US 3742930 A
In the formation of vesicular, matte or semi-matte material useful for imparting continuous cleaning properties to a food cooking oven wherein an admixture of a frit-containing enameling composition and particulate elemental aluminum, magnesium, a high alloy of either, or a mixture of same is fired, increased surface area of said vesicular material can be generated by incorporating into said admixture an additional proportion of copper, manganese, vanadium, iron, nickel, cobalt, chromium, or mixtures of same.
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Description (OCR text may contain errors)
United States Patent [1 1 Ott PROCESS FOR GENERATING VESICULAR MATERIAL HAVING POROUS, SORPTIVE SURFACE STRUCTURE, AND APPARATUS COATED WITH SAME Inventor: Robert E. Ott, Baltimore, Md.
Assignee: SCM Corporation, Cleveland, Ohio Filed: Apr. 8, 1971 Appl. No.: 132,555
Related US. Application Data Continuation-impart of Ser. Nos. 817,128, April 17,
1969, Pat. No. 3,580,733, and Ser. No. 67,996, Aug. 28, 1970.
US. Cl. 126/19 R, 117/129 Int. Cl A2lb 1/00 Field of Search 126/19 R, 273 R;
' References Cited UNITED STATES PATENTS 12/1970 Lee 126/19 R [111 3,742,930 [4 1 Jul '3,1973
3,587,556 6/1971 Moreland 126/19 R Primary Examiner-Edward G. Favors Attorney-Merton H. Douthitt [5 7 ABSTRACT 3 Claims, No Drawings This application is a continuation-in-part of the two below-mentioned patent applications. It relates to an improvement in the ceramic coating technology of copending U. S. Pat. application Ser. No. 817,128 filed Apr. 17, 1969, and now US. Pat. No. 3,580,733 to the discrete pieces of such ceramic material of copending U. S. Pat. application Ser. No. 67,996 filed Aug. 28, 1970, to agglomerates and coatings made from the latter, and to apparatus made with either having continuous cleaning properties, particularly to food cooking apparatus such as ovens having interior surfaces so coated. Such surfaces have been characterized as vesicular with porous, sorptive structure, usually of matte (or mat) finish, and may be referred to herein simply as vesicular for convenient identification purposes.
l-leretofore it has been proposed to make cooking ovens with roughened interior surfaces containing artrecognized oxidation catalyst materials deposited upon, imbedded into, and smelted into a ceramic coating to enhance the continuous cleaning properties thereof. Such surfaces can vary from quite resistant to abrasive action to relatively poorly resistant to abrasive action.
' Typically the oxidation catalyst suggested for such incorporation includes the elemental metals, e.g., ruthenium, palladium, platinum, the oxides, cerates, manganates, manganites, chromates, chromites or vanadates of cobalt, nickel, cerium, ruthenium, palladium or platinum, various iron, copper, nickel or cobait-providing compounds, and smelted-in oxidationinducing oxides such as those of cobalt, manganese,
copper and chromium. The resulting matte to semimatte surfaces usually are dark, and are effective for continuous cleaning by which is meant that, when heated to 350-550F. over an extended period as in normal on-going food cooking operations, there is apparent to the naked eye a diminishing and/or retarding of stains resulting from accumulations of carbonaceous residues thereon which in turn resulted from spattering of grease from the cooking of food. U. S. Pat. representative of this art include: No. 3,266,477; No. 3,460,523; No. 3,513,109; No. 3,536,056; No. 3,547,098; No. 3,549,419 and No. 3,556,077. Pertinent to this art also is the Japanese Kamado pot, made of sorptive, iron-containing Mikawa pottery, heretofore described in connection with Ser. No. 817,128, such pot being used with charcoal heating therein for traditional cooking of foods, e.g., broiling of meats, in the manner of a covered, vented smoke cooker.
The incorporation of oxidation-inducing oxides of the iron subgroup of elements fused into frit for the production of matte, semi-matte, or otherwise rough, porous surfaces of vesicular structure is old in the ceramic art and is shown in U. S. Pat. Nos. 2,053,244 and 2,135,544. Such inventions were directed towards providing primarily heat insulation for non-visible areas in domestic appliances or formation of rough surfaces for making laminated building structural units.
The copending U. S. Pat. applications, Ser. No. 817,128 and Ser. No. 67,996, above mentioned, brought to the art of generating porous, sorptive, continuous cleaning ceramic surfaces the addition of particulate elemental metal selected from the group consisting of aluminum, magnesium, high alloys of either, and mixtures of the same in an enameling composition comprising glass frit. The teachings of these applications are incorporated herein by reference, particularly as to the generation of a dark, matte continuous cleaning surface for continuous cleaning purposes.
Such teachings represented a distinct departure from the previous processes and uses of aluminum metal particulates to make ceramic coatings or refractories for heat resistance and other uses non-analogous to continuous cleaning coatings and surfaces. Relevent U. S. Pat. concerning the use of elemental aluminum and other metals in the formation of such various previous ceramic materials include: No. 2,843,507; No. 2,889,238; No. 2,898,236; No. 2,898,253; No. 2,900,276; No. 2,977,251; No. 3,041,206; No.
3,203,815 and No. 3,271,322. Representative refractory materials and processes of even remoter pertinency, using aluminum metal as a reactant for various purposes, include those described in U. S. Pat. Nos. 1,856,303; 3,248,241; 3,338,995; 3,397,154; 3,473,987; 3,471,413 and 3,492,148.
The instant invention is an improvement in the processes, products and apparatus of U. S. Ser. Nos. 817,128 and 67,996 wherein it already is shown expressly that the iron subgroup metals (iron, nickel and cobalt) as well as other Period IV metals, e.g., vanadium, copper and chromium, typically in the form of metal oxides, frit components, or even in the form of the elemental metal, e.g., alloys, can be used in appreciable proportion in an enameling composition containing frit particles for generating the dark, matte surface having desirable continuous cleaning properties.
Thus, to increase the surface area of the so-generated surface using particulate aluminum, magnesium, high alloys of either, and mixtures of same in an admixture with frit for firing, the instant improvement comprises: incorporating into such admixture, in the form of a mill additive or a component of the frit, an additional proportion of the Period 1V metal selected from the group consisting of copper, manganese, vanadium, iron, nickel, cobalt, chromium, or mixtures of same. Such additional proportion refers to and should be understood clearly as that Period 1V metal (or metals) that is over and above the proportion of the Period IV metals used in a specific formulation made in accordance with the express teachings of Ser. No. 817,128 and Ser. No. 67,996. Thus, if such specific formulation has, e. g., 15 weight percent iron (measured as metal) in it, (basis weight of the solids content of the enameling composition as measured in Ser. No. 817,128-which normally is preponderantly frit) and the new counterpart hereunder has 20 percent on the same, basis, then such additional proportion is 5 percent; similarly, if such specific addition" herein means a material added to an enameling composition other than as a component of the frit in such composition. Preferably and usually such additive or addition is blended in, e.g., simply blunged into a slip. It can be an individual component of the composition, or, if a Period IV metal, such metal alloyed with the aluminum or magnesium. Aluminum and magnesium metal and alloys thereof generally are not ground with frit for safety reasons; other mill additives or mill additions can be and often actually are ground (or milled) with frit in the preparation of the instant enameling compositions. Such additional Period 1V metal, when referred to herein, should be understood to embrace such elemental metal or the elemental metal equivalent of the oxide-providing substance thereof. However, when a percentage of such additional Period 1V metal is stated herein, it should be understood that such percentage is calculated for convenience as the elemental Period 1V metal rather than an oxide of such metal. Additionally, like in Ser. No. 817,128, the aluminum, magnesium, and/or high alloy thereof used herein should be understood for convenient measuring purposes to be one entity, and the enameling composition solids content another, (even though they are mixed to make the composite composition or admixture for firing), and the percentages or fractions of aluminum, magnesium and/or alloys thereof (collectively light metal) called for herein are weight percentages or weight fractions of the non-volatile (on ordinary drying) solids in the enameling composition. Thus, while such light metal materials are mill additions or mill additives, the solids content of the enameling composition is calculated without-water, volatile solvents, and such light metal for convenience, and the light metal material is expressed as weight parts of same per 100 weight parts of these other non-volatile solids contained in such enameling composition.
Where a significant proportion of such Period 1V metal already is incorporated into the smelted frit, or is in admixture with the frit (as, for example, from an alloy of aluminum and/or magnesium added in particulate form, or as other mill additive distinct therefrom), as little as about 3-10 percent additional of such metal, particularly cobalt, based on the weight of enameling solids, can noticeably increase the surface area of the matte surface to be generated by firing. On the other hand, where an extremely minute proportion of or no such Period IV metal is present in the frit (e.g., as a conventional adherence-promoting or coloring oxide) or is in the admixture used for generating by firing the coating or the discrete particles, as much as about 40 weight percent of such Period 1V metal based on the weight of the enameling solids (i.e., exclusive of the actual light metal or metals used) used can be employed. Thus, the total Period 1V metal in the enameling composition can be as much as about 3 or 4 to about 60 percent and generally will be about 5-40 percent of the enameling solids. For efficiency and economy, such additional proportion of Period 1V metal is in the form of an iron oxide-providing substance, advantageously an iron oxide, but also iron metal or a carbonate of iron.
When the additional Period 1V metal is added as a smelted-in-the-frit component'or as a mill addition (including as when it is added as a component of an alloy of aluminum or magnesium) to the enameling composition to give an especially high total content of such metal in the admixture, the additional percentage of such metal in such composition, based on the weight of solids content in the enameling composition, can be as much as about 22 percentage units greater than the maximum fraction taught in Ser. No. 817,128 and at least about 20 percentage units greater than the maximum fraction taught in Ser. No. 67,996; furthermore, the total content of such metal in such special enameling composition then can be from about 40 percent to about 60 percent, based on the weight of the solids in such enameling composition. Ordinarily more Period IV metal can be added as a mill addition than can be conveniently smelted into a frit.
Ser. No. 67,996 teaches that the oxide equivalent content of such Period IV metal smelted into the frit or used as a mill addition with the enameling composition can amount to a maximum value approaching, but being less than, 50 percent by weight of the solids of the enameling composition. Converting such oxide figure gravimetrically to its elemental metal computes on one extreme to about 40 weight percent metal for cupric oxide, on the other extreme to about 28 weight percent metal for vanadium pentoxide, and to about 35 weight percent metal for ferric oxide. The differences between the two extreme values and the aforesaid 60 weight percent maximum total of such metal are 20 weight percent and 32 weight percent, respectively.
Ser. No. 817,128 teaches that such Period 1V metal, added as the metal alloy ingredient with an aluminum and/or magnesium alloy can amount to approaching, but being less than, 50 percent by weight of such alloy. As Ser. No. 817,128 teaches also that such alloy can amount to as much as about weight parts per weight parts of porcelain enameling composition solids as therein called for, the greatest total of such Period 1V metal therein taught to be so added then can be reckoned at about 37.5 weight parts, which as a maximum relative then to the solids of the enameling compositions of Ser. No. 817,128 would amount to about 37.5 weight percent. Even when this value is variously adjusted, as for example, for the customary minute addition of adherence and coloring oxides of Period [V such as cobalt, manganese, and iron, the total amount of the subject Period IV metals at maximum in Ser. No. 817,128 would be slightly less than the maximum weight percentage value for same which can be understood from Ser. No. 67,996. Accordingly, when an admixture for the instant purpose is to be made especially high in total content of the subject Period lV metals, and even richer in such metals than previously has been contemplated in these two foregoing patent applications, one can use up to about 20 weight percent more of the subject Period IV metal or metals in the admixture than previously so contemplated, this amounting to a total of about 40-60 weight percent of such metal or metals in the admixture, either as smelted into the frit, or as a mill additive, or as a mixture from both modes of addition.
The porous, sorptive, matte surface of the ceramic coating or particle thereof has been shown to be fundamental for imparting truly effective, visible continuous cleaning properties to such ceramic; see, for example, Ser. No. 817,128 and Ser. No. 67,996. Such surfaces usually will have about 20-80 percent of voids per volume of the resulting ceramic in the form of coating or particulate (computed from measurements made with a conventional mercury porosimetry method using an Aminco-Winslow Porosimeter). Even higher porosities have been noted in continuous cleaning coatings. The distribution of the pores generally is bimodal with far more very small pores than large ones, the center of the mode when plotted forthe smaller pores being about four microns, and the center of the mode for the larger pores being about 40 microns, with average pore size between about 3 and about 7 microns. This porosity correlates well with surface area, i.e., the higher the porosity, the greater the surface area of the resulting coating or particulate. Such surface areas also have been measured by the conventional Brunauer-Emmett- Teller (B.E.T.) method; in terms of square centimeters per square centimeter of coated metal area they run about 80+ cm /cm Generally an increase in such surface area enhances the continuous cleaning effectiveness of the material. The additional Period lV metal incorporation assists in generating more such effective surface area in such material.
Catalytic effects, e.g., oxidation catalysis, of such ceramic materials, even those with significant proportions of art-recognized oxidation catalyst Period IV metal compounds in such materials, often are much more difficult to assess definitively and to distinguish from simply the effects of the dark, matte surface area generated. Nevertheless, adding known oxidation catalysts to such material logically would be expected to help oxidize carbonaceous soils and vapors in contact with such surface in the presence of hot air, and such expected effect should not be discounted as a possible benefit, along with the spreading and sorption effects of soils, etc. in thin layer on the matte surface, the localization of heat trapped with oxidizing soil, etc. in pores (furnace effect), the enhancement of volatilization, and other likely mechanisms.
The teachings of U. S. Pat. application Ser. No. 817,128 apply here for the generation of the instant improved. vesicular coatings, particularly as to the useful enameling compositions including slips thereof, particle size, finely divided elemental aluminum and magnesium metals and alloys thereof, the blending and mixing procedures, the application of the resulting admixtures to metal substrates, the firing ranges and times for the admixtures, and the test procedures for the resulting coatings. Where the additional proportion of Period IV metal is being added to the improved coating composition in the form of smelted-in to frit material, the frit can be made in accordance with conventional fritmaking practice. Where the additional proportion of Period 1V metal is being added as a distinct mill additive, the frits of Ser. No. 817,128 can be used as such in the admixture. The use of halides in the enameling compositions as taught in Ser. No. 817,128 and Ser. No. 67,996 generally is desirable in most cases, but is not required to practice this improvement.
Similarly, the teachings of U. S. Pat. application Ser. No. 67,996 apply here in the generation of the instant improved vesicular particles, agglomerates, and coatings, particularly as to the useful enameling compositions, particle sizes, the finely divided elemental aluminum and magnesium metals and alloys thereof, the blending and mixing procedures, the generation of the discrete particles, their preparation into shapes, placing into containers or other apparatus, and application of the particles to metal substrates, the firing ranges and times for the admixtures, and the test procedures for the resulting coatings.
For economy and safety aluminum metal is the preferred particulate elemental metal of the aluminum, magnesium, and said highalloy (light metal) group of additives for making the enameling composition for use in this improvement. Preferably it is in paste form as described in Ser. No. 817,128. The proportion of light metal used broadly is from 2-75 parts per parts of solids in the enameling composition. Advanta geously such proportion is from 5-40 parts, and preferably from about l5-35 parts. The additional Period lV metal advantageously is from 5--40 percent by weight of the frit used and generally can be somewhat less, e.g., 5-30 percent with good effect on the surface area.
The following examples show ways in which the improvement has been practiced, but should not be construed as limiting it. In this specification, all temperatures are in degrees Fahrenheit, all percentages are by weight, and all parts are parts by weight, unless other wise specified.
In the preparations tabulated in Examples 1, 2 and 3, the porosity and surface area of the resulting ceramic coatings was substantially increased over those similar ceramic coatings made in the same way, except that no iron, cobalt, nickel, chromium, manganese, vanadium, or copper was added to the slip as a mill additive, nor was any smelted into the frit, except for minute amounts indigenously present in the raw materials or used as an adhesion-promoting oxide (about 1 percent or less) in the frit. Correspondingly, in such exemplary preparations tabulated, the rate of or effectiveness of visual removal of soil (continuous cleaning) appeared to be increased also. It should be noted, additionally, that the applications and firings in this improvement are done in accordance with conventional enameling practice.
EXAMPLE 1 Frit was prepared by smelting the material set forth in Table l at a temperature of 2150 and quenching the melt through water-cooled rolls.
TABLE 1 Silica Soda Ash Fluorspar Zinc Oxide Sodium Silicofiuoride Lithium Carbonate Zirconium Silicate Barium Carbonate Rasorite Rutile Cobalt Oxide Total:
Table 11 gives the calculated oxide composition.
TABLE 11 Percent This frit was ground in a pebble mill until substan tially all of it would pass through a Tyler Standard 325 mesh sieve. The slip of Table 11], below, was prepared by blunging the milled frit with the other ingredients can also be used in the form of a paste consisting of 69% elemental aluminum flake having a fatty acid lubricant thereon and particle size of about 325 mesh in a vehicle of: l3% mineral spirits (preponderantly aliphatic hydrocarbons boilin between 300 and 390), 12% 1- nitropropane, and 6% of an yl aryl sulfonate wetting agent, (e.g., sold by the Aluminum Company of America, under the trademark Hydropaste").
The slip was sprayed onto thin steel sample panels which previously had been cleaned, pickled and treated in conventional manner for receiving porcelain enameling compositions. These panels were fired at l450 for three minutes to yield a hard, finely-porous, sorptive, matte, adherent coating.
The sample panels, thus coated, were soiled by first heating to 350 in a conventional household oven, then lightly brushing on their hot ceramic surface various pronounced stripes of soil, i.e., refined lard, cottenseed, and corn oils, and finally heating for two hours, at 450. Upon cooling, the panels were examined visually and found to have no observable stain on them (in sharp contrast to similarly soiled and treated conventional, glossy, vitreous, porous enamel coatings like those used in many conventional food cooking ovens, which panels retain a heavy, vamish-like deposit).
in a similar preparation and test, except that the iron oxide used was 1 percent of the frit, very good continuous cleanability was observed.
EXAMPLE 2 The frit used was prepared like that of Example 1 from the materials listed in Table IV, and this frit was quenched in cold water.
TABLE IV Parts Potassium Carbonate 180 Rutile 120 Silica 755 Iron Oxide 800 Rasorite 259 Wollastonite 102 Spodumene 254 Total: 2470 2244 parts were recovered, and the frit had the calculated oxide composition listed in Table V.
TABLE V Percent A slip was made like that of Example 1, except that no iron oxide was added as a mill additive to the admixture by blunging, but rather only as a component of the frit. No usual adherence-promoting oxides were added to this frit. Sample plates were prepared using a firing temperature of 1550", then soiled and tested like those of Example l. No residual soil was visible on the cleaned sample plates. Adherence of the coating was good.
Similar results can be obtained when an equivalent amount of iron is added an iron oxide mill. additive instead of being' smelted into the frit.
EXAMPLE 3 A frit was prepared by smelting the material set forth in Table Vl at 2l50 and quenched by pouring it into cool water.
TABLE VI Parts Soda Ash 276 Silica 526 Sodium Nitrate 47 Magnesium Carbonate 34 Sodium Silicofluoride 59 Zirconium Silicate 365 Manganese Dioxide 4 Barium Carbonate 18 Aluminum Oxide 28 Copper Oxide 294 Total: 1651 Table Vll gives the calculated oxide composition for this frit.
TABLE Vll Percent N3 0 l3.l SiO, 43.9 A1 0; 2.l MgO l.0 ZrO, 16.3 BaO 1.0 MnO, 0.2 CuO 21.2 F: 1.2
After smelting, 1495 parts of the frit were recovered. The frit was ground and prepared into a slip like that of Example 1, except that no iron oxide was used as a mill additive. Sample plates were prepared, soiled and cleaned in the manner of Example 1. N 0 soil was visible on the cleaned sample plates, and the coating was observed to adhere well to them.
In similar preparations, V 0 CrO and M 0, can be added in proportions of 2-20 percent, suitably in the form of these oxides as mill additions or combined in the frit, or combined in the frit as a smelt or addition of various ores, fractions thereof, or refined products therefrom, e.g., chromite, vanadinite, nickel-bearing pyrrohotite, etc.
In summation, if conventional glass frit low in Period lV metal oxides is used in this improvement, then the additional Period lV metal is added (preferably as the metal oxide) as a mill additive during preparation of the slip or other bisque coating preparatory to the firing. If the frit is rich in such Period IV metal oxides, no such special mill addition need be made. The frit preferably is milled with water and conventional suspending agents used in making a porcelain enamel. Conventional binders can be used in the bisque mixture. Subsequently the light metal is blunged or otherwise mixed in, as mixed into the slip. The application and firing on conventionally prepared metal substrates is in accordance with the usual porcelain enameling methods. Care should be taken to use properly vented equipment when grinding or otherwise handling the light metalcontaining materials because of the general evolution of hydrogen in aqueous alkaline suspensions and because of the pyrotechnics which can ensue in their firmg.
A typical basic glass frit for generating the improved vesicular material generally will have a calculated oxide composition selected from those oxides listed in following Table Vlll (such frit also can have other wellknown constituents of glass such as alkaline earth metal compounds, if desired), the selection being made so that the oxide composition totals 100 percent.
TABLE VIII Percent ALO, BaO -20 5,0, 0-35 CaO 0-30 K,-, o-25 L50 0-20 MgO 0-20 as 045 sio 8-65 TiO, 0-25 ZnO 0-15 Zi'O 0-20 104 F, 0-10 Period lV metal, i.e., Fe, Co,
Ni, V, Cu, Cr, and/or Mn for porosity development 5-40 If the Period lV metal (or metals) last listed for porosity development are to be used in the admixture solely as a mill additive in the form of the metal or compound(s) thereof, the total of the tabulated first l5 oxides and fluorine will be selected to total 100%. If, however, such Period lV material for porosity development is to be added all or in part as a component of the frit, then such selected percentages of the other frit components will be lowered correspondingly for adjustment of the frit formula to 100%.
feet on the physical properties of the coating, depending on the choice of the Period lV metal-providing material and the proportions to be used-due usually to some solubility of such material, particularly iron or an face structure being the product of firing an admixture of frit-containing enameling composition and a particulate elemental metal selected from the group consisting of aluminum, magnesium, high alloys of either, and mixtures of same, said admixture containing in the form of a mill additive,-or in the form of a smelted-in component of the frit, or in both said forms, for generating increased surface area of said material, a proportion of a Period lV metal selected from the group consisting of copper, manganese, vanadium, iron, cobalt, nickel, chromium, or mixtures of the same, said proportion amounting to about 4 to 40 weight percent of the solids content of said enameling composition and being in addition to any customary Period lV metal which may be contained in appreciable amount in said composition, but constitutingwith said customary Period lV metal not substantially in excess of about 60 weight percent of said solids content.
2. An oven wall panel comprising a metallic substrate having adhered thereon a vesicular material, the surface structure of which is porous and sorptive and has the property of continuous cleaning, said surface structure being the product of firing an admixture of fritcontaining enameling composition and a particulate elemental metal selected from the group consisting'of aluminum, magnesium, high alloys of either, and mixtures of same, said admixture containing in the form of a mill additive, or in the form of a smelted-in component of the frit, or in both said forms, for generating increased surface area of said material, a proportion of Period lV metal selected from the group consisting of copper, manganese, vanadium, iron, cobalt, nickel, chromium, or mixtures of the same, said proportion amounting to about 4 to 40 weight percent of the solids content of said enameling composition and being in addition to any customary Period IV metal which may be contained in appreciable amount in said composition, but constituting with said customary Period IV metal not substantially in excess of about 60 weight percent of said solids content.
iron oxide, in the frit. Hence, adjustment of the frit composition or the slip composition can be required to achieve optimum application and end use properties of the coating. The light metal addition to a slip also can have similar effect, and thus can require some compositional adjustments for such optimization. The final fired product is a porous, sorptive composite of residual metal, crystalline structures derived from various added or generated substances, particularly iron oxide when iron is used, and it is held together by a glassy matrix; this, in combination with the Period lV metals and the light metal and the various resulting products, provides adherence to a metal, e.g., steel, substrate to which said product is directly applied as a coating.
What is claimed is:
1. A food cooking range part comprising a metallic substrate having adhered thereon a vesicular material, the surface structure of which is porous and sorptive and has the property of continuous cleaning, said sur- 3. In a cooking device having means for heating the cooking area, means for supporting food to be cooked, and a wall that is partially enclosing said cooking area, the inner surface of said wall comprising an oven wall panel including a' metallic substrate having adhered thereon a vesicular material, the surface structure of which is porous and sorptive and has the property of continuous cleaning, said surface structure being the product of firing an admixture of frit-containing enameling composition and a particulate elemental metal selected from the group consisting of aluminum, magnesium, high alloys of either, and mixtures of same, said admixture containing in the form of a mill additive, or in the form of a smelted-in component of the frit, or in both said forms, for generating increased surface area of said material, a proportion of a Period lV metal selected from the group consisting of copper, manganese, vanadium, iron, cobalt, nickel, chromium, or mixtures of the same, said proportion amounting to about 4 to 40 weight percent of the solids content of said enameling composition and being in addition to any customary Period lV metal which may be contained in appreciable amount in said composition, but constituting with said customary Period-lVmetal not substantially in excess of about weight percent of said solids content.
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