Abstract:
A transparent glass exhibiting a light green color, the glass, as analyzed, consisting essentially of 40-200 ppm chromium oxide in a soda lime silicate base glass that contains as impurities no more than about 400 ppm MnO 2  and 500 ppm Fe 2  O 3 .

Description:
This application claims the benefit of U.S. Provisional Application No. 60/013,658 filed Mar. 19, 1996, entitled LIGHT GREEN GLASSWARE, by W. Duane Amundson, Jr. 
     FIELD OF THE INVENTION 
     Transparent, colored glasses having a soda lime silicate base, and glassware produced therefrom. 
     BACKGROUND OF THE INVENTION 
     Glass ovenware was introduced commercially about 1915 under the trademark PYREX. This glassware was dear, water white, and molded from a borosilicate glass. 
     Subsequently, use of glassware became wide spread for the preparation and serving of food. As this occurred, a desire for glassware having color developed. 
     One method of meeting this need was with opal ware. This glassware is rendered opaque by development of light scattering particles dispersed in the glass. Opal ware was used as such, was colored by colorants added to the glass batch, or was given a colored coating, for example, an enamel applied to the surface of the ware. The latter procedure was of particular interest because it permitted either solid color or patterns. 
     The development of opal ware led to a need for increased mechanical strength as well as heat resistance. It was found that this need could be met by tempering the opal ware. Tempering is a procedure of glass treatment in which the surface of a glass article is placed in compression, thereby increasing the force necessary to induce fracture. Numerous methods have been described and used to temper glass articles. These include thermal treatment by chilling from an elevated temperature, and chemical treatment by ion exchange. 
     The tempering treatment can also be employed to strengthen, and render thermally resistant, transparent glassware, either plain or colored, produced with a soda lime silicate base glass. This glass base has a significantly less expensive batch, and is easier to melt than the borosilicate glasses originally employed. 
     U.S. Pat. No. 5,204,293 (Amundson, Jr. et al.) describes a family of transparent, soda lime silicate glasses having a burgundy color. The glass compositions consist essentially of 0.3-2.2 percent by weight manganese oxide in a soda lime silicate base glass. The glass has impurity levels for NiO and Fe 2  O 3  not exceeding 100 parts per million (ppm) and 500 ppm, respectively. 
     The present invention arose in the course of developing a set of transparent, solid-color bowls having such light colors as to be termed blush colors. The color intensity is so low that the color is barely visible in a thin wall section. The color becomes more like a highlight being visible in thicker rim and foot areas, as well as through an edge. The invention is concerned directly with a member of the set having a light green color. 
     A primary purpose of the invention then is to provide a transparent, soda lime silicate glass embodying an acceptable light green color and a bowl molded therefrom. Another purpose is to provide a complete line of such glassware for use in preparation and serving of food. A further purpose is to provide such glassware that can be strengthened and rendered heat resistant by tempering. Another purpose is to provide a combination of colorants that can be melted as part of a glass batch, or can be incorporated in an uncolored base glass by a color cell procedure. Such a procedure involves adding colorant to a glass as the molten glass passes through a forehearth. 
     SUMMARY OF THE INVENTION 
     The invention resides in part in a transparent glass exhibiting a light green color and having a composition, as analyzed, consisting essentially of 40-200 ppm chromium oxide (Cr 2  O 3 ) in a soda lime silicate base glass, the glass containing as impurities no more than about 400 ppm MnO 2  and 500 ppm Fe 2  O 3 . 
     The invention further resides in a tempered glass article that is useful in the preparation and serving of food, and that is molded from the glass just described. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The single FIGURE in the accompanying drawing is a graphical representation of the invention in terms of x and y color coordinates. 
    
    
     PRIOR ART 
     Literature of possible relevance is set forth in an accompanying document. 
     DESCRIPTION OF THE INVENTION 
     A key feature of the present invention resides in discovery of certain glass colorant combinations. These colorant combinations provide a relatively specific, light green color in a soda lime silicate base glass. 
     The term &#34;soda lime silicate glass&#34; is generally understood in the glass art to mean glasses having compositions consisting essentially of 70-75% SiO 2 , 5-15% Na 2  O and 5-15% CaO. Minor amounts of other oxides may optionally be present, such as alumina as a stabilizer and antimony as a fining agent. 
     The single FIGURE in the accompanying drawing is a graphical representation defining, in terms of x and y color coordinates, the color feature of the invention. The color coordinates refer to, and are based on, Illuminant C in accordance with the CIE color coordinate system. The x coordinates are plotted on the horizontal axis. The y coordinates are plotted on the vertical axis. 
     The polygon ABCDA encompasses coordinate values that represent the area within which a green color can be obtained that is suitable for present purposes. x and y coordinate values for the points A, B, C and D are as follows: 
     
         ______________________________________        x     y______________________________________A              0.3118  0.3196B              0.3136  0.3255C              0.3116  0.3255D              0.3112  0.3196______________________________________ 
    
     A smaller area was established as a target, or preferred, area, for introduction of colorant by a color cell procedure. This preferred area is designated as AEFG in the drawing and has color coordinates as follows: 
     x=0.3115-0.3118 
     y=0.3196-0.3223 
     CAP Y=87.7-88.9. 
     The desired light green color is obtained by incorporating chromic oxide in the glass in an amount of 40-200 parts per million (ppm). Lesser amounts fail to provide an adequately perceptive color. Amounts over about 200 ppm impart a color that is too intense. 
     The light green color of the invention may be obtained in a reasonably pure form. However, care must be taken to either avoid, or carefully limit, the presence of other colorants that may produce either an off-color or a grayish tint. In order to use cullet from a glass containing manganese oxide, it has been found that this oxide may be tolerated in amounts up to about 400 ppm. However, it is preferred to keep the content below 350 ppm since larger amounts tend to have an undesirable effect on the color. Nickel oxide (NiO) should be kept below about 30 ppm, to avoid undesirable color effects. 
     Iron oxide is almost inevitably present as an impurity. A small amount, for example on the order of 250 ppm, is useful for heat retention in the glass melt during the melting operation. However, this oxide should not exceed 500 ppm, and preferably is maintained under 400 ppm. 
     SPECIFIC EMBODIMENTS 
     A commercially melted, water white, soda lime silicate glass was selected as a base glass for development purposes. A typical analysis of this glass, in approximate weight percent, is: 
     
         ______________________________________SiO.sub.2    75.5         CaO        9.5Na.sub.2 O    12.9         Al.sub.2 O.sub.3                            1.7K.sub.2 O    0.4          Sb.sub.2 O.sub.3                            0.02Li.sub.2 O    0.02         Fe.sub.2 O.sub.3                            290 ppm.______________________________________ 
    
     Antimony oxide functions as a fining agent and alumina as a stabilizer. K 2  O, Li 2  O and Fe 2  O 3  are present only because they occur in soda and silica raw materials. 
     Batches were prepared based on this commercial glass composition. Various amounts of colorants were added to the batches to provide a range of glass colors for establishing color feasibility and color selection. Melts were made in silica crucibles in a gas-fired furnace that was held at 1500° C. for 4 hours. The melts were poured in molds to form patties for test purposes. 
     It is apparent that both colorant retention and the color obtained will be dependent on furnace conditions. Thus, the amount of colorant required may depend on whether it is included in a batch that is tank melted, or whether it is added by the color cell procedure. Also, the oxidation state of a glass, which is influenced by the atmosphere, may affect the state of the colorant as well as the amount. In general, reducing conditions may be detrimental to certain furnace components. Accordingly, we prefer to maintain neutral or mildly oxidizing conditions. A strongly oxidizing atmosphere may alter the oxidation state of the glass and make it difficult to achieve a given color. 
     TABLE I sets forth in parts per million (ppm) the colorant amounts batched in several melts, together with the color coordinates measured on samples of the batch melts. The colorants added were Cr 2  O 3  alone, or in combination with MnO 2 . The color coordinates were measured in terms of the CIE system. 
     
                       TABLE I______________________________________ Cr.sub.2 O.sub.3           MnO.sub.2Ex.   (ppm)     (ppm)   x       y     Cap Y______________________________________1     100       350     0.3120  0.3214                                 88.92     150       350     0.3129  0.3239                                 87.13     200       350     0.3131  0.3257                                 86.84     100       700     0.3118  0.3211                                 89.45     150       700     0.3148  0.3253                                 83.56     200       700     0.3131  0.3254                                 86.87     100       --      0.3113  0.3209                                 89.98     150       --      0.3119  0.3232                                 88.99     200       --      0.3126  0.3255                                 87.5______________________________________ 
    
     Subsequently, with these crucible melts as a guide, a trial run was carried out on glass from a commercial unit melting the water white base glass. Colorant was added in increasing amounts to the glass in a forehearth via a color cell system. In this procedure, a granulated, sintered color concentrate was added to the glass melt in the forehearth. Samples were taken periodically and 4 mm thick test pieces were prepared for measurement of x and y coordinates as well as colorant contents. 
     On the basis of the color samples thus produced, a set of limits for preferred analyzed colorant content and x, y and CAP Y coordinates was established. These were: 
     
                       TABLE II______________________________________   Cr.sub.2 O.sub.3 (ppm)           x         y       Cap Y______________________________________Light     43        0.3115    0.3196                               88.9Preferred 73        0.3116    0.3211                               88.2Dark      87        0.3118    0.3223                               87.7______________________________________