Guideposts in Selecting Porcelain Enamels and Ceramic Coatings—A Summary

The papers presented in this Symposium indicate that ceramic coatings and porcelain enamels are indeed highly versatile materials. They are thermally and electrically insulating; they are thermally and electrically conducting. They are refractory and resistant to flow at high temperatures; they are plastic and capable of deformation without sustained stress at moderate temperatures. They are essentially glassy and brittle at room temperature; but a porcelain enameled sheet-steel angle under optimum conditions can be twisted through upwards of 180 deg per ft without failure. They are subject to abrasive wear and will lose their gloss with continued scrubbing or gouging forces; yet they are so resistant to abrasion that the life of steel coal chutes is decisively lengthened by coating and recoating with porcelain enamel. They are subject to attack by acids, alkalies and hypochlorites; on the other hand, they are so resistant to chemical attack that there is a substantial and time-tested industry based on the manufacture of procelain enamel lined (glass lined) chemical and food processing equipment, the prime purpose of which is the prevention of corrosion and the preservation of aclean, sanitary surface. That these apparently contradictory statements are true is due in part to the application of different definitions of terms and limits, but it is due even more to the fact that this is not a single material of fixed composition, but rather a whole class of materials that could properly be placed under the general heading of inorganic coatings. As a matter of convenience this class of materials has been subdivided into two groups that have become generally known as porcelain enamels and ceramic coatings, respectively. In a strict sense, from the viewpoint of the lexicographer, it must be admitted that the general term “ceramic coatings” includes the finishes that have long been known in this country as “porcelain enamels.” But in justice to both groups of finishes some distinguishing terminology was needed. Since the more descriptive term “refractory ceramic coatings” was too long for general use, the abbreviated expression “ceramic coatings” has become prevalent, especially in industries for which the introduction of this type of protective finish is a recent innovation,as for example in the aircraft industry. The distinction between these groups of coating materials has been ably presented by G. H. Spencer-Strong earlier in this Symposium. Essentially, porcelain enamels fused on metals above a red heat are the conventional type of inorganic coating that is used on the more durable advertising signs, on steel architectural panels, on kitchen ranges, sanitary fixtures, and other familiar objects. Their function is often as much decorative as protective, and normally the top operating temperatures do not exceed those used in food processing. Ceramic coatings, on the other hand, serve primarily the purely functional purpose of protection, and only incidentally give the coated objects an attractive appearance. The range of operating temperatures is approximately from the threshold of redness on the low end up to the maximum temperatures at which they can be made effective. Both procelain enamels and ceramic coatings are susceptible of endless modifications in composition by means of which they can be adapted to specialized uses. It is the purpose of these remarks to bring the diverse aspects of this general class of materials, as presented in the papers of this Symposium, into perspective as an integrated picture. It is the hope of the authors that this Symposium will give you a firm grasp of the range of essential properties of these materials, the type of applications in which they are currently serving effectively, and—most important—a resolve to explore the possibilities of putting this information to use in the solution of your own problems. If this type of finish proves to be the best available material in a substantial fraction of the potential uses for which it is tried, the effort of investigation will be amply repaid. So many new applications have been found in the past few years, and are continuing to be found, that it is obvious the ore-vein of new applications is far from exhausted.