RELATION OF THICKNESS TO IMPACT RESISTANCE OF PORCELAIN ENAMEL*

Chipping of porcelain enamel on the flanges of table tops as a result of impact was studied and compared with the results obtained from the Porcelain Enamel Institute tentative standard impact test. A correlation was found to exist between the results using the P.E.I, test on standard 1-in. tubes and those obtained on the corner of a table-top flange. The impact resistance of enamels applied over a radius also increased as the thickness of the enamel was increased.     

EFFECT OF COEFFICIENT OF EXPANSION OF GROUND- AND COVER-COAT ENAMELS ON THERMAL-SHOCK AND IMPACT RESISTANCE *

Four antimony cover-coat enamels, four fluoride cover-coat enamels, and four ground-coat enamels were formulated to give varying coefficients of expansion. The calculated values were 250, 275, 300, and 325 × 10^–7. The observed coefficients of expansion are included in the paper. The enamels were applied to standard 2-quart pudding pans and were tested for thermal shock and impact resistance using the standard methods of the Enameled Utensil Manufacturers’Council. The results show that thermal-shock resistance is controlled chiefly by the coefficient of expansion of the ground-coat and cover-coat enamels. The best results were obtained by using a cover-coat enamel with a low coefficient of expansion and a ground coat with a high coefficient of expansion. No conclusive evidence was found to indicate any effect of the coefficient of expansion of the ground- and cover-coat enamels on impact resistance. The solubility resistance and reflectance curves for the enamels are also included.     

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.     

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.     

RELATION OF COEFFICIENT OF EXPANSION TO CROSS-BENDING STRENGTH OF SHEET-STEEL ENAMELS1

Two series of enamels were made with calculated cubical coefficients of expansion varying from 361 to 306. In both series of enamels, the cross-bending values were inversely proportional to the coefficient of expansion. This is due to the fact that all enamels on sheet steel have lower coefficients of thermal expansion than steel. In cooling, after the enamel is fused to the metal, strains are set up in the enamel; the lower the expansion of the enamel the greater the strains. When the enameled piece is bent, the greater the stresses present, the farther the pieces can be bent before there is any real tensile strain on the enamel. High bending ability is dependent primarily on high compressive strains induced by lower coefficient of expansion of the enamel than of the metal base. Compressive strains cause warping, fishscaling (in ground coat), and chipping; therefore the bending strength should not be too high. In all control work both the minimum and the maximum bending strength should be specified.     

RESISTANCE OF ENAMELS TO THERMAL SHOCK*

Thermal-shock resistance tests were made on enamels of varying thermal expansions applied on test pans. Increased weight of application and high expansion of the cover coat decreased the thermal-shock resistance, whereas high expansion of the ground coat increased the resistance.     

RELATION OF METAL THICKNESS,ENAMEL THICKNESS,AND BOTTOM RADIUS TO IMPACT RESISTANCE OF PORCELAIN ENAMELED UTENSILS*

Three series of standard two-quart pudding pans of different metal thicknesses, coated with one, two, and three coats, respectively, of enamel were tested for impact resistance, using the Enameled Utensil Manufacturers' Council standard impact test. The impact resistance of the pans was increased with increase (1) in metal thickness of the pans, (2) of the enamel thickness, and (3) in the size of the bottom radius. The size of the chip formed on impact also increased with increase in enamel thickness. The literature dealing with the fracture of glass and enamels is reviewed, and the factors influencing spontaneous chipping and chipping caused by bending and impact are discussed.     

ELECTROSTATIC SPRAYING OF PORCELAIN ENAMELS *

The application of the process of electrostatic spraying to the porcelain enamel industry is described. The equipment which is used and the method of using it are discussed in detail. Particular emphasis is placed on the preparation of enamel for electrostatic spraying and on theoretical considerations which have a direct bearing on the spraying process. The applicability and limitations on the use of the equipment in the porcelain enamel industry are considered.     

RELATION OF HYDROGEN TO ADHERENCE OF SHEET-STEEL ENAMELS*

The principal theories of adherence are reviewed, and a new theory involving hydrogen gas is discussed. A review of the literature and experimental data show that hydrogen is a factor in the adherence of sheet-steel enamels and that adherence, reboiling, and fish scaling are closely related.     

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.     

RELATION OF THE PARTICLE SIZES OF THE FRIT AND COLOR OXIDES TO THE COLOR PROPERTIES OF PORCELAIN ENAMELS*

The effect of variations in the particle size of typical green and blue color stains in sized fractions of a representative clear frit was studied. Microscopic examinations were made of the sized fractions, and photomicrographs of the respective fields are presented. The color characteristics of the enameled panels were determined on a continuously recording spectrophotometer. Frit particles of 45 to 75 μ, combined with color particles of less than 5μ, were found to give the optimum or most efficient color characteristics, and color particles larger than 5 μ, seriously impaired the resulting color.     

THE THERMAL EXPANSION OF SHEET-IRON GROUND-COAT ENAMELS*

The thermal expansions of sheet-iron ground-coat enamels were determined by use of the interferometer. The results showed that the calculated expansions based on the factors of Mayer and Havas were close to the actual expansions. It was also shown that variations of borax, feldspar, and quartz resulted in a fairly uniform change in the expansion. It was possible to interpolate between values for these constituents. The softening of the enamels in the interferometer test proved to be a characteristic change depending on the fusibility of the enamel. These data showed promise as a test for fusibility.     

FUNDAMENTAL STUDY OF FACTORS INFLUENCING COLOR OF PORCELAIN ENAMELS*

The fundamental basis of color specification, namely, brilliance, purity, and dominant wave length, has been determined from spectrophotometric curves by means of an integrating calculator. The effect of variations in the particle size of the frit and the coloring agents on the purity, brilliance, and dominant wave length has also been studied.     

TESTS FOR HOT-WATER RESISTANCE OF TANK ENAMELS*

The resistance to attack by hot water of a representative group of commercial enamels, designed especially for use on hot-water tanks, and of several nontank enamels was measured by the following three types of test: (A) loss of gloss after periods up to several hours in a conventional autoclave with distilled water, (B) loss of gloss after one day to two weeks in apparatus which kept the specimens in contact with constantly changing, boiling distilled water, and (C) loss in thickness after periods up to 5000 hours (208 days) in a modified autoclave with circulating aerated tap water under pressure. The tests are described in detail, comparisons are made, and the loss-of-gloss test with boiling distilled water as used in Commercial Standard CS115–44 is discussed.     

THE RELATION OF FINENESS OF FELDSPAR USED IN COMPOUNDING A PORCELAIN ENAMEL TO RESISTANCE TO DEFLECTION1

The effect of changing the fineness OF the feldspar in an enamel was investigated, from which test it was concluded that resistance to deflection is lowered about 9% with a slight increase in fusion temperature when substituting 40-mesh (glassmakers') feldspar for 120-mesh feldspar, both of the same chemical composition.     

THE RELATION OF FINENESS OF GRINDING TO OPACITY IN WHITE ENAMELS

The problem of securing a uniform opacity by making observations of smelting, testing of raw materials, etc., led to the conclusion that the most important process was the mill treatment given the mixture. A table showing screen analyses of white enamels is given.     

EXPERIMENTS WITH AN ACCELERATED WEATHERING TEST FOR PORCELAIN ENAMELS*

Experiments were conducted on porcelain enamels using the accelerated weather test developed at the National Bureau of Standards. A comparison of results indicates this test to be a promising solution to the problem of the rapid determination of the weather resistance of porcelain enamel.     

EFFECT OF SIZE AND SHAPE OF TITANIUM OXIDE CRYSTALS ON SPECTROPHOTOMETRIC PROPERTIES OF TITANIUM-BEARING PORCELAIN ENAMELS*

Color changes of three titanium enamels given varied firing treatment were investigated. The size and shape of titanium oxide particles were studied with the electron microscope, the relative amounts of anatase and rutile were determined from X-ray analyses, and spectrophotometric curves of the fired panels were made. Electron micrographs showed that rutile particles recrystallized as needles or “sticks,” whereas anatase appeared as ill-defined, irregularly shaped, rounded particles. With increasing firing temperature or firing time (1) rutile particles showed a greater increase in size than anatase particles, (2) the amount (by weight or volume) of anatase crystals decreased and the amount of rutile crystals increased, and (3) a color change from blue-white to cream-white was observed. The change in color seemed to be related to the scattering of blue light by small particles at lower temperatures or shorter periods of firing. As the size of opacifying particles increased, the scattering of blue light decreased, and the characteristic absorption of visible light in the shorter wave lengths was more apparent. In enamels containing both anatase and rutile, the smaller anatase crystals were predominant at lower and the larger rutile crystals at higher temperatures, so that sharp changes of color were observed as the firing temperature was increased.