US2425032A - Enamel for resistors - Google Patents

Description

Aug. 5, 1947.

' A. J. DEYRUP ENAMELS FOR RESISTORS Filed Aug. 24, 1944 Alden (I. DQJI'UP Inventor Aitorne y Patented Aug. 5, 1947 ENAMEL FOR RESISTORS Alden J. Deyrup, Westfield, N. J., assignor to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware Application August 24, 1944, Serial No. 551,041

12 Claims.

This invention relates tothe manufacture of wire-wound electrical resistors, such as are commonly used in radio and other electrical circuits. In particular it relates to the manufacture of that general type of resistor which comprises a base or tube of ceramic material, a winding of resistance wire thereon, suitable clips, clamps, ferrules, tabs, or other metallic fasteners at the ends to secure the ends of the resistance wire and to provide means for establishing electrical contact therewith, and a coating of vitreous enamel applied over the entire unit for the purpose of holding the resistance wire firmly in place, insulating the turns thereof from each other and from out side contact, and protecting the winding against deteriorating influences such as moisture, electrolytes, or atmospheric oxidation. This invention also relates to new enamel compositions particularly suitable for use in insulating the electrical resistance wires in this type of wire-wound electrical resistor.

This is a continuation-in-part of my co-pending application, Serial No. 512,151, filed November 29, 1943.

In preparing electrical resistors of this type, great difiiculties were formerly encountered in manufacturing an enameled unit which would withstand satisfactorily the severe stresses inherent both during its manufacture and during subsequent service conditions. These severe stresses are partly attributable to the fact that the resistor is a power dissipating unit and, as ordinarily used in most electrical circuits, is required to dissipate a considerable amount of heat which is developed in the resistance wire by the flow of electricity therethrough. The heat developed in this way may develop uniformly over a period of time, or it may be very rapidly developed under some circumstances. In all cases it gives rise to mechanical stresses of the severest sort because of the fact that the electrical resistor unites, in one mechanical assembly, a number of elements all of which possess different coefficients of thermal expansion, as well as different heat conductivities. These elements include in the usual wire-wound electrical resistor, the ceramic base, generally tubular in form, the resistor winding, generally formed of some metal such as a nickel-chromium or nickel-copper alloy, terminal tabs, or other connections for establishing electrical communication with the resistor winding, and the vitreous insulating coating. Due to the difierence in expansion coefficients and heat conductivities of these various parts, the stresses developed in the usual type of electrical resistor are sometimes exceptionally severe.

In my copending application S. N. 498,001, filed August 9, 1943, now U. S. Patent No. 2,351,252, Feb. 1, 1944, I have disclosed improved enameled electrical resistor units, and enamel compositions suitable for use in connection therewith,

which enamel compositions possess superior properties for the purpose for which they are designed. By the use of the enamels disclosed in that application for insulating the resistor windings, it is possible to produce complete resistor assemblies which will withstand quenching from an operating temperature of about 275 C. by immerslon in cold water at a temperature of 0 C. without cracking, crazing, or spalling off of the enamel. In preparing resistor assemblies which utilize such enamels, however, it was found generally preferable to utilize as the tubular base upon which the resistance wire is wound a core of steatite porcelain having a thermal expansion coefficient of 9 10 or higher. Unfortunately, steatite porcelains having expansion coefficients of this order are not readily available on the market, except certain steatite porcelains which possess such an objectionably high degree of porosity as to render them unsatisfactory as cores or bases where it is possible that the resistor unit may come into contact with water. When the enamel compositions disclosed in my copending application are utilized in conjunction with readily available nonporous steatite porcelains having linear expansion coefficients falling within the range 6x 10- to '7 X 10* per degree centigrade, the results were generally not very satisfactory, as the resistors so enameled had but limited resistance to thermal shock. Moreover, it was observed that such electrical resistors were superficially attached or etched upon prolonged exposure to water.

It is an object of this invention to provide improved vitreous enamel resistor units of superior ruggedness andremarkable stability to thermal shock and attack upon. exposure to moisture. Another object of this invention is to provide certain new and improved vitreous enamel compositions, which be utilized conjunction with steatite porcelain, magnesium silicate, and magnesium aluminum silicate, ceramic support ing bodies having linear expansion coefiicients within the range Exit) to 7x10" to produce superior electrical resistor units. Still another object of this invention is to provide certain new and improved vitreous namel compositions characterized by mocha... cal toughness, sufficiently high to permit their utilization in conjunction with ceramic supporting bodies of the type described to manufacture superior enameled resistor elements. These, and still further objects of this invention, will be apparent from the ensuing disclosure wherein certain preferred embodiments thereof are described.

My invention is best illustrated with reference to the attached drawing which shows a complete electrical resistor unit comprising a ceramic base; a winding of metallic resistance wire thereon; suitable clips, clamps, ferrules, tabs, or other ma talllc fastenings at the ends of the resistance wire to permit the resistance wire being incorporated in an electrical circuit; and a layer of insulating enamel of the improved type herein described surrounding and insulating all portions of the resistor unit.

Referring more specifically to the drawing, I denotes the ceramic refractory base or supporting body, which is generally tubular in form. This base may be formed of steatite porcelain, magnesium silicate, or magnesium aluminum silicate possessing a linear expansion coefficient within the range 6x10 to 7 10- As previously pointed out, my improved insulating en amel compositions permit the use of such ceramic supporting bodies, which have the advantage of being commercially available, as well as being free from an objectionable degree of porosity.

On the ceramic supporting base, I, there is wound the electrical resistance wire 2, generally in the form of a helix. This wire may be made of any suitable metallic material, it being usual, in practice, to employ nickel-chromium or nickelcopper alloys such as Nichrome or constantan.

The tabs 3 and 4 are provided at each end of the ceramic supporting base, the ends of the re" sistor wire being secured thereto at 5 and 3 by brazing, welding, or other suitable expedient. As shown, the tabs 3 and 4 are provided with apertures 1 and 8 in which electrical conductors may be secured, thereby permitting incorporation or" the electrical resistor in an electrical circuit.

My new and improved insulating enamel com position is, designated by the numeral 9 in the drawing. As shown, it entirely covers the ceramic base I, the resistance wire 2 wound thereon, and the tabs 3 and 4, leaving exposed only the projecting portions of the tabs in which the apertures T and 8 are formed.

In preparing my improved vitreous enamel compositions used to insulate such units, as shown at 9 in the drawing, I first melt together ingredients which will yield fused compositions having the constituents given below, these constituents being present within the percentage ranges speclfied. After ingredients which will yield the desired composition are melted, the fused enamel may be fritted in the usual manner by pouring it into some liquid such as cold water or by pouring the melt on a cool surface. The resulting frit is then ground into a fine state of subdivision, to getber with added materials, to make a slip of suitable working properties.

In preparing an electrical resistor element utilizing my improved Vitreous enamels for insulating the various parts thereof, I first wind the electrical resistance wire upon a tube or support of smooth shape, this support being a ceramic fractory having a linear expansion coefficient within the range 6 10 to 7 lO Tabs, fcrrules, or other connections per-mittinp, incorporation in an electrical circuit are then provided, as shown at 3 and 3 in the drawing. The entire as sembly is then dipped or otherwise coated with the previously prepared enamel-containing slip in order to leave a coating on the outer surface of the resistor unit which, when fired, will insulate the various parts thereof. The coating is then dried and fired at a temperature within the range 650 C. to 750 C. to produce a smooth, tightly adhering, insulating enamel coating over the ex terior parts of the resistor, this coating being designated by the numeral 9 in the drawing.

I have found that enamel compositions containing the oxides of lead, zinc, boron, aluminum, and silicon provide superior properties as regards resistance to crazing or other thermal shock i"ail-- ures, provided that these oxides are present in the enamel composition within certain relatively narrowly defined proportions. These proportions are given in the following table:

It has been found that the combined use of the oxides of aluminum and zinc in enamel compositions of the type given above imparts great mechanical toughness to the resulting enameled products. It is indeed surprising that enamels containing these constituents adhere both to the ceramic base having an expansion coefiicient within the range 6 1()- to 7 10 as well as to the metallic resistance wire, such as constantan, which has an expansion coefficient corresponding approximately to l4 10- even when the resulting resistor units are subjected to severe thermal stresses. As indicated above, in addition to the oxides of aluminum and zinc, my improved enamel compositions will contain the oxides of lead, boron, and silicon. In some cases they may also contain cobalt oxide, C00, in amounts ranging from 0.5% to 3.0%, although the inclusion of cobalt oxide is not essential to the practice of my invention. All percentages given are by weight, based on the total weight of the enamel.

As examples of various batch compositions w ich may be melted to yield enamels having the constituents and relative percentages as given in Table I, the following are illustrative. The various materials present in the batch are thoroughly mixed and melted until substantially homogene. ous, glassy melts are secured.

Batch Constituent A B C D E F G H Read Lead (Pbzoi) 23.8 32.4 28.6 23.8 24.1 30. 5 31.0 15.9 Zinc Oxide (2110)--.. 11.8 5.7 9. 5 11.9 12.0 7.6 7.8 19.8 Boric Acid (H;BO 35. 6 34. 5 34. 5 35. 7 36.3 34. 5 35.1 35. 7 Flint (S101) 8.0 10.2 10.2 7.9 8.0 10.2 10.6 7.9 K801i!) (39.5% A1205; 46.5% $102) 19. 2 15. 6 15. 6 16.1 16. 1 15. 6 16. 5 15.9 Cobalt Oxide (C0304) 1.6 1.6 1.6 1.5 1.6 l. 1.6 Sodium Zirconium Silicate (14% N820; 29% SiO 65% ZrO 3. 3. 2 Ammonium Phosphate (NE4H2P04) All percentages are by Weight, based on the total weight of the batch.

When melted the batch compositions given in Table II yield enamel compositions having the constituents given below in the percentages specified:

All percentages are by weight, based on the total Weight of the enamel.

My improved vitreous enamels of the novel composition specified are prepared in the manner usually employed in preparing vitreous enamels. This involves mechanically mixing the powdered batch constituents, and melting the batch in a suitable furnace to produce uniform fluid glass of viscosity sufliciently low to permit of its being poured. The melting temperature may vary somewhat, but will generally fall within the range 1000 to 1300 C. Melting is continued for a sufliciently long time to bring the melt to a condition of uniformity. After melting, the fused composition is conveniently poured in a thin stream into water. The resulting frit, or granular product, is thereafter reduced in particle size by grinding, e. g., milling in a ball mill or in any other convenient apparatus,

The ground enamel frit may then be incorporated into a slip with water, preferably with the addition of suitably chosen dispersing and setting-up agents and temporary binders. Such materials may include lead formate, ammonium formate, ammonium hydroxide, water-soluble gums, clay, bentonite, and/or ethylene glycol. It is also preferable to add to the frit either during or after grinding the same from 5 to 50 per cent of lead oxide (PhD), and from 2 to 7 per cent of silica (S102), in the form of flint, these percent ages based on the total dry weight of the resulting enamel. These additions are valuable in securing the maximum resistance to thermal stresses in the resulting enamel, and I find that they are more effective than if the same'materials were incorporated in the frit batch before melting. It should be noted that these additions are not shown in Tables II and III as part of the lead oxide and silica percentages given therein. They are, however, taken into consider; ation in the previous statement of the ranges of composition of the finished enamel as given in Table I, since they are present in the finished enamel as part of the lead oxide and silica contents thereof and may be analytically determined therein.

Two examples of enamel slip compositions giving very satisfactory results are as follows:

EXAMPLE I Partsby weight Enamel frit H of Table III 74.3 Litharge 13.4 Lead formate 2.0 Flint 5.4 Sodium nitrite 0.2 Vallendar clay 4.6 Ammonium hydroxide 0.5 Water 39.0

EXAMPLE II Parts by weight Enamel frit H of Table III 47.9 Litharge 36.3 Lead formate 1.3 Flint 3.6 Sodium nitrite 0.1 Vallendar clay 4.4 Ammonium hydroxide 0.5 Water 33.0

After preparing a suitable enamel slip, I wind the proper length and size of resistance wire for the electrical resistor required upon a suitable ceramic base, preferably a tube of steatite ceramic material having a linear thermal ex-, pansion :coefiiclent of from 6 l0- to 'lxlO-t The tube or base is selected as to diameter and length in accordance with the amount of power which it is intended that the finished resistor will be-required to dissipate as heat. In making this selection the usual criteria employed in the electrical resistor art are followed as to the approximate number of square inches of external surface required by the wattage to be handled. The resistance wire 2, wound on the base I, is provided, either before or after winding, with suitable tabs, ferrules, or other connecting members, and the ends of the winding are connected thereto. This connection may be coveniently made by spot-welding or brazing, although of course other means of attachment may be utilized. It is preferable to utilize as the connecting members metals or metal alloys which have linear expansion coeflicients within the range 6X10- to 8X10- Suitable metals for this purpose are available among the nickel-iron alloys. The wound tube is then held in any convenient holder and dipped into the slip, a sufficient quantity adhering thereto to make a satisfactory coating which completely covers the wire and tube. After cleaning the terminals and drying the enamel-containing slip, the unit is placed in a furnace and fired at a temperature within the range 650 C. to 750 C. for a period of time sufficient to mature the enamel properly in place on the tube. One or more coats of insulating enamel may be employed, as desired.

In some cases enamels of diifering composition within the range stated may be used for successive coats. To obtain the maximum resistance to thermal shock combined with corrosive or moist conditions we find, it preferable to conduct the enameling in such a way that turns of the winding do not draw together but rer .in spaced apart, This may be done by ti ht winch ing of the resistor wire, or corrugation or rou hening of the tube surface before winding, or careful control of enamel application and firing, It may be done most advantageously by dipping the resistor in an aqueous suspension of clay or clay and bentonite, followed by drying, before coating with enamel, as disclosed in the co-pending applications of Burton 0, Briclrer S. N. 550,635, and Jason M. Zander S. N. 547,529.

In utilizing specific compositions other than those given in Tables II and III, yet remaining within the general composition range defining the invention insofar as the enamel composi-- tions are concerned, as given in Table I, it is sometimes desirable to adjust the expansion cceflicient of the enamel in order to adopt it for use in connection with a particular ceramic body. This is most readily accomplished by varying the ratio of lead oxide content to zinc oxide content. When ceramic bodies of higher expansion coefficient are utilized, the ratio of lead oxide to zinc oxide may be increased, staying however within the range of percentages given above. Such minor adjustments may be made, within the composition ranges of Table I, without losing the high resistance to thermal shocks which is characteristic of these improved enamels. As above indicated, the enamel may contain other ingredients than those listed in Table I. In departing from the specific compositions given in Table I addition of oxides of the alkali metals and/ or compound of fluorine should be avoided where possible because they increase the expansion coefficient of these enamels to a dangerous degree. Small amounts may be tolerated, as in enamel frits D and H in Tables II III, where sodium oxide was a minor constituent of c an pound containing 2102, which was desired as a melt constituent in these cases, but large amounts of alkali metal oxides or fluorine com-pounds such as are used in enamels for sheet steel or cast iron, are detrimental and defeat the objects of this invention. I'hus, alkali oxides when presout should not exceed 1% of No.20 or its chemically equivalent amount of other alkali oxides. When fluorine compounds are present they should not exceed that amount equivalent to 0.5% by weight of fluorine. The above limiting percentages of No.20 and I. are based on the total weight of the enamel. Vie find there is considerable latitude in choice of the portion of lead oxide and flint in Fable I that can be omitted from the batch composition and subsequently be added to the frit during or after the grinding operation, but it is preferable to incorporate as large a portion by addition to the 8 frit as practical and it is necessary that the total final stoichiometric composition of the enamel as applied to the resistor be as defined in Table I. Complete enameled resistors as described above have been prepared, utilizing windings of 10,000

ohms resistance formed of Nichrome wire of 0.0025

inch diameter. These have been enameled with my improved enamel compositions, and then tested by heating them to 275 0., followed by quenching in water at 0 C. Upon examination it has been impossible to detect any evidence of failure, and the products, even after this severe heat treatment, have been entirely free from cracking, crazing, or spalling off. These enamels are of outstanding value to the art insofar as the fabrication of improved electrical resistors is concerned, since they permit the production of satisfactory enameled electrical resistors, able to stand up under the severe test conditions described, which utilize for the supporting base or tube readily available, non-porous, ceramic compositions. The improved electrical resistors are thus much less likely to fail when used as component parts of communication or other electrical circuits.

Various modifications may be made in the novel enamel compositions as described without departfrom my invention, certain preferred embodiments of which have been given herein as illustrative. Thus, the exact compounds employed as the source of the oxidic materials in my final melted compositions may be varied widely, as by selecting readily available, inexpensive sources. The exact manner of melting the batch constituents may be varied, as desired, provided a homogenous fluid glass melt is secured before fritting. While dipping is a suitable means of applying the enamel to the resistor, it is of course possible to employ any other convenient method. Various alternative sources of the lead oxide preferably added to the frit composition in the mill during grinding, in addition to the litharge given as illustrative, include lead formats, red lead, or any other suitable form of lead oxide. Similarly, other forms of silica, in addition to silica in the form of flint, may be added as mill additions during the grinding or milling of the frit to form the desired slip.

If desired, other oxidic constituents may also be present in my improved enamel compositions. Thus, the oxides of nickel, NiO, and chromium, ClzOs, which materially enhance adhesion of the enamel to resistor alloy wire, may be present in amounts ranging from 1% to 4%. They are not, however, essential and may be omitted if desired. My invention is therefore to be construed in accordance with the appended claims, it being understood that various alternative additions. and procedures may be adopted Without departing from the spirit or scope of my invention.

I claim:

l. A vitreous enamel composition suitable for use in enameling electrical resistors which comprises: lead oxide in amounts ranging from 28 to 55%; silica in amounts ranging from 11 to 32%; zinc oxide in amounts ranging from 5 to 20%; alumina in amounts ranging from 4 to 1.0%; and boric oxide in amounts ranging from to 30%; said enamel containing less than that amount of alkali metal oxide chemically equivalent to 1% sodium oxide and less than that amount of fluorine compound equivalent to .5% of fluorine; said percentages being by weight based-'onthe total weight of said vitreous enamel.

2. A vitreous enamel composition suitable for 9 use in enameling electrical resistors which comprises: lead oxide in amounts ranging from 28 to 55%; silica in amounts ranging from 11,to 32%; zinc oxide in amounts ranging from to 20%;

alumina in amounts ranging from 4 to I boric oxide in amounts ranging from 10 to 30%; and cobalt oxide in amounts ranging from 0.5 to 30%; said enamel containing less than that amount of alkali metal oxide chemically equivalent to 1% sodium oxide and less than that amount of fluorine compound equivalent to .5% of fluorine; said percentages being by weight based on the total weight of said vitreous enamel.

3. A vitreous enamel composition suitable for use in enameling electrical resistors which comprises: lead oxide in amounts ranging from 28 to 55%; silica in amounts ranging from 11 to 32% zinc oxide in amounts ranging from 5 to alumina in amounts ranging from 4 to 10%; boric oxide in amounts ranging from 10 to and nickel oxide; said enamel containing less than that amount of alkali metal oxide chemically equivalent to 1% sodium oxide and less than that amount of fluorine compound equivalent to .5% of fluorine; said percentages being by weight based on the total weight of said vitreous enamel.

4. A vitreous enamel composition suitable for use in enameling electrical resistors which comprises: lead oxide in amounts ranging from 28 to 55%; silica in amounts ranging from 11 to 32%; zinc oxide in amounts ranging from 5 to 20%; alumina in amounts ranging from 4 to 10%; boric oxide in amounts ranging from 10 to 30%; and chromium oxide; said enamel containing less than that amount of alkali metal oxide chemically equivalent to 1% sodium oxide and less than that amount of fluorine compound equivalent to .5% of fluorine; said percentages being by weight based on the total weight of said vitreous enamel.

5. A vitreous enamel composition suitable for use in enameling electrical resistors which comprises: lead oxide in amounts ranging from 28 to 55%; silica in amounts ranging from 11 to 32%; zinc oxide in amounts ranging from 5 to 20%; alumina in amounts ranging from 4 to 10%; boric oxide in amounts ranging from 10 to 30%, nickel oxide; and chromium oxide; said enamel containing less than that amount of alkali metal oxide chemically equivalent to 1% sodium oxide and less than that amount of fluorine compound equivalent to .5% of fluorine; said percentages being by weight based. on the total weight of said vitreous enamel.

6. A vitreous enamel composition suitable for use in enameling electrical resistors which comprises: lead oxide in amounts ranging from 28 to 55%; silica in amounts ranging from 11 to 32%; zinc oxide in amounts ranging from 5 to 20%; alumina in amounts ranging from 4 to 10%; boric oxide in amounts ranging from 10 to 30%; cobalt oxide in amounts ranging from 0.5 to 3.0%; and nickel oxide; said enamel containing less than that amount of alkali metal oxide chemically equivalent to 1% sodium oxide and less than that amount of fluorine compound equivalent to .5% of fluorine; said percentages being by weight based on the total weight of said vitreous enamel.

7. A vitreous enamel composition suitable for use in enameling electrical resistors which com prises: lead oxide in amounts ranging from 28 to 55%; silica in amounts ranging from 11 to 32%: zinc oxide in amounts ranging from 5 to 20%; alumina in amounts ranging from 4 to 10%; boric oxide in amounts ranging from 10 to 30%; cobalt oxide in amounts ranging from 0.5 to 3.0%

and chromium oxide; said enamel containing less than that amount of alkali metal oxide chemically equivalent to 1% sodium oxide and less than that amount of fluorine compound equivalent to .5% of fluorine; said percentages being by weight based on the total weight of said vitreous enamel.

8. A vitreous enamel composition suitable for use in enameling electrical resistors which comprises: lead oxide in amounts ranging from 28 to 55%; silica in amounts ranging from 11 to 32%; zinc oxide in amounts ranging from 5 to 20%; alumina in amounts ranging from 4 to 10%; boric oxide in amounts ranging from 10 to 30%; cobalt oxide in amounts ranging from 0.5 to 3.0%; nickel oxide; and chromium oxide; said enamel containing less than that amount of alkali metal oxide chemically equivalent to 1% sodium oxide and less than that amount of fluorine compound equivalent to .5% of fluorine; said percentages being by weight based on the total weight of said vitreous enamel.

9. The method of preparing a vitreous enamel composition suitable for use in enameling electrical resistors which comprises fusing ingredients which, when melted, will yield a fluid glass having the following oxidic constituents in the percentages noted:

. Per cent Lead oxide 28 to 55 Silica 11 to 32 Zinc oxide 5 to 20 Alumina 4 to 10 Boric oxide 10 to 30 said enamel containing less than that amount of alkali metal oxide chemically equivalent to 1% sodium oxide and less than that amount of fluorine compound equivalent to .5% of fluorine; said percentages being by weight based on the total weight of said composition, pouring said fused composition into a liquid in order to frit the same, and grinding said frit to comminuted form sufficiently fine to permit said enamel to be utilized for coating said electrical resistors.

10. The method of preparing vitreous enamel compositions suitable for use in enameling electrical resistors which comprises fusing ingredients which, when melted, Will yield a fluid glass having the following oxidic constituents in the proportions noted:

Per cent Lead oxide 28 to 55 silica 4 to 30 Zinc oxide 5 to 20 Alumina 4 to 10 Boric oxide 10 to 30 said enamel containing less than that amount of alkali metal oxide chemically equivalent to 1% sodium oxide and less than that amount of fluorine compound equivalent to .5% of fluorine; pouring said melted composition into a liquid in order to frit th same, grinding said frit to comminuted form and adding during said grinding from 2 to 7% of silica, said amount being so selected as to make the total silica content not less than 11% and not more than 32%, all percentages specified being by weight based on the total weight of the composition.

11. The method of preparing vitreous enamel compositions suitable for use in. enameling elec trical resistors which comprises fusing a batch which, on melting, will yield a fluid glass containlog the following oxidic constituents in the proportions noted:

Per cent Lead oxide 1- 18 to 50 Silica V 11 to 32 Zinc oxide to 20 Alumina 4 to Boric oxide 10 to 30 said enamel containing less than that amount of alkali metal oxide chemically equivalent to 1% sodium oxide and less than that amount of fluorine compound equivalent to .5% of fluorine; pouring said melted composition into liquid in order to frit the same, grinding said frit to comminuted form, and adding during said grinding from 5 to 10% of lead oxide, said amount added being so selected as to make the total lead oxide content not less than 28% and not more than 55%, all percentages specified being by weight based on the total weight of the composition.

12. The method of preparing vitreous enamel compositions suitable for use in enameling electrical resistors which comprises fusing a batch composition which, on melting, will yield a fluid glass containing the following oxidic constituents in the proportions noted:

Per cent Lead oxide w 18 to 50 Silica 4 to 30 Zinc oxide 5 to 20 Alumina 4 to 10 Boric oxide 10 to said enamel containing less than that amount of alkali metal oxide chemically equivalent to 1% sodium oxide and less than that amount of fluorine compound equivalent to .5% of fluorine;

pouring said melted composition into liquid in order to fri-t said composition, grinding said frit to a finely comminuted form, and adding during said grinding from 5 to 10% of lead oxide and from 2 to 7% of silica, said amounts of lead oxide and silica added during grinding being so selected as to make the total lead oxide content not less than 28% and not more than 55%, and the total silica content not less than 11% and not more 10 than 32%, all percentages specified being by weight based on the total weight of the composition.

ALDEN J. DEYRUP.

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

UNITED STATES PATENTS OTHER REFERENCES 35 Technologic Papers of the Bureau of Standards,

No. 246 (1923), page 715.

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