PL117108B1 - Method of glass heat treatment - Google Patents

Description

The subject of the invention is a method of heat treatment of glass, and in particular thermal toughening of flat glass or curved glass panes, for example panes for use individually as windshields of motor vehicles or as part of laminated windshields of motor vehicles, side and rear windows, lights, or for use in the construction of windshield assemblies in aircraft and locomotives. Polish patent description No. 111643 describes a method of heat treatment of glass products by heating any glass object to a temperature above the temperature of stress disappearance and subsequently rapidly cooling the glass products in a fluidized, gas-assisted bed of fine-grained material in a quiescent, uniformly expanded state of fluidization in the fine-grained material (the rate is regulated by the The flow of gas through this material is between the rate corresponding to initial fluidization and the rate corresponding to maximum expansion of the fine-grained material. The fluidized bed is in a state such that agitation of the fluidized fine-grained material is induced on the hot immersed surfaces of the glass as the glass cools in the fluidized bed, but any transiently developed tensile stress on the surface of the hot glass as its leading edge comes into contact with the fluidized bed is not so severe as to expose the glass to damage. Therefore, the process is highly efficient. The degree of tempering of a glass sheet immersed in such a fluidized bed depends on the rate of heat exchange between the particles of the fluidized material. material and the hot sheet immersed in them, and on the rate of transfer of hot particles, on the proximity of the glass sheet to the co-current neck and on the bringing of cooler particles from the fluidized bed near the glass sheet. The motion of particles at the glass surface is faster than the motion of particles in the bulk bed due to the rapid mixing of the fluidized fine-grained material which is induced on the hot immersed glass surfaces due to the heating of the fine-grained material by the glass continuing as the glass cools in the fluidized bed. The mixing of the fine-grained material at the glass surface is greatly enhanced if a fine-grained material having latent gas-evolution properties is selected, so that when this material is in the fluidized bed, it At a short distance from the glass surface, gas is rapidly evolved. Surprisingly, three factors have been found to dominate the thermal toughening of glass in fine-grained material fluidized with gas, and in particular, to regulate the degree of toughening of a hot glass sheet during contact with fine-grained material fluidized with gas. These factors are as follows: 1. The gas-producing property of the fine-grained material. 2. The thermal capacity per unit volume of the fine-grained material at minimum fluidization. This is derived from the specific heat of the material, measured at 50°C, and the density m of the bed material measured at minimum fluidization of the material. 3. The fluidity of a fine-grained material, according to the following definition, which is the sum of the designations of four data assigned to the material 15 based on the evaluation of four properties of the fine-grained material with respect to its flow properties. The term fluidity used herein has the meaning given. These four properties of the fine-grained material with respect to its flow properties and the method of selecting the four starting points are described in the article by Ralph L. Carr Jr. "Evaluating Flow Properties of Solids", Chemical Engineering, Vol. 72, or 2, dated January 18, 1965, and are as follows: 1. Compressibility = 100 /P - A/ 30 In the given formula, P denotes the density of the packed mass and A denotes the density of the aerated mass. 2. Angle of repose: it is the angle in degrees between the horizontal line and the slope of the cone. fine-grained material falling from a point above the level, until the measured angle is constant. 3. Vane angle: The vane is inserted horizontally into the bottom of the mass of dry fine-grained material and the material is lifted straight up from the top. The average angle in degrees between the horizontal and the side line of the pile of material on the vane is the vane angle. 4. Particle size distribution, referred to in the above article as the uniformity coefficient. In the above article, it is described as the numerical value derived from dividing the width of the sieve opening (i.e., particle size) through which 0% of the fine-grained material will pass by the width of the sieve opening through which only 1.0% of the ground material will pass. All particle size distribution values given in the description were measured The known method using a Coulter counter determines the particle diameter according to the retained cumulative amounts by weight of 40% and 90%, corresponding to the widths of the sieve openings through which 60% and only 10% of the fine-grained material will pass. The numerical values of compressibility, angle of repose, and blade angle were measured on a Hosakawa Powder Tester from Hosakawa Micrometrics Laboratory, The Hosakawa Iron Works, Osaka, Japan. The above Powder Tester is specifically designed to determine the flowability of powders, as defined above. The flowability of the ground material is based essentially on such factors as the average particle size, particle size distribution, and shape. The particle size, which is sometimes referred to as the angularity of the particle, i.e., whether it is rounded or angular. The fluidity value increases with increasing average particle size, with narrowing of the particle size distribution, and with decreasing angularity. The heat capacity per unit volume at minimum fluidization depends on the specific heat of the material and on the density of the fluidized bed at minimum fluidization, with density increasing with narrowing of the particle size distribution. A high value of toughening stress is created in glass when it is rapidly cooled in a fluidized bed—optimum fluidity. Some materials that produce the required toughening stresses may be commercially available. Other commercially available materials can be modified to create the required The problem is that there is a limit to the extent to which the degree of quench stress induced in the glass can be adjusted by changing the fluidity of commercially available materials. Materials with the required fluidity may not be commercially available. Producing a large quantity of material with the required fluidity may involve sieving a large quantity of fine-grained material. Furthermore, when using a single material, the only way to modify the heat capacity of the fluidized bed is to narrow the particle size distribution, so that there is no possibility of modifying the heat capacity independently of the change in fluidity that is created by reducing the size distribution. particles. It has now been found that a fine-grained material with optimum gas generation, heat capacity, and fluidity properties for creating the required toughening stresses in glass articles can be produced by using a mixture of fine-grained materials, each of which contributes to imparting the desired properties to the mixture. By selecting the fine-grained materials and the proportions in which they are mixed, a fine-grained gas-fluidized material can be prepared to provide any required toughening stresses over a wide range. According to the invention, a method of heat treating glassware in which glass is heated to a predetermined temperature and contacted with the fine-grained gas-fluidized material comprises using a fine-grained material consisting of a mixture of a plurality of selected fine-grained materials, at least one of which is At least one of the materials has the property of producing a gas when heated from hot glass, and these materials are mixed in selected proportions to impart to the mixture of fine-grained material fluidized with gas such a heat capacity and fluidity as to achieve the desired degree of heat treatment. In the method according to the invention, the above materials are mixed in selected, predetermined proportions so that the required tempering stresses are induced in the glass as it cools in the fine-grained material fluidized with gas from a temperature above its stress breakdown temperature. In the method according to the invention, the fine-grained gas-producing material is a material capable of releasing gas in an amount of 4 to 37 percent of its weight when heated to constant weight. at a temperature of 800°C. Fine-grained materials are mixed in predetermined proportions to give the mixture a heat capacity per unit volume at minimum fluidization in the range of 1.02-1.73 MJ/m2K and a fluidity in the range of 60-86. In thermal toughening of a sheet of soda-lime-silica glass 2-2.5 mm thick (in which the glass sheet is heated to a temperature in the range of 10-680°C and the mixture is kept in a state of quiescent, uniformly expanded fluidization of the fine-grained material), a fine-grained material is used which induces a central tensile stress in the glass sheet in the range of 35-57 MPa. In the production of glass for vehicle windscreens by the method according to the invention Fine-grained materials are mixed in predetermined proportions to give the mixture a fluidity in the range of 71-83 and a heat capacity per unit volume at minimum fluidization in the range of 1.09 MJ/7m2K - 1.38 MJ/m2K. γ-alumina may be used as the gas-producing material. γ-alumina may be mixed with α-alumina. The mixture may contain 7% - 86% by weight of α-alumina. Another method of carrying out the invention comprises cooling a hot glass sheet in a gas-fluidized mixture of a fine-grained gas-producing material and at least one fine-grained metal oxide having a heat capacity per unit volume at minimum fluidization of 40 The heat capacity per unit volume at minimum fluidization is 1.76 MJ/m2 K - 2.01 MJ/m2 K. The fine-grained materials are mixed in predetermined proportions to give the mixture a heat capacity per unit volume at minimum fluidization in the range of 1.27 MJ/m2 K - 1.76 MJ/m2 K and a fluidity of 45°C in the range of 71 - 82°C. Spheroidal iron oxide (α - Fe2O3) can be used as the fine-grained metal oxide. The mixture can contain 30 - 70% by weight of spheroidal iron oxide. Furthermore, the mixture can contain 70 - 30% by weight of alumina as a gas-forming material. Another method of carrying out the invention is to use a mixture of 28 - 35% by weight of spheroidal iron oxide -iron, 45-56% by weight of alumina and the remainder is alumina as a gas-producing material. The zirconium compound ZrC2 • SiO2 can be used as a fine-grained metal oxide. The mixture may contain 10-70% by weight of alumina monohydrate Al2O3 • 1H2O as a gas-producing material and 90-30% by weight of the above zirconium compound. Another example of a fine-grained gas-producing material is aluminosilicate. The aluminosilicate can be converted into zeolite, in which case 8-10% by weight of zeolite is mixed with 90-92% by weight of alumina to form Alumina monohydrate (Al2O3 • 1H2O) may be used as the fine-grained gas-forming material. Further, in the process of the invention, the mixture may comprise silicon carbide SiC mixed with the fine-grained gas-forming material. In one embodiment, the fine-grained gas-forming material is alumina monohydrate Al2O3 • 1H2O and the mixture comprises 17% by weight of alumina monohydrate mixed with 83% by weight of silicon carbide. Still another fine-grained gas-forming material used in the process of the invention may be alumina trihydrate Al2O3 • 3HgO). The mixture may comprise two fine-grained gas-forming materials: alumina trihydrate (Al2O3 • 3HgO) and alumina γ in equal parts. Still another fine-grained gas-producing material used in the method of the invention may be sodium bicarbonate (NaHCO3). The mixture may consist of 10% by weight of sodium bicarbonate mixed with 90% by weight of alumina. The method is carried out in a device for the heat treatment of glass comprising a container of fine-grained material fluidized with a gas, means for feeding gas to the container to maintain the fluidization of the particulate material, and means for positioning hot glass in the container, wherein in the device the container is filled with a gas-fluidized mixture of selected fine-grained materials, at least one of which has the property of generating gas when heated by hot glass. These materials are mixed in selected predetermined proportions so as to impart to the gas-fluidized mixture such heat capacity and fluidity that the required heat treatment takes place. In order to better illustrate the invention, examples of its implementation are presented below, with reference to the attached drawings, in which: Fig. 1 shows a vertical section through a device for thermally toughening a glass sheet using the method according to the invention; Fig. 2 - a graph of the dependence of the central tensile stress on the proportional content of fine-grained materials in the mixture constituting the gas-fluidized bed, showing changes in the stress as a result of changes in these proportions; Fig. 3 - a graph similar to the graph shown in Fig. 2, illustrating changes in the central tensile stress depending on the proportions of the components in another mixture of fine-grained materials. 4 is a graph similar to that shown in Fig. 3 illustrating changes in the surface compressive stress for 2.3 mm thick glass as a function of changes in the composition of the gas fluidized bed, Fig. 5 is a graph similar to that shown in Fig. 3 illustrating the central tensile stress induced in 6 mm thick glass, Fig. 6 is a graph similar to that shown in Fig. 4 illustrating changes in the surface compressive stress induced in 6 mm thick glass, Fig. 7 is a graph similar to that shown in Fig. 3 for 12 mm thick glass, Fig. 8 is a graph similar to that shown in Fig. 4 for 12 mm thick glass, and Figs. 9, 10 and 11 illustrate changes in the central tensile stress. tensile strength depending on the composition of the mixture and fine-grained material for three different embodiments of the invention. Referring to Figure 1, the attached drawings show a vertical hardening furnace 1, which has side walls 2 and a roof 3. The side walls 2 and the roof S are made of ordinary fire-resistant material, while the bottom of the furnace has an open outlet in the form of an elongated slot 4 in the base of the plate 5 on which the furnace 1 rests. A slidable shutter, shown, is provided for closing the opening 4 in a known manner. The glass sheet intended for bending and subsequent thermal hardening is suspended in the furnace 1 by means of tongs 7, which grip the upper edge of the glass sheet 6. The tongs 7 are suspended on a hanging rod 8, which is suspended from a conventional hoist, not shown, and which runs on vertical guides downward from the furnace to guide, lower, and raise the hanging rod. A pair of bending dies 18 and 11 are positioned directly below furnace opening 4 in a heating chamber 12, maintained at such a temperature that the dies have the same temperature as the hot glass they are to bend. Chamber 12 is heated by hot gases introduced through passage 12a. When the dies are open, they are positioned on either side of the path of the glass sheet 6. Die 18 is a fixed convex die mounted on the press slide. 13 and has a curved outer surface which imparts a curvature when applied to a hot glass sheet. The die 11 is an annular concave frame guided by compressible rods 14 mounted on a rear plate 15 which is mounted on a press slide 16. The curvature of the die frame 11* is matched to the convex curvature of the die front face 18. A guide rail 8 extends downward from both sides of the bending dies towards a container containing a fluidized bed 17 of fine-grained fireproof material in which the hot bent glass sheet is to be cooled by introducing the sheet down into the bed. Container The fluidized bed comprises an open-topped rectangular tank 18 mounted on a scissor-type support platform 18. When the platform 18 is in the raised position, the upper edge of the tank 18 is just below the bending dies 18 and 11. A microporous membrane 28 with a high pressure drop passes through the base of the tank 18. The edges of the membrane 28 are attached between a flange 21 on the tank and a flange 22 on the pressurized air chamber 28, which forms the base of the tank. The flanges and edges of the membrane 20 are bolted together as indicated by 24 in the drawing. An air inlet channel 25 is connected to the pressurized air chamber and the fluidizing air. is supplied through a regulated high pressure channel. There is a high pressure drop, at least 60% of the pressure of the pressurized air chamber, across membrane 28, resulting in a uniform distribution of fluidizing air in the fine-grained material at a gas flow rate through the fine-grained material between the rate corresponding to minimal fluidization with particles barely suspended in the upward flowing air and the rate corresponding to maximum expansion of the fine-grained material in which a dense fluidization phase is maintained. The expanded bed is essentially free of bubbles in the quiescent state of fluidization of the fine-grained material with a quiescent horizontal surface through which the glass strip enters the bed. * Membrane 28 may contain A steel plate has holes for adjustable distribution and multiple layers of strong microporous paper lying on this plate. For example, 15 sheets of paper can be used. The membrane assembly is supplemented by a woven wire mesh lying on top of the paper layers, for example, stainless steel mesh. A basket for collecting cullet can be placed around the membrane 28, which is designed so as not to interfere with the uniform flow of fluidizing gas upwards from the membrane. A guide rail 8 extends down to a point below the bending dies and terminates near the upper edge of the tank. A fixed frame 27 is mounted on the tank 18 and has an upwardly curved foot 28 at the bottom to receive the lower edge of the glass sheet immersed in the fluidized bed when The hanging rod is lowered below the bending dies by means of a lifting device. To load the glass sheet into the device, the scissor-type support table 18 and the hanging rod are lowered to their lowest position at the bottom of the guide rail and the glass sheet to be bent and tempered is placed in the tongs 7. The lifting device lifts the suspended glass sheet into the furnace 1, which is maintained at a temperature of, for example, 850°C, so that the glass sheet is quickly heated to a temperature close to the temperature of stress decay, for example in the range of 810°C to 680°C. When the entire glass sheet reaches the required temperature, the shutter closing the opening 4 is opened and the hot glass sheet is lowered down. ana, by means of a jack, to a position between the open bending dies 18 and 11. The press slides 13 and 16 operate so that the dies are closed in order to bend the sheet to the desired curvature. Once the desired curvature has been imparted to the glass, for example to enable it to be used as a component of a laminated windshield for motor vehicles, the dies are opened and the hot curved glass sheet is rapidly lowered down into the fluidized bed in tank 18, which is raised to a cooling position by raising the scissor-type support table 19 while the glass sheet is being heated in furnace 1. The fluidized bed is maintained at a temperature between 30 and 150°C by means of a water cooling jacket permanently mounted to the flat long wall of tank 18. The fluidized bed 17 consists of a fine-grained material fluidized by a gas which is a mixture in a predetermined ratio of a number of different substances. fine-grained materials, at least one of which has gas-forming properties and is capable of evolving gas when the fluidized material comes into contact with hot glass. A suitable fine-grained gas-forming material is capable of evolving gas in an amount of 4.0 to 37% of its own weight when heated to constant weight at 800°C. Suitable materials are γ-alumina (γ-r-λOi) which is a porous material and contains water absorbed in its vapors, aluminosilicates which are porous and contain water absorbed in their pores, hydrates of alumina such as alumina trihydrate (Al^)3* •3H^) containing bound water of crystallization, and alumina monohydrate (Al^Os-1H^). containing water of crystallization, which is also a porous material and has absorbed water in its pores, and materials that produce gases other than water, for example sodium carbonate NaHCO3. In order to produce the required toughening stresses in glass, the components of a mixture of fine-grained materials are mixed together in predetermined proportions to give the mixture a fluidity in the range of 60-86 and a heat capacity per unit volume in the range of 1.0 (2-1.75 MJ/m*K. Other components of the mixture that are mixed with the fine-grained gas-producing material are fine-grained materials that are inert in the sense that they do not substantially produce gas when the material is heated. Examples are alumina and (lot—Al/)3), a zirconium compound ZrO2 • SiO2, silicon carbide, and spheroidal iron oxide (a—Fe^). These fine-grained materials are in dense non-porous form and so selected as to have a fluidity and a heat capacity different from that of the fine-grained gas-producing material, so that, depending on the proportional amount of dense non-porous material, they are effective in modifying the fluidity and heat capacity of the mixture of fine-grained materials to such an extent as to cause the formation of the required degree of toughening stress in the glass. It is assumed that sheets of hot glass are cooled in a gas-fluidized bed of such a mixture of fine-grained materials, and the gas liberated from the fine-grained gas-producing material is rapidly released and spread. By heating the fine-grained material near the glass surface, local mixing of the mixture of fine-grained materials is increased on the glass surfaces in a manner similar to boiling a liquid. As a result, mixed gas layers are formed and the fine-grained material flows in a stream over the glass surfaces as the glass cools in the fluidized bed. By mixing the components of the mixture in predetermined proportions, optimal heat transfer from the glass surface to the bulk of the bed can be achieved, which induces the desired stresses in the glass. Furthermore, the heat removed from the glass by "mixing the fluidized fine-grained material directly with the fluidized bed" is continuously dispersed towards more distant parts of the bed. surrounding the glass sheet. The water-cooled jacket 29 keeps the more distant parts of the bed cooled so that they act as a heat sink. The vigorous mixing of the fine-grained material on the surfaces of the glass 20 continues after the glass has cooled below its stress break point, ensuring that the temperature gradient from the center to the surface of the glass is maintained. Initially induced in the glass in the fluidized bed, and as it cools through the stress break point, the required tempering stresses develop as the glass continues to cool while still immersed in the bed. The lower edge of the hot glass sheet is cooled uniformly as it enters the fluidized bed. to the horizontal resting surface of the expanded fluidized bed. Substantially the same tensile stresses are generated in different areas of the glass sheet's edge surface, so that there is a very low crack rate. As the lower edge of the glass is lowered into the bed, each part of the lower edge is always in contact with the fluidized material, which is in a state of uniformly expanded fine-grained fluidization, and such uniform treatment of the lower edge, regardless of the streaming of fine-grained material formed on the hot glass surfaces by gas evolution from the gas-producing component in the mixture, significantly prevents cracks and the consequent problems of glass fragments in the bed. This, together with Avoiding glass losses due to shape changes and/or losses due to surface deterioration ensures industrially acceptable performance of tempered glass sheets. Some examples of the operation of the process with selected mixtures of fine-grained material are given below. In each of these examples, the numerical values of the product particle density in g/cm2 and the average particle size in jim of each component of the mixture are below 220. These criteria, which are useful in assessing whether a given fine-grained material is suitable for fluidization in the quiescent, uniformly expanded state of fine-grained fluidization, when working with air at normal temperature and pressure. The mixture of individual fine-grained materials is then capable of fluidizing in a quiescent, uniformly expanded state of fine-grained fluidization. Example 1. A fluidized bed 17 was prepared consisting of a mixture of γ-alumina as a fine-grained gas-producing material and α-alumina. The γ-alumina used was a microporous material having pores with diameters in the range of 2.7-4.9 mm and 20-40% free pore space. The pores contained absorbed water, which was released as a gas by heating the material. The γ-alumina had the following properties: average particle size 119 \mu ... Heat capacity per unit volume under minimum fluidization: 1.09 MJ/m²K. The alumina used was in a dense, non-porous form with the following properties: average particle size: 30 µm, particle size distribution: 1.22, fluidity: 70, heat capacity per unit volume under minimum fluidization: 1.3 MJ/m²K. Tests were conducted with a mixture of alumina and alumina alumina, which were mixed together in predetermined proportions. Sheets of soda-lime-silicate glass, 2.3 mm thick, were cut, and the edges of the cut sheet were finished by rounding using a fine-grained diamond wheel. Each sheet was heated to 660°C in furnace 1 before bending. cooled in a fluidized mixture, the sphere was in a state of uniformly expanded fluidization of fine-grained material. Table 1 gives the properties of various mixtures of these materials in the range of 30%-90% by weight of α-alumina and 70%-10% by weight of γ-alumina, as well as the central tensile stress induced in the glass sheet during cooling. For the comparison, the central tensile stress* generated using α-alumina alone and γ-alumina alone is also given. Figure 2 shows a graph of the variation of central tensile stress* depending on the composition of the mixture. γ-alumina alone has a fluidity that is too high to produce maximum stress. The average particle size and the fact that the particles are relatively smooth and non-angular in shape. Increasing the proportion of alpha alumina, which has a lower fluidity than gamma alumina due to the smaller average particle size of alpha alumina and the angularity of its individual particles, lowers the fluidity of the mixture. The fluidity of the mixture decreases as the amount of alpha alumina in the mixture increases, and there is a corresponding increase in the central tensile stress generated. A maximum tensile stress of 49 MtPa is achieved when the fluidity has been adjusted to the optimum value of 74 and the mixture contains about 70% by weight of alpha alumina and 30% by weight of gamma alumina. Alpha alumina has higher heat capacity than γ-alumina. As the proportion of γ-alumina in the mixture increases, there is a progressive increase in the thermal heat capacity of the mixture, which contributes to the increase in stress achieved. Further addition of γ-alumina to amounts above 70% by weight increases the heat capacity somewhat more and maintains good fluidity but reduces the induced central tensile stress because the content of the gas-producing component, γ-alumina, has been reduced to too low a level. Example II. A fluidized bed was prepared by mixing γ-alumina as the fine-grained gas-producing material and γ-alumina. The γ-alumina used was a microporous material with a pore size in the range 2.7—4.9 mm and 20—40% of free pore space. Absorbed water was contained in the pores, which Table 1 Alumina and alumina y 1 Fluidity of the mixture Heat capacity of the mixture per unit volume at minimum fluidization (MJ/m*K) Central tensile stress (MPa) % by weight in mixture 1; 0% 100% 90.25 1.09 41 30% 70% 81.5 1.16 43 50% 50% 75 1.20 49 50% 30% 74 1.24 49 90% 10% 72.25 1.28 47 100% L 0% f 70 [ b 1.3 \ h and 32 [117 108 13 11 :ra was released as a gas when the material was heated. The γ-alumina used had the following characteristics: average particle size, particle size distribution, fluidity, water content (weight loss at 800°C), heat capacity per unit volume at minimum fluidization 64 {im 1.88 84 4%, 1.06 MJ/m»K. The α-alumina used was the same as in Example I. Tests were carried out with mixtures of γ-alumina and α-alumina, which were mixed together in predetermined proportions from 100% γ-alumina and 0% α-alumina to 0% γ-alumina and 100% Table 2 gives the heat capacity per unit volume with minimum fluidization and fluidity of the mixtures used: Table 1 Mixture % by weight alumina y 100 86 j 61 40 1 22 7 0 1 alumina a 0 14 39 60 78 93 100 ica 2 Heat capacity 1 MJ/m*K ' 1.05 1.09 1.15 1.20 1.25 1.29 1.30 Fluidity 84 82.75 79 76 73.25 71 70 Sheets of soda-lime-silicate glass with a thickness of 2.3 mm were cut and the edges of the cut sheets were finished by rounding with using a fine-grained diamond wheel. Each sheet was ground using tongs 7 and heated in furnace 1 before being bent and cooled. The results are illustrated in Figures 3 and 4. The abscissa of each curve represents the composition of the mixture in weight percent. Each of Figures 3 and 4 contains four curves corresponding to the central tensile stress (Figure 3) and the superficial compressive stress (Figure 4) induced in a 2-3 mm thick glass sheet which was heated at 10°C, 630°C, 650°C or 670°C and then cooled in a fluidized bed 17 which was maintained in a quiescent, uniformly expanded fluidized state. fine-grained material in the temperature range 60-80°C. The curves show that it was advantageous to use from about 7% to about 86% by weight of alumina mixed with γ-alumina. As the alumina content in the mixture increases, the central tensile stress and the surface compressive stress induced in the glass during the thermal toughening process increase, to a maximum which is achieved with an amount of alumina in the mixture of about 70% to 80% by weight. Generally, the highest stress is 55-85% by weight. A higher alumina content in the mixture causes a decrease in the induced stresses. By appropriately selecting the proportions of γ-alumina and α-alumina, the mixture has gas-forming properties, a heat capacity per unit volume with minimal fluidization at 50°C, and a fluidity that produce consistently high values of central tensile stress and surface compressive stress in a 2.3 mm thick glass sheet. For example, when glass is heated to 670°C and then cooled, the necessary central tensile stress in the range of 42 MPa - 49 MPa and a corresponding surface compressive stress in the range of 83 MPa - 103 MPa can be induced in the glass by selecting the proportions of γ-alumina and α-alumina in the mixture in the range of 7 - 86% by weight of oxide. Example III. Sheets of soda-lime-silica glass 6 mm thick were cut, the edges rounded, then heated and cooled in a fluidized bed in a quiescent, uniformly expanded fluidized state of a fine-grained material consisting of a mixture of the same fine-grained γ-alumina and α-alumina as described in Example II. Figures 5 and 6 are graphs similar to Figures 3 and 4, which illustrate the results obtained for glass sheets heated to temperatures of 610°C, 630°C, 650°C and 670°C and then cooled. The results indicate that the required toughening stresses can be induced in the glass, which are a function of the proportion of γ-alumina. γ-alumina and α-alumina in a mixture. Maximum stresses were achieved when the mixture contained about 65-95% by weight of α-alumina. For example, when the glass was heated to 670°C and then cooled in a fluidized mixture of 22% by weight of γ-alumina and 78% by weight of α-alumina, the central tensile stress induced in the glass was 91 MPa and the surface compressive stress was 216 MPa. This very strong 6 mm thick glass was used to produce window assemblies for aircraft and locomotives. Similar results were obtained when toughening 10 mm thick sheets of soda-lime-silica glass. Such sheets were used to produce window units for aircraft, which could, for example, consist of two sheets of tempered glass 10 mm thick and an outer sheet 3 mm thick. The glass sheets were laminated together with intermediate layers of plastic of a known type. Example IV. Sheets of soda-lime-silica glass 12 mm thick were cut, the edges were finished, then heated and cooled in a fluidized bed consisting of a mixture of γ-alumina and α-alumina in predetermined proportions in the same manner as described in Example II. Results obtained for glass sheets heated to temperatures of 610°C, 630°C, 650°C and 670°C for the given proportions of γ-alumina and α-alumina are illustrated by the curves in Figures 7 and 8. The measurements showed maximum stress values when the fluidized mixture contained about 65-85% by weight of α-alumina. When the sheets were heated to 670°C and cooled in a fluidized bed consisting of a mixture of 22% by weight of γ-alumina and 78% by weight of α-alumina, the central tensile stress in the gas was 124 MPa and the surface compressive stress was 261 MPa. Figures 7 and 8 illustrate how wide a range of toughening stress values can be induced in the glass, if necessary, by selecting the proportions of the components of the mixture of fine-grained materials and The results shown in Figs. 3-8 show that higher quenching stresses are achieved as the proportion of the higher heat capacity component (α-alumina) in the mixture increases, to the point where further increases in the proportion reduce the gas-forming component (γ-alumina) to an insufficient level. The proportion of the gas-forming material and the other component or components in the mixture provides a fluidity of the mixture in the range 60-06 which is such that the nature of the mixing favors cooling the glass at a rate at which the desired stress values are achieved in the glass. Cooling of the glass occurs by rapid mixing of the fine-grained material in the vicinity of the glass surface, the mixing being essentially due to from the evolution of water vapor from the γ-alumina, which is the mixed component. A higher α-alumina content increases the rate of heat transfer from the glass and also modifies the fluidity of the mixture. Example V. The fluidized bed consisted of a mixture of γ-alumina as the gas-producing component with the addition of spheroidal iron oxide (α-Fe^) and one or two types of α-alumina. The γ-alumina had the following characteristics: average particle size, particle size distribution, fluidity, water content (weight loss at 800°C), heat capacity per unit volume at minimum fluidization 24 µm 1.94 87.25 6% 1.063 MJ/m2K The spheroidal iron oxide had the following characteristics: Average particle size Particle size distribution Fluidity Heat capacity per unit volume at minimum fluidization 41 µm 1.69 76.5 2.01 MJ/m2K The first alumina a was as used in Example 1. The second alumina a had the following characteristics: Average particle size Particle size distribution Fluidity Heat capacity per unit volume at minimum fluidization 24 µm 1.25 66 1.192 MJ/m2K Sheets of soda-lime-silica glass 2.3 mm thick were heated to 660°C and cooled in a fluidized bed mixture of the above materials which were in a state of rest uniformly expanded fluidization of the particulate material. I alumina % spheroidal iron oxide alumina a (1) 1 alumina a (2) fluidity of the mixture heat capacity of the mixture per unit volume at minimum fluidization MJ/m*K central tensile stress (MPa) Table (1) 70% 30% — — 82 l.r347 45 •; 3 % by weight in the mixture (2) 50% 50% • — 79 1.54 49. (a) 30% 70% — • — 78 1.726 50 (4) 20% 35% 45% — 7)4% 1.502 57 (5) 16% 28% 1 36% f 20% 1 73.5 | r 1.44 [ 53/) 1117 108 17 18 The properties of the mixture and the resulting central tensile stresses generated in the glass sheets are given in Table 3. Figure 9 illustrates the variation of the central tensile stress as a function of the composition of the mixtures (1), <2) and (3) of γ-alumina and α-Fe2O3 in Table 3. The central tensile stress resulting from the use of γ-alumina alone and α-Fe2O3 alone was 41 MPa and 32 MPa, respectively. As in Example 1, the γ-alumina used in this example had a fluidity too high to generate the maximum hard stress. glass sheet. Spheroidal iron oxide has a lower fluidity than γ-alumina, particularly because of its smaller average particle diameter. Adding increasing amounts of spheroidal iron oxide to γ-alumina in mixtures (1), (2), and (3) of Table 3 has the effect of progressively lowering the fluidity of the mixture due to the progressively decreasing average mixture size as the amount of spheroidal iron oxide in the mixture increases. As the fluidity of the mixture decreases, there is a progressive increase in the central stress generated in the glass sheet. A maximum central tensile stress of 50 MPa is reached when the mixture contains about 70% spheroidal iron oxide and 30% γ-alumina. Fluidity The fluidity of the spheroidal iron oxide is not as low as the alumina used in Example 1 because of its larger average particle size and the smooth rounded particles compared to the angular alumina particles. Thus, the spheroidal iron oxide is not as effective in reducing the fluidity of the mixture as the alumina of Example 1. The mixture (3) of this Example containing 70% by weight of the spheroidal iron oxide and 30% by weight of the alumina, which produces a maximum central tensile stress of 50 MPa, has a fluidity of 78, which is higher than the optimum fluidity of 74 of the mixture of 70% by weight of the alumina and 30% by weight of the alumina, which produces a maximum central tensile stress of 50 MPa. The maximum central tensile stress produced by mixture (3) of this Example is, however, approximately the same as the maximum central tensile stress produced by mixture (3) of Example I. This is because, although the fluidity of mixture (3) is slightly higher than the optimum fluidity at which the maximum stress is produced, the spheroidal iron oxide used in mixture (3) has a significantly higher heat capacity than the alumina α used in Example I. Since it was suspected that the fluidity of mixture (3) was slightly too high, mixture (4) was supplemented to contain some of the alumina α used in Example I. This lowered the fluidity of the mixture to an optimum value of 74 and the mixture formed further increased the central tensile stress to 57 MPa, despite the reduction in heat capacity. Mixture (4) had the same optimum fluidity value of 74 as the mixture of Example 1 consisting of 30% γ-alumina and 70% γ-alumina, producing a maximum central tensile stress of 49 MPa. The fact that mixture (4) produces a higher central tensile stress of 57 MPa is due to the higher heat capacity of mixture (4) being 1.502 MJ/m³K as compared to 1.42 MJ/m³K of the mixture of Example 1. The further reduction in the fluidity of the mixture (4) was due to the reduction in the heat capacity of the mixture (4) of 1.502 MJ/m³K as compared to 1.42 MJ/m³K of the mixture of Example 1. The fluidity resulting from the inclusion of part of the second alumina in mixture (5) causes a reduction in the heat capacity of the mixture in evaporation with mixture (4) with a concomitant slight reduction in the central tensile stress. Example VI. The fluidized bed consisted of a mixture of a fine-grained gas-producing material, being alumina monohydrate Al 2 O 3 • 1 H 2 O, and a zirconium compound Zr 2 O • Si 2 O. The alumina monohydrate was in the form of boehmite, which is a porous material containing 15% by weight of water of crystallization and 13% by weight of pore water. The main effect was due to the water absorbed in the pores, which was released during the cooling of the glass, ensuring the production of gas causing increased mixing of the fine-grained material in the vicinity of the glass surface. '' 4 The alumina monohydrate used had the following characteristics: average particle size 51 [µm • particle size distribution 1.70 fluidity 78 water content (weight loss -^ - at 800°C) 28.4% heat capacity per unit volume at minimum fluidization 1.18 MJ/m*K The zirconium compound ZrO2 • SiO^, which is a neutral non-porous orithosilicate - zirconium oxide with a heat capacity higher than that of alumina, had the following characteristics: 30 35 40 45 50 55 average particle size 34 µm particle size distribution 1.73 fluidity 67 heat capacity per unit volume 1.75 MJ/m*K. Glass sheets of thickness 2*3 mm were heated to 660°C and cooled in a mixture of alumina monohydrate and a zirconium compound as given in Table 4, which shows the characteristics of the mixtures and the central tensile stress developed in the glass sheets. Figure 10 illustrates the changes in the central tensile stress with a change in the composition of the mixture. Alumina monohydrate has good gas-forming properties and a lower fluidity value than the aluminas discussed in Examples I and V. Jed-19 117 108 Table 4 20 alumina monohydrate ZrOj* SiOa fluidity of the mixture heat capacity of the mixture per unit volume at minimum fluidization MJ/m*K J Central tensile stress (MPa) i * i weight % In the mixture [ 100% 0% 78 1.005 37 70% 30% 75.5 1.277 42 50% 50% 74 1.41 44 20% 80% 73 1.62 46.5 10% 90% 71 1.70 39 0% 100% | k h 67 1 1.76 | 23 f However, the fluidity of alumina monohydrate is higher than the optimum fluidity, which produces maximum central tensile stress and the heat capacity is relatively low. ZirO2 • SiO2 has a lower fluidity and a higher heat capacity than alumina monohydrate, and as the proportion of zirconium compound in the mixture increases, there is a progressive increase in the central tensile stress developed in the glass due to both the progressive reduction in fluidity and the increase in the heat capacity of the mixture. ZrOa • SiO2 has a high heat capacity, which contributes significantly to the increase in central tensile stress in the glass sheet in the same way as the spheroidal iron oxide of Example V. Since the zirconium compound has a lower fluidity than the spheroidal iron oxide of Example V, it is more effective in reducing the fluidity of the mixture and therefore contributes more to the increase in central tensile stress induced by the reduction in the value of The fluidity of the mixture. A maximum central tensile stress of 46.5 MPa is achieved when the mixture contains about 20% alumina monohydrate and 80% zirconium compound. This mixture has an optimum fluidity of 73. Further addition of zirconium compound of about 80% by weight increases the heat capacity of the mixture but results in a lower central tensile stress due to the fluidity being significantly reduced below the optimum value and the proportion of the gas-producing component, alumina monohydrate, being reduced to a less effective level. Example VII. A fluidized bed was prepared which consisted of a mixture of alumina and equal amounts of each of the four aluminas. Table Average particle size (µm) Particle size distribution Fluidity Water content (% weight loss at 800°C) Heat capacity per unit volume at minimum fluidization (MJ/m»K) 5 aluminas 1 A 70 U7 88.5 7 1.16 B 1 61 1.67 88 7 1.16 C 57 1.66 85 7 1.12 D 72 \ 1.65 1 86 ! 7 1 1.12 l I Mixture of 4 aluminas y alumina and mixture fluidity heat capacity of the mixture per unit volume assuming minimum fluidization (MJ/m*K) central tensile stress (MPa) Table 6 % by weight in the mixture 100% 0% 87 1.14 39 40% 60% 70 1.20 40 20% 80% 67 1.22 35 l€% 90% 65 1.23 31 0% 100% | 63 | 1.24 1 25 117 108 and 21 22 n y marked A, B„ C and D in Table V, which gives the characteristics of the alumina y. The alumina had the following characteristics: average particle size 22 µm particle size distribution 1.69 fluidity 63 heat capacity per unit volume at minimum fluidization 1.24 MJ/m³K. Sheets of soda-lime-silica glass with a thickness of 2.3 mm were heated to 660°C and cooled in a mixture of the above materials fluidized with gas, in a state of quiescent uniformly expanded fluidization of fine-grained material. The properties of the mixtures and the resulting central tensile stresses in the glass The stresses in the sheets were as given in Table 6. Figure 11 illustrates the variation of the central tensile stress as a function of the mixture composition. The previous examples have shown how much higher stresses can be produced by mixtures of fine-grained gas-producing material with an inert material than those produced using the fine-grained material alone. However, lower stress values than can be obtained using the fine-grained gas-producing material alone may be desirable. In this example, this was achieved by using alumina a with a smaller average particle size and a relatively broad particle size distribution, which resulted in significantly lower fluidity than that of the alumina a used in the previous examples. The maximum central stress produced The tensile stress was 40 MPa using a mixture of 40% γ-alumina and 60% α-alumina with a fluidity of 70. This is only slightly higher than the central tensile stress of 39 MPa produced using γ-alumina alone. Progressive addition of further amounts of α-alumina to the mixture quickly reduced the fluidity of the mixture to such a low range that the central tensile stress produced in the glass sheet was lower than that produced using γ-alumina alone. Example VIII. A fluidized bed was prepared consisting of a mixture of 9% by weight of zeolite, which is a porous, crystalline aluminosilicate with water absorbed in its pores, and 91% by weight of alumina. a. The zeolite had the following characteristics: average particle size 24 µm particle size distribution 4 fluidity 51 water content (weight loss at 800°C) 20% heat capacity per unit volume under minimal fluidization / 0.8 MJ/m²K. The alumina a had the following characteristics: 10 20 25 30 35 40 45 50 55 average particle size 37 µm particle size distribution 1.682 fluidity 70 heat capacity per unit volume under minimal fluidization 1.4 MJ/m²K. The heat capacity per unit volume under minimal fluidization of the mixture was 1.34 MJ/m²K and the fluidity of the mixture was 60. A 2.3 mm thick glass sheet was heated to 660°C and cooled in a fluidized mixture. A central tensile stress of 41 MPa was generated in the sheet. By changing the proportions of the mixture components, a central tensile stress in the range of 23 MPa to 41 MPa could be induced in the glass sheet. Example IX. The fine fluidization material mixture consisted of 201% by weight of γ-alumina and 4% by weight of each of the two α-aluminas, which were readily available and which were used to replace the single missing α-alumina. The γ-alumina had the following characteristics: average particle size, particle size distribution, fluidity, water content (weight loss at 800°C), heat capacity per unit volume at minimum fluidization, 57 m³, 1.66 85 7%, 1.18 MJ/m³K. The characteristics of the two aluminas A and B are given in Table 7. Table 7 Average particle size, particle size distribution, fluidity, and capacity heat capacity per unit volume at minimum fluidization (MJ/m8K) Alumina a 1 A 38 1.19 75 1.14 B 24 | 1.25 1 66 1 Ii 65 The fluidity of the mixture was 73.5 and its heat capacity per unit volume at minimum fluidization was 1.25 MJ/m*K. Glass sheets of 2.3 mm thickness were heated to 660°C and cooled in the fluidized mixture. The induced central tensile stress in the sheet was 48 MPa. By changing the relative proportions of the mixture components, it was possible to induce a selected central tensile stress in the glass pane in the range of 34 MPa - 48 MPa. Example X. The versatility of the method according to the invention is further illustrated by preparing a mixture by mixing several gas-producing components, each of which was a readily commercially available and relatively cheap material, to form mixtures with gas-producing properties and optimum fluidity and thermal capacity, which induced the desired stresses in glass cooled in these mixtures in a state of quiescent uniformly expanded fluidization of fine-grained material. In this example, the mixture contained 5% by weight of each of the four carbon oxides A, B, C and D, of which characteristics are given in Table 8. 1 I average particle size |im particle size distribution |fluidity |water content (weight % loss at aO^C) |I heat capacity |per unit volume at i | (minimum fluidization MJ/m*K) Table 8 1 γ alumina A 70 1.47 88.5 7 1.16 B 61 1 67 88 7 1.16 C 57 1.66 85 7 1.12 D 72 1.65 86 7 1.12 117 108 24 temperature e60°C and cooled in a fluidized mixture. A central tensile stress of 49 MPa was induced in the plate. By changing the selected proportions of γ aluminas constituting 20% by weight of the mixture or by changing the proportions of α aluminas constituting 80% by weight of the mixture or by changing the relative proportions of the total amount of alumina y to the total amount of alumina a in the mixture, a selected central tensile stress in the range of 32-49 MPa can be induced. In Example XI. The fluidized fine-grained material consisted of 17% by weight of the alumina monohydrate of Example VI mixed with 83% by weight of silicon carbide having the following characteristics: average particle size .40 particle size distribution 1.32 fluidity 72.75 heat capacity per unit volume at minimum fluidization 1.21 MJ/m8K. A total of 20% by weight of alumina a was mixed with 26.67% by weight of each of the three aluminas a E, F, and jr, whose The characteristics are given in Table 9. Table 9 1 Average particle size 1 µm Particle size distribution 1 Fluidity 1 Heat capacity per unit volume 1 at minimum fluidization (MJ/m»K) alumina a 1 E 38 1.19 75 1.38 F 30 1.22 70 1.3 G 24 1 1.25 | 66 | 1.19 1 10 15 20 25 3fr 35 45 50 55 60 The fluidity of the mixture was 74 and its thermal fluidity per unit volume at minimum fluidization was 1.26 MJ/m»K. (Glass panes of 2.3 mm thick were heated to 100°C.) The fluidity of the mixture was 75 and its thermal fluidity per unit volume at minimum fluidization was 1.26 MJ/m»K. The heat capacity per unit volume under minimum fluidization was 1.02 MJ/m2K. A 23 mm thick glass sheet was heated to 660°C and cooled in the fluidized mixture. A central tensile stress of 51 MPa was induced in the sheet. These materials provided the facility of induced selected central tensile stress in a wide range of 32 MPa - 51 MPa by selecting predetermined proportions of the components in the mixture to prepare a fluidized mixture that produced the required stress in the glass. Example XII. In a binary mixture, both fine-grained materials were required to have gas-producing properties. The mixture was made from equal weights of γ-alumina and γ-alumina trihydrate. alumina (AhO•• 3H2O). During heating, water of crystallization was released from the alumina trihydrate, which enhanced the effect of the water released from the pores of the γ-alumina. The γ-alumina had the following characteristics: average particle size, particle size distribution, fluidity, water content (weight loss at 800°C), heat capacity per unit volume at minimum fluidization 60 µm 1.9 84 8% 1.05 MJ/m•K The alumina trihydrate had the following characteristics: average particle size, particle size distribution, fluidity, water content (weight loss at 800°C), heat capacity per unit volume at minimum fluidization fluidization 86 nm 1.42 86 34% 1.56 MJ/m2K The fluidity of the mixture was 85.25 and its thermal capacity per unit volume at minimum fluidization was 1.31 MJ/m2K. Glass sheets of thickness 2.3 mm were heated to 660°C and cooled in the fluidized mixture. The induced central tensile stress was 47 MPa. A central tensile stress in the range of 42 MPa - 47 MPa could be induced in the glass sheet by appropriate selection of the relative proportions of the two gas-producing materials. Example XIII. Fine-grained materials were used which produce a gas other than water upon heating, for example sodium bicarbonate, which releases carbon dioxide and also water. A mixture of 10% by weight of sodium bicarbonate containing 0.6% by weight of colloidal silica to improve its fluidity was used with 90% by weight of alumina A according to Example 9. The sodium bicarbonate/colloidal silica mixture had the following characteristics: Average particle size Particle size distribution Fluidity H2O + CC2 content (weight loss at 800°C) Heat capacity per unit volume at minimum fluidization 70 µm 1.98 75 37% 1.41 MJ/m3K 30 35 The fluidity of the fluidized mixture was 75 and its heat capacity per unit volume at minimum fluidization was 1.38 MJ/m8K. A 2.3 mm thick glass sheet was heated to 660°C and then cooled in the fluidized mixture. A central tensile stress of 53y5 MPa was induced in the sheet. By appropriately selecting the relative proportions of the mixture components, selected central tensile stresses in the range of 34 MPa to about 55 MPa could be induced in the glass sheet. In many examples, an indication of the stresses induced in the glass during cooling in a fluidized mixture of fine-grained materials was given as the stresses induced in a 2.3 mm thick glass sheet made of glass Soda-lime-silica heated to 660°C and then cooled. In this manner, as described in Examples II, III and IV, different stresses could be achieved by varying the temperature to which the glass was heated. Proportionately higher stresses were induced in the thicker glass. All examples illustrate how to select a proportion of a fine-grained gas-producing material that is capable of releasing 4-47% of its weight of gas when heated to constant weight at 800°C, and then mixing the gas-producing material in predetermined proportions with other fine-grained gas-producing materials or with inert materials, the mixture being prepared to produce the desired fluidity in the range of 60-86 and capacity Thermal energy per unit volume in the range of 1.02-1.75 MJ/m³K, which ensures that the glass sheet cooled in the mixture is thermally toughened to the desired degree, indicated in the examples by the central tensile stress. As is normal in thermally toughened glass, the ratio of surface compressive stress to central tensile stress is of the order of 2:1, and the induced surface compressive stress is usually twice the central tensile stress. By using the method according to the invention, the toughening conditions can be easily reproduced, and it is possible to use a wide range of fine-grained materials as purchased, or it is possible to use cheaper and more readily available mixtures instead of the rarer and more expensive individual components of the mixture, which reduces the costs of running the process. Furthermore, by appropriate selection of materials By combining fine-grained particles and the proportions in which they are mixed, it is possible to produce higher tempering stresses in the glass than could be achieved by any of the components in the mixture used alone. Some of the particulate materials described above were commercially available with suitable average particle size, particle size distribution, fluidity, and heat capacity. When commercially available materials with the desired characteristics, e.g., alumina, are not available, screening of the materials is employed to obtain purified particulate materials with the desired characteristics for mixing with other components to form a mixture which, in the fluidized state, can induce the desired tempering stresses in the glass. As found, the fluidized mixtures of Examples I-IV and XIII were particularly suitable for thermally toughening glass sheets into laminates for the production of automotive windshields. The fluidity of such mixtures is in the range of 71-83, their gas content expressed as weight loss after heating to constant weight at 800°C is in the range of 4-37% and their heat capacity per unit volume with minimal fluidization is in the range of 1.09-1.38 MJ/m3K. The selection of the proportions of the components in the mixture allows for the generation of lower stresses in the glass than those generated by the gas-producing component alone. This is illustrated by Example VII. Patent Claims 1. A method of thermally treating glass in which the glass is heated to a predetermined temperature and contacted with a fine-grained material fluidized by a gas, characterized in that the fine-grained material consisting of a mixture of a plurality of selected fine-grained materials, at least one of which has the latent property of releasing a gas upon heating: from hot glass, and mixing these materials in selected proportions to give the mixture of fine-grained material fluidized with gas such a heat capacity and fluidity as to achieve the desired degree of heat treatment. 2. A method according to claim 1, characterized in that the materials are mixed in proportions so selected as to induce the required hardening stresses in the glass sheet as it is cooled in the fine-grained material fluidized with gas from a temperature higher than the stress decay temperature. 3. A method according to claim 2, characterized in that the fine-grained material generating the gas is selected to be A material which, when heated to a constant weight at a temperature of 80 K)°C, is capable of producing a gas in an amount of 4-37% of its weight and the fine-grained material is mixed in predetermined proportions, giving the mixture a heat capacity per unit volume at minimum fluidization in the range of 1.02-1/73 MJ/m3K and a fluidity in the range of 60-86. 4. A method according to claim 3, characterized in that in the case of thermal toughening of a soda-lime-silica glass sheet of thickness 2-2.5 mm, in which the glass sheet is heated to a temperature in the range of 610-680°C and the mixture is maintained in a state of quiescent, uniformly expanded fluidization of the fine-grained material, a fine-grained material is used which induces a central tensile stress in the glass pane in the range of 35-57 MPa. 5. A method as claimed in claim 3 or 4, characterised in that the fine-grained material is mixed in predetermined proportions to give the mixture 39 a fluidity in the range of 71-83 and a heat capacity per unit volume at minimum fluidisation in the range of 1.09-1.38 MJ/m3K. 6. A method as claimed in claim 1, characterised in that alumina α is used as the fine-grained gas-forming material 35. 7. A method as claimed in claim 6, characterised in that alumina γ mixed with alumina α is used as the fine-grained gas-forming material. 8. A method as claimed in claim 7, characterised in that the mixture comprises 7-86% by weight of γ-alumina and 93-14% by weight of alumina. a. 9. A method according to claim 3 or 4, characterized in that the hot glass sheet is cooled in a gas-fluidized mixture of a fine-grained gas-producing material and at least one fine-grained metal oxide having a heat capacity per unit volume at minimum fluidization of 1176-2.01 MJ/m³K, wherein the mixed fine-grained materials in predetermined proportions impart to the mixture a heat capacity per unit volume at minimum fluidization of 1.27-1.76 MJ/m³K and a fluidity of 71-82. 11. A method according to claim 10, characterized in that a mixture containing 30-70% by weight of spheroidal iron oxide is used as the fine-grained metal oxide. 12. A method according to claim 10, characterized in that a mixture containing 70-30% by weight of alumina α is used as the gas-generating material. 13. A method according to claim 10, characterized in that a mixture consisting of 28-35% by weight of spheroidal iron oxide, 45-56% by weight of alumina γ and the remainder being alumina γ is used as the gas-generating material. 14. A method according to claim 9, characterized in that 15. A method according to claim 14, wherein the mixture comprises 10-70% by weight of alumina monohydrate (A12O3•1H2O) as the gas-forming material and 90-30% by weight of the above zirconium compound. 16. A method according to claim 1, wherein the fine-grained gas-forming material is an aluminosilicate. 17. A method according to claim 16, wherein the aluminosilicate is a zeolite, the mixture being formed from 8-10% by weight of zeolite and 90-92% by weight of alumina. 18. A method according to claim 1, wherein the fine-grained gas-forming material is an aluminosilicate. alumina monohydrate (A1203 • 1H2O). 19. A process as claimed in claim 1, wherein the mixture comprises silicon carbide mixed with a finely divided gas-forming material. 20. A process as claimed in claim 19, wherein the finely divided gas-forming material is alumina monohydrate (A1203 • 1H2O). The mixture comprises 17% by weight of alumina monohydrate mixed with 83% by weight of silicon carbide. 21. A process as claimed in claim 1, wherein the finely divided gas-forming material is alumina trihydrate (A1203 • 3H2O). 22. A process as claimed in claim 1, wherein the mixture comprises alumina trihydrate (A1203 • 3H2O). 23. A method according to claim 1, wherein the fine-grained gas-producing material is a material containing sodium bicarbonate. 24. A method according to claim 23, wherein the fine-grained gas-producing material is a material containing sodium bicarbonate. 25. A method according to claim 23, wherein the mixture is composed of 10% by weight of sodium bicarbonate and 90% by weight of alumina. α.117 108 V.7-Al2O3 100 90 80 70 60 50 40 30 20 » O V.7-Al^3 m/.a-Al2O3 O W 20 30 40 50 60 70 80 90 100 'Aa-At^ Composition of the mixture (Z nagane) rlG.c.117 108 Y*7-Al203 Y. ot-Al203 Glass 2.3 mm Mixture composition (wt%) 10 O % 7-ai^03 90 100 Y.oc-Al203 F/G.J.Szkto 2y3mm 670 °C %7-M203 ioO Y.a-Al203 90 10 80 20 70 30 60 40 50 50 40 60 30 70 20 80 Mixture composition (wt%) 10 o -0/- r-Ai203 90 100 Y.cx-Al203 F/gA117 108 7^ i .8 •55*1 O) .«• P £ Sb Vj c F & c: ?« V c^ v. *v £ ,*J 95- l 90- 85- 60i 75- /O- "A7-M203 KO 3) 3 % S5 50 35 30 lo 10 O %1-M& '/M-AI2O3 O 10 20 30 40 50 60 10 80 90 100 Ka-At&s w* • • r* . FLGS octaa mixed f wt% V.7-Al203 100 90 60 70 60 50 40 V.a-At203 O 10 20 30 40 50 60 30 20 70 80 Mixture composition (wt%) 10 0 CA7~AI203- 90 100 %a-Al203 F/ad117108 fy.7-Al203 100 .90 60 70 60 50 40 30 20 10 O %7-Al203 %a-Al203 O 10 20 30 40 50 60 70 60 90 100 V.tt-M&3 Mixture composition (wt%) F/aZ V.7-Al2O3 100 90 60 70 60 %ol-A12O3 O 10 20 30 40 50 40 30 20 50 60 70 60 Composition of the mixture (% by weight) 10 o V. 7-Al2O3 90 100 y.cc-A(203 F/G.8.117 108 I I £3 50] 40] ! 30] '/•7-Al2O3 204 100 0 90 10 80 20 70 30 60 40 50 50 40 60 30 70 20 Composition of the mixture Cfawagone) 10 O V. r-Al2O3 90 100 '/.a-Fe^Oj Fig. 9. 7.Al203.IHp 100 9080706050403020 10 O '/. Al203.IH20 V.Zr02.SiO2 O 10 20 30 40 50 60 70 80 90 100 °/.Zr02.SiO^ SKLaol mixture (% rtogone) Fig./O.117 108 C3 i I ! 40] 30] 20\ io] % 7-Al203 % u-Al203 0-1 . ¦ , 1 ¦ r- 100 90 80 70 60 50 40 O 10 20 30 40 50 60 30 20 10 0 V.7-AI203 70 80 90 100 '/.a-AI203 Cement composition (% by weight) F/G.//.ZGK 5, Btm. ordered 9293 — 95 copies C«na 109 il PL PL PL PL PL PL PL PL PL

Claims (1)

1.