Abstract:
A method of making glass in which a first glass composition is fed to a furnace and the densities of such glass is determined. The composition is then changed to produce a second glass which has substantially different properties from the first glass, the change to the composition being such that the desired properties are produced in the second glass while simultaneously maintaining the densities of the second glass substantially equal to that of the second glass. The first glass may be a clear glass while the second glass may be a tinted glass. In such a case, the tinted glass may contain iron and the densities are maintained substantially equal by utilizing the iron to replace calcium oxide in the clear glass composition.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of making glass. More particularly, the present invention relates to a method of making glass preferably, but not essentially, by the float process which permits the changeover time from making a clear glass to a tinted glass and vice versa to be considerably shortened. 
     BACKGROUND OF THE INVENTION AND PRIOR ART DISCUSSION 
     It is well known to make both clear and tinted glasses by the float process. At the present time, tinted glasses are much used in automotive vehicles. Such tinted glasses are not provided simply for aesthetic reasons. Clear glass does, of course, provide excellent luminous radiation transmission but has the consequential drawback of permitting other, unwanted radiation to enter the interior of the vehicle. Thus solar heat radiation, in both the visible and infra-red regions of the electromagnetic spectrum, can cause the temperature in the interior of the vehicle to become sufficiently high as to cause discomfort to the occupants and/or damage to goods being transported. Moreover, clear glass allows a high proportion of ultraviolet radiation to pass therethrough and it is well known that such radiation causes the embrittling of plastics materials and the fading of the colours of fabrics in the interior of the vehicle. 
     To alleviate these problems, it is well known to add colorants to the basic, clear glass composition. In particular, it is extremely well known to employ iron oxide as the colorant Iron oxide (Fe 2  O 3 ) is particularly beneficial because it contains iron in both the ferric and ferrous states. Ferric iron reduces the amount of ultraviolet radiation that is transmitted through the glass whilst ferrous iron absorbs radiation in the near infra-red region at about 1050 nm. 
     However, the use of iron oxide does have disadvantageous side effects. In particular, it increases the densities of the glass. The most obvious drawback of this is that the weight of the vehicle in which the glass is incorporated is increased. From a manufacturing point of view, particularly on a float line, a far more serious problem manifests itself. For commercial reasons float lines are not usually dedicated to making one particular type of glass. For example, a float line may be used for making clear glass, then for making a tinted glass and then restored to making clear glass. It will be readily appreciated that the changeover cannot be effected instantaneously. 
     When a changeover from clear glass to a tinted glass is made, the tinted glass will not be of the desired quality until all of the colorant is uniformly incorporated in, that is to say, mixed into, the glass. It takes longer for this to be achieved if certain colorant materials, such as iron oxide, are used. Similarly, when the reverse changeover is made, the clear glass must be completely clear. In other words, there must not be any streaks of colour in the glass. 
     When the changeover from clear to tint is effected, the changeover is retarded by the fact that the colorant has a higher densities than the molten clear glass and tends to sink to the bottom of the melter. The mixing of the colorant with the clear glass constituents is, therefore, somewhat inefficient and takes a considerable length of time to optimise itself. Until optimal mixing is attained, the tinted glass will not be of an acceptable commercial quality. When the run of making tinted glass is completed, the colorant again tends to settle on the bottom of the tank and is only removed slowly therefrom. These changeover times, particularly the changeover from tinted glass to clear glass can be very long, of the order of weeks. From a commercial point of view, this represents a significant disadvantage. 
     A further problem is that of devitrification. When making a tinted glass, particularly one containing iron oxide, there is a greater tendency for a build-up of devitrified glass on the bottom of the furnace at the working end to occur. 
     The problems of incomplete mixing and devitrification are, in glass-making terms, problems on a macro-scale. There is also a minor-scale problem which occurs when the composition of the glass is changed, such as when there is a change-over in a furnace from making one kind of glass to a second kind of glass. This is the problem of changes occuring in the refractive index within the glass being formed and is commonly known as &#34;ream&#34;. This manifests itself in the form of optical distortion when one looks through the glass. 
     Still further, and alluded to above, vehicle manufacturers require the glass which they use in their vehicles to have ever lower weight but still to have the particular radiation transmission or absorption properties. Accordingly, glass manufacturers are attempting to make thinner glass having the desired properties. According to the Lambert-Beer law, the thickness of a glass (or the concentration of a colorant species) is linked with the visible transmission of the glass. In essence, this states that as the thickness of the glass is halved, the internal optical density of the glass will also halve. Accordingly, in order to make a glass having a thickness of 2 mm having the same radiation transmission or absorption properties as a glass having a thickness of 4 mm, it is usually necessary to double the concentration of the colorant. Of course, if the concentration of the colorant is doubled, the problems outlined above caused by the higher densities of the colorant compared with that of the clear glass composition are exacerbated. 
     OBJECTS OF THE INVENTION 
     The present invention therefore seeks to provide a method of making glass in which the above-described problems are at least minimized. In particular, the present invention seeks to provide a method of making glass so that the changeover time when a float line is changed from making a first type of glass to a second type of glass is substantially reduced. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is therefore provided a method of making glass comprising the steps of producing a first glass in a furnace, measuring the densities of the glass thus produced and altering the composition fed to the furnace such that a further glass having significantly different properties from the first glass is produced, wherein the alteration to the composition is so selected as to produce the desired properties in the further glass whilst simultaneously maintaining the densities of the further glass substantially equal to that of the first glass. 
     We have surprisingly found that if the compositions of the different glasses made in a particular furnace are selected so that the densities of all of the glasses made in that furnace, measured at 25° C., are substantially the same, the changeover from making one commercially acceptable glass to another commercially acceptable glass can be effected much more rapidly. The term &#34;substantially the same densities&#34; is defined as meaning that the densities are within 0.01, preferably within 0.005, of one another. Moreover, we have found that if the densities of the different glasses are matched, the refractive indices tend also to be more closely matched. Accordingly, we have found that, by carrying out the method of the present invention, the problem of ream may also be alleviated. 
     Preferably, the first glass is a clear glass and the further glass is a tinted glass containing iron. 
     Alternatively, both said first and second glasses contain Fe 2  O 3  characterised in that the densities of the two glasses are made substantially constant by reducing the CaO content in the glass containing more Fe 2  O 3  relative to the glass containing less Fe 2  O 3 . 
     Glass used in vehicles and buildings is usually of the soda-lime-silica type and the major constituents thereof are SiO 2 , Na 2  O, CaO, MgO, K 2  O and Al 2  O 3 . Of these, the lightest component is SiO 2  which, when fused, has a densities of approximately 2.2. Most glass manufacturers have a basic clear glass composition which they maintain substantially consistent. If they then wish to make a tinted glass containing iron, this is effected in one of two ways. Firstly the composition may be modified by selecting the appropriate amount of iron oxide to be added (in weight %) and reducing the amount of SiO 2  present in the composition by the same amount. Alternatively, the iron oxide may simply be added to the other constituents of the clear glass composition. It will be readily apparent that, if the amounts are calculated in weight %, this latter procedure has the effect of slightly reducing the weight percentage of all of the components originally present 
     We have found that each of the constituents of a glass making mixture have an effect on the densities of the glass which is produced. Whilst this may appear, at first sight, to be self-evident, we have surprisingly found that the effects produced are not predictable. For example, the densities of iron oxide, in the form of Fe 2  O 3 , and CaO are approximately 5.2 and 3.2 respectively. However, for each weight percent of Fe 2  O 3  used to replace one weight percent of SiO 2 , the densities of the glass rises by approximately 0.0150. For each weight percent of CaO used to replace one weight percent of SiO 2 , the densities rises by 0.0149. This is totally unexpected on the basis of the densities of the Fe 2  O 3  and the CaO. Similarly, from their densities, one would expect K 2  O to have a greater effect on the density of the glass than Na 2  O. However, again referring to the replacement of one weight percent of SiO 2  by one weight percent of each of these alkali metal oxides, we have found that the sodium oxide increases the densities of the glass by 0.0078 whilst the potassium oxide only increases it by 0.0056. 
     From the above, it will be readily apparent that if the tinted glass is to contain Fe 2  O 3  in relatively large amounts--say from 0.7 to 5 weight %--the densities of the glass would be substantially increased if the usual procedure of reducing the amount of SiO 2  in an amount corresponding to that of the iron oxide added were adopted. However, it will be equally apparent from the above discussion that if the amount of CaO is reduced by an amount corresponding to the amount of Fe 2  O 3  added, the effect on the densities will be very small (an increase of approximately 0.0001 per weight percent of Fe 2  O 3  added). 
     However, particularly if there is a large amount of iron in the tinted glass, a straight exchange of Fe 2  O 3  for CaO can give rise to problems. In particular, although lowering the amount of CaO lessens the chance of devitrification due to wollastonite (calcium silicate) occurring, too low an amount of calcium present tends to encourage the formation of phases such as diopside or silica-rich primary phases such as tridymite. Both of these are harder to remove than wollastonite, particularly the silica-rich phases. 
     Accordingly, it may be preferred if the MgO content is simultaneously reduced along with the CaO. The removal of calcium oxide and/or magnesium oxide can lead to a change in the working temperature (also known as the log4 viscosity because this is the temperature at which the glass has a viscosity of 10 4  poise). Since a change in the working temperature is not usually regarded as desirable by glass manufacturers because it may necessitate physical changes to other parameters it may therefore be desirable if this is counterbalanced by a slight increase in the amount of Na 2  O and K 2  O in the composition. 
     In a particularly preferred embodiment of the present invention, the first glass contains A% CaO, B% MgO, C% Na 2  O plus K 2  O and D% SiO 2  plus Al 2  O 3 , and the second glass contains X% total iron (expressed as Fe 2  O 3 ), (A-0.5X)% to (A-1.5X)% CaO, B% to (B-X)% MgO with the proviso that the amount of MgO tends to (B-X) when the change in CaO is least, C to (C+0.5X) Na 2  O plus K 2  O and D% SiO 2  plus Al 2  O 3  with sufficient changes to cause the total contents to be 100 weight %. 
     As previously mentioned, the tinted glasses are intended, in most cases to be so-called &#34;high performance glasses&#34;, that is to say, they have low infra-red and ultraviolet radiation transmittance. In a particularly preferred embodiment of the present invention, the glasses are primarily intended for use as either privacy glazing or as the glazing for a sunroof in a vehicle. To produce such glazing, the iron content in the composition needs to be relatively high and the method of the present invention is particularly useful in preparing glass wherein the iron oxide content is in excess of 0.7 weight percent and may be as high as 5 weight percent. 
     Other colorants may be used in association with the iron oxide. Examples of such colorants are selenium, nickel oxide and cobalt oxide. Even though these have relatively high molecular weights, the amounts which are used are relatively small, of the order of 500 ppm or less and such small amounts have such a small effect on the densities of the finished glass that, for the practical purposes of the present invention, they can be ignored. 
     If the amount of calcium oxide is reduced as the amount of iron oxide is increased, the working temperature of the glass remains substantially the same. However, the liquidus temperature, that is to say, the temperature at which the glass will begin to crystallise under equilibrium conditions, will drop. This is because magnesium calcium silicate (diopside) is then formed as the primary or highest temperature phase as the amount of calcium in the glass is reduced. This aspect of the present invention therefore helps to prevent a build-up of devitrified glass. Devitrification is a common problem in glasses containing high amounts of iron oxide. The working range, which is the difference between the working temperature and the liquidus temperature, is thereby increased. This latter is of importance because it means that it is easier to form the glass without crystallisation occurring and the amount of devitrification is reduced or even eliminated completely. 
     In another aspect of the present invention, there is provided a high soda-lime-silica glass for the production of vehicle windows having a low Direct Solar Heat Transmission by virtue of the presence of colorants including the iron, characterised in that the composition contains a reduced proportion of alkaline earth metal oxides. In a preferred embodiment, the glass contains more than 2.0 weight % iron calculated as Fe 2  O 3  and less than 12 weight % alkaline earth metal oxides. Advantageously, the alkaline earth metal oxide content is below 11% and, desirably, the iron content is less than 2.5 weight %, calculated as Fe 2  O 3 . Glasses of this type preferably have a working range of at least 9C. 
     Although high iron content glasses have been made hitherto, such glasses have either not had the desired optical properties or have caused production difficulties of the types detailed hereinbefore. We have surprisingly found that, by reducing the amount of alkaline earth metal oxides present in high iron glasses to below, for example 12%, the production difficulties are overcome whilst the optical properties are attained. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The invention will be further described, by way of example only, with reference to the following non-limitative Examples appearing in the following Tables. In the tables: 
     The ferrous state is the amount of FeO (expressed as Fe 2  O 3 ), divided by the total iron oxide (expressed as Fe 2  O 3 ). 
     Illuminant A transmission represents the power distribution of tungsten filament according to the 1931 Observer set. 
     DSHT (Direct Solar Heat Transmission) refers to solar heat transmission integrated over the wavelength range 350 to 2100 nm according to the relative solar spectral distribution Parry Moon for air mass 2 direct, as detailed in ISO 9050. 
     UVT (ISO) for the measurement of ultraviolet radiation over the wavelength range 280-380 nm (air mass 2 global) is from standard ISO 9050. 
     UVT (PM) is the measurement of ultraviolet radiation over the wavelength range 300 to 400 nm derived from the factors published by Parry Moon in the Journal of the Franklin Institute 230 pp 583-617, 1940. 
     a* and b* are colour co-ordinates derived using the CieLAB co-ordinate system. 
     The log 4 viscosity temperature is the temperature in degrees Celsius at which the glass has a viscosity of 10,000 poise. 
     The liquidus temperature is determined by placing glass grains in a gradient furnace for 16 hours then measuring the hottest temperature at which crystal growth occurs. 
     Density is the mass per unit volume in grams per cubic centimeter at 25° C. measured using the Archimedes method. 
     W=Wollastonite 
     Di=Diopside 
     De=Devitrite 
     Si=Silica 
     Nd=Refractive Index 
     
                                           TABLE 1__________________________________________________________________________Example 1   2a  2b  3   4   5   6   7__________________________________________________________________________SiO.sub.2   72.7       71.77           71.4               72.7                   72.7                       72.2                           72.7                               72.7Al.sub.2 O.sub.3   1.0 0.987           1.0 1.0 1.0 1.0 1.0 1.0Na.sub.2 O   13.0       12.833           13.0               13.0                   13.0                       13.5                           13.0                               13.0K.sub.2 O   0.6 0.592           0.6 0.6 0.6 0.6 0.6 0.6MgO     4.0 3.949           4.0 4.0 3.8 3.8 3.4 2.8CaO     8.4 8.292           8.4 7.2 7.4 7.4 7.8 8.4SO.sub.3   0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2Fe.sub.2 O.sub.3   0.1 1.3 1.3 1.3 1.3 1.3 1.3 1.3Ferrous state   20% 22% 22%III. A TransDSHTUVT (ISO)UVT (PM)a*b*dom λ nmColour Purity %log4 visc.   1037       (1033)           (1028)               1037                   1038                       1030                           1034                               1032temp. °C.Liquidus   1000 W       1000 W  982 Di                   982 W                       980 W                           990 W                               998 WTemp. (T.sub.l)       998 Di  964 De                   976 Di  963 DeT.sub.w - T.sub.l   +36° C.       +33° C.           +28° C.               +55° C.                   +56° C.                       +50° C.                           +44° C.                               +34° C.Densities   2.4910       2.5086           2.5103               2.4925                   2.4941                       2.4987                           2.4963                               2.4996Refractive   1.5180       1.5224           1.5230               1.5188                   1.5188                       1.5194                           1.5197                               1.5207Index (Nd)__________________________________________________________________________Example 8   9   10  11  12  13  14  15__________________________________________________________________________SiO.sub.2   71.14       72.7           72.7               72.2                   72.4                       72.7                           72.7                               72.7Al.sub.2 O.sub.3   0.98       1.0 1.0 1.5 1.5 1.0 1.0 1.0Na.sub.2 O   12.72       13.0           13.0               14.1                   14.2                       13.0                           13.0                               13.0K.sub.2 O   0.587       0.6 0.6 0.6 1.0 0.6 0.6 0.6MgO     3.91       3.4 3.4 4.0 1.0 3.6 3.0 0CaO     8.22       6.8 6.8 5.1 7.4 7.1 5.4 8.4SO.sub.3   0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2Fe.sub.2 O.sub.3   2.2 2.2 2.2 2.2 2.2 1.7 4.0 4.0Ferrous state   22% 22% 30% 22% 33% 33% 22.4%                               21%Co.sub.3 O.sub.4   295 275 235 285 235 90  500 500Se      15  16  17  24  18  10  20  13NiO     250 250 75  150 0   50  400 400III. A Trans   28.1       31.5           32.0               28.5                   34.1                       51.5                           11.0                               16.0DSHT    20.9       24.1           21.4               21.1                   22  32.5                           9.0 14.4UVT (ISO)   4.9 6.6 6.1 6.1 8.7 14.4                           0.5 0.8UVT (PM)   15.4       19  17.6               17.4                   22.6                       32  3.4 5.6a*      -7.0       -6.4           -6.3               -5.8                   -7.1                       -7.6                           -7.7                               -8.9b*      -0.6       -1.5           -0.4               -1.7                   -6.4                       -0.5                           +2.2                               -2.8dom λ nm   493.7       492 494 491 486 494 507 491Colour Purity %   5.7 6   4.8 6.1 11.4                       4.2 4.5 11.1Log 4 visc.   (1027)       1042           1042               1038                   1012                       (1040)                           1044temp ° C.Liquidus   1016 Di       970 970 990 Di                   958 De                       972 980 SiTemp. (T.sub.l)       W/Di           W/Di        W/DiT.sub.w - T.sub.l   +11° C.       +72° C.           +72° C.               +48° C.                   +54° C.                       +68° C.                           +64° C.Density 2.5189       2.4951           2.4965               2.4884                   2.4995                       2.4944                           2.4983                               2.5153Refractive Index   1.5253       1.5197           1.5197  1.5202                       1.5194                           1.5216                               1.5259__________________________________________________________________________Example 16   17  18   19  20   21  22__________________________________________________________________________SiO.sub.2   72.0 72.7            72.7 72.7                     72.7 72.7                              72.7Al.sub.2 O.sub.3   1.9  1.0 1.0  1.0 1.0  1.0 1.0Na.sub.2 O   15.0 13.0            13.0 13.0                     13.0 13.0                              13.0K.sub.2 O   0.6  0.6 0.6  0.6 0.6  0.6 0.6MgO     0    4.0 3.45 3.8 3.7  3.6 3.7CaO     6.2  4.4 6.8  7.5 7.4  7.1 7.4SO.sub.3   0.2  0.2 0.2  0.2 0.2  0.2 0.2Fe.sub.2 O.sub.3   4.0  4.0 2.15 1.1 1.3  1.7 1.3Ferrous state   20.5%        21% 26%  27% 23%  34% 27%Co.sub.3 O.sub.4   500  500 200  113 130  170 130Se      43   12  26   19  15   21  10NiO     400  400 75   50  50   75  50III. A Trans   9.3  16.7            32.5 52  52.2 38.9                              53.4DSHT    8.8  15.1            23.4 43  43   26  41.5UVT (ISO)   0.5  0.6 5.4  18  16   11  18.4UVT (PM)   3.2  4.6 16   36.2                     34.6 24.1                              37.7a*      -2.8 -10.0            -3.9 -1.6                     -3.2 -5.5b*      5.9  -0.2            +4.9 +3.2                     +0.8 +0.9dom λ nm   568  495.8            561  586 498  498 490Colour Purity %   10.7 8.4 5.1  2.7 1.5  3   4.2log4 visc.   1005temp. °C.Liquidus   955 DeTemp. (T.sub.l)   950 SiT.sub.w - T.sub.l   +50° C.Densities   2.5074        2.4899            2.4933                 2.4938                     2.4941                          2.4947                              2.4944Refractive Index   1.5214        1.5202__________________________________________________________________________Example 23   24   25   26   27   28__________________________________________________________________________SiO.sub.2   72.7 72.7 72.7 72.7 72.7 72.7Al.sub.2 O.sub.3   1.0  1.0  1.0  1.0  1.0  1.0Na.sub.2 O   13.0 13.0 13.0 13.0 13.0 13.0K.sub.2 O   0.6  0.6  0.6  0.6  0.6  0.6MgO     3.6  3.7  3.8  3.6  3.6  3.4CaO     7.1  7.4  7.5  7.1  7.1  6.8SO.sub.3   0.2  0.2  0.2  0.2  0.2  0.2Fe.sub.2 O.sub.3   1.7  1.3  1.1  1.7  1.7  2.2Ferrous state   27%  27%  28%  36.5%                       37.5%                            28.4%Co.sub.3 O.sub.4   10   45   13   170  170  255Se      56   26   31   10   5    12NiO     75   50   50   75   75   150III. A Trans   35   53   56   43.3 44.7 32.7DSHT    27   39   43   28   28.6 23UVT (ISO)   5.3  13.3 15   13.9 15.4 7UVT (PM)   12   29.5 31.5 31   33.4 19.6a*      +8.0 -1.0 +1.4 -7.9 -8.7 -7.7b*      +26.9        +10.0             +13.4                  -5.4 -7.3 -4.0dom λ nm   582  575  578  488  487  489Colour Purity %   38.6 11   16   10.0 12.1 9.3log4 visc.temp. °C.Liquidus               976 WTemp. (T.sub.l)        970 DiT.sub.w - T.sub.lDensities   2.4940        2.4949             2.4944                  2.4961                       2.4955                            2.4967Refractive index__________________________________________________________________________Example 29      30      31      32__________________________________________________________________________SiO.sub.2   72.6    72.6    70.6    70.6Al.sub.2 O.sub.3   0.1     0.1     0.91    0.91Na.sub.2 O   13.9    13.9    13.7    13.7K.sub.2 O   0.03    0.03    0.57    0.57MgO     4.1     3.5     4.0     3.4CaO     8.9     7.3     9.76    8.16SO.sub.3   0.23    0.23    0.27    0.27Fe.sub.2 O.sub.3   0.11    2.2     0.1     2.2Ferrous state   20%     24%     20%     24%Co.sub.3 O.sub.4           275             275Se              23              22NiO             250             250III. A Trans    29.4            29DSHT            23.5            23.1UVT (ISO)       5.4             4.5UVT (PM)        16.4            14.6a*              -5.1            -5.4b*              +1.4            +2.7dom λ nm 503             522.2Colour Purity % 2.4             2.1log4 visc.   1014    1020    1000    1010temp. °C.Liquidus   1008 W  996 Si  1028 W  990 WTemp. (T.sub.l) 988 WT.sub.w - T.sub.l   +6      +24     -28     +20Densities   2.5017  2.5017  2.5176  2.5237Refractive index   1.5201  1.5221  1.5249  1.5268__________________________________________________________________________ 
    
     
                                           TABLE 2__________________________________________________________________________Example 1   8   9   10   11  12  13   14__________________________________________________________________________SiO.sub.2   72.7       71.14           72.7               72.7 72.7                        72.4                            72.7 72.7Al.sub.2 O.sub.3   1.0 0.98           1.0 1.0  1.5 1.5 1.0  1.0Na.sub.2 O   13.0       12.72           13.0               13.0 14.1                        14.2                            13.0 13.0K.sub.2 O   0.6 0.587           0.6 0.6  0.6 1.0 0.6  0.6MgO     4.0 3.91           3.4 3.4  4.0 1.0 3.6  3.0CaO     8.4 8.22           6.8 6.8  5.1 7.4 7.1  5.4SO.sub.3   0.2 0.2 0.2 0.2  0.2 0.2 0.2  0.2Fe.sub.2 O.sub.3   0.1 2.2 2.2 2.2  2.2 2.2 1.7  4.0Ferrous state   20% 22% 22% 30%  22% 33% 33%  22.4%Co.sub.3 O.sub.4       295 275 235  285 235 90   500Se          15  16  17   24  18  10   20NiO         250 250 75   150 0   50   400III. A Trans       8.7 10.7               11.2 9   12.9                            29.2 1.4DSHT        5.6 7.1 6.3  5.8 7.2 14.9 1.1UVT (ISO)   0.7 1.1 1.0  0.9 1.9 4.6  0UVT (PM)    3.9 5.5 4.9  4.8 7.8 14.7 0.3a*          -9.1           -8.7               -8.6 -7.5                        -9.4                            -12.1                                 -7.3b*          -0.6           -1.9               -0.4 -2.1                        -9.1                            -0.6 +2.4dom λ nm       494.9           492.3               495.2                    491.4                        486 495.1                                 510.7Colour Purity %       10.1           11.0               8.6  10.9                        20.8                            8.9  8.2Log 4 visc.   1037    1042     1.038                        1012     1044temp °C.Liquidus   1000 W       1016 Di 970 W/Di                    990 Di                        958 De                            972 W/Di                                 980 SiTemp. (T.sub.l)T.sub.w - T.sub.l   +36° C.   +48° C.                        +54° C.Densities   2.4910       2.5189           2.4951               2.4965                    2.4884                        2.4995                            2.4944                                 2.4983Refractive Index   1.5180__________________________________________________________________________Example 15  16   17   18   19   20   21__________________________________________________________________________SiO.sub.2   72.7       72.0 72.7 72.7 72.7 72.7 72.7Al.sub.2 O.sub.3   1.0 1.9  1.0  1.0  1.0  1.0  1.0Na.sub.2 O   13.0       15.0 13.0 13.0 13.0 13.0 13.0K.sub.2 O   0.6 0.6  0.6  0.6  0.6  0.6  0.6MgO     0   0    4.0  3.45 3.8  3.7  3.6CaO     8.4 6.2  4.4  6.8  7.5  7.4  7.1SO.sub.3   0.2 0.2  0.2  0.2  0.2  0.2  0.2Fe.sub.2 O.sub.3   4.0 4.0  4.0  2.15 1.1  1.3  1.7Ferrous state   21% 20.3%            21%  26%  27%  23%  34%Co.sub.3 O.sub.4   500 500  500  200  113  130  170Se      13  43   12   26   19   15   21NiO     400 400  400  75   50   50   75III. A Trans   2.9 1.0  2.7  11.6 29.9 29.9 16.6DSHT    2.7 1.1  2.5  7.0  21.6 21.4 9.2UVT (ISO)   0   0    0    0.8  6.7  5.7  2.6UVT (PM)   0.6 0.2  0.4  4.1  18   17   8.7a*      -9.5       -2.5 -10.8                 -5.5 -2.6 -5.3 -8.1b*      -3.2       5.1  0    +6.9 +5.3 +1.2 +1.4dom λ nm   491 568  496.8                 561.4                      568  501.2                                500.9Colour Purity %   20.2       22.5 15.4 10.9 6.2  2.7  5.1log4 visc.  1005temp. °C.Liquidus    955 DeTemp. (T.sub.l)       950 SiT.sub.w - T.sub.l       +50° C.Densities   2.5153       2.5074            2.4899                 2.4933                      2.4938                           2.4941                                2.4947Refractive Index__________________________________________________________________________Example 22  23   24   25   26   27   28__________________________________________________________________________SiO.sub.2   72.7       72.7 72.7 72.7 72.7 72.7 72.7Al.sub.2 O.sub.3   1.0 1.0  1.0  1.0  1.0  1.0  1.0Na.sub.2 O   13.0       13.0 13.0 13.0 13.0 13.0 13.0K.sub.2 O   0.6 0.6  0.6  0.6  0.6  0.6  0.6MgO     3.7 3.6  3.7  3.8  3.6  3.6  3.4CaO     7.4 7.1  7.4  7.5  7.1  7.1  6.8SO.sub.3   0.2 0.2  0.2  0.2  0.2  0.2  0.2Fe.sub.2 O.sub.3   1.3 1.7  1.3  1.1  1.7  1.7  2.2Ferrous state   27% 27%  27%  28%  36.5%                           37.5%                                28.4%Co.sub.3 O.sub.4   130 10   45   13   170  170  255Se      10  56   26   31   10   5    12NiO     50  75   50   50   75   75   150III. A Trans   31.2       13.7 31.3 34.2 20.8 22.3 11.9DSHT    20.6       8.9  18.8 21.6 11.4 12.2 7.1UVT (ISO)   7.1 0.5  4.3  4.9  4.3  5.1  1.2UVT (PM)   19.7       2.1  13   13.8 13.8 15.8 6.1a*      -7.2       +13.3            -1.2 +3.2 -11.6                           -12.5                                -10.3b*      -2.5       +35.7            +15.9                 +21.9                      -8.2 -11.4                                -5.4dom λ nm   490.4       584.1            575  578.8                      487.5                           486.3                                489Colour Purity %   7.5 67.1 21.8 30.7 18.3 22.1 16.9log4 visc.                 976 Wtemp. °C.           970 DiLiquidusTemp. (T.sub.l)T.sub.w - T.sub.l          2.4961                           2.4955                                2.4967Densities   2.4944       2.4940            2.4949                 2.4944Refractive Index__________________________________________________________________________Example      29     30     31      32__________________________________________________________________________SiO.sub.2    72.6   72.6   70.6    70.6Al.sub.2 O.sub.3        0.1    0.1    0.91    0.91Na.sub.2 O   13.9   13.9   13.7    13.7K.sub.2 O    0.03   0.03   0.57    0.57MgO          4.1    3.5    4.0     3.4CaO          8.9    7.3    9.76    8.16SO.sub.3     0.23   0.23   0.27    0.27Fe.sub.2 O.sub.3        0.11   2.2    0.1     2.2Ferrous state        20%    24%    20%     24%Co.sub.3 O.sub.4    275            275Se                  23             22NiO                 250            250III. A Trans        9.5            9.3DSHT                6.8            6.5UVT (ISO)           0.8            0.6UVT (PM)            4.4            3.6a*                  -6.9           -7.2b*                  +2.1           +3.8dom λ nm     507.7          530Colour Purity %     4.2            5log4 visc.   1014   1020   1000    1010temp. °C.Liquidus     1008 W 996 Si 1028 W  990 WTemp. (T.sub.l)     988 WT.sub.w - T.sub.l        +6            -28     +20Densities    2.5017 2.5017 2.5176  2.5237Refractive Index        1.5201        1.5249__________________________________________________________________________ 
    
     In Table 1, there are shown the compositions and spectral properties of a variety of glasses in a 2 mm thickness. In Table 2, the spectral properties of the glasses having the compositions shown in Table 1 are given, but at a thickness of 4 mm. 
     In Table 1, Example 1 is an example of a clear float glass containing an amount of iron oxide at an impurity level. Examples 2a and 2b are glasses containing 1.2 weight percent Fe 2  O 3  added to the basic composition of Example 1. In Example 2a, in accordance with one of the known techniques described hereinbefore, the total weight percentage of the composition is now notionally 101.2 and this must obviously be re-calculated to bring the total back to 100%. Example 2b shows a similar composition to Example 2a but, in this case, the extra 1.2 weight percent Fe 2  O 3  has been added at the expense of SiO 2 . All of the other Examples up to and including Example 28 with the exceptions of Example 8 (similar to Example 2a), Example 15 (in which the iron oxide has been added at the expense of MgO rather than CaO) and Example 16 (in which all of the MgO and some CaO has been removed and has been replaced by the iron oxide) are compositions for use in the present invention in which the densities of the iron oxide containing glass has been balanced with the densities of the clear glass shown in Example 1. Examples 9 to 11 and 13 to 17, in both Tables, are green grey glasses; Examples 18 to 22 are grey glasses; Examples 23 to 25 are bronze glasses and Examples 12 and 26 to 28 are blue glasses. As mentioned hereinbefore, there is no &#34;standard&#34; clear glass composition because, for example, the various raw materials used for making glass have varying compositions depending upon the source from which they are obtained or the glass manufacturer has a particular composition which he prefers to use. Accordingly, Examples 29 and 31 show clear glasses having different compositions from that shown in Example 1 and Examples 30 and 32 show tinted glasses, which are grey-green, having densities matched to those of these two clear glasses respectively. 
     From the Tables, it will be clearly seen that the densities of the tinted glasses are closely matched with the corresponding clear glass. Moreover, the working range, that is to say, the difference the working temperature and the liquidus temperature has been increased. We have found that, by carrying out the method of the present invention, the changeover from making a clear glass to a tinted glass can be shortened by hours and the changeover from tinted glass to clear glass can be shortened by days. Furthermore, from the spectral values given, it can clearly be seen that glasses having the requisite luminous, infra-red and ultraviolet transmissions can be obtained. 
     Various modifications may be made to the method of the present invention without departing from the scope thereof Thus, for example, although the above description is primarily concerned with effecting a changeover from clear glass to tinted glass and vice versa, the method may equally easily be used for effecting a changeover between two different tinted glasses.