Light Blue Glassware

This application claims the benefit of U.S. Provisional Application No. 60/012,417 filed Feb. 28, 1996, entitled LIGHT BLUE GLASSWARE, by W. Duane Amundson, Jr.

Transparent, colored glasses having a soda lime silicate base, and glassware produced therefrom.

Glass ovenware was introduced commercially about 1915 under the trademark PYREX. This glassware was clear, 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 (Aroundson, 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.sub.2 O.sub.3 not exceeding 100 parts per million (ppm) and 500 ppm, respectively.

The present invention arose in the course of developing a set of three 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 blue color.

A primary purpose of the invention then is to provide a transparent, soda time silicate glass embodying an acceptable light blue 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.

The invention resides in part in a transparent glass exhibiting a light blue color and having a composition, as analyzed, consisting essentially of 100-250 ppm copper oxide in conjunction with 5-15 ppm cobalt oxide, calculated as Co.sub.3 O.sub.4, in a soda lime silicate base glass, the glass containing as impurities no more than 500 ppm MnO.sub.2 and 500 ppm Fe.sub.2 O.sub.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.

The single FIGURE in the accompanying drawing is a graphical representation of the invention in terms of x and y color coordinates.

Literature of possible relevance is set forth in an accompanying document.

A key feature of the present invention resides in discovery of a certain glass colorant combination. This colorant combination provides a relatively specific, light blue color in a soda lime silicate base glass.

The term "soda lime silicate glass" is generally understood in the glass art to mean glasses having compositions consisting essentially of 70-75% SiO.sub.2, 5-15% Na.sub.2 O and 5-15% CaO. Minor mounts 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 rectangle ABCDA encompasses coordinate values that represent the area within which a blue 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:

A smaller area was established as a target, or preferred area, for introduction of a colorant by a color cell procedure. This preferred area is designated as EFGH in the drawing and has color coordinates as follows:

The desired light blue color is obtained only with a combination of 100-250 ppm copper oxide (CuO) and 5-15 ppm cobalt oxide (Co.sub.3 O.sub.4) as glass colorants. Neither oxide alone produces an acceptable light blue color. It was observed that cobalt oxide is the more sensitive of the two colorants. Thus, within the indicated limits, the cobalt oxide has the dominant effect on color. Hence, it must be more closely monitored in order to obtain a consistent color in ware.

The light blue color of the invention may be obtained in a reasonably pure form with the colorant combination of copper and cobalt oxides. However, care must be taken to observe the indicated composition limits. Thus, an excess of cobalt oxide tends to push the color toward a deep blue. Likewise, too much copper oxide tends to produce a green cast in the glass article.

Care must also 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 500 ppm. However, it is preferred to keep the content below 400 ppm since larger amounts tend to have an undesirable effect on the color. Chrome (Cr.sub.2 O.sub.3) and molybdenum oxide (MoO.sub.3) should each be kept below about 10 ppm, while 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.

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:

Antimony oxide functions as a fining agent and alumina as a stabilizer. K.sub.2 O, Li.sub.2 O and Fe.sub.2 O.sub.3 are present only because they occur in soda and silica raw materials.

Batches were prepared based on this commercial glass composition. Various mounts of colorants were added to the batches to provide a range of glass colors for establishing color feasibility and color selection. Crucible melts were made in silica crucibles in a gas-fired furnace that was held at 1500.degree. C. for 4 hours. The melts were poured in molds to form patties for test purposes.

TABLE I sets forth in parts per million (ppm) the colorant amounts hatched in several melts, together with the color coordinates measured on samples of the batch melts. The colorants added were copper oxide alone, or in combination with cobalt oxide. The color coordinates were measured in terms of the CIE system.

Subsequently, with these crucible melts as a guide, a trial run was carded 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: