Method of coating glass or ceramics with a colored frit

This invention is directed to glazes exhibiting a strong yellow-to-brown coloration or a bright red color which are stable at temperatures of 700.degree.-950.degree. C., the latter glazes being stable in non-oxidizing or inert environments. Both glazes utilize frits consisting essentially, in weight percent on the oxide basis as calculated from the batch, of ______________________________________ SiO.sub.2 35-47 B.sub.2 O.sub.3 5.5-9 BaO 24-42 TiO.sub.2 1.5-4 ZrO.sub.2 6-10 Li.sub.2 O 1-5 MgO 0-5 CaO 0-4 SrO 0-8 ZnO 0-10 Bi.sub.2 O.sub.3 0-8 SrO + MgO + CaO + ZnO + Bi.sub.2 O.sub.3 0-10 ______________________________________ The yellow-to-brown coloration is produced through the addition of 0.5-10% by weight Fe.sub.2 O.sub.3 and 0-4% TiO.sub.2. The red coloration is produced through the addition of 2-10% cadmium sulfoselenide.

BACKGROUND OF THE INVENTION 
The decorating of glass and ceramic material has been a developing art over 
the space of hundreds of years. One important mode of such decoration has 
involved the use of glazes. Glazes are, in essence, vitreous or glassy 
coatings which not only improve the aesthetic appearance of the articles 
but also, in the case of ceramic bodies, provide a non-porous, impermeable 
surface coating therefor. Since the sale of many glass, ceramic, and, more 
recently, glass-ceramic products and, in particular, those products 
designed for culinary ware and tableware is highly subject to the visual 
appearance thereof, research has been continuous to develop ever more 
attractive surface finishes for such products. In common parlance, glazes 
are considered to be clear glasses, colored glazes are regarded as clear 
glasses with colorants dissolved therein, and enamels are deemed to be 
glazes having pigments suspended therein to impart color thereto. Both 
glazes and enamels are commonly applied to the surface of a glass, 
glass-ceramic, or ceramic body in the form of very finely-divided 
particles or powder, known to the art as "frit", suspended in a volatile 
vehicle to provide a paste or slurry. The frit is thereafter fired to 
first remove the vehicle and then at a higher temperature to fuse the 
powder to form a strongly-adherent, continuous film on the surface of the 
body. 
Colored glasses have been the subject of extensive research with every 
color of the visible spectrum being produced. Coloured Glasses, W. W. 
Weyl, Dawson's of Pall Mall, London, 1959, is a monograph devoted to a 
study of the effects of various cations and anions in developing colors in 
glasses. The interactions between ions to form colors are essentially 
limitless and are still under investigation even today. 
The use of iron with or without the inclusion of titanium to obtain 
yellow-to-brown colorations in various glass compositions, especially 
those glasses containing lead, has been practiced for many years. Weyl, 
supra, devotes a number of pages to such phenomena, e.g., page 108 ff and 
page 212 ff. Likewise, the development of a bright red coloration in 
glasses (ruby glasses) via the inclusion of cadmium sulfoselenide in the 
composition is well known to the art. Inasmuch as glazes are essentially 
glasses applied as a thin surface coating on a substrate, pigments 
containing iron and, optionally, titanium have been formulated and sold 
commercially for additions to frit compositions, as have pigments 
containing cadmium sulfoselenide. 
To be suitable as a glaze, the frit therefor must possess certain physical 
characteristics among which are: (1) the firing or maturing temperature 
thereof, i.e., the temperature at which the frit will flow sufficiently to 
produce a smooth uniform coating on a substrate, must be sufficiently low 
to prevent thermal deformation of the ware being coated; and (2) the 
coefficient of thermal expansion thereof must be compatible with that of 
the substrate to inhibit crazing and/or spalling of the resultant glaze. 
In the preferred practice, the coefficient of thermal expansion of the 
frit will be somewhat less than that of the substrate since that 
circumstance places the coating under compressive stress which, in turn, 
enhances the mechanical strength of the final product. 
The most prevalent, commerically-marketed glazes for decorating glass, 
glass-ceramic, and ceramic articles have included substantial levels of 
lead oxide (PbO) therein and, in some instances, cadmium oxide (CdO). 
Those ingredients have comprised part of the frit compositions for two 
primary purposes: first, to soften the frit, i.e., to reduce the melting 
point thereof sufficiently to permit its fusion onto the surface of ware 
without causing thermal deformation of the ware; and, second, to raise the 
refractive index of the glaze. CdO has also been utilized as a colorant in 
some frits. Unfortunately, however, lead and cadmium are extremely toxic 
such that glazes and enamels containing those metals which are to be used 
in contact with foods must demonstrate superior resistance to attack by 
acidic and alkaline materials to preclude the release of any significant 
amount of those metals. Hence, the Food and Drug Administration (FDA) has 
promulgated standards for cadmium and lead release which must not be 
exceeded by surfaces which come into contact with food. 
In general, the release of lead and cadmium from glazes, or, stated 
differently, the chemical durability of the glazes, is a function of the 
firing temperature employed in the glazing operation. Thus, as a general 
rule, the higher the firing temperature, the lower will be the level of 
lead and cadmium release. However, when frits were prepared of 
lead-containing compositions for coloring with iron+titanium pigments and 
fired to temperatures in excess of 700.degree. C., the desired strong 
yellow-to-brown coloration was not developed. Instead, only a washed-out, 
pale yellow coloration was produced. In like manner, the firing of 
lead-containing frits with conventional cadmium sulfoselenide pigments at 
high temperatures did not yield the desired, customary bright red 
coloration but only dull gray hues. 
Lead-containing glazes commonly exhibit high coefficients of thermal 
expansion, viz., higher than about 85.times.10.sup.-7 /.degree.C. and, 
frequently, in excess of 90.times.10.sup.-7 /.degree.C. Recent 
developments, particularly in the field of glass-ceramics, have led to the 
production of ware suitable for food service applications having 
coefficients of thermal expansion less than those of the lead-containing 
glazes. Such products are not compatible with the conventional glazes, 
Moreover, as noted previously, glazes free from lead would be desirable 
for surfaces which contact food. 
U.S. application Ser. No. 121,745, filed concurrently herewith by the 
present applicant under the title Lead-Free and Cadmium-Free Frits, now 
U.S. Pat. No. 4,282,035 is directed to compositions essentially free from 
lead and cadmium and consist essentially, by weight, of 
______________________________________ 
SiO.sub.2 35-47 
B.sub.2 O.sub.3 5.5-9 
BaO 24-42 
TiO.sub.2 1.5-4 
ZrO.sub.2 6-10 
Li.sub.2 O 1-5 
MgO 0-5 
CaO 0-4 
SrO 0-8 
ZnO 0-10 
Bi.sub.2 O.sub.3 0-8 
SrO + MgO + CaO + ZnO + Bi.sub.2 O.sub.3 
0-10 
______________________________________ 
Those frits demonstrate relatively low coefficients of thermal expansion 
(20.degree.-300.degree. C.) of about 65-75.times.10.sup.-7 /.degree.C. and 
can be fired onto glass, glass-ceramic, and ceramic substrates at 
temperatures between about 700.degree.-950.degree. C. The compositions 
manifest excellent resistance to attack by acids and bases and, where 
employed as glazed surfaces for use in food service applications, to 
attack by detergents utilized in commercial dishwashers. The presence of 
BaO, TiO.sub.2, and ZrO.sub.2 imparts a high refractive index to the 
frits, thereby providing a glaze exhibiting a high gloss. Various 
compatible metal oxides may be added, if desired, to modify the properties 
thereof. For example, although contrary to the overall purpose of the 
frits, CdO can be included to raise the refractive index even higher. 
Their lower coefficients of thermal expansion, when compared with 
conventional leadcontaining frits, have recommended their utility as 
glazes for the above-mentioned, lower expansion substrate materials. 
However, the higher temperatures required for maturing such glazes has 
required an investigation into pigments suitable for imparting colorations 
thereto. 
SUMMARY OF THE INVENTION 
I have discovered that strong thermally-stable, yellow-to-brown colors, 
most commonly orange-to-brownish red colors, can be developed with such 
frits through additions thereto of 0.5-10% by weight of Fe.sub.2 O.sub.3 
and 0-4% by weight of TiO.sub.2. Furthermore, a bright red color can be 
produced with such frits when mixed with 2-10% by weight cadmium 
sulfoselenide and fired in a non-oxidizing or inert atmosphere.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A frit exhibiting a coefficient of thermal expansion 
(20.degree.-300.degree. C.) of about 66.4.times.10.sup.-7 /.degree.C. was 
prepared having the following approximate composition, expressed in weight 
percent on the oxide basis as calculated from the batch, of 
______________________________________ 
BaO 28.57 
MgO 2.50 
Li.sub.2 O 
3.35 
SiO.sub.2 
46.19 
B.sub.2 O.sub.3 
7.33 
ZrO.sub.2 
9.30 
TiO.sub.2 
2.76 
______________________________________ 
and comminuted to pass a No. 325 United States Standard Sieve (44 microns). 
The resultant powder was blended with 1% by weight Fe.sub.2 O.sub.3 and 2% 
by weight of TiO.sub.2 in powder form, silk screened onto dinner plates of 
PYROCERAM.RTM. ware, a glass-ceramic tableware marketed by Corning Glass 
Works, Corning, New York, having a coefficient of thermal expansion 
(0.degree.-300.degree. C.) of about 97.times.10.sup.-7 /.degree.C., and 
the coated plates fired at 750.degree. C. for 15 minutes. The glaze 
exhibited high gloss and a strong brownish red color. 
Where a conventional lead-based frit was combined with Fe.sub.2 O.sub.3 and 
TiO.sub.2 in like amounts to the above illustrative example and fired onto 
dinnerware plates of PYROCERAM.RTM. ware under the same conditions, only a 
very pale yellow hue was observed. 
A frit exhibiting a coefficient of thermal expansion 
(20.degree.-300.degree. C.) of about 66.8.times.10.sup.-7 /.degree.C. was 
prepared having the following approximate composition, expressed in weight 
percent on the oxide basis as calculated from the batch, of 
______________________________________ 
BaO 33.86 
SiO.sub.2 
40.80 
Li.sub.2 O 
1.31 
ZnO 4.37 
B.sub.2 O.sub.3 
6.47 
ZrO.sub.2 
8.22 
TiO.sub.2 
2.44 
CdO 2.53 
______________________________________ 
and comminuted to pass a No. 325 U.S. Standard Sieve (44 microns). The fine 
particles were then mixed with 5% by weight of a powdered commercial 
cadmium sulfoselenide pigment, silk screened onto dinner plates of 
PYROCERAM.RTM. ware, and the coated plates fired at 850.degree. C. for 15 
minutes in a nitrogen atmosphere. The resulting glaze displayed high gloss 
and a bright red color. 
Firing in other non-oxidizing or inert atmospheres, e.g., argon and helium, 
or slightly reducing environments, e.g., forming gas (hydrogen+nitrogen) 
and mixtures of CO and CO.sub.2, will also yield bright red colors. Yet, 
when the glazing is undertaken in air or other oxidizing atmosphere, the 
red coloration is not thermally stable with only dull gray hues being 
produced. 
A lead-containing frit, an alkali borosilicate frit, and a zinc 
borosilicate frit were each blended with 5% by weight of the cadmium 
sulfoselenide pigment and fired onto PYROCERAM.RTM. brand dinner plates in 
a nitrogen atmosphere in like manner to that described above. Only dull 
gray colors resulted evidencing the uniqueness of the frits disclosed in 
Ser. No. 121,745, supra, for the present inventive purposes.