US2495837A

US2495837A - Enameled articles

Info

Publication number: US2495837A
Application number: US90049A
Authority: US
Inventors: Frank R Porter
Original assignee: GEORGE N HIBBEN
Current assignee: GEORGE N HIBBEN
Priority date: 1949-04-27
Filing date: 1949-04-27
Publication date: 1950-01-31
Anticipated expiration: 1967-01-31

Description

Patented Jan. 31, 1 950 Frank R.. Porter Crown Point, Ind., .assig,nor to. GeorgeN. Hibben, Chicago, Ill'.,.as trustee.

N'o Drawl'ng; -Application:AzprileZfl, 1949,; Serial No. 90,0.49

This application is a continuationein-part ofcopending application Serial. No. 591,603, filed June 4,1945.

This. invention relates to light-colored. enameled'e steel articles and. more: particularly to a means of eliminatin the: use of the customary dark-colored ground coat in enameling, articles. of this character.

In. order to obtain: on. a; steel oriron article. a: smooth uniformly adhering, coat of. light-coleorect porcelain or vitreous enamel, it has. been the. practice in. the enameling trade: for many years. to. apply or fusezto the steel. article. a dark-colored ground coat containing; an oxide such. as cobalt oxide, nickel oxide or manganese. oxide. Over the darkecolored ground coat one or more finishing or cover coats of the desired light-colored enamel were, then applied. and; separately fused. In most instances. because. of, the. in'.-.- herent dark. color of the ground coat, sit has been considered necessary. to. employ: several cover coats of light-colored, enamelih order to' achieve a satisfactory light-coloredfinishg By, the. term light-colored. I mean white. or pastel shades.

The. conventionalv ground coat is dark or blue: black. in color and. is usually fused to thesteel'. base at temperatures of the'order. of from about. 1580? F1. to about I6Q0 F. The dark color of' the ground. coat is due to thepresence of the cobaItfnickel. or manganese oxides which possess the'property of' promoting adherence of the cover coats to. the steel base. The-cover coats are separately fused over the ground coat at temperatures which may range from about 1 1500" F; toabout 1600 F.,'dependent upon the. composition and properties of the enamel. The cover coats may, of course, be of any color. but most of the two or three coat articles, for example, parts of stoves or refrigeratorsareflnished in white or pastel shades.

, The purpose of the dark-colored ground coat istwofold: (l) to secure satisfactory adherenceof the cover coats, and (2) to eliminate or minimize in the finished article such objectionable surface defects as blistering; pitting, blackspecking, and fish-scaling.

According to widely accepted theory, the ad. herence of vitreous enamel to steel is accomplished through an iron oxide layer or film formed on the steel surface and under the enamel coating. Apparently the iron oxide dissolves in both the steel and enamel at enamel fusing tem-. peratures. This iron oxide may begin to form factory unitorm. adherence.

assoon. as. the, enamel is: applied to. he; S eel surface. because of. the moisture, in the. enamel.

However, the quantity of iron, oxide, so formed;

orrplain. carbon, steellis. not suificient for sail longbeen knownt'opromote porcelain .1.. vitreous. enamel adherence and it is believed; that, this.

property is, due. to its, relative, unstability; i. e'.,

it releases its oxy en comparatively easi1y,, par.-

, ticularly at. enameling temperatures and when,

in,...the..presence of. some. element. or compound which. will. unite. with. oxygento form. a more stable oxide. which in this. case; is, irorr. When;

the. cobalt has given. up. its. oxygen. to. the. steel.

at enamelfusing.temperatures, it then combines.

with. oxygen from. the atmosphereand. is; ready again to furnish oxygen to the steel; Thecobalt.

therefore. serves as. an. oxygen, carrier. or, in. e1.- fect,.,as acatalyst. Nickel and, manganese oxides. arev known. tov behave in 'a. similar manner..., In, accordance with. the foregoing explanation it will. be. evident. that the ground, coat. constitutes the medium whereby sati'siaetory bond obtention.

is. realized, between. the, steelhase. andlthe finish or cover coatsoff enamel..

It has. also been. established. that one; ofjthe. principal. causes of. surface defects; in. enameled;

articles. particularly blistering. pitting andv blacksspecking, is the. formation and. evolution of. gaseous carbon. products which are. released; from the steel base during. firin oi the enamel; coats, These. gaseous carbon. products. apparently result from the reaction of the. carbon. in the. steelwithmetallic oxides in the enamel or withhydjrogen which is. sometimes formed at enameli'ng. temperatures by thejreduction 'Qfwater,

contained. in the enamel The surface defect.

known as fish-scaling is considered to be the result of hydrogen escaping from the enameled metal upon cooling from enameli'ng tempera,-

tures to room temperature; The hydrogen whichmay be formed during enameling dissolves to a substantial extent in thesteeli at elevated" tem-. peratures; and as the enameled metal. coolsga. supersaturated condition exists. with the result; that free hydrogen is released from solution, in the steel. The'hydrogen'thus released-tends; to create excessive pressures under the enamel coating which force the enamel off the steellin' fish sca'le shaped pieces. "The 'applicationiof a ground coat. to the'base steel tends to 'trap'the various gaseousproducts which may form and thereby minimizes the magnitude and frequency. of surface defects of the type described}.

Cobalt. oxide has;

3 However, the use of the ground coat does not completely eliminate surface defects of the above type and the enameling trade has long been faced with the annoying problem of rejecting and reprocessing a substantial proportion of the found that the use of a ground coat introduces another type of surface defect known as reboiling. In other words, during the firing of the outer cover coats, the underlying and previously applied ground coat has a tendency to soften and boil thereby disturbing the adherence and smooth appearance of the cover coats. In order to. overcome the reboiling problem, it has been necessary to choose enamel compositions for the cover coats which can be fired at temperatures appreciably lower than the softening temperature of the ground coat.

For the past twenty years or more extensive efforts have been made and considerable experimentation carried out by ceramic engineers and others skilled in the enameling arts with the object in view of obtaining a light-colored enameled steel. article which is free of undesirable surface defects without the necessity of using the conventional dark-colored ground coat. Moreover, it has been the hope of these investigators that the elimination of the dark-colored ground coat would thereby permit the production of enameled steel articles with only a, single light-colored coat of enamel thereby greatly reducing production costs both from the point of view of materials used and the elimination of one or more process steps. It has been found that articles having only a single coat of enamel are highly resistant to chipping and crazing of' the enamel surface as compared with multi-coate d articles having two or three or even a greater number of cover coats. Certain of the prior art workers have attempted to solve the problem by devising special compositions for the enamel frit or by employing particular techniques in the preparation of the enamel slip. Others in the prior art have resorted to nickel fiash or nickel dip techniques for the purpose of improving bond obtention. However, none of these proposals has provided a satisfactory solution to the problem of surface defects in enameled articles, and the enameling trade has. consistently employed the conventional dark-colored ground coat even whenspecial enamel compositions or nickeling techniques have also been used. 7 g

Accordingly, the principal object of my invention is to provide light-colored enameled steel articles which can be prepared without the use of the'conventional dark-colored'ground coat and with only a single light-colored enamel coat, which articles are substantially free of objectionable surface defects.

, Broadly, I accomplish this object by utilizing a killed steel base having alloyed therewith a sufficient quantity of zirconium, columbium, or vanadium to chemically combine with and stabilizethe carbon present in the steel. These metals posses's'the common chemical character'- 4 istic of forming relatively stable carbides. By thus tyingup or stabilizing the carbon content of the steel, the formation of gaseous carbon products is inhibited with the result that surface defects of the type hereinbefore described are eliminated.

It is important in the practice of my invention that a killed or substantially completely deoxidized steel be employed. Deoxidation may be accomplished by the use of well known deoxidizing agents such as aluminum, titanium, and the like. If the base steel contains oxygen to any appreciable extent, the zirconium, columbium, or vanadium added as an alloying and carbon stabilizing constituent will tend to react preferentially with the oxygen content of the steel and thereby fail to react completely with the carbon. The steels employed in my invention should have carbon contents as low as consistent with good steelmaking practice since it will be obvious that the lower the carbon content the less alloying metal will be required. In general, the carbon content of the steel should not be in excess of about 0.15%. In addition to carbon and the alloying metal, the steels may contain the usual amounts of the other common ingredients such as manganese, phosphorus, sulfur, and silicon.

The killed steel may be rolled into strips, sheets, or plates and the desired articles formed therefrom by methods well known in the art. The steel articles are then cleaned by any desired method which will insure a proper surface for adherence of the enamel coat, and a light-colored enamel is then applied directly to the steel base and fused thereto by firing at an appropriate temperature. As hereinafter described in greater detail, the present invention is directed particularly to the use of a light-colored enamel containing an adherence-promoting oxide selected from the group consisting of the oxides of antimony, molybdenum, and arsenic. Preferably, the adherence-promoting oxide is incorporated as part of the enamel frit.

If desired, the steel article may be subjected to :a nickeling process, by methods well known in the art, whereby to deposit upon the steel a nickel film which has the effect of assisting bond obtention and also of lowering the requisite firing temperature for light-colored enamels of the abovementioned type. Such a nickel deposit may comprise from about .02 gram to about .08 gram of nickel per square foot of surface although greater amounts may also be used, if desired. The use of a nickel treatment prior to the application of the enamel coat to the steel article permits the firing temperature of the enamel coat to be reduced by as much as 40 F. to 50 F. below the usual firing temperature required for enamels of this type. Thus, in the case where it is desired to employ the nickeling pretreatment, the steel base comprises the nickeled steel article, and the lightcolored enamel coat is applied directly to the base without the conventional dark-colored ground coat.

As mentioned above, the alloying metal, i. e. zirconium, columbium, or vanadium, is employed in sufficient quantity to combine chemically with the carbon content of the particular steel being used. On the basis of experimental investigation, it has been determined that in the case of zirconium and colurnbium the weight ratio of alloying metal to carbon should be within the range of from about 7:1 to about 15:1. In the case of vanadium, the weight ratio of vanadium to 'cara sess? :bon should be withinmthe range oftfromwabout 8:1 to about 1 18:1. Thet-upperzvlimits: onithe amounts of alloying metal are determined primarily by economic considerations, but for all .practical purposes the above-indicated maximum quantities will suffice. Furthermorepbythepse of the alloying metal in the above amounts, sufii- Jcient Zirconium, columbium, or vanadium ispro- -vided to compensate for the usual processlosses inzexcess of the stoichiometric quantitiesrequired, rand also such amounts will be sufficient t0 ..com- -.bine with any free hydrogen which maybe formed incidentally duringfiring of the enamel coatings as previously described.

I, have found by extended experimentsthat @there is an excellent correlation between the amount of alloying metal needed to eliminate surface defects during subsequent enameling and the:amount necessary to eliminate the yield point ofthe steel as indicated by the conventional,

:stress-strain curves. Accordingly, the yield point :ItBSt affords a highly convenient physical tech- :nique for determining the appropriate amount of alloying metal for stabilizing the carbon content of any given steel. Thus, in order to practice my invention, the amount of alloying metal to be added to a given quantity of steel may be ascertained by either of two methods:

(1) The carbon content or the steel maybe determined quantitatively by chemical or metallurgical methods and the quantity of alloying metal then calculated by selecting a Weight ratio of alloying metal to carbon Within the ranges disclosed above; or

(2) A series of sample steels may be prepared containing difierent known quantities of alloying metal and the samples then tested for yield point by conventional test methods. The samples in which the yield point has been substantially eliminated contain suflicient alloying metal ,to

- stabilize the carbon content and consequently any steel of corresponding composition will enamel satisfactorily without the conventional ground coat.

It will thus be seen that in practicing my invention the quantity of alloying metal to be added to a batch of steel in any given instance may. be;determined in advance by either purelychemical or by essentially physical methods.

Although the advantages of enameling. steels 50 containing zirconium, columbium or vanadium. in the amounts herein disclosed are obtained-with a wide variety of enamel compositions, my invention is particularly directed to the use of lightcolored enamel compositions containing as part of ..the' frit an adherence-promoting oxide selected :from the group consisting of the oxides of anti- :.mony, molybdenum, and arsenic. Thus, the .in-

: vention contemplates the use of a steel base con- 1 taining any or" the aforementioned alloying metals (zirconium, columbium, vanadium) :in combina- .tion with an enamel coat containinganyrofithe --previously mentioned adherence-promoting oxides (antimony, molybdenum, and arsenic oxides).

In order to further illustrate the invention, the following specific examples are presented, but it will be understood that thescop ofthe invention is by no means limited to-the details of these-car amples.

EXAMPLEI A zirconium steel having a weight ratio of ezzirconium to carbonof 7.7 :1:-' was,-:enamel'ed;- dieu'ectly, i-. e. without-an .interveninggroundsooat,

using aswhitezenamel containing. antimony oxide as the adherence-promoting;oxide.

,flha analysis of the steel was as follows (the ;balance being iron):

Specimens of thissteel were hot-rolledto one- ,-.quarter 3inch-plates and-samples were cut for enameling. Thespecimens were cleaned thoroug-hly, pickled; dried, and then enameled. The

enamel frit had .the following composition:

Weight percent SbzOs 6.45 SiOz 42.22 A1203 1.80 AlF3 10.00 C'aFz 7.15

ZnO 3.15 'JNaO 12,11 ,K'zO 2.35 "B 14.48

Undetermined portion 0.29

The enamel slip was prepared by combining the following mill additions:

,, Frit(having-above'analysis) grams 3,400 -qoiay do 238 Commercial opacifier do 68 Sodium nitrite do 4.25 Water cc 1,400

The composition of the commercial opacifier re- "..ferred to above was as follows: ZrOz 79.21 SiO-Z 14.20 %','Na'2O 2.23%,2120 25%, Geo 2.0 3%, F'ezOz 1.03%, A1203 .63%, and undetermined portion The enamel was applied to the surface of the "steel samples'by'spraying, the thickness of the "sprayed coating varying from .006 inch to .010 "inch-Thespray'ed enamel coating was dried and fired for approximately six minutes at about 1560" F.

The samples resulting from this treatment were free of blisters, black-specks, pin-holes, and fishscaling, and the enamel had an excellent bond, 5 i: e. a high" degree of adherence to the steel sur- "face.

EXAMPLE II Following substantially the same procedure as described above inconnection with Example I, a Fzirconium-containing steel having a zirconium to carbon ratio of approximately 9.6:1 was enameled with a somewhat similar white enamel con- ;tainingantimony oxide as: the adherence-promotwingoxide.

The analysis of the steel was as follows (rewmainder .being iron plus sulfur, phosphorus, and

other impuritiesrin the *usual amounts) Weight per cent Carbon .05 Manganese .21 Silic on. 1.62 ;-;Aluminum .026

nmzirconium p.48

enamel frit was sub- An enamel slip was prepared using the above same proportions as disclosed in connection with Example I. The enamel coating was applied directly to the steel without the use of an intervening ground coat. Excellent adherence was obtained with no objectionable surface defects.

EXAMPLE III In place of the zirconium-containing steels such frit and other mill additions in substantially the all?) as those described in Examples I and II, my inven- I tion also contemplates the use of steels containing columbium in similar amounts. Typical steel compositions which I' have found will enamel satisfactorily without the use of an intervening ground coat are as follows:

Sample No l 2 3 Carbon (weight percent) .05 .07 .09 Manganese .28 .32 34 Phosphorus .011 ll .ili'i Sulfur I 026 028 017' Silicon .04 .06 .07 Oolumbium. .37 .01 1.20 Aluminum 011 084 .015 Cb/C (weight ratio) 7. 4 8. 7 13.4

Any of the light-colored enamels containing antimony,

EXAMPLE IV In addition to the zirconiumand columbiumcontaining steels, the invention also embraces the use of vanadium-containing steels of which the following steel analyses are typical of those which. have been found to enamel satisfactorily with only a single light-colored enamel coat applied directly to the steel base:

If; (i

typical molybdenum oxide enamel frit analysis and a slip composition using this frit:

Frit

Weight Percent Sodium oxide, NazO 12.04

Potassium oxide, K 3.58 Barium oxide, BaO 23.40 Alumina, A1203 3.83 Boric oxide, B203 8.87 Silica, S102 30.33 Antimony oxide, Sb203 2.90 Calcium fluoride, CaFz 9.95 Molybdenum oxide, M003 5.10

Mill additions Paris Frit 100.00 Clay 6.00 Sodium nitrite 0.25 O-pacifier 4.00 Water 45.00 Other typical molybdenum oxide enamels which may be employed are as follows, the ingredients being given on the basis of the smelter batch percentage used in producing the irit:

s it B t 11, w 1 m Ingredient me f 0 g Borax (hydrous) 10. 0 20. 0 20. s 20. 5 eldspnr 10. 0 11. 5 22.2 10. 3 Quartz 13.1 17.4 10. 0 12.6 oda Ash .3 8.7 Sodium nitrate .3 3. 5 3. 0 Fiuorspar .8 6.4 4.1 Cryolite 4. 4 4.9 Galcspar 3.5 Barium carbonate... l7. 2 Zincoxidemnuu- 1.4 Molybdic oxide. 4.1 Antimony trioxid 2. 0 Titanium dioxide 1.4 100.0 100.0 100.0 100.0

EXAMPLE VI In place of the molybdenum oxide and antimony oxide enamels hereinbefore disclosed, arsenic oxide-containing enamels may also be employed in practicing the invention. A typical arsenic oxide enamel frit and a typical slip composition are as follows: a

i k Frit Sample-No. i 1 5 i 0 7 8 I i 1 Weight Percent Carbon(weigiitpercent).\ .05 .10 .04 .05 4 sodium oxlde Nazo 17373 %ihang:i1le se I Potassium oxide, K20 4.80 sufiiijiffi1131:111 183i .032 i031 1058 022% Alumina, A1203 Silicon... .0 5 .00 255 W Boric oxide, B203 23.60 :m. .-i- "I \l il l' l l i i l uln .0021 .001 .014 .02 .015 slhcai S102 38-22 e (weight ratio) 8.0 1 11.7 I 12.0 15.0 17.0 Arsenic oxide, AS203 2,42 1 i Calcium fluoride, CaFz 8.03

The foregoing steels may be enameled successfully without the use of a ground coat by loo-00 v means of any of the light-colored enamels herein Mm additions disclosed containing antimony, molybdenum or arsenic oxides as part of the frit. Part3 Frit 100.00 EXAMPLE v Clay 7.00 In Examples I and II, typical antimony-con- 352mg taining enamel compositions were given. In Water place of such antimony enamels, molybdenum o trioxide enamels may also be employed with other typical arsenic-containing enamel frits equally satisfactory results. The following is a which may be used are as follows, the ingredients being reported onztheibasisvof the smelter batch p rcentages employed in forming theirits:

Ingredient:

Borax Quartz 'Oryolita: 6.7 Arsenic oxide l 4. 6 4. 9

'Iin oxide 9. White lead It will be seen that my invention provides a means of eliminating the conventional dark-colored ground coat from enameled steel articles. This result is accomplished by the combination of a particular steel base with a preferred class of light-colored enamels. The presence of zirconium, columbium, or vanadium in the disclosed amounts results in substantially complete chemical combination of the carbon present in the steel to form stable carbides, and in addition, any free hydrogen formed during enameling may react with the alloying metal to form the corresponding hydride. Thus, the formation and release of gaseous products during the enameling step is avoided and the cause of such surface defects as blistering, pitting, black-specking and fish-scaling is eliminated.

The elimination of the conventional dark-colored ground coat offers a number of important advantages. Obviously, the process becomes more economical as a result of the saving in material and also as a result of the elimination of a process step. Moreover, the conventional ground coat is fired at a relatively high temperature whereas the cover coats are usually fired at somewhat lower temperatures. By eliminating the application of the ground coat altogether, I avoid subjecting the enameled articles to excessively high temperatures thereby minimizing sagging and warping of the articles during the firing cycle. In addition. with the dark-colored ground coat omitted, I am able to employ a relatively thin enamel coat which is much more resistant to crazing and chipping than the thick multi-coat wear of the prior art.

I claim:

1. A light-colored enameled steel article comprisin 1) a steel base comprising a killed steel containing alloyed therewith a metal selected from the group consisting of zirconium. columbium. and vanadium. the carbon content of said steel being not in excess of about 0.15% and the wei ht ratio of alloying metal to carbon present in the steel being from about 8:1 to about 18:1 in the case of vanadium and from about 7:1 to about :1 in the case of zirconium and columbium. whereby to provide suflicient alloying metal to eliminate the yield point of the steel: and. (2) a coat of light-colored vitreous enamel applied directly to said base and containing as part of the frit a light-colored adherence promoting oxide selected from the group consisting of the oxides of antimony, molybdenum, and arsenic.

2. A light-colored enameled steel article comprising: (1) a steel base comprising 2. killed steel containing alloyed therewith a metal selected rom the group consisting of zirconium, columbium, and vanadium, the carbon content of said steel being not in excess of about 0.15% and the weight ratio of alloying metal to carbon present in the 1 steel-being .from about 8:1 -to about 18: 1 -:in the case of vanadium and from about 7:1 to

about 15:1 in the case ofzirconium and columbium; and (2) a coat of light-colored vitreous enamel applied directly to said base and con *tainingas part of the frit a light-colored adherence promotingoxide selected from the group consisting of the .oxides .of antimony, molybdenum, and arsenic.

-3. A light-:colored enameled steel article comprising: 1) a steel base comprising a killed steel containing vanadium alloyed therewith, the carbon content of said steel being not in excess of about 0.15% and the weight ratio of vanadium to carbon present in the steel being from about 8:1 to about 18:1; and (2) a coat of light-colored vitreous enamel applied directly to said base and containing as part of the frit a light-colored adherence promoting oxide selected from the group consisting of the oxides of antimony, molybdenum, and arsenic.

4. A light-colored enameled steel article comprising: 1) a steel base comprising a killed steel containing zirconium alloyed therewith, the carbon content of said steel being not in excess of about 0.15% and the weight ratio of zirconium to carbon present in the steel being from about 7:1 to about 15:1; and (2) a coat of light-colored vitreous enamel applied directly to said base and containing as part of the frit a lightcolored adherence promoting oxide selected from the group consisting of the oxides of antimony, molybdenum, and arsenic.

5. A light-colored enameled steel article comprising: 1) a steel base comprising a killed steel containing columbium alloyed therewith, the carbon content of said steel being not in excess of about 0.15% and the weight ratio of columbium to carbon present in the steel being from about 7:1 to about 15:1; and (2) a coat of lightcolored vitreous enamel applied directly to said base and containing as part of the frit a lightcolored adherence promoting oxide selected from the group consisting of the oxides of antimony, molybdenum, and arsenic.

6. A light-colored enameled steel article comprising: (1 )a steel base comprising a killed steel containing alloyed therewith a metal selected from the group consisting of zirconium, columbium, and vanadium, the carbon content of said steel being not in excess of about 0.15% and the weight ratio of alloying metal to carbon present in the steel being from about 8:1 to about 18:1 in the case of vanadium and from about 7:1 to about 15:1 in the case of zirconium and columbium; and (2) a coat of light-colored vitreous enamel applied directly to said base and containing antimony oxide as part of the frit.

7. A light-colored enameled steel article comprising: (l) a steel base comprising a killed steel containing alloyed therewith a metal se lected from the group consisting of zirconium, columbium, and vanadium, the carbon content of said steel being not in excess of about 0.15% and the weight ratio of alloying metal to carbon present in the steel being from about 8:1 to about 18:1 in the case of vanadium and from about 7:1 to about 15:1 in the case of zirconium and columbium; and (2) a coat of light-colored vitreous enamel applied directly to said base and containing molybdenum oxide as part of the frit.

8. Alight-colored enameled steel article comprising: (l) a steel base comprising a killed steel containing alloyed therewith a metal selected 5 from the group consisting of zirconium, columin the case of vanadium and from about 7:1 to.

about 15:1 in the case of zirconium and columbium; and (2) a coat of light-colored vitreous enamel applied directly to said base and containing arsenic oxide as part of the frit.

FRANK R. PORTER.

REFERENCES CITED The following references are of record in the file of this patent:

5 UNITED STATES PATENTS Number Name Date 1,360,317 Rieser Nov. 30, 1920 2,396,856 King Mar. 19, 1946 2,414,633 Bryant Jan. 21, 1947

Claims

1. A LIGHT-COLORED ENAMELED STEEL ARTICLE COMPRISING: (1) A STEEL BASE COMPRISING A KILLED STEEL CONTAINING ALLOYED THEREWITH A METAL SELECTED FROM THE GROUP CONSISTING OF ZIRCONIUM, COLUMBIUM, AND VANDIUM, THE CARBON CONTENT OF SAID STEEL BEING NOT IN EXCESS OF ABOUT 0.15% AND THE WEIGHT RATIO OF ALLOYING METAL TO CARBON PRESENT IN THE STEEL BEING FROM ABOUT 8:1 TO ABOUT 18:1 IN THE CASE OF VANADIUM AND FROM ABOUT 7:1 TO ABOUT 15:1 IN THE CASE OF ZIRCONIUM AND COLUMBIUM, WHEREBY TO PROVIDE SUFFICIENT ALLOYING METAL TO ELIMINATE THE YIELD POINT OF THE STEEL; AND (2) A COAT OF LIGHT-COLORED VITREOUS ENAMEL APPLIED DIRECTLY TO SAID BASE AND CONTAINING AS PART OF THE FRIT A LIGHT-COLORED ADHERENCE PROMOTING OXIDE SELECTED FROM THE GROUP CONSISTING OF THE OXIDES OF ANTIMONY, MOLYBDENUM, AND ARSENIC.