MXPA00007812A - A medium gray colored glass with improved uv and ir absorption and nitrate-free manufacturing process therefor - Google Patents

Abstract

A medium grey glass having a composition comprising by weight:68 to 75%SiO2, 10 to 18%Na2O, 5 to 15%CaO, 0 to 10%MgO, 0 to 5%A12O3, and 0 to 5%K2O, where CaO + MgO is 6 to 15%and Na2O + K2O is 10 to 20%;and colorants consisting essentially of:Fe2O3 in an amount greater than 0.5 and less than 0.9 wt.%;0.1 to 1.0 wt.%Mm02;0.002 to 0.010 wt.%Co;0.0005 to 0.003 wt.%selenium;and 0-1.0 wt.%Ti02, the glass having at a 4.0 mm thickness:485-570 dominant wavelength, less than 5.0%purity of excitation, less than 40%ultraviolet transmittance at measured over 300-400 nm, less than 45%infrared transmittance measured over 760-2120 nm, with 35-60%light transmittance using Illuminant A.

Description

A GLASS OF MEDIUM GRAY COLOR WITH IMPROVED UV AND IR ABSORPTION AND PROCESS FOR MANUFACTURING THE SAME FREE OF NITRATE The invention is directed to a medium gray glass composition, which has improved UV and IR absorption, while maintaining high visible light transmission and which can be manufactured without the use of sodium or potassium nitrate. More particularly, it is a soda-lime-silica glass whose colorants include iron oxide, selenium, manganese oxide, cobalt oxide and optionally titanium oxide. Gray glass has found particular utility for architectural applications such as construction glass and has been considered for automotive glass applications. Glass is generally defined by certain spectral properties such as dominant wavelength (color), excitation purity and light transmission. The lower the purity of excitation of a color, the closer it will be to a so-called neutral color, which does not distort the tones of objects seen through it. Those skilled in the art know that the dominant wavelength, purity, and light transmission all vary in unpredicted ways. Consequently, developing a new glass composition having a particular value of color, purity and light transmission is generally difficult. For example, an experimental change in the relative amounts or proportions of one or more dyes in a intended glass composition to bring one of these numerical values closer to a target value may cause either or both of the other values to shift. Of the objective. Therefore, particular portions of the dyes are critical to developing a particular glass composition. Numerous different patented gray glasses have been manufactured using iron oxide, cobalt oxide and chromium oxide, nickel oxide or selenium as colorants. These same dyes have also been used to produce glasses of colors other than gray, as will be appreciated, however at least one or more dyes are in different amounts. For example, a brown or bronze glass can be made from iron oxide, cobalt oxide and selenium while the glass can be made blue, if the proportion of a component, for example cobalt is increased. Therefore, proportions of dyes are critical for the spectral properties of glass. Considering gray glass, the patent of the U.S. No. 4,104,076 discloses a gray glass composition whose colorants consist essentially of: iron oxide, cobalt oxide and selenium, wherein to avoid the use of nickel oxide, either or both of chromium oxide or uranium oxide are employed. Cheng in U.S. Pat. No. 5,278,108, in contrast, forms a gray glass with dyes of iron oxide, cobalt oxide and selenium, specifically avoiding constituents that are described to cause manufacturing difficulty, i.e. chromium and manganese. In the patent of the U.S.A. No. 4,873,206, a dark gray nickel free glass is manufactured using iron oxide, cobalt oxide and selenium, "there it is described that the composition will be essentially free of chromium, titanium and manganese." Another approach is taken in the Patent of the No. 5,346,867, where the neutral gray glass is made using the dyes iron oxide, cobalt, selenium, manganese oxide and optionally titanium oxide, yet another glass is illustrated in the US Patent using Serious Oxide. , iron oxide, selenium and optionally cobalt oxide, nickel oxide and titanium oxide to obtain a gray color In U.S. Patent No. 5,656,560, a gray or bronze dye glass whose color is obtained is described by using at least a relatively high amount of manganese oxide and optionally any of iron oxide, vanadium oxide, nickel oxide, copper oxide and cobalt oxide. This patent is to form its gray or bronze glass without the use of selenium, a very volatile and expensive dye. This latter patent again demonstrates the non-predictable nature of the art of coloring glass, where by using for example iron, manganese, nickel and cobalt results in several exemplary embodiments of a bronze-colored glass. In the present invention, a new medium gray glass composition different from those described above has been developed, containing in specific proportions iron oxide, manganese oxide, cobalt oxide and selenium, which provides a glass with excellent ultraviolet absorbing properties. (UV) and infrared (IR) and very low excitation purity. This low purity of excitation makes glass ideally suited for both automotive and architectural applications as it has a neutral appearance favored by designers. Its excellent UV and IR absorption properties make it ideally suited to prevent color fade damage to components within a car or construction and keep the interior cool. As will be appreciated, the absorbing properties of UV and IR light are especially valuable when the glass is for buildings or constructions since the heat is absorbed by the glass, the load on air conditioners of the building is reduced. Therefore, developing a glass with these spectral properties is very important. Iron oxide exists in two forms in the glass melting, the oxidized form of iron oxide (Fe203) absorbs the UV and the reduced form of iron oxide (FeO) absorbs infrared light. In this way, these two forms of iron oxide in the glass reduce the UV transmittance in IR through the glass products. Adding iron oxide to a soda-lime-silica glass under normal oven conditions improves both the UV and infrared absorption of glass since the concentration of the iron forms is correspondingly increased. This improvement is at the cost of visible transmittance, however. That is, as iron oxide is added, the color of the glass is obscured in such a manner that the visible transmittance is correspondingly decreased. In the present invention, the incorporation of particular dyes in combination has been found to allow improved UV-absorbing properties in IR when more iron oxide is added without a proportional dimming of the glass color. Therefore, it retains good visible transmission properties. The present invention is a medium gray soda-lime-silica glass composition having excellent ultra violet and infrared absorbing ability. The colorants of the glass composition essentially consist of: more than 0.5 and less than 0.9% by weight of total iron oxide as Fe203; 0.1 to 1.0% by weight of the manganese compound as Mn0; 0.0005 to 0.003% _in selenium weight as Se; 0.002 to 0.010% by weight of cobalt oxide as Co; and optionally up to 1.0% by weight of titanium oxide; the glass composition has a thickness of 4.0 mm: 485 to 570 of dominant length, less than 5% of excitation purity, 35 to 60% of light transmittance using A illuminant, less than 40% of ultraviolet transmittance measured over 300 a 400 nm, and less than 45% infrared transmittance measured over 760 to 2120 nm. The medium gray composition comprises by weight: 68 to 75% Si02, 10 to 18% Na20, 5 to 15% CaO, 0 to 10% MgO, 0 to 5% A1203, and 0 to 5% K20, wherein CaO + MgO is 6 to 15% and Na20 + K20 is 10 to 20%. The glass can also include trap materials that sometimes enter the glass with the raw materials or as a result of exchanging one glass composition with another in a glass oven. For example, this will include up to about 0.005% by weight of nickel oxide as Ni02. Advantageously, the glass composition has improved UV absorption in IR and a good shading coefficient, while good visible light transmittance is maintained. And the new composition of the invention can be made with little or no added sodium or potassium nitrate during the melt processing. By far it has been illustrated in the industry that these nitrates are going to be added to maintain oxidation conditions and to decrease the volatilization of selenium. Avoiding the use of nitrates advantageously reduces the production of NOx emissions from these nitrates. According to another aspect of the invention, it is a commercially convenient method to manufacture a gray glass composition without the addition of sodium or potassium nitrate as discussed above and to maintain high visible transmittance, while enhancing the absorption properties of UV e? .R. According to this method, additional amounts of iron oxide can be added to improve these properties without the expected decrease in visible light transmission that is ordinarily expected. These and other advantages of the present invention will be apparent from the detailed description. The flat soda-lime-silica glass, used in the automotive and architectural industries and conveniently made by the well-known float glass process, is generally characterized by the following basic composition shown in Table I, the amounts of the components being based on the percentage by weight of the total glass composition. TABLE I The medium gray glass composition of the present invention employs this basic glass composition of soda-lime-silica wherein additionally, CaO + MgO is 6 to 15% and Na20 + K20 is 10 to 20%. In addition, the coloring components of the gray glass composition essentially consist of: total iron oxide as Fe203 in the amount greater than 0.5 and less than 0.9% by weight, 0.1 to 1.0% by weight of manganese oxide as Mn02, 0.002 to 0-010% by weight of cobalt oxide as Co, and 0.0005 to 0.003% by weight of selenium as Se. In addition, the composition considered at a thickness of 4.0 mm has the following spectral properties: 485 to 570 dominant wavelength, less than 5% excitation purity, 35 to 60% light transmission using A illuminant, less than 40% of ultraviolet transmittance measured over the range of 300 to 400 nanometers, and less than 45% infrared transmittance measured over the range of 760 to 2120 nanometers. In general, as the amounts of the dyes increase, the transmittance of% LTA,% UV and% IR will be reduced. Similarly, as the glass thickness increases for a given glass composition, the transmittance of the thicker glass decreases. Preferably, the dominant wavelength is between 485 and 560 nanometers, more preferably 485 to 520. As will be known to a person skilled in the art, the mixed materials diffuse together to form the glass. In general, laminar glass is made according to the well-known float glass process. Fusion and refining aids are routinely included in glassmaking and can also be employed here. A refining aid generally employed to remove bubbles from the glass is sodium sulfate which results in S03 in the glass. Preferably, S03 is present in the glass composition of 0.10 to 0.30% by weight, more preferably 0.14 to 0.25% by weight.

A required dye, iron oxide as total iron oxide as Fe203 is present in the gray glass composition of the invention, in amounts in a range greater than 0.5 but less than 0.9% by weight, preferably from 0.6 to 0.85% by weight, in particular from 0.7 to 0.85% by weight. All percentages by weight herein are based on the total weight of the glass composition of the invention. Typically, this dye is added to the batch ingredients in the Fe203 oxide form. As discussed above, iron oxide exists in two forms in glass melting. The oxidized form absorbs UV light and the reduced form absorbs infrared light, therefore reducing the UV transmittance in IR through the glass products. Both absorbing fusions of iron oxide are especially valuable when the glass product is used in architectural applications, particularly in geographical areas that have significant sunlight. The ratio of reduced iron oxide to total iron oxide, FeO / total iron as Fe203, is called the redox ratio: of iron oxide. In the present glass, it is preferable from 0.18 to 0.29, more preferably from 0.20 to 0.26. The optimum redox ratio in the final product is determined by the particular spectral properties of the desired glass. Another essential dye in the gray glass composition is a manganese oxide such as Mn02. The manganese compound is present in the composition, of the invention in an amount of 0.10 to 1.0% by weight based on MnO ^, more preferably 0.2 to 0.8% by weight. This manganese compound can be added to the glass components of the batch in a variety of ways, for example but not limited to Mn02, Mn304, MnO, MnC0, MnS04, MnF2, MnCl_, etc. It is preferably more convenient to use the manganese oxide or manganese carbonate compounds in the batch. As will be appreciated, a mixture of these compounds can also be employed. According to a particularly important aspect of the gray glass manufacturing process of the present invention, a mineral of natural origin, pyrolusite, can be used to supply manganese dioxide to the batch. This mineral is advantageously used because the impurities within the mineral deposits are also those included in the glass of the present invention such as Fe203, A1_03, SiO2, BaO, and CaO. Pyrolusite in this way can be effectively employed in the batch mixture as the source of manganese oxide and with significant savings in the cost of the batch. In the glass composition, this dye is generally present in the Mn + 2 and Mn * 3 state, although it may additionally be present in other states such as Mn +. A form of manganese oxide absorbs in the same area as the selenium dye in such a way that it can be conveniently employed to replace selenium in providing the desired gray color. Selenium is expensive and easily volatilized from glass melting. Manganese oxide is expensive and is not subject to such volatility in such a way that it is optimal as a colorant in the present gray glass composition. The manganese dye has oxidant capacity in such a way that when it is added it is able to displace the redox balance of the iron oxide towards its less colored oxidized form. Correspondingly, the manganese oxide is converted to the more colorless MnO reduced. We consider that this allows the improved UV and IR properties without the proportional reduction in visible transmittance that is ordinarily expected when more iron oxide is added. In this way, while the properties of UV and IR can be improved in conventional glass by increasing the iron oxide dye, if a glass of high visible light transmission is desired, it would not be achieved. Neither the validity nor the understanding of these are nevertheless necessary for a practice of the invention. The following example shows the improved properties of the medium gray glass of the present invention compared to conventional gray glass. For example, a conventional gray glass that includes iron oxide as a colorant, ordinarily has a UV transmittance of about 44.4% and an IR transmittance of 53.6-% to 56.8% LTA (visible light transmission). One embodiment of the present invention with a more intense gray color can be made to have almost identical 56.3% LTA, while also exhibiting a significantly improved (ie diminished) UV transmittance of only 34.3% and IR transmittance of only 36.4%. This particular example of an embodiment of the present invention makes visible the advantage of the glass of the present invention for architectural applications, which is apparent. We have found that when sodium nitrate (or potassium nitrate) routinely used in the glass industry to provide oxidizing conditions to glass melting, used in the batch, sodium sulfate (a de-refining agent) must be reduced, which may adversely affect the action of refining sodium sulfate. While the composition of this invention is not required to exclude the use of sodium nitrate, we have found that the use of nitrates is less than desirable both from the interaction point of sodium sulfate and nitrogen oxide emissions that can be result. Therefore, we found that it would be more convenient to limit or exclude the addition of nitrates to glass melting during the glass production process. We have found that the use of the manganese oxide dye, which additionally provides oxidizing benefits, allows the elimination of sodium nitrate as a raw material component in the manufacture of the gray glass of the present invention. Manganese oxide, which we consider to help in adjusting the oxidation state of iron oxide in the glass, also allows the use of anthracite coal to improve the decomposition of sodium sulfate and improve the refining characteristics (removal of gaseous inclusions) from the glass batch.

Typically, a person skilled in the art of glass production will not use a batch mixture containing manganese oxide, which has oxidizing properties, together with a reducing agent such as anthracite charcoal. A preferred embodiment of the present invention is to combine manganese oxide with anthracite coal or other similar reductants such as graphite, slag from coal-fired furnaces, blast furnace slag, coke or carbonaceous materials. In theory, increasing manganese oxide will cause the iron oxide balance in the melt to shto the oxidized form of iron while the reductant reacts with sodium sulfate to decompose it into sodium oxide which becomes a part of the glass and sulfur trioxide that provides the refining action at lower temperatures. Sodium sulfate requires higher melting temperatures when a reducer is absent in the batch. The use of a reducing agent in the batch allows lower oven operating temperatures while producing a glass product with the same quality as the glass produced without the reducer at those higher temperatures. This methodology we consider, has significant commercial and environmental benefits. Cobalt is a dye used in the gray glass composition of the present invention.

Typically it is added to the batch ingredients as an oxide, and is present as a coloring component in the glass in an amount of 0.002 to 0.01% by weight as Co, preferably in the amount 0.004 to 0.009% by weight. Cobalt works to absorb light in the range of 580 to 680 nanometers of the visible spectrum. The cobalt has the function of reducing the% LTA and balancing the amount of absorption of Se, Mn02 and both FeO and Fe203 to achieve the desired gray appearance of the present glass composition. The glass composition also includes selenium in an amount of 0.0005 to 0.003% by weight as Se, which is an essential ingredient for the gray color because selenium has an absorption maximum of approximately 500 nanometers, and is also combined with oxide of iron to form an iron-selenium complex with a stronger absorption peak at approximately 490 nanometers. As discussed above, the manganese oxide in the Mn * 3 form also has an absorption peak of approximately 490 nanometers such that the manganese oxide can partially replace the selenium in the composition and provide the absorption required for the gray color of glass. Selenium can be added to glass in a variety of ways, including: elemental material and in any compound form such as sodium selenite, selenite of barium, selenium oxide, sodium selenate, etc. The present glass composition is capable of achieving excellent UV properties without the expensive additives often employed in other glass compositions for this purpose such as titanium oxide, chromium oxide, cerium oxide or vanadium pentoxide. However, if it is desired to increase the UV absorption further, the titanium oxide in an amount of up to 1.0% by weight can be added. It is known in the glass industry that often titanium dioxide enters glass compositions as a trap material or impurity with raw materials when producing "sosacal glass-silica" compositions, for example with dolomite sand or limestone. The level of impurity is generally present in the range of typically from about 0.015 to about 0.05% by weight, depending on the source of the individual raw materials.Thus, typically 0.02% by weight of the titanium oxide will be present in a manufactured glass, even when titanium oxide has not been intentionally added as a dye to the glass melt It is known that foreign fragment materials can also enter the glass batch by exchanging the glass melting furnaces of a composition The glass composition can also include materials of foreign fragments or impurities that often accompany the material s employed premiums. These foreign fragment materials are a material of foreign fragments or impurities that are expected to be in small amounts, for example up to 0.005% by weight of nickel oxide as NiO. As described by the present inventors in U.S. Pat. No. 5,725,628 the inclusion of a manganese compound in the glass has been found to reduce the formation of nickel sulfide stone. Still other materials of extraneous fragments such as chromium, cobalt, which can enter into the glass production operation and which may be present in the gray glass of the invention described, will be apparent to those skilled in the art in view of the present description. . The following table lists ingredients that are preferably used to form the embodiments of the gray glass compositions according to the present invention. TABLE II NEFELINA SIENITA OR A 68.1 (OR A 150; The glass is made by mixing and melting the components and is expected to be manufactured in accordance with well-known glass production techniques, usually including continuously supplying the components to a melting furnace and heating the materials in general at approximately 1500 °. C to melt them and refine the glass. Then, this molten glass is formed in a glass sheet having a predetermined thickness by a float process or the like. Well known in the industry is the use of molten tin baths to float the glass. It will be noted that all glass compositions made in the examples are produced according to the manufacturing process of the preferred invention, ie without the use of sodium nitrate as components. It is unexpected and more convenient that the gray glass composition can be manufactured without the use of the commonly employed oxidant, sodium (or potassium) nitrate however as discussed above the invention is not so limited. Glass can also be manufactured with the use of nitrate. We consider that the use of manganese compound dye allows the manufacture of glass without the addition of "nitrate" since the manganese compound dye is also capable of providing convenient oxidation conditions.The fact that manganese compounds such as manganese dioxide They are relatively inexpensive allowing additional commercial convenience to the present invention Gray glass compositions made in accordance with the present invention can potentially be used for architectural applications or automotive applications Glasses containing manganese and iron oxide have been known to solarize or discolor when The glasses of the present invention have been found not to undergo any appreciable solarization.The glass of the present invention can also be provided with a coating to vary the spectral properties as it is so often In the industry, for example, to modify the appearance color or vary the reflectance of glass with materials such as chromium, iron, cobalt or titanium. Still other coatings that can be applied to glasses for these purposes will be apparent to those skilled in the art in view of the present disclosure. - - - - Examples of glass composition embodiments of the present invention were made in the laboratory according to the following procedure: batches were weighed, placed in a glass jar approximately 5.08 cm (2") high and 5.08 cm (2") of smaller diameter and mixed dry for 10 minutes each in a turbula mixer, the dry batch is placed in a crucible of 80% platinum / 20% rhodium, which has a height of 5.08 cm (2") ) and has an inside diameter at the top of 6.35 cm (2.5") and tapers to the base, which has an inner diameter of 4.45 cm (1.75"). An amount of 4.5 ml of water is added to the dry batch at the crucible and mix with a metal spoon.After this preparation, a group of six different batches are melted in a gas / air burned oven at the same time for one hour at 1,426.7 ° C (2,600 ° F) and each crucible is In its turn, it removes from the oven and fried, the fritting of the glass involves the coating of the inside of the crucible of p Latin / rhodium with the molten glass and then sink the crucible in cold water. After removing the crucible from the water and draining it, the broken glass particles are removed from the sides of the crucible and mechanically mixed inside the crucible. All six samples are similarly fried and all the crucibles are placed back from the oven for another one hour interval at 2626.7 ° C (2,600 ° F) and the fritting procedure is repeated. After the second sintering procedure, the crucibles are returned to the oven for four hours at 1,426.7 ° C (2,600 ° F). Each crucible is removed in turn from the furnace and each sample of molten glass is emptied into a graphite mold with an internal diameter of 6.35 cm (2.5"). Each glass is slowly cooled and labeled and placed in an annealing furnace where the temperature rises rapidly to 565.6 ° C (1, 050 ° F) holds for two hours and then cools slowly when the oven is turned off and the samples removed after 14 or more hours. The samples are milled and polished to approximately 4.0 mm in thickness and subsequently the spectral properties are measured for each sample. All laboratory mergers made with the above procedure use a base composition of 100 grams of sand, 32.22 grams of commercial soda, 8.81 grams of limestone, 23.09 grams of dolomite, 1.2 grams of sodium sulfate, 0.075 grams of carbocita, 2.64 grams of nepheline syenite, and the rest of the lot includes red, selenium, manganese dioxide and cobalt oxide in some exemplary mergers. Sodium nitrate was not included as a component in any of the examples. Table III shows the improvement of the ultraviolet and infrared absorption of embodiments of the glass compositions of the present invention as the iron oxide is increased and displaced to its oxidized form by the addition of varying amounts of manganese dioxide. All the examples show the calculated amounts of each batch dye. For comparison, Example 1 is a commercially produced product of Ford Motor Company called Sunglas ™ Gray. All the selenium contents of the examples are the target values of the batch based on 10 to 20% selenium retention. Iron oxide, manganese dioxide and cobalt oxide in the examples are also the target values of the batch. TABLE III From Table III, it can readily be seen that the addition of manganese dioxide together with the increased iron oxide significantly improves both the ultraviolet and infrared absorption of the gray glass composition embodiments of the present invention as illustrated in the examples of the table. More conveniently, in addition to the significant improvement in ultraviolet absorption of the glass, the invention can also achieve essentially the same visible transmittance of the glass as evidenced by% LTA. Examples 2 to 7 show the improvement in ultraviolet and infrared absorption while maintaining a similar transmittance to the commercial product. Also note in Example 7 that the iron oxide is significantly increased against the commercial product in Example 1, with little loss of visible transmittance.

Table IV shows other examples of glasses where ultraviolet and infrared absorption has been improved. In Tables III, IV, and V, TiO is not added; to the glass, but it was present as an impurity in the glass at a level of approximately 0.02% by weight that arrived with the raw materials. TABLE IV The results of Table IV, for the glass composition embodiments of the present invention, demonstrate again that increasing the Fe.sub.0.sub.2 correspondingly increases the ultraviolet absorption, Table IV also shows that as it increases The concentration of Mn02 ultraviolet absorption is also increased by a significant amount. Since both manganese oxide and iron oxide are increased, infrared absorption is also improved while maintaining the gray color. Table V further shows the ultraviolet and infrared absorption improvements, which are achieved when Mn02 and Fe203 are uniformly increased in exemplary embodiments of the invention. TABLE V Example 17 of Table V represents an important embodiment of the present invention where manganese dioxide has been increased, the glass has remained gray, evidenced by the low purity of excitation in%, and both the ultraviolet absorption and Infrared has been improved uniformly. It should also be noted that manganese dioxide can replace some of the selenium as in Examples 17 and 19, while maintaining the gray color. While certain preferred embodiments of the invention have been described above, it is expected that another variation as known to those skilled in the art may be resorted to, without departing from the scope of the invention as defined by the claims that follow.

Claims (8)

1. - A medium gray glass having improved ultraviolet and infrared absorption properties, the glass composition comprises by weight: 68 to 75% of SiO2, 11 to 18% of Na20, 5 to 15% of CaO, 0 to 10% of MgO, 0 to 5% of A1203, and 0 to 5% of K20, where CaO + MgO is 6 to 15% and Na20 + T? 20 is 10 to 20%; and colorants that essentially consist of: total iron oxide as Fe203 in an amount greater than 0.5 and less than 0.9% by weight; 0.1 to 1.0% by weight of manganese oxide as Mn02; 0.002 to 0.010% by weight of cobalt oxide as Co; .0005 to 0.003% by weight of selenium; and 0 to 1.0% by weight of Ti02, the glass has a thickness of 4.0 mm: 485-570 of dominant wavelength, less than 5.0% excitation purity, less than 40% of ultraviolet transmittance as measured over 300- 400 nm, less than 45% infrared transmittance measured over 760-21-20 nm, with a light transmittance using A illuminant of more than 35% and less than 60%.

2. A gray glass composition according to claim 1, characterized in that the dominant wavelength is between 485 and 560 nanometers.

3. - A gray glass composition according to claim 2, characterized in that the amount of total iron iron expressed as Fe203 is within the range of 0.6 to 0.85% by weight.

4. - A gray glass composition according to any of the preceding claims, characterized in that the amount of the manganese compound expressed as Mn02 is 0.2 to 0.8% by weight.

5. - A gray glass composition according to any of the preceding claims, characterized in that the total amount of iron expressed as Fe203 is between the range of 0.7 to 0.85% by weight.

6. A gray glass composition according to any of the preceding claims, characterized in that the amount of cobalt oxide as Co is within the range of 0.004 to 0.009% by weight.

7. A gray glass composition according to any of claims 1 or 2, characterized in that the dominant wavelength is between approximately 485 and 520 nanometers.

8. An enameling made from the composition according to any of claims 1 or 7, characterized in that a coating has been applied to further improve the spectral properties of the glass substrate.