EFFECT OF SOLUBLE SALTS ON THE PROPERTIES OF ENAMELS1

Enamels, in common with other types of glasses, are more or less soluble in water, depending on their compositions. The nature and quantity of salts dissolved from the enamel glass during milling have a very definite bearing on the suspension, work ability, and other properties of the enamel, such as pitting and crawling. Those enamels which produce mill liquors which are excessively alkaline have very poor floating qualities. The ratio of free alkali to boric oxide dissolved from the enamel frit is an important factor in the flotation properties of the enamel. The poor floating powers of certain enamels can be corrected by the addition of suitable electrolytes. The kind and quantity of salts dissolved from the frit will determine the selection of the proper electrolyte. Enamel compositions having minimum solubility for good suspension and other properties are given. The value of an enamel clay cannot be determined by floating the clay in water. Tests must be made with the clay in the type of enamel with which it is to be used. Any studies on the effect of aging of enamels on their working properties must give consideration to the constitution and quantity of the individual salts dissolved from the enamel frit, and to the action of carbon dioxide in neutralizing part of the free alkali.     

GOUGE RESISTANCE OF PORCELAIN ENAMELS*

An exploratory study was made to determine possibilities of the Porcelain Enamel Institute rolling-ball gouge test as well as to discover some of the factors influencing the gouge resistance of enamels as shown by the test. The test results compare favorably with deep-scratch and gouge defects encountered in actual enamel practice. The accuracy of test results, moreover, compares favorably with other such physical tests designed for use with porcelain enamel. The gouge resistance is shown to be a function of enamel thickness and of bubble structure. Factors which increase or decrease bubble structure, such as mill additions, frit compositions, and firing treatment, are shown to affect gouge resistance.     

WET PROCESS ENAMELS FOR CAST IRON1

Wet process enameling of cast iron is becoming of importance because of its application to the enameling of stove parts, sanitary fittings and hardware. This paper is a report of an extended investigation of wet process enamels for cast iron, in connection with the use of a ground coat. Compositions used in dry process enameling have served as a basis for the work, which includes a study of ground coats and white cover enamels. The effect of varying methods of preparing the frits, mill additions, and the relation of composition of the enamels to such properties as adherence, texture and opacity have been carefully studied. Ground Coats. —Sintering of most ground coat frits has been found desirable in order to develop best adherence on the castings. Clay gave best results as a mill addition for the groundvt and feldspar were found to be less satisfactory. Excessive additions of clay or flint produced flaking of the ground coat while feldspar tended to develop blistering. About 15 per cent of clay or 10 per cent each of clay and flint are recommended as mill additions for the ground coats. Excessive additions of any one flux were not desirable, and best results were obtained when sodium, lead and boric oxides were combined in more or less definite pro-portions. Sodium oxide in excess of about 10 per cent gave rise to blistering. Boric oxide increased the firing range of the ground coats and was preferable to lead oxide on this account, although it tended to promote crawling of ground coats high in boric oxide if they were applied somewhat heavily. The best ground coats developed are Rg-26, 17, 1, 25 and 18. Cover Enamels. —Cover enamels were smelted in the usual manner. The mill additions consisted of 5 per cent of clay, 8 per cent of tin oxide, and 45 per cent of water, all based on the weight of the dry frit. For best results, the cover enamel must he adapted to the ground coat in refractoriness. Boric acid increased the firing range of the enamels but tended to promote crawling, although this was less pronounced in the case of the more fusible compositions. Boric oxide improved the opacity when suhsituted for such fluxes as sodium and lead oxides. Cryolite increased opacity but additions above 10 per cent, based on the melted weight, tended to promote crawling. Considering all factors, cover enamels R-14 11, 28, 1 and 18 gave best results.     

STUDY OF THE EFFECT OF SOLUBLE SALTS PRESENT IN ENAMEL MILL LIQUORS*

Studies were made of the solution of the frit during milling, the soluble salts derived from the frit were identified and studied, and some properties of the slips and mill liquors were investigated. The influence of some soluble salts present in cover-coat enamels on enamel film strength is shown, and a theory regarding the mechanism of tearing has been developed. The function of some salts, particularly sodium hydroxide and sodium nitrite, in eliminating tearing was studied, and the manner in which these salts improve film strength is reported.     

Microstructure of Titania-Opacified Porcelain Enamels

The microstructure of TiO₂−opacified porcelain enamels was examined by optical microscopy and by transmission electron microscopy. The mosaic pattern found appears to correspond to the positions of the frit particles from which the enamel was made. Significant differences were observed in the crystallization behavior of adjoining frit particles. There is evidence that rutile grows from frit particle surfaces, whereas anatase is body-nucleated. The anatase-to-rutile inversion is a ripening process rather than a phase transformation. Processes occurring during the firing of a TiO₂ enamel include crystallization from single-phase glass, crystallization from multiphase glass, and a ripening process.     

THE PRODUCTION OF SOME WHITE ENAMELS FOR COPPER1

The investigation deals with the development of some white enamels for copper watch dials, thermometer scale plates, and signs. The effect of varying Na₂O, PbO, cryolite, SiO₂ and B₂O₃ on the properties of the enamels was studied in twenty enamel compositions. The following conclusions were reached: (1) Correct melting is extremely important in the preparation of copper enamels; (a) Slow air cooling of the frit is preferable to quenching in water. (b) Repeated melting tends to promote opacity and to eliminate dissolved gases. (2) A somewhat reducing atmosphere during firing avoids oxidation of the copper. (3) Slight changes in enamel composition have a decided effect on the resultant properties. (4) Sodium oxide promotes gloss but reduces opacity very decidedly. (5) Lead oxide promotes fusibility without materially reducing opacity, except by dilution. (6) Cryolite is not a desirable flux or opacifier as it tends to develop a matt finish. (7) It is desirable to keep the boric oxide content low and good enamels may be produced without it. (8) By using “white arsenic” as an opacifier tin oxide is unnecessary. (9) The following were the best enamels obtained and these appear to be equal to a number of watch dial enamels examined: C-19 and C-20. 20. (10) A second group of compositions which appear to promote results equal to those in use for thermometer scales, advertising letters, etc., are C-10. 10. C-21, C-22, and C-24. In selecting these compositions, all factors such as opacity, gloss, texture, fit and resistance to attack of copper have been considered.     

ABRASION RESISTANCE OF PORCELAIN ENAMELS*

The abrasion resistances of enamels, differing in composition and properties, were determined by the test for resistance of porcelain enamels to surface abrasion (a standard of the Porcelain Enamel Institute, March, 1942). Four to five classes of abrasion resistance were obtained, but for the most part, the results fell within relatively narrow limits. Differences in abrasion index, in general, were not distinguishable by visual inspection. No one class or kind of enamel was superior. While changes in abrasion index were affected by changes in frit formula, it seems that many compositions will give comparable results.     

CHEMICAL DURABILITY OF PORCELAIN ENAMELS *

The resistance of representative porcelain enamel surfaces to the chemical attack of different concentrations of various solutions was investigated in considerable detail. Cylindrical cup-shaped samples were coated with (1) a ground coat, (2) a white fluoride cover enamel, (3) a white antimony cover enamel, (4) a white zirconium cover enamel, (5) an acid-resistant white cover enamel, (6) a sign blue cover enamel, (7) a blue zirconium enamel, and (8) a chemical acidproof blue cover enamel. The loss in weight of these enamel surfaces, after exposure to chemical attack, was determined at definite time intervals. The solutions studied consisted of different concentrations of the inorganic acids, alkalis, several organic acids, as well as selected salt solutions at both room and boiling temperatures. Numerous curves are presented showing the comparative chemical durability of the various porcelain enamel surfaces in which the cumulative loss in weight is plotted against time. Several photomicrographs show the nature and type of chemical attack on the different surfaces. The porcelain enamel surfaces showed considerable variation in their resistance to the chemical attack of the various solutions. All of the porcelain enamels were, in general, resistant to attack at room temperature by the alkali and salt solutions. The acid-resistant and acidproof enamels were resistant to the action of both inorganic and organic acids at room and at boiling temperatures; a wide variance, however, was shown in the comparative acid resistance of the non-acid-resisting enamel to the attack of either organic acids or inorganic acids at room temperature. AU porcelain enamels showed equally low resistance to boiling alkali solutions, but some of the enamel surfaces were attacked markedly by the boiling salt solutions.     

ELECTROLYTES IN ACID-RESISTING ENAMELS*

The effect of various electrolytes on the acid resistance, “set,” and defects of acid-resisting enamels was studied. The defects encountered were scumming, pitting, and shorelining. It was found that (1) insufficient set caused scumming which was aggravated by aging the enamel; this defect may be eliminated by increasing the set of the enamel; (2) one of the enamels studied had a marked tendency to shoreline whenever a sulfate or an electrolyte containing the sulfate radical was used; and (3) the pitting tendency of one enamel was traced to the high soluble salt content of the mill liquor; this defect was eliminated by spraying the enamel at a high gravity or by applying the acid-resisting enamel in a heavier coating. Some of this work has been continued in regular plant practice, and a discussion of this phase of the work is included.     

MOLYBDENUM IN ENAMELS: III,TYPICAL MOLYBDENUM ENAMELS*

Compositions and characteristics of various types of enamels using the most successful mill additions in Parts I and II are reported. The enamels used in this paper are (1) clear enamels (regular) for ground or colored coats, (2) clear enamels (acid resisting) for colored coats, (3) high lead clear enamels maturing below 1000°F., (4) raw or partially fritted enamels for ground or colored coats, and (5) white antimony enamels for direct application to the metal base. Reported for the first time are some white enamels containing molybdenum in the frit, which adhere to sheet steel without additions of molybdenum compounds to the mill.     

X-RAY INVESTIGATION OF THE OPACIFYING COMPOUNDS PRESENT IN SHEET-IRON COVER ENAMELS*

By the use of X-ray methods, sheet-iron cover-enamel frits and fired enamels were investigated. The results obtained show that tin oxide exists as tin dioxide in the enamel, antimony as antimony pentoxide, fluorspar as calcium fluoride, and cryolite as sodium fluoride. Zirconium oxide does not exist as the crystalline zirconium oxide even when present as a mill addition. Variations in the compositions of the ordinary sheet-iron cover enamels do not affect the identity of the crystals.     

熔加色素法搪瓷色釉结构的研究

用ＳＥＭ、ＷＤＳ、ＩＲＲＳ、ＸＰＳ和ＡＳＳ对搪瓷色釉结构进行了研究熔加法的搪瓷色釉具有均匀分布的小尺寸、数量小的气孔。分析了搪瓷色釉的Ｏ１ｓ谱，并分解了成桥氧、非桥氧、及自由氧。熔加法的桥氧浓度远大于磨加法，而熔加法的非桥氧的浓度远低于磨加法，这表明熔加法的搪瓷色釉的结构强度高于磨加法。     

PROPERTIES OF ENAMEL SLIPS: I,GENERAL PROPERTIES OF CLAYS AND ENAMEL SLIPS*

Five clays used to suspend porcelain enamels and their properties were studied. Fundamental data were first obtained on the clays, which were then milled in different proportions with several types of porcelain enamel frits. The properties of the resulting slips were studied. The pickup weight, that is, the amount of enamel retained on a ground-coated plaque which had been dipped in the slip and allowed to drain, was used as a basis of comparison.     

Nature of Adherence of Porcelain Enamels to Metals

An investigation of the fundamentals of the adherence of porcelain enamels to metals indicated that good adherence is the result of metal-to-metal bonds between the atoms in the base metal and the proper metallic ions in the enamel. To accomplish this type of bond, the enamel must meet certain chemical and thermodynamic requirements: (1) The enamel at the interface must be saturated with an oxide of the metal and (2) this oxide must be one which, when in solution in the glass, will not be reduced by the metal. In the case of iron, the oxide is FeO. Many of the phenomena observed in commercial enameling were investigated and found to be related to adherence, but not essential for its development. An example is the precipitation of metallic particles in the enamel. Much of the complexity in commercial enameling arises from the limitations imposed by practical considerations. For example, because enamels usually are fired in air, the heavy scale developed during the early stages of firing must be removed before adherence can be developed. Likewise, as the conditions of the enamel-metal interface change rapidly during firing, "adherence-promoting oxides" are used to help maintain the necessary conditions for the time required in commercial enameling. Surface roughness, although not necessary for excellent adherence, was found to improve the apparent adherence when the bond between the enamel and the metal was relatively weak.     

Stability of Acid-Resistant Red Porcelain Enamels

The comparative acid resistance of a series of enamels was determined by the loss-in-weight test and by the spot test. The stability of the red color oxide in these enamels was also studied by varying the alumina, fluoride, soda, and potassia contents in the frit. Improved acid resistance was obtained with no alumina and no fluoride in the frit. The stability of the red color was markedly influenced by the fluoride content; the absence of fluoride in the frit gave very poor stability.     

EFFECT OF VARIATIONS IN COLOR STAINS AND FRITS ON COLORS PRODUCED IN PORCELAIN ENAMELS*

A representative series of colors, blue, brown, green, red, and yellow, was selected for this investigation. Variations of the constituents for these five stains were made and the resulting colors produced in the enamels were measured by means of spectrophotometric curves. The influence of additions of barium oxide, lead oxide, and zinc oxide to the frit was also studied. The addition of lead oxide to the frit tended to give greater solution of the blue stains. The barium oxide additions dissolved the lead antimonate yellow the most, while the lead oxide had the least effect on the yellow stains. Variations in calcination temperatures of the red stains produced decided changes in color. The additions of lead oxide darkened the color produced with the red stains.     

WET-PROCESS LEADLESS CAST-IRON ENAMELS1

Several promising compositions have been developed in the laboratory in an extensive investigation for the development of satisfactory leadless wet-process cast-iron enamels. Each series investigated is described. It is concluded that such an enamel is feasible as several fair enamels and one good one were developed during the investigation. Best results were obtained when the enamels were washed and although this is impractical in plant practice it is hoped that an insoluble frit may be obtained thus eliminating the necessity for washing in order to obtain the required gloss.     

EFFECT OF ZINC OXIDE ON SHEET-IRON COVER-ENAMEL PROPERTIES*

The effect of ZnO on sheet-iron cover-enamel properties was studied by evaluation of the properties of a typical sheet-iron cover enamel in which ZnO was used as single- and double-partial replacements for the various ingredients. The fluxing action of ZnO was found to be pronounced and it was markedly effective in decreasing frit solubility in the mill liquor. Improvement in gloss resulted from its use without any adverse effect on enamel extensibility and resistance to thermal shock.     

THE EFFECT OF MILL PRACTICE ON THE WORKING PROPERTIES AND FINISH OF VITREOUS ENAMELS

Variations in mill or grinding practice are shown to affect the working properties and finish of vitreous enamels. Uniform mill practice from day to day is necessary for exact color matches from day to day in some shades of enamels. TOO small a mill charge contaminates the enamel with abraded pebble and mill lining material, and may therefore decrease the gloss and alter the color. Attention is called to the possibility of obtaining too much “set-up” in an enamel charge ground in a new or newly relined pebble mill. The particle fineness of the enamel may be tested by the usual screen test, or in a cone screen, which is described.     

Role of P2O5 in TiO2-Opacified Porcelain Enamels

Phosphorus pentoxide is known to affect the reflectance and color of TiO₂-opacified porcelain enamels. It has been postulated that P₂O₅, impedes the anatase-to-rutile phase transformation. This hypothesis was tested by applying recently developed analytical procedures based on X-ray diffraction analysis to a typical porcelain enamel cover coat system. The inversion of anatase to rutile was not significantly affected by P₂O₅. Instead, in the range 0.5 to 4%, as P₂O₅ concentration was increased in the frit, the rate of crystallization of both anatase and rutile also increased, with the rate of increase of anatase being about twice that of rutile.