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.     

RELATION OF COEFFICIENT OF EXPANSION TO IMPACT RESISTANCE OF PORCELAIN ENAMELS*

The tentative standard impact test procedure, adopted by the Porcelain Enamel Institute, was used to test and compare cylinders coated with several experimental and commercial enamels. The important facts noted were (1) the effect of cubic thermal expansion on the impact resistance, (2) the importance of the ground coat to the resistance of any one cover enamel, and (3) the wide variation between cover enamels in resistance to impact.     

STANDARD THERMAL-SHOCK TEST FOR PORCELAIN ENAMELED UTENSILS*

The equipment and method used in testing porcelain enameled utensils for thermal-shock resistance is described. Test results are given which show that increased enamel thickness and decreased metal thickness decrease the thermal-shock resistance of enameled ware. The mechanics of thermal-shock failures is discussed. This study is one of a series sponsored by the Enameled Utensil Manufacturers' Council.     

ENAMELED UTENSIL MANUFACTURERS' COUNCIL TENTATIVE STANDARD SOLUBILITY TEST FOR PORCELAIN ENAMELED COOKING UTENSILS*

The various methods for determining the degree of resistance of porcelain enameled cookingware to acid solubility are discussed. A method that overcomes many of the difficulties encountered in such a test and offers greater accuracy and reproducibility is described in detail.     

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.     

TESTING RESISTANCE OF ENAMELED SURFACES TO SCRATCHING GOUGING,AND ABRASION*

Seven types of cover-coat enamels were tested for resistance to scratching, gouging, and abrasion by various methods. These methods were (a) P.E.I. abrasion test, (b) P.E.I. gouge test, (c) variations of the gouge test, (d) Taber Abraser, and (e) a scratch test. Chemical durability and reflectance were found to have little or no correlation with the resistance of the enamels to abrasion. The bubble structure of the enamel layer seemed to have the greatest effect on abrasion and scratch resistance of the enamel layer.     

RELATION OF PARTICLE SIZE AND CHARACTERISTICS OF LIGHT REFLECTED FROM PORCELAIN ENAMEL SURFACES*

Various opacifiers were added in increasing percentages to clear, regular white, very opaque white, and acid-resisting enamels. These were ground to various degrees of fineness from coarse down to a few microns or finer; they were applied to sample plates and their reflectances were measured. Increased opacity is obtained by extremely fine milling because of the excellent dispersion of the opacifiers.     

NOTE ON THE ACID RESISTANCE OF ENAMELED COOKING UTENSILS1

Apparent segregation of the constituents of an enamel under the influence of gravitation. —Experiments with trial pieces with poured samples, and with enameled ware of various mfgr's. showed that the surface of the layer of enamel which is uppermost during firing and cooling is readily attacked by acid, thus indicating, apparently, that the more soluble materials tend to rise to the top of the melt. It is therefore recommended that enameled kitchen utensils be fired in an inverted position. In the discussion by Bryan A. Rice a series of similar experiments are described using small shallow cups. Quantitative tests failed to show any difference in acid resistance and it is suggested that the author's results were due to the nature of the gases in contact with the enamel surface or to volatilization. In his reply the author accepts this explanation of his results.     

NICKEL FLASHING AND ITS RELATION TO ENAMEL ADHERENCE*

Nickel flashing as applied to enameling stock is shown to be primarily metallic nickel. The role of nickel in developing enamel adherence is apparently bound up with the retardation of oxidation which the nickel flash imparts in the enameling cycle. The problem of adherence may be considered to be a corrosion phenomenon of the base iron developed by the action of gases and other agents which may be present at enameling temperatures. As such, the degree of adherence is a function of oxygen pressure at the interface when the glass is fused. The equilibria developed are strongly affected by the presence of nickel. Using cover coats directly on iron, the amount of nickel required will vary with each particular enamel composition. Şome evidence as to the nature of the adherence-promoting oxide is presented.     

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.     

FRIT SOLUBILITY: II,CONTROLLED VARIABLES OF FRIT COMPOSITION AND THEIR RELATION TO ENAMEL CONSISTENCY *

The sodium carbonate and borax of a sheet-steel ground-coat composition were varied to give variable frit solubility. Definite relationships between enamel consistency and Na₂O and B₂O₃ content of the mill liquor are shown. The ratio of soluble Na₂O to B₂O₃ is of more importance than their total solubility.     

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.     

PRACTICAL THICKNESS OF PARTIAL COATINGS OF ACID-RESISTANT ENAMEL*

The effect of the firing cycle, fluidity of enamel glasses, water content of enamel slips, method of application, fineness of grind, and opacifiers on the actual thickness of partial acid-resistant enamel coatings, measured by the Shartsis and Harrison method, is discussed. The results of this study appear to explain the failure of highly acid-resistant enamels to produce satisfactory acid resistance in some enameling plants.     

A NEW METHOD FOR STUDYING FRACTURES OF PORCELAIN ENAMELED SPECIMENS*

A process is described which locates microscopic cracks in an enamel layer. This method shows great promise in the study of different types of fractures of porcelain enameled specimens. An analysis of the stress conditions in the enamel-iron-enamel system before, during, and after thermal shock is also included.     

THE RELATION OF DEFECTS IN ENAMEL COATINGS TO HYDROGEN IN STEEL*

The effects of hydrogen as functions of time, temperature, and impurities in steel are described and experimentally demonstrated. Experimental evidence is presented to show that the blistering and boiling action over carbide areas in steel are due principally to hydrogen that associates with the carbon. Steel enameling stock contains quantities of hydrogen which may effuse during firing to cause or aggravate such phenomena as “boiling,”“primary boiling,”“rebelling,”“blistering,” and “bubbling.” Low-temperature effusion of hydrogen contributes to other defects, including “fishscaling,”“delayed fishscaling,” and possibly “pop-offs,”“jumping,”“shiners,” and some cases of “chipping” and, perhaps, “bursting” of enamel on cooking utensils. These defects have seldom been identified with hydrogen evolution. An indirect effect of hydrogen on “copperheads” and “black specks” is also identified. An exhaustive review of both English and German literature on enameling defects is included. Many observations recorded in the literature are shown to agree with the hydrogen theory. Certain types of inclusions in steel are shown to react with occluded hydrogen to form compounds that will not dissociate appreciably at some enamel-firing temperatures, and critical quantities of the hydrogen are therefore prevented from reaching the enamel coating during firing to cause blistering and related defects.     

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.     

EFFECT OF RESIDUAL COLOR ON CONSUMER ACCEPTANCE OF PORCELAIN ENAMEL WHITES*

Reflectometers are widely used to give a numerical rating for the comparative whiteness of porcelain enamels, but these instruments occasionally show values in disagreement with a careful visual comparison. To find the reason for this disagreement, a survey was made to determine the effect of residual color on the consumer rating of porcelain enamel whites. A series of 12 white plaques was made ranging from 67 to 75% in reflectance, from 478 to 485 m/x in dominant wave length, and from 1.6 to 5.6% in residual color. These specifications were determined from curves run on a Hardy G. E. recording spectrophotometer. The 12 plaques were submitted to 36 persons who arranged them in order of whiteness. The order of arrangement was compared statistically, and the whites were rated in their order of acceptance. Residual color was found to have an important effect on apparent whiteness, and of these typical porcelain enamel whites, those most highly colored (blue) were preferred.     

DETERMINATION OF COMPRESSIVE STRESS PRESENT IN PORCELAIN ENAMEL ON SHEET IRON*

A method for determining comparative compressive stress produced in porcelain enamel on sheet iron is described. The method consists of preparation of laboratory samples under carefully selected and controlled conditions and measurement of the warpage produced in the sample due to application of enamel on one side. Data obtained by the above method are compared with expansion curves. Several methods of analyzing the curves fail to produce correlation between expansion as determined on frit samples and compression in the porcelain enamel as indicated by the warp of sample plates. It is concluded that expansion curves on frit samples are not a satisfactory means of evaluating compressive stress in the porcelain enamel coating on sheet iron.     

金属基烤瓷牙冠残余应力产生的原因与控制方法

详细分析了金属基烤瓷牙冠残余应力产生的原因，认为金属与烤瓷热膨胀系数不匹配，制备工艺不合理以及金属与烤瓷结合欠佳等是产生残余应力和导致烤瓷破坏的主要原因，并对如何控制残余应力，关少烤瓷开裂和肃落进行了论述。     

APPLICATION OF GARDNER MOBILOMETER TO ENAMEL SLIP CONSISTENCY STUDIES AND CONTROLS*

A brief review of the type of instrument used by previous investigators is given. The Gardner mobilometer is described and its application to enamel-slip consistency problems and control is discussed.