This invention is directed to a glass frit composition essentially free from lead and cadmium exhibiting a linear coefficient of thermal expansion (25.degree. C.-300.degree. C.) between about 75-85.times.10.sup.-7 /.degree.C., good flow at temperatures between about 950.degree.-1000.degree. C., a softening point between about 650.degree.-725.degree. C., and excellent resistance to attack by alkaline detergent and hydrochloric acid, said frit consisting essentially, expressed in terms of weight percent on the oxide basis (except for the fluorine content), of ______________________________________ SiO.sub.2 49-55 MgO 0-2 B.sub.2 O.sub.3 13-20 CaO 1-7 Al.sub.2 O.sub.3 4.5-7 SrO 0-8 Na.sub.2 O 5-8.5 BaO 0-11 K.sub.2 O 5-8.5 ZnO 0-2 Na.sub.2 O + K.sub.2 O 11-16 F 0.25-3. ______________________________________

BACKGROUND OF THE INVENTION 
The use of glazes to decorate glasses and ceramics has its roots in 
antiquity. In essence, glazes are transparent glasses which may be colored 
or clear which are applied to the surfaces of articles to provide 
decorative designs and finishes thereto and so enhance the aesthetic 
appearance thereof. Customarily, glazes are applied to the surface of an 
article in the form of a slurry or slip of finely-divided glass particles, 
termed "frit" in the art, and, after drying the slip, the frit is fired at 
a temperature and for a time sufficient to cause the particles to fuse and 
flow over the surface to form a continuous glassy coating thereon. 
The most widely marketed commercial frits have contained high 
concentrations of lead oxide (PbO) and, less frequently, substantial 
amounts of cadmium oxide (CdO). Both of these oxides exhibit two 
characteristics which render them especially desirable for inclusion in 
glazing frits. First, they lower the melting point of the glass, thereby 
enabling it to be fused to flow along the surface of the base body to be 
coated at a temperature which is sufficiently low to forestall thermal 
deformation of the base body. Second, they raise the refractive index of 
the frit, thereby increasing the gloss displayed by the glaze. CdO has 
also been used as a colorant in certain frits. Nevertheless, because both 
CdO and PbO are highly toxic, very stringent regulations have been 
promulgated by the Federal Food and Drug Administration strictly limiting 
their release when compositions containing those compounds come into 
contact with comestibles. 
Pressure from both the public and private sectors has been continuous and 
in increasing volume to totally eliminate lead and cadmium from materials 
which come into contact with food. Therefore, numerous research programs 
have been initiated in recent years to develop lead- and cadmium-free 
frits suitable for glazing articles of glass, glass-ceramic, and ceramic. 
To perform satisfactorily as a glaze, a frit must satisfy at least the four 
following basic criteria: 
(a) the frit must demonstrate good glass stability; i.e., the frit must not 
devitrify during the firing of the frit to fuse the glass particles into a 
flowing mass to coat the surface of the article; 
(b) the frit must exhibit excellent resistance to attack by acids and bases 
to avoid corrosion of the glaze resulting in loss of gloss, the generation 
of haze and/or iridescence, the development of porosity, or other defects 
deleterious to the appearance and/or physical character of the glaze; 
(c) the firing or maturing temperature of the frit, i.e., the temperature 
at which the frit demonstrates sufficient flow to yield a smooth 
homogeneous coating, must be low enough to avert thermal deformation of 
the article being coated; and 
(d) the linear coefficient of thermal expansion of the frit must be 
compatible with that of the article being glazed in order to avoid crazing 
and/or spalling, with the preferred frits exhibiting a linear coefficient 
of thermal expansion somewhat lower than that of the article being glazed, 
thereby placing the matured coating under compressive stress when the 
glazed article is cooled to room temperature. 
Corning Incorporated, Corning, N.Y., markets a glass-ceramic dinnerware 
product as Corning Code 0308 under the trademark SUPREMA.RTM.. That 
product is encompassed within U.S. Pat. No. 4,608,348 (Beall et al.) in 
that it contains potassium fluorrichterite as the predominant crystal 
phase with cristobalite as a secondary phase in an amount of about 10-20% 
by volume and consists essentially, expressed in terms of parts by weight 
on the oxide basis, of about 
______________________________________ 
SiO.sub.2 
66.8 K.sub.2 O 
4.75 Sb.sub.2 O.sub.3 
&lt;0.1 
Al.sub.2 O.sub.3 
1.7 Na.sub.2 O 
3.3 NiO 0.014 
MgO 14.5 Li.sub.2 O 
0.8 Co.sub.3 O.sub.4 
0.0015 
CaO 4.4 P.sub.2 O.sub.5 
1.15 Fe.sub.2 O.sub.3 
0.02. 
BaO 0.21 F 3.74 
______________________________________ 
Because the sum of the recited components closely approximates 100, for all 
practical purposes the individual values may be deemed to represent weight 
percent. The precursor glasses for the dinnerware are crystallized in situ 
to the glass-ceramic state through heat treatment at temperatures of about 
950.degree.-1000.degree. C., with the glass-ceramic exhibiting a linear 
coefficient of thermal expansion over the temperature range of 
25.degree.-300.degree. C. between about 120-140.times.10.sup.-7 
/.degree.C. 
U.S. Pat. No. 5,204,291 (Nigrin) discloses lead- and cadmium-free glazes 
expressly designed for coating SUPREMA.RTM. dinnerware. The frits therefor 
consisted essentially, expressed in terms of weight percent on the oxide 
basis, of 
______________________________________ 
SiO.sub.2 51-59 CaO 0-7 
Li.sub.2 O 0-2 SrO 0-12 
Na.sub.2 O 3.5-7 BaO 0-9 
K.sub.2 O 6-8.5 ZnO 0-10 
Li.sub.2 O+Na.sub.2 O+K.sub.2 O 
10-15 CaO+SrO+BaO+ZnO 8-18 
B.sub.2 O.sub.3 
9-12 Al.sub.2 O.sub.3 
4.5-7. 
______________________________________ 
The frits demonstrated linear coefficients of thermal expansion over the 
temperature range of 25.degree.-300.degree. C. between about 
70-85.times.10.sup.-7 /.degree.C. and good flow at temperatures between 
about 900.degree.-1000.degree. C. The preferred glaze compositions 
consisted essentially of 
______________________________________ 
Li.sub.2 O 
1-2 SrO 2.5-6.5 
Na.sub.2 O 
4-7 B.sub.2 O.sub.3 
10-11.5 
K.sub.2 O 
6.5-8 Al.sub.2 O.sub.3 
5-6.5 
ZnO 7-9.5 SiO.sub.2 
55.5-58.5. 
______________________________________ 
Whereas those glazes generally exhibited excellent resistance to attack by 
acids and bases, good gloss at the firing temperatures, and good 
compatibility in thermal expansion with the SUPREMA.RTM. ware, they 
interacted with the SUPREMA.RTM. ware surface during glaze maturing, 
thereby producing a crystalline interlayer with very large crystals, those 
crystals comprising mainly magnesium, potassium, zinc, and silicon oxides. 
The crystals exhibit a roedderite structure and, consequently, demonstrate 
a low linear coefficient of thermal expansion (estimated to be about 
30.times.10.sup.-7 /.degree.C. over the temperature range of 
25.degree.-300.degree. C.). Because those large crystals exhibiting such 
low coefficients of thermal expansion were sandwiched between SUPREMA.RTM. 
ware with a linear coefficient of thermal expansion of about 
120.times.10.sup.-7 /.degree.C. and a glaze with a linear coefficient of 
thermal expansion of about 80.times.10.sup.-7 /.degree.C., stresses are 
created in the interface which can weaken the body/glaze bond lead to 
spalling. 
Laboratory experiments have indicated that roedderite crystals are 
developed only with frits containing more than about 1% by weight ZnO, and 
that Li.sub.2 O acts as a mineralizer in the system. Removal of those two 
components, however, raised the melting temperatures of the frits too high 
to be operable for glazing SUPREME.RTM. dinnerware. 
Accordingly, the primary objective of the present invention was to devise 
CdO- and PbO-free frits which are preferably also free of Li.sub.2 O and 
ZnO suitable for glazing SUPREMA.RTM. dinnerware. 
SUMMARY OF THE INVENTION 
That objective can be achieved with glass frits which are essentially free 
of CdO, Li.sub.2 O, PbO, and ZnO, which exhibit a linear coefficient of 
thermal expansion (25.degree.-300.degree. C.) between about 
75-85.times.10.sup.-7 /.degree.C., which exhibit good flow at 
950.degree.-1000.degree. C., which exhibit a softening point between about 
650.degree.-725.degree. C., and which exhibit excellent resistance to 
attack by acids and detergents consisting essentially, expressed in terms 
of weight percent on the oxide basis (except for the fluorine content), of 
______________________________________ 
SiO.sub.2 49-55 CaO 1-7 
B.sub.2 O.sub.3 
13-20 SrO 0-8 
Al.sub.2 O.sub.3 
4.5-7 BaO 0-11 
Na.sub.2 O 5-8.5 F 0.25-3. 
K.sub.2 O 5-8.5 
Na.sub.2 O+K.sub.2 O 
11-16 
______________________________________ 
As employed herein, the expression "consisting essentially of" renders the 
glass composition open only for the inclusion of unspecified ingredients 
which do not materially affect the basic and novel characteristics of the 
glass. For example, whereas the preferred glass compositions will be 
essentially free of additional constituents, minor amounts, perhaps up to 
2% each, of such oxides as MgO and ZnO, depending upon the remainder of 
the base composition, and up to 0.5% Li.sub.2 O may be present, the 
amounts of the latter two components being sufficiently low to avoid the 
development of roedderite crystals. Also, whereas the above compositions 
yield a transparent glaze, where a colored glaze is desired, colorants 
conventional in the glass art, e.g., CoO, Cr.sub.2 O.sub.3, CuO, Fe.sub.2 
O.sub.3, MnO.sub.2, NiO, V.sub.2 O.sub.5, selenium, and the rare earth 
metal oxides, can be incorporated in customary amounts, commonly up to 
about 2%. In general, however, the total of all such optional inclusions 
will be less than about 5%. As utilized herein, the expression 
"essentially free of" indicates that no substantial amount of a particular 
constituent is intentionally included in the glass composition. 
When the Li.sub.2 O and ZnO were eliminated from the frits of U.S. Pat. No. 
5,204,291, it was necessary to increase the B.sub.2 O.sub.3 content to 13% 
to produce a glaze exhibiting adequate flow at about 
950.degree.-1000.degree. C. to be operable with SUPREMA.RTM. dinnerware. 
Those glasses, however, displayed very poor resistance to acids. The 
addition of fluorine improved this resistance very significantly. 
The more preferred compositions consist essentially, expressed in terms of 
weight percent on the oxide basis, of about 
______________________________________ 
SiO.sub.2 
50-55 Na.sub.2 O+K.sub.2 O 
12-15 
B.sub.2 O.sub.3 
13-16 CaO 1.5-6 
Al.sub.2 O.sub.3 
5-7 SrO 2-8 
Na.sub.2 O 
5.5-7.5 BaO 4-11 
K.sub.2 O 
6-8 F 0.5-2.25. 
______________________________________ 
In addition to U.S. Pat. No. 5,204,291, the following patents are of 
interest in providing background technology in the field of lead- and 
cadmium-free glazes: 
U.S. Pat. No. 4,224,074 (Reade) discloses frits exhibiting linear 
coefficients of thermal expansion (20.degree.-300.degree. C.) between 
about 50-110.times.10.sup.-7 /.degree.C. and a viscosity suitable for 
firing at about 650.degree.-775.degree. C., the frits consisting 
essentially, in weight percent, of 
______________________________________ 
SiO.sub.2 
29- Al.sub.2 0.sub.3 
2- Na.sub.2 O 
4-20 Na.sub.2 O+Li.sub.2 O 
6- 
55 8 24 
B.sub.2 O.sub.3 
7- ZrO.sub.2 
5- Li.sub.2 O 
0-7 F 0.75- 
31 16 4. 
______________________________________ 
The total absence of alkaline earth metal oxides and the high ZrO.sub.2 
content place those frits outside the present inventive composition 
system. 
U.S. Pat. No. 4,493,900 (Nishino et al.) describes enamels devised for 
coating metals consisting essentially, in weight percent, of 
______________________________________ 
SiO.sub.2 
31-39 ZnO 5-20 ZrO.sub.2 0-5 
B.sub.2 O.sub.3 
13-21 Al.sub.2 O.sub.3 
0-5 Al.sub.2 O.sub.3 +TiO.sub.2 +ZrO.sub.2 
2-9 
Na.sub.2 O 
14-21 TiO.sub.2 
0-5 F 2-10. 
K.sub.2 O 
1-5 
______________________________________ 
The total absence of alkaline earth metal oxides and the high 
concentrations of Na.sub.2 O and ZnO place those frits outside of the 
subject inventive composition. 
U.S. Pat. No. 4,590,171 (Nigrin) is directed to frits demonstrating 
softening points between 600.degree.-625.degree. C. and linear 
coefficients of thermal expansion (20.degree.-300.degree. C.) between 
about 57-62.times.10.sup.-7 /.degree.C., the frits consisting essentially, 
in weight percent, of 
______________________________________ 
Li.sub.2 O 
3-4 B.sub.2 O.sub.3 
14-17.5 
ZrO.sub.2 
6.75-10.5 
Na.sub.2 O 
0.75-3 Al.sub.2 O.sub.3 
6.75-8.75 
F 3-4. 
BaO 3.5-9.5 SiO.sub.2 
48-55 
______________________________________ 
The low alkali metal oxide content, the high level of Li.sub.2 O, and the 
high concentration of ZrO.sub.2 place those frits outside of the instant 
inventive compositions.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Table I records a number of glass compositions, expressed in terms of 
weight percent on the oxide basis (except for the fluoride content), 
illustrating the composition parameters of the subject invention. Because 
it is not known with which metal cation(s) the fluorine is combined and 
the quantity thereof is small, it is merely reported as fluorine in 
accordance with conventional glass analysis practice. The actual batch 
ingredients may comprise any materials, either oxides or compounds, which, 
when melted together, will be converted into the desired oxide in the 
proper proportions. For example, CaCO.sub.3 and Na.sub.2 CO.sub.3 can 
constitute the source of CaO and Na.sub.2 O, respectively. The fluorine 
can be conveniently incorporated as an alkali metal or an alkaline earth 
metal fluoride. 
The batch ingredients were compounded, thoroughly blended together to 
assist in obtaining a homogeneous melt, and charged into platinum 
crucibles. The crucibles were introduced into a furnace operating at about 
1400.degree. C. and the batches melted for about 3 hours. Thereafter, one 
portion of each melt was poured into a steel mold to form a glass slab 
having dimensions of about 7.62.times.7.62.times.1.27 cm 
(3".times.3".times.0.5") and that slab was transferred immediately to an 
annealer operating at about 450.degree. C. The remainder of each melt was 
poured as a fine stream into a bath of tap water, a practice termed 
"drigaging" in the glass melting art, to yield finely-divided glass 
particles which, after drying, were further comminuted by milling to 
produce particles averaging about 15 .mu.m in size. 
Whereas the above description is directed to a laboratory melting and 
forming practice, it must be recognized that the glass compositions 
recorded in Table I falling within the ranges of the present invention can 
be melted and formed utilizing conventional commercial glaze smelting 
units with standard equipment and techniques. It is only necessary that 
glass batches of appropriate formulations be prepared, those batches fired 
at a temperature and for a time sufficient to secure homogeneous melts, 
and those melts subsequently cooled and ground to a frit of desired grain 
sizes. 
TABLE I 
______________________________________ 
1 2 3 4 5 6 
______________________________________ 
SiO.sub.2 
54.1 53.5 52.6 52.1 51.1 50.6 
Na.sub.2 O 
6.5 6.4 6.5 6.4 6.5 6.4 
K.sub.2 O 
7.4 7.3 7.4 7.3 7.4 7.3 
CaO 3.7 3.7 1.8 1.8 1.8 1.8 
SrO 3.4 3.4 6.8 6.7 3.4 3.4 
BaO 5.1 5.1 5.1 5.1 10.0 9.9 
B.sub.2 O.sub.3 
14.0 13.9 14.0 13.9 14.0 13.9 
Al.sub.2 O.sub.3 
5.8 5.7 5.8 5.7 5.8 5.7 
F -- 1.0 -- 1.0 -- 1.0 
______________________________________ 
7 8 9 10 11 
______________________________________ 
SiO.sub.2 
52.7 52.1 52.3 51.3 50.3 
Na.sub.2 O 
6.5 6.4 7.3 7.3 7.3 
K.sub.2 O 
7.4 7.3 6.8 6.8 6.8 
CaO 2.4 2.4 5.5 5.5 5.5 
SrO 4.5 4.5 -- -- -- 
BaO 6.7 6.6 -- -- -- 
B.sub.2 O.sub.3 
14.0 13.9 19.5 19.5 19.5 
Al.sub.2 O.sub.3 
5.8 5.7 5.4 5.4 5.4 
ZnO -- -- 1.7 1.7 1.7 
MgO -- -- 1.5 1.5 1.5 
F -- 1.0 -- 1.0 2.0 
______________________________________ 
Bars were cut from the glass slabs for use in determining the linear 
coefficient of thermal expansion (Exp) over the temperature range of 
25.degree.-300.degree. C., expressed in terms of .times.10.sup.-7 
/.degree.C., the softening point (S.P.) expressed in .degree.C., a 
qualitative assessment of resistance to attack by alkaline detergents 
(Deter), a qualitative assessment of resistance to attack by hydrochloric 
acid (Acid), and a qualitative assessment of resistance to attack by 
coffee (Coffee). 
The assessment of resistance to detergent attack involved the following 
procedure: 
(1) a 0.3% by weight aqueous solution of SUPER SOILAX.RTM. detergent, 
marketed by Economics Laboratories, St. Paul, Minn., is prepared; 
(2) that solution is heated to 96.degree. C.; 
(3) samples are immersed into the hot solution; and 
(4) after an immersion of 72 hours, the samples are withdrawn from the 
solution, washed in tap water, dried, and inspected visually to observe 
any change in the gloss of the glaze. 
The assessment of resistance to attack by hydrochloric acid involved the 
following procedure: 
(1) an aqueous 5% by weight HCl solution is prepared; 
(2) samples are immersed into the solution maintained at room temperature; 
and 
(3) after an immersion for 24 hours, the samples are withdrawn from the 
solution, washed in tap water, dried, and inspected visually to observe 
any change in the gloss of the glaze. 
The assessment of the resistance to attack by coffee involved the following 
procedure: 
(1) a solution of percolated coffee is prepared; 
(2) with the solution at 96.degree. C., samples are immersed therein; and 
(3) after an immersion of 24 hours, the samples are withdrawn from the 
solution, washed in tap water, dried, and inspected visually to observe 
any change in the gloss of the glaze. 
In Table II, samples were deemed to fail the test when visual observation 
descried even a minor change in the appearance of the gloss. 
The softening points and linear coefficients of thermal expansion were 
determined employing measuring techniques conventional in the glass art. 
TABLE II 
______________________________________ 
1 2 3 4 5 6 
______________________________________ 
S.P. 703 685 704 678 699 674 
Exp. 81.9 82.5 82.3 83.6 83.9 83.0 
Deter Pass Pass Pass Pass Pass Pass 
Acid Fail Pass Fail Pass Fail Pass 
Coffee Fail Pass Fail Pass Fail Pass 
______________________________________ 
7 8 9 10 11 
______________________________________ 
S.P. 698 681 677 669 659 
Exp 83.0 81.8 76.4 78.1 79.3 
Deter Pass Pass Pass Pass Pass 
Acid Fail Pass Fail Pass Pass 
Coffee Fail Pass Fail Pass Pass 
______________________________________ 
The criticality of composition control is clearly evidenced through a 
comparison of Examples 2, 4, 6, 8, 10, and 11 with Examples 1, 3, 5, 7, 
and 9. 
Example 4 is considered to be the most preferred embodiment of the 
inventive compositions.