed such that in case of a failure of one heating element, the heating power of the adjacent heating element is immediately increased. The method decreases the thermal stress the forming body is exposed to due to the failure of the heating element.

Abstract: Methods are provided for controlling the formation of defects in sheet glass produced by a fusion process which employs a zirconia melting unit. The methods comprise controlling the temperature profile of the glass as it passes through the finer, finer to stir chamber connecting tube, and stir chamber to minimize both the amount of zirconia which diffuses into the glass and the amount of secondary zirconia based defects which comes out of solution in the stir chamber.

Abstract: In an optical element manufacturing method for press molding in which primary molten glass droplets are caused to collide with a plate to separate some of the droplets and fine droplets of a secondary molten glass that have passed through an opening are dropped onto a lower die to perform press molding, by setting the diameter of the opening of the plate in the range of 50-100% of the effective diameter of the optical functional surface provided for the lower die, manufacturing conditions for the secondary molten glass droplets can be set easily and properly, and optical elements with satisfactory quality of both appearance and optical performance can be manufactured reliably.

Abstract: Systems and methods are provided for controlling temperature in a glass forehearth. In one implementation, a system includes at least one burner disposed in said forehearth, a manifold coupled to said burner, a combustion fuel supply coupled to said burner, a combustion air blower for delivering ambient air under pressure to said manifold, and a controller coupled to said burner for controlling operation of said burner. The system may include a temperature sensor operatively coupled downstream of the blower for providing to the controller a temperature signal indicative of temperature of air delivered to the manifold by the blower. The controller may be responsive to the temperature signal for controlling operation of the burner as a function of current temperature of air fed to the manifold. Operation of the burner may also be controlled as a function of an average air temperature over a preceding time duration.

Abstract: To provide glass for a data storage medium substrate, whereby high heat resistance can be obtained. Glass for a data storage medium substrate, which comprises, as represented by mol percentage based on the following oxides, from 55 to 70% of SiO2, from 2.5 to 9% of Al2O3, from 0 to 10% of MgO, from 0 to 7% of CaO, from 0.5 to 10% of SrO, from 0 to 12.5% of BaO, from 0 to 2.5% of TiO2, from 0.5 to 3.7% of ZrO2, from 0 to 2.5% of Li2O, from 0 to 8% of Na2O, from 2 to 8% of K2O and from 0.5 to 5% of La2O3, provided that the total content of Al2O3and ZrO2(Al2O3+ZrO2) is at most 12%, and the total content of Li2O, Na2O and K2O (R2O) is at most 12%.

Abstract: The method of making thin glass panes includes conveying a glass melt through a vertical inlet to a drawing tank with a slit nozzle; drawing a glass ribbon downward from the slit nozzle; setting a total throughput by setting length and cross section of the inlet and by heating and cooling the inlet to control glass melt viscosity so that pressure in the inlet decreases; and setting a throughput per unit of length in a lateral direction along the glass ribbon via nozzle system geometry and by heating and cooling of the drawing tank and slit nozzle to control melt viscosity, so that glass does not wet an underside of the slit nozzle near a breaking edge. The setting of the total throughput and the throughput per unit length are largely decoupled to simplify process control. An inventive apparatus for performing the method is also disclosed.

Abstract: In the case of a method for manufacturing glass, with which molten glass is enclosed at least partially by precious metal walls or refractory metal walls, and with which the oxygen partial pressure of the molten glass is influenced by a treatment means to prevent disturbances, gas bubbles or other disturbances often form as a result of over-compensation. This over-compensation can be prevented by locating at least one probe (20) for determining the oxygen partial pressure at the interface—or close to the interface—of the glass melt and metal wall, and by regulating the influencing of the oxygen partial pressure in a safe range of the oxygen partial pressure with the treatment means using a regulating system (39, 45).

Abstract: Feed materials are melted in a furnace to form a glass melt at a first temperature T1, the glass melt containing at least one fining agent. The glass melt is cooled to a second temperature T2 less than T1, and an oxygen-containing gas is bubbled through the cooled melt. The glass melt is then re-heated to a third temperature T3 equal to or greater than the first temperature T1.

Abstract: The invention relates to an apparatus for contactless temperature measurement of a melting charge located in a melting crucible 2 inside a melting furnace, in particular a furnace for precision casting, by means of a pyrometer 5 with an optical system 8 and at least one sensor 6 optically connected to said optical system 8, wherein said optical system 8 can be directed by means of a sight glass 18 onto at least one section of the melting crucible 2. In order to reduce soiling caused by smoke gas deposits on the sight glass, a tube is provided which is connected at its upper end to the sight glass 18, extends into a melting chamber 11 of the melting furnace and can be pointed in the direction of the melting crucible 2.

Abstract: A method of a stirring a molten glass which includes an analytical model which may be used to optimize the stirring conditions to minimize refractory metal inclusions which may result from the-stirring operation.

Abstract: A system and method for preparing chalcogenide glass are provided that allow for larger quantities of glass to be produced with lower production costs and less risks of environmental hazards. The system includes a reaction container operable to hold chalcogenide glass constituents during a glass formation reaction, a stirring rod operable to mix the contents of the reaction container, a thermocouple operable to measure the temperature inside the reaction container, and a reaction chamber operable to hold the reaction container. The method includes placing chalcogenide glass constituents in a reaction container, heating the chalcogenide glass constituents above the melting point of at least one of the constituents, promoting dissolving or reaction of the other constituents, stirring the reaction melt, maintaining an overpressure of at least one atmosphere over the reaction melt, and cooling the reaction melt to below the chalcogenide glass transition temperature.

Abstract: A molten glass supply device is provided, which can solve unavoidable problems for high viscosity characteristics in connection with the conventional molten glass supply device for high viscosity glass. Such problems include improperly high heating cost caused by excessive heat radiation in a melting furnace, reduction in the grade of products deriving from an excess amount of an erosion foreign material and reduction in the product yield. High viscosity molten glass having a property in which a temperature at which the molten glass exhibits a viscosity of 1000 poise is 1350° C. or higher is supplied to a forming device through a melting furnace, a distribution portion in communication with the outlet of the melting furnace, and a plurality of branch paths branching from the distribution portion. In the branch paths, distribution resistance providing portions that provide distribution resistance to molten glass passed through the branch paths are provided.

Abstract: A structural component for a device for the treatment of melts, especially of glass melts, having a base body of metal or of a metal alloy and a cooling system in which a cooling medium is led through the structural component for the leading-off of heat. The base body is provided with a coating of a material the decomposition temperature of which lies below the temperature of the melt, and the cooling system is designed and arranged in such manner that the temperature of the boundary layer of the melt that immediately surrounds the structural component lies below the decomposition temperature of the coating material.

Abstract: The present invention alters the flow path at the inlet of the sheet glass forming apparatus to improve quality. The bottom of the downcomer pipe is preferably shaped to alter the character of the vortex flow in the quiescent flow zone between the pipes. In another embodiment, a bead guide provides hydraulic stresses that are in opposition to the surface tension stress and thus reduces the influence of surface tension on the formation of thick beads on the edges of the sheet. The present invention also measures the temperature of the glass by immersing thermocouples in the glass, at locations where any defects caused by the immersion are in the glass that forms the unusable edges of the sheet. In another embodiment, the support structure for the trough is altered to substantially reduce the aging of the trough due to thermal creep.

Abstract: A blown body or slice from differently colored or saturated millefiori glass of a given design of 5 to 30 times enlarged over the entire area while maintaining its proportionality is sealed inside a basic transparent glass 0.5 to 3 mm thick, and pre-heated so that during its attachment to a hot glass bottle made of basic transparent glass the viscosity of its contact surface is in the range of 104 to 109 Pas, whereafter, the bottle is shaped by enlarging the millefiori design while maintaining the proportionality of its design. The blank glass bottle with the millefiori is overlaid by basic glass, shaped further by heating, rolling and blowing it into a final shape and into the enlarged millefiori design while maintaining the proportionality of its design and while obtaining the final thickness of glass of the product of 0.5-3 mm in the area of the sealed millefiori.

Abstract: A system and method for preparing chalcogenide glass are provided that allow for larger quantities of glass to be produced with lower production costs and less risks of environmental hazards. The system includes a reaction container operable to hold chalcogenide glass constituents during a glass formation reaction, a stirring rod operable to mix the contents of the reaction container, a thermocouple operable to measure the temperature inside the reaction container, and a reaction chamber operable to hold the reaction container. The method includes placing chalcogenide glass constituents in a reaction container, heating the chalcogenide glass constituents above the melting point of at least one of the constituents, promoting dissolving or reaction of the other constituents, stirring the reaction melt, maintaining an overpressure of at least one atmosphere over the reaction melt, and cooling the reaction melt to below the chalcogenide glass transition temperature.

Abstract: A method for equalizing temperature differences in molten glass in at least one temperature equalization zone that is in the form of a channel for transporting a glass melt. The equalization zone is located upstream from a tapping point, at which the glass is tapped into a mold in a forming machine, or the like. Resistor heating elements are provided in the temperature equalization zone side walls, bottom wall, and roof. The temperatures of the surfaces of the respective side walls, bottom wall, and roof that are in contact with the resistor heating elements are measured. The resistor heating elements are controlled by an electric controller so that the temperatures of the surfaces are substantially equal to a predetermined tapping temperature of the glass melt.

Abstract: A conveyance apparatus, including:

Abstract: The invention relates to a method for producing thin glass panes, especially glass panes with a thickness of below 1 mm, by drawing a thin glass strand vertically downwards. According to the inventive method, a glass melt is fed from a melting bath via a feed to the glass drawing tank that comprises a nozzle system with at least one slotted nozzle. The length and the diameter of the feed as well as the viscosity of the glass melt present in the feed determine the overall throughput. The throughput per length unit in the transverse direction to the glass strand (so-called line throughput) is adjusted via the geometry of the nozzle system and via the viscosity of the glass melt present in the nozzle system. These parameters are adjusted in such a manner that the glass, when leaving the nozzle system, does not wet the bottom of the slotted nozzle in the range of the tearoff edge.

Abstract: The temperature of cooling air to a manifold for distribution to molds of a glass manufacturing machine is controlled by passing untreated air through an indirect heat exchanger in indirect heat exchange relationship with water flowing through a coil in the indirect heat exchanger before treated air from the indirect heat exchanger is passed through a blower for distribution to the manifold. Water for treating the air flowing through the indirect heat exchanger is pumped through the coil from a water cooling tower that is used in a glass manufacturing plant, either at a first temperature from a first outlet of the water cooling tower or at a second temperature from a second outlet of a water cooling tower, or a mixture of water from the first outlet and the second outlet. In a first embodiment, water from the first outlet and water from the second outlet pass through a temperature controlled three-way mixing valve before delivery to a pump for delivery to the indirect heat exchanger.

Abstract: Glass melting furnace, of the type consisting of a melting chamber, inside which is found the liquid glass mass, with an outlet, and at least one flue gas recuperative chamber, a means for supplying the influx of fuel, featuring at least one mass temperature gauge immersed in the liquid glass mass and placed approximately at a height (h2) with respect to the bottom of the melting chamber of ⅓ to ⅕ of the height (h) of the level of the liquid glass mass, and ⅕ h≦h2≦⅓ h on the longitudinal axis of the melting chamber at a distance (11) with respect to the wall of the chamber opposite the outlet, of between 0.6 and 0.85 of the length of the melting chamber 0.6 l≦11≦0.85 l. For application in the manufacture of glass containers.

Abstract: The present invention relates to a vitrification process for a pulverulent material and an apparatus for implementing said process. The process consists in introducing the pulverulent material into the fusion area of a furnace via injection means, where it is melted by means of at least one plasma torch to obtain a melt, and where the melt is removed from the furnace via a casting zone, the material being introduced laterally into the fusion zone in a direction comprising a horizontal component, and the melt being removed from the furnace by overflowing via the said casting zone, more or less along said horizontal component and away from the means for injecting the material into the furnace, in relation to the fusion zone.

Abstract: A glassware manufacturing system includes at least one blank mold and a loading funnel for feeding molten glass gobs in sequence into the blank mold. A temperature sensor is operatively coupled to the external surface of the funnel for providing an electrical signal indicative of temperature at the funnel due to heat imparted thereto by the molten glass gobs falling through the funnel. Electronics are responsive to the sensor signal for indicating an increase in gob diameter as a function of an increase in temperature at the funnel external surface.

Abstract: A process controls oxy-boost firing and air-fuel burner firing in furnaces, including large glass furnaces such as float furnaces. The large side-fired regenerative float furnaces use oxy-boost firing for a variety of reasons, including production increase, improved glass quality, lowering of superstructure temperatures, and reduction of emissions. An adaptive controller receives input data from process parameter sensors throughout the furnace, and adjusts its control logic for controlling both the oxy-boost burners and the air-fuel port burners.

Abstract: A glassware forming machine system includes an individual section glassware forming machine having a plurality of sections, each with at least one blank mold, and a gob distributor for distributing molten glass gobs to the blank molds of each machine section in sequence. The molten glass gobs are delivered to the blank molds of each section through deflectors on which the glass gobs slide to each blank mold. At least one liquid coolant passage is integral with each deflector, and the several coolant passages for the entire machine are connected in parallel between source and return liquid coolant manifolds. Variable flow control valves are individually connected between each liquid coolant passage and the return manifold for controlling flow of liquid coolant through the passages and thereby balance temperatures among the parallel gob deflectors.

Abstract: Disclosed is a process for producing a glass substrate for an information recording medium by press-shaping a molten glass which gives a glass containing 0.1 to 30 mol % of TiO2, 1 to 45 mol % of CaO, 5 to 40 mol % of total of MgO and the above CaO, 3 to 30 mol % of total of Na2O and Li2O, 0 to 15 mol % of Al2O3 and 35 to 65 mol % of SiO2 and having properties of a liquidus temperature of 1,360° C. or lower and a viscosity of at least 10 poise in a shaping-allowable temperature range, or by preparing a preform formed of a glass which contains 0.1 to 30 mol % of TiO2, 1 to 45 mol % of CaO, 5 to 40 mol % of total of MgO and the above CaO, 3 to 30 mol % of total of Na2O and Li2O, 0 to 15 mol % of Al2O3 and 35 to 65 mol % of SiO2 and has properties of a liquidus temperature of 1,360° C. or lower and shaping the preform in the form of a disc by a re-heat pressing method.