US2724526A - Tin plate baking pan - Google Patents

Description

Nov. 22, 1955 J. J. RUSSELL ETAI- 2,724,526

TIN PLATE BAKING PAN Filed April 18, 1950 2 Sheets-Sheet l NOV. 22, 1955 J, J, RUSSELL ET AL 2,724,526

TIN PLATE BAKING PAN Filed April 18, 1950 2 Sheets-Sheet 2 United States Patent() TIN PLATE BAKING PAN John James Russell, Des Plaines, William A. Beck, Itasca, and Jack Kollman, Chicago, Ill., assignors `to Ekco Products Company, Chicago, Ill., a corporation `of Illinois Application April 18, 1950, Serial No. 156,672`

2 Claims. (Cl. 220-64) i This invention relates to a composite metallic structure which may be fabricated into shaped metallic articles having outer surfaces rendered highly absorptive to radiant energy while retaining a relative high resistance to corrosion.

An object of our invention is the provision of a simple, direct, and thoroughly practical process for chemically treating tin and tin alloys by anodic treatment in various electrolytes, in order that the outward surfaces of the tin and tin alloys exhibit excellent heat absorption.

Another object of our invention is to `provide a fabricated article which has been anodically treated wherein the surfaces of said article shall be characterized by excellent heat absorption, high abrasion resistance, and continuity and uniformity of the`treated tin or tin alloy surfaces.` l

Another object of our invention is the provision that the heat absorbing coating produced by the chemical process for treating the surfaces of tin or tin alloy shall be adherent and sutiiciently ductile in order that articles may be fabricated by mechanical, drawing, i forming, and/or bending operations.

Another object of our invention is `the provision that a shaped composite sheet metal fabricated article, such as a baking pan or a process tray, possess outerand inner surfaces having `relatively similar heat absorbing properties.`

Other objects and advantages of this invention will be made more apparent as this description proceeds, particularly when considered in connection with the accompanying drawings, in which:

Fig. l is a plan view of a baking pan embodying the invention;

Fig. 2 is a fragmentary cross-sectional view taken along the lines 2 2 in Fig. l showing a schematic crosssection of the oxide coatings, alloy layers, tin layers, and steel base;

Fig. 3 is a photomicrographic cross-sectional View of a portion of a tin plate `area showing the oxide coating, tin layer, alloy layer, and steel base (2500)( magnification);

Fig. 4 is a photomicrographic cross-sectional view showing the relative thickness of the oxide coating wherein the time of chemical treatment approaches about one minute (2500 magnification); and

Fig. 5 is a photomicrographic cross-sectional view showing the relative thickness of the `oxide coating which is substantially greater than the tin layer wherein the time of chemical treatment approached a few minutes (2500)( magnification). j p

Referring now to the illustrated embodiments ofthis invention, the metallic shaped article, such as a baking pan, is designated generallyby reference numeral 2, its lower inside surface being shown at 4, and its inner side surfaces by numeral 6. The outer side surfaces are designated by numeral S and the outer bottom surface thereof by numeral 10.

The shaped article 2 as shown, is of the conventional folded end type baking pan; although this type is being `used for illustration, the invention shall not `be limited 2,724,526 Patented Nov. 2.2, 1955 ICC to such construction. The shaped article 2 is formed from a ilat composite sheet metal structure by the process of blanking into any desired dimensions and then forming the blank into a conventional folded end baking pan in a manner well known in the art.

As conducive to a clearer understanding of certain features of the invention, it may be noted that at this point tin articles having surfaces of tin or tin alloys shall also include tinplate, although this invention may be adapted to shaped articles formed of other base materials, other than steel, such as copper, brass and the like.

Hot dipped tin plate is generally considered to be steel base metal coated on its exterior surfaces with metallic tin wherein intermediate to the tin and steel interface, an iron-tin alloy composition is formed. Tin plate may be formed by either the conventional method of hot dipping or by electrolytically depositing the tin on the surface. The coating weight of the tin is usually specified in pounds of tin per basis box, or in grams of tin per square metre of tin. The conversion of weight per basis box to linear thickness depends on an assumed density which is compensated by the fact that in. the hot dipped process there is a greater proportional thickness of the alloy layer formed than in the electrodeposited process.

it is generally accepted that one pound per basis box is equivalent to 0.0000606 inch thick of tin on each face of the tin plate. One and one-half pounds tin plate is generally assumed to be about 0.0000909 inch thick. The proportional ratio between thickness and total weight of a tin plate can be approximated from the relative proportion above mentioned for hot dipped tin plate.

In the case of the electrolytic process, tin plate thickness coatings of 8 ounces to l0 ounces per basis box is generally accepted, although some applications of tin plate may use less than 11/2 pounds but generally not greater than 11/1. pounds per basis box.

ln the present invention, the anodic chemical treatment of tin plate is primarily directed toward tin plate having the tin content greater than about 1% pounds per basis box which would primarily be adaptable to hot dipped tin plate, although the scope of the invention is also applicable to electrolytic tin plated articles.

The formation of oxide films of tin, as heretofore known may be produced by converting the tin into the oxide by subjecting the tin layer under oxidizing conditions, such as air, at elevated temperatures. This process of subjecting the tin to oxidizing conditions at elevated temperatures, has been conventionally used by the baking industry in converting the tin layers to oxides of tin by placing formed articles of tin plate in baking ovens at an elevated temperature of approximately 400 to 425 F. for a period in excess of about 4 to l2 hours. This has been conventionally called the burning in or burning out process, wherein shaped articles 2, baking pans acquire a color ranging from the interference films of light iridescent hues ranging in color from yellow, blue through greens, and subsequently a surface coating having a degree of coloring may be obtained depending upon the chemical and physical. characteristics of the tin plate surface in conjunction with variable atmospheric conditions during the burning in cycle.

it has been found that these oxide films possess wide variations in color characteristics; and it is highly de-` sirable to eliminate these Wide variations in order to produce uniform and consistent bread crust color. In addition, the tin-iron alloy intermediate to the tin layer and the steel base is brittle and less corrosion resistant than the tin layer. Normally, the tin-iron alloy is' substantially increased with respect to the available metallic tin layer by these lengthy burning in operations; and it is. highly desirous to reduce the "burning in time to about 30 minutes to one hour. The baking industry finds it economically undesirable to subject the bread pan to long period of burning in because it ties up the baking ovens, baking pansas well as personnel in a non-productive operation and also produces variable results.

In addition, the temperature controls of the oven may vary considerably. It has been found that oven temperatures generally will exceed the melting point of the tin layer on the surface; as a result the tin-iron alloy layer will be substantially increased and the baking pans will be subsequently destroyed by the evaporation and decomposition of tin surface layer. Also complete uniformity of crust color of bread is a definite sales factor, and a baker who can produce a uniform loaf of bread in the initial baking operation without the conventional burning in of a new set of pans, will increase his production capacity.

In addition, the adherence of oxide of tin, by the conventional burning-in method, has been found to be very poor. Chemical factors as well as atmospheric factors which control the adherence of oxides of tin are not well defined, and it is known that variations of the chemical and physical composition of the surface layers of the tin will produce variation in adherence of the oxide coatings. Therefore, it is desirous to secure oxide coating formation having heat absorption which `is uniform and reproducible and at the same time to have a surface oxide coating which will not tend to reflect heat energy that impinges upon the surface of objects made of tin plate.

An outstanding object of the invention accordingly is the provision of an economical industrially practical process for anodically treating tin plated articles wherein work of widely varying quality with respect to chemical and physical tin surface composition, as well as shapes of manufactured articles rnay be employed. In addition, the condition of the surface of the tin plate may be physically varied by mechanical operation; and under these conditions a uniform and constant color oxide film having excellent adherence and continuity, showing substantial reduction of spangling due to the breakdown of large crystal growth on the tin surfaces may be produced within a pre-determined range of wholly practical conditions.

Referring now more particularly to the practice of this invention, the tin plate will normally have wide variations in the amount of tin on the surface, and will vary widely with respect to surface conditions, such as chemical composition, the presence of embedded organic foreign matter, porosity of the tin layer, and the crystalline structure of the tin surface. It is undesirable to have a heat reflecting exterior surface in a bread baking operation, and the conventional burning in process will reproduce oxides of tin corresponding to the initial surface condition.

The anodic chemical process of the present invention for treating tin surface layers will substantially reduce the heretofore mentioned objections; and in addition, will substantially remove objectionable carbonaceous deposits within the porous tinplate, such as grease and oil. In addition, the anodic chemical process will tend to seal the pores of the tinplate, thus reducing the tendency toward porosity corrosion. In the chemical anodic treatment of tin plate, article 2, or products of various shapes and configurations, the tinplate layer is electrolytically treated by using one or more of the plates, articles or products as the anodes in an electrolytic bath.

The electrolytic bath contains at least one or more or combinations thereof, of a complexing reagent consisting of substantial amounts of (l) polybasic organic acids, such citric acid, picric, tartaric acid, oxalic acid, malic acid, maleic acid, and succinic acid; and (2) monobasic organic acids, such as acetic acid, lactic acid, propionic acid, benzene, sulphonic acid, trichloracetic acid, land salicyclic acid; and (3) non-oxidizing inorganic 4 acids, such as phosphoric acids, boric, molybdic, tungstic, and hydrofluoric acid; and (4) aqueous soluble salts, such as the alkali metal and/or alkaline earth salts of the above mentioned organic acids or inorganic acids and other metallic salt compositions. It has been found that combinations of the complexing reagents in aqueous solutions may be employed such as phosphoric acid,

combined with citric acid, phosphoric acid combinedl with sodium phosphate; phosphoric acid combined with oxalic acid, acetic acid combined with sodium citrate, and sodium phosphate combined with sodium citrate. The scope of this invention shall not be limited to the chemical composition of the complexing reagents.

In the anodic treatment of tin plated articles and products in the electrolyte, it has been found advantageous to maintain current densities ranging from about 4 amperes per square foot to about 60 amperes per square foot of tin surfaces undergoing treatment together with a solution temperature of at least C.; and usually more, up to the maximum temperature which falls below the boiling pointof the solution so as not to cause excessive evaporation. Under the chemical conditions specified together with time of. immersion, it is possible to obtain a uniform meta stable tin oxide coating on tin plate having thickness ranging from about 3 to 5 micro inches to almost complete conversion of the free tin surface.

It is preferred to employ an electrolyte which by weight consists of at least 0.5% up to about 30% of the complexing reagents wherein the pH of the solution lmay be adjusted if necessary, with the corresponding acid or combination of other acids so that the pH range may vary from about a pH of 2 to a pH of about 8; and the remaining parts needed to form 100% by Weight being substantially of water. It has been found that the anionic stanno-complexes of tin are more readily formed when the pH is Within the range of about two and one-half (2/2) to five (5).

One or more other tin plated articles 2, such as tin formed baking pans are made the anodes of the electrolytic solution and are subject to anodic treatment while maintaining a preferred solution bath temperature of about 80 to 98 degrees C. and a current density of about 20 to 40 amperes per square foot. The cathode may consist of shaped metallic compositions, such as stainless steel or the lead lined tank may be used as the cathode. Under such temperature conditions, the solution is found to remain stable to the extent that the complexing reagent is substantially retained in the bath with respect to weight' percentage. The complexing reagent is not substantially chemically removed from the solution.

A uniform meta stable oxide of tin may be imparted to the tin surface in a short interval of time. The time of immersion together with solution conditions are importantfactors with regard to the amount of meta stable tin oxidey formed by the anodic treatment. It has been found that interference films of about 2 to 6 micro inches thick may be formed by anodic treatment, in the electrolyte for an interval of time corresponding to a few seconds. In addition, it has been found that in a matter of minutes the entire tin layer can be converted into a meta stable oxide of tin.

As examples of other exceptionally stable, conductive and highly effective electrolyte solutions, and the related operating conditions which are employed for rapidly obtaining a uniform continuous meta stable oxide of tin, the following treatments may be employed:

Treatment A Any remaining parts needed with the above to total 100% (percent) by weight, being substantially Water.

Bath temperature 90-100" C. Minimum current density 15 amperes per sq. foot.

Time of immersion 30 to 120 seconds.

Treatment B Percent Total t Weight of Bath Electrolyte Potassium Tartrate KQCiHrO Tartaric Acid HgCtHlO Any remaining parts needed with the above to total 100 percent by weight, being substantially water.

Treatment C Percent Total Weight of Bath Electrolytc 5-20 Phosphoric Acid Any remaining parts needed with the above 100 percent by weight, being substantially water.

Bath temperature 80 to 100 C. Minimum current density 15 amperes per sq. foot. Time of immersion 30 to 120 seconds.

t total Treatment D Percent Total Weight of Bath Elcctrolyte Tartaric Acid C OOI-I- (CHOH)2-C 00H 5-5l) Disodium Hydrogen Phosphate Any remaining parts needed with the above 100 percent by Weight, being substantially Water.

Bath temperature Sil-100 C. Minimum current density amperes per sq. foot. Time of immersion to 120 seconds.

to total Treatment E Percent Total W eight of Bath Electrolyte Sodium Tartrate N a2C4H4O-2H2O 3-30 Disodium Hydrogen Phosphate NagHP Or.

Any remainingparts needed with the above to total 100 percent by weight, being substantially water.

Bath temperature 90-100 C. Minimum current density l5 amperes per sq. foot. Time of immersion 15 to 120 seconds.

Treatment F Any remaining parts needed with `the above to total 100` percent by weight, being .substantially water.

Bath temperature -100 C. Minimum current density 20 amperes per sq. ft. Time of immersion-; 30 to 120 seconds.

Treatment G Percent of Bath Sodium Oxalate NanCzOi 2-15 Phosphorlc Acid H3130; 3-10 Any remaining parts needed with the above to total 100 percent by weight, being substantially water.

Bath temperature 70-100" C. Minimum current density 10 amperes per sq. ft. Time of `immersion 60 to 250 seconds.

Treatment H Any remaining parts needed with the above to total 100 percent by'weight, being substantially Water.

Bath temperature l00 C. Minimum current density 20 amperes per sq. ft. Time of immersion 60 to 150 seconds.

An excellent meta stable oxide layer of tin was obtained on tin plate in the instance of using Treatment A through TreatmentH, inclusive. The scope of the invention should not be bound by any such quality of chemical composition, nor by the specific proportions of acids, salts, and water given in the several illustrative examples of the treatment.

The `treated tin plate articles are then removed from the electrolyte bath and rinsed with water in order to remove any occluded salts. The treated articles are then dried at slightly elevated temperature in order to remove the adherent water. The treated tin plate articles upon which the meta stable `oxide of tin .is deposited thereon are then subjected to an oxide conversion temperature condition whereby the meta stable oxide of tin coating is then converted at elevated temperatures to form the stable oxide of tin coating, primarily stannous oxide. The elevated temperature range shall include approximately the melting point .of tin; although the preferred temperature range is from about 200 C. to about 230 C. This step in the process may be obtained by direct heat application to formulate the oxide conversion, although exposure to `air or to other oxidizing conditions will s0mewhat accelerate the oxide conversion at elevated tempep atures.

Theformation of the anionic stanno-complex of tin under electrolytic treatment will vary depending upon solution` compositions and conditions. Since tin exhibits amphoteric properties, it has been noted that the meta stable oxide `of tinmay be formed. under alkaline conditions, although the electrolytic step in the process is preferred under acid conditions (pH from 21/2 to 5), successful results have been obtained under alkaline conditions. In addition it has been noted that the formation of the meta stable` oxide of tin may be formed rst by immersion in an alkaline media and then subsequently into` an acid media or combinations thereof.

The complexing reagent forms the anionic stanno com plex which in turn is converted into the hydrate lof stannous oxide. The hydrate is unstable in fonn and is then partially converted into the meta stable form of stannous oxide. The meta stable stannous oxide coating exhibits the interference color on the surface of tin, and when the Vthickness of the meta stable stannous oxide appears to approach about seven (7) micro inches, opacityof the lm begins to occur. By varying the time of immersion in the electrolytic bath, the blue-black meta stable stannous oxide is formed and at about 2 to 10 minutes immersion time under the aforementioned conditions it appears that the tin layer may be converted totally into the meta stable stannous oxide. Therefore, it is desirable to reduce the time of electrolytic immersion to about fifteen to ninety seconds in orderk to retain a relatively high percentage of free tin.

The time interval for electrolytic immersion will vary depending upon solution conditions. By reducing the current density and/ or the temperature as well as the concentration of the complexing reagent, it is possible to increase substantially the time interval of immersion of the tin layer in the electrolytic bath. Therefore, the scope of the invention shall not be bound by the specific time interval of immersion.

The meta stable stannous oxide exhibits various transitional interference colors approaching the blue-black. The blue-black is then considered the end point of opacity; and it has been found that the interference colors of red, purple, green, and blue-green will form, upon subjecting the then treated meta stable stannous oxide coating to conversion conditions at elevated temperature, the stable stannous oxide coating, exhibiting a uniform olive green color. When using the term olive-green oxide of tin in the claims, it shall be understood that said terminology shall define the color of the oxide per se.

Referring to Figure 2 in the drawing, the schematic fragmentary cross-sectional view represents a composite structure section of a processed shaped article 2, wherein the diagram illustrates schematically, the relative proportional thicknesses of the oxide tin coatings 22e, the tin layers 24e, the tin-iron alloy layers 26C, and the steel base 28C.

It shall be noted that the oxides of tin coatings 22 This photomicrograph illustratesV the thickness of thel oxide coating 22a wherein a substantial portion of the tin layer 24a remains. The corrosion resistance of the tin plate has not been reduced substantially while heat have approximately the same thickness on the inner surl faces 6 as on the outer surfaces 8; thus, exhibiting similar heat absorption properties.

Figs. 3, 4, and 5, are single sectionphotomicrographs of a composite tin plate structure, such as a sheet of tin plate, or a section of the surface of a shaped` article 2, wherein the photomicrograph corresponds to a magnification of about 2500. It shall be noted that the crosssectons as shown in Figs. 3, 4, and 5, represent variable thicknesses of the oxide coatings 22 (a, b, and c), which may be obtained depending upon -the chemical composition, concentration of the electrolytes as well as the time of immersion in the electrolytes'. These photomicrographs represent tin composition layers 24 (a, b, and c) including the tin alloy layers 26 (a, b, and c) respectively corresponding to one and one-half pound tin plate, having combined thicknesses of approximately 0.000090 inch. The relative proportion of the amount of oxide coatings 22, with respect to the total amount of the initial composite thickness of the combined tin layer 24 and the alloy layer `26, will depend on the initial coating Weight of the tin plate, and the chemical treatment to which the tin layer 24 has been subjected.

In Figs. 3, 4, and 5, the mounting 20 (a, b, and c) is the same. For the purpose of securing a photomicrographic cross section, a copper mounting is generally used in order to back up the tin plate so that a well defined boundary may be secured. In these instances, the mountings 20 are used to back up the oxide coatings 22. l

Fig. 3 exhibits an opaque oxide coating 22avwhich has a thickness of approximately 15 micro inches thick. The oxide coatingi22a was formed on one and onehalf pound tin plate, using the aforementioned treatabsorptive properties of the tin plate has been greatly enhanced. In addition the oxide coating 22 tends to reduce porosity corrosion of the tin layer 24, by substantially sealing the pores of the tin layer with the oxide of tin coating 22, which exhibits chemical stability.

Fig. 5 is an illustrative photomicrographic cross. section of one and one-half pound tin plate, wherein the oxide coating 22e is substantially greater than the tin layer 24C. The oxide coating 22e was formed by the electrolytic treatment of one and one-half pound tin plate using Treatment C as previously described. The stable oxide coating 22e has a thickness of approximately 60 micro inches as compared to the tin layer 24e of approximately 25 micro inches.

In the accompanying Figures 3, 4, and 5, the relative thickness of the stable form of the oxide of tin coatings, 22 (a, b, and c), are proportional to thicknesses of the combined tin composition layers 24 (a, b, and c), plus the alloy layers 26 (a, b, and c) respectively. As the thicknesses of the oxide coatings 22 (a, b, and c), increase the relative thicknesses of the combined tin layers 24 (a, b, and c), and alloy layer 26 (a, b, and c) decrease, respectively. This process is applicable to other tin coating weights.

In examining the photomicrographic cross-section of the tinplate that has been subjected to anodic treatment and temperature conversion steps, we have found that the stannous oxide coatings 22, may be varied in thickness ranging from a few micro inches to a thickness equivalent to the entire layer 24, of the available free tin. Micro cross section examinations of a one and one-half pound per basis box of untreated tin plate has a thickness of free tin layer 24 plus the tin-iron alloy layer 26 of about 0.000090 inch. The tin-iron alloy layer 26 on hot dipped tin plate will be approximately equivalent to about .0.000010 to 0.000015 inch thick. The amount of the tin layer 24 on each surface of one and one-half pound tin plate per basis box will approximate from 0.000060 to 0.000080 inch thick. Allowing a sucient time interval to elapse in the electrolytic immersion step, the tin layer 24 can be substantially converted into the meta stable form of the stannous oxide. This appears to be undesirable and chemical control ofthe amount of the meta stable stannous oxide coating 22 to be formed is imperative. It has been found that a tin oxide coating of approximately 0.000007 to 0.000030 inch thick will exhibit the preferred heat absorption on the surface of tin plate. Experimentally, it has been found that excellent bread baking was produced by converting up to about of the free tin layer 24, to the tin oxide coatings 22 without substantially decreasing the corrosion resistance of the tin plate. Thus, in one and one-half pound dipped tin plate, the conversion of 80% of the free tin `layer would leave at least 12 micro-inchesv of free tin under the oxide.

`The thickness of the iron-tin alloy portion remains substantially unchanged from its thickness in the original tin plate. Although the meta stable oxide of tin coating may be used in the baking operation, we have found that the stable oxide of tin coating 22 is the preferred tin oxide coating.

.The novel chemical treatment described in this process may be further accomplished by using a fused salt, such as disodium hydrogen phosphate dodecahydrate or `magnesium phosphate, as the electrolyte, or combinations thereof. One may consider this to be substantially an aqueous solution when the temperature of the electrolyte bath is within 80 to 100 C. We have found that satisfactory results can be obtained by holding the bath temperature between 80 and 90 C. operating at a minimum current density of about amperes for a square foot for a period of time of about to 90 seconds. When the current density is held at about amperes per square foot, it has been found that the voltage applied dropped to about 2 volts. In this invention, the term complexing reagent shall also include a fused salt as described above.

`It shall be noted that the process as described may be applied to tin plate in sheet form wherein the anode may be a sheet of tin plate or a shaped article as defined by numeral 2. The tin plate or composite sheet structure may be chemically treated by the treatments aforementioned; and then fabricated into shaped articles.

An alternative method of manufacturing these shaped articles 2, having heat absorbing oxide coatings 22, formed thereon, may be accomplished by forming the tin plate and then subjecting the formed shaped 2, to the anodic chemical treatment by immersing the shaped article 2 in the electrolyte. The cathode generally will have to be shaped into various forms in order that a uniform oxide of tin coating 22 may be formed on the tin layer 24; especially wherein the surfaces of the shaped article 2 are deeply formed.

Another embodiment of this invention comprises the coating of the oxide of tin coatings 22 with a stable high temperature organic bread releasing film. The organic bread releasing film may be applied prior to the final temperature conversion step wherein the meta stable stannous oxide 22 may be coated with a stable high temperature organic bread releasing film such as alkyl aryl silicones, polytetrauoroethylene, polytrifluoromonochloroethylene and other high polymer lm forming organic substance and/or mixtures thereof. The film coated tin plated article 2, having either the inner, outer, or both surfaces coated, is then subjected to the temperature conversion steps wherein independently but simultaneously the meta stable oxide coatings are converted into the opaque stable oxide of tin coatings 22, and the organic lm is cured.

Excellent adhesion of the organic bread releasing film to the treated tin oxide coatings 22 has been obtained. It shall be noted that the organic lm may be applied to either or both surfaces of the article 2, which has been previously converted into the stable oxide of tin coatings 22, and then be cured. The application of the organic bread releasing iilm to the meta stable form of the oxide of tin coating and then subjecting the combined meta stable oxide and the uncured organic iilrn to the temperature conversion step will eliminate one step in the process.

As many possible embodiments may be made of our invention and as many changes may be made in the embodiments hereinbefore set forth, it is to be understood that all matter described herein, is to be interpreted as illustrative and not as a limitation.

We claim:

1. A tin plate baking pan comprising; a shaped sheet structure including a steel supporting base having an outer surface and tin composition layers bonded coextensively to said surface, said tin composition layers comprising an inner portion consisting of an iron-tin alloy having a thickness of approximately 10 to 15 micro-inches, an intermediate portion consisting of a metallic free tin layer bonded coextensively to said inner portion, an outer portion consisting of a uniform continuous heat absorbing tenaciously adherent opaque olive-green oxide of tin bonded coextensively to said intermediate portion, said outer portion having a thickness of about 7 micro-inches to 30 micro-inches.

2. A tin plate baking pan comprising a shaped sheet structure including a steel supporting base having an outer surface and tin composition layers bonded coextensively to said surface, said tin composition layers comprising an inner portion consisting of an iron-tin alloy having a thickness of approximately 10 to 15 micro-inches, an intermediate portion consisting of a metallic free tin layer bonded coextensively to said inner portion, said metallic free tin layer having a thickness of at least 12 micro-inches, an outer portion consisting of a uniform continuous heat absorbing tenaciously adherent opaque olive-green oxide of tin bonded coextensively to said intermediate portion, said outer portion having a thickness of about 7 micro-inches to 30 micro-inches.

References Cited in the file of this patent UNITED STATES PATENTS 1,553,617 Katzinger Sept. 15, 1925 2,024,951 Schutte Dec. 17, 1935 2,103,119 Romanolf Dec. 21, 1937 2,215,165 Sumner Sept. 17, 1940 2,245,561 Nelson June 17, 1941 2,283,171 Botcheller May 19, 1942 2,327,127 Rath Aug. 17, 1943 2,391,660 Ward Dec. 25, 1945 2,462,728 Debs Feb. 22, 1949 2,606,510 Collings Aug. 12, 1952 2,606,837 Clark Aug. 12, 1952 FOREIGN PATENTS 300,641 Great Britain Apr. 7, 1930 416,608 Great Britain Mar. 21, 1934 479,746 Great Britain Feb. 7, 1938 535,670 Great Britain Apr. 17, 1941

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