PORCELAIN ENAMEL: ITS MANUFACTURE AND APPLICATION M. J. BAHNSEN Ferro Enamel Corporation, Cleveland, Ohio
T m s PAPER is an attempt to show the most important steps in a manufacturing process. In order to give you a clear idea of the process we will actually show you some of these different operations. What is porcelain enamel? Porcelain enamel is a glass coating actually fused to a metal base. We will confine our discussion to commercia1 practice in which the metal base is either cast iron or sheet steel. , The present day uses of porcelain enamel are on bathtubs, sinks, wash basins, stoves, clothes-washing machine tubs, pots and pans, signs, and other objects which require a hard, easily cleanable decorative finish on cast iron or sheet steel. Demonstration No. 1. We have here a piece of cast iron and a piece of window glass. We place the glass on the cast iron. Now we have the cast iron covered with the grass. This fits our previous defmition of porcelain enamel except for the fact that the glass must be fused or melted on to the cast iron. The analogy of window glass on cast iron fails, because we cannot fuse window glass to cast iron. One reason is that window glass melts a t such a high temperature that the cast iron begins to soften and it droops out of shape. Demonstration No. d . This shows a piece of cast ironon which we attempted to fuse some window glass. You will note that the cast iron plate is no longer flat, but it has sagged in the middle where it was not supported by the burning bars. You will also note that the glass has largely cracked away from the cast iron. This is because cast iron contracts more on cooling than does window glass. When this sample was removed from the furnace a t a temperature of 1800°F. the glass was soft and completely covered the iron. But as they cooled, the iron contracted more rapidly than the glass, and the glass cracked away. This makes an excellent lecture demonstration, but neither the time nor the furnace was available for it. You will have already answered the next question. If we cannot use window glass, what kind can we use? We must make a special glass, one that softens a t about 1300°F. and one that expands and contracts with temperature change a t about the same rate as cast iron. To make glasses that meet these requirements we are forced to use a number of chemical compounds which, when melted together, form a glass. You are all aware that window glass can be desig-
nated as soda-lime-silica glass. If we use the same type of terminology we can say that porcelain enamels are alkali-alumina-boro-fluo-silicates (with various other elements added, depending on the particular properties desired). A typical enamel composition is as follows: Feldspar (NaKO, AIL&, 6(S10p) Borax (NctaB207.10H20) Quarts (SiO.) Soda Ash ( N a C 0 3 Sodlum N~trate(NaNOJ Fluorspar (Cah) Cryolite (N%AIFJ
30 25 20 5 5 5 10 100
To illustrate this composition better we show this chart which gives the comparative volumes of the ingredients (Demonstration No. 3). These ingredients are melted together to form a glass. This glass is one example of porcelain enamel. There are hundreds of compositions in actual use today, each having special properties. Your next question might be: is this glass cast or molded in the form of a bathtub and then fused to the metal base of the tub? The answer is, no! We grind this glass very fine and then apply it to the metal. To aid in the grinding of the glass, it is fritted; that is, the molten glass is poured into water. Demonstration No. 4. We have in this furnace a crucible which contains the raw mix for a low-melting enamel. This has been melting while I have been talking and should be melted by now. We will pour this molten glass into a beaker of water. You will notice that the glass shatters into small friable particles. This operation is called the fritting operation and the product-the fritted porcelain enamel glass-is called frit. Now that we have actually smelted and fritted some porcelain enamel, we must proceed with the application of this frit to the metal base. There are two major methods of application being used commercially. The simpler and older procedure is "dry process enameling." Most of the enameled cast iron in use today was finished in this manner. Such articles include cast iron sinks and bath tubs. The next step in dry process enameling is the grinding of the frit. This operation is carried out in a ball mill. This is an example of a small size mill (Demonstration No. 6 ) . These mills are lined with porcelain and half filled with porcelain balls. A charge of dried frit is introduced. ( A normal charge for this size mill is 100 g.) The cover is then placed on (actually a porcelain cover
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is used, but we are using a plastic cover so that you can After grinding for two hours, we will have this much see the operation) and the mill is rotated for a sufficient enamel slip (100 ml.). We now return to the preparation of the metal for the period of time to powder the frit. I n this case, rotating the mill a t 50 r. p. m. for 4 hours would grind the frit t o application of the wet-process enamel. After the metal a fineness of 85 per cent through a 200-mesh screen. has been formed into the desired shape, it is cleaned by This powdered enamel frit is now ready t o be used in dry placing in a strongly alkaline boiling cleaning solution. processing enameling. The cleaned metal is pickled or etched in an acid solution The cast iron is now prepared for enameling. This (usually 7 per cent sulfuric acid a t about 160°F.). This means that aU dirt must be removed from the metal. is followed by a rinse and a neutralizing solution of some This is usually accomplished by carefully sandblasting dilute alkaline mixture. When the ware has been dried it is ready for application of the porcelain enamel slip. the object t o be enameled. The cast iron is then coated (usually by spraying) The cleaned, pickled, and dried steel is then dipped with a thin water "slip" of a clay and glass mixture. into a ground-coat enamel slip. The object is heated a t slightly above the boiling point Demonstration No. 9. You are probably wondering of water until this thin coating is dry. This coating why this enamel slip has a dark blue color. This is be(as shown here, Demonstraiion No. 6) serves mainly t o cause the glass contains some cobalt and nickel oxide prevent the cast iron from oxidizing during the next smelted into the glass. These metal oxides give the operation. enamel the property of adhering t o the steel more tenaThe coated iron is now placed in a furnace, and heated ciously than if they are omitted. slowly to a temperature of 1300' t o 1400°F. Before After the steel has been completely immersed in the this lecture started we placed a slip-coated piece of cast ground-coat slip, it is allowed to drain (or is shaken) t o cause the excess material to flow off. The dipPed and iron in the furnace. Demonstration No. 7. You will note that this red-hot drained piece is then dried and looks like this (Demonpiece of metal has a matte, or duU coating, due to the stration No. 10). This metal is now covered by a coatslip coat we first applipd. Now we must work rapidly, ing of glass separated into millions of minute particles as we do not want the cast iron piece to become cold. and held to the metal by the adhesive qualities of the We dust Borne of the powdered frit we previously pre- clay which was milled into the enamel slip. You can pared on the hot surface. This enamel powder tends t o see that this powdered glass is held firmly enough that melt as it strikes the hot metal and thus forms a glassy the ware can be handled (Demonstration No. 11). The enameled surface. However, t o make certain that all of ware is now ready t o he fired. This is accomplished in the powdered frit has melted t o form a continuous glass much the same way as the cast iron ware is fired, but in surface, we put the coated piece back in the furnace to this case we usually fire a t 1500' t o 1600°F. Also, a be reheated. much shorter time is required because there is a smaller You have now witnessed the complete operation of mass of metal t o be heated. . cast iron enameling. This method ie actually in use Demonstration No. 12. On removal from the furnace today. However, the industrial operations are not the ware is red hot. I t cools rather rapidly and when quite as simple as shown here, due largely to the size and cold will look like this. This is a typical example of a weight of the objects being enameled. ground-coated piece of ware. We will now proceed to the "wet process" method of However, we all think of porcelain enamel as a gleamenameling. Fundamentally, this method differs from ing white finish. This is obtained by applying a second the "dry process" method in several ways. coat of a white enamel over the fired and cooled ground coat. 1. I n this case the powdered enamel is applied to the The white enamels are opacified with antimony oxide, metal in the form of a wet slurry. zirconium oxide, or titanium oxide. These ingredients 2. The metal is always a t room temperature when are mixed into the raw batch and are smelted in the glass the enamel slip is applied. so that they form an integral part of the enamel glass. 3. Wet process enamels are usually applied to sheet The smelting, fritting, and millmg operations are persteel or small cast iron articles. formed in the same manner as described previously. Demonstration No. 13. The milled enamel is either The successive steps in the application of wet process dipped or sprayed over the 6red ground coat. Dipping enamels follow: The frit is first powdered in a ball mill, but in this case water is added so that a wet slurry of the is used when both sides are to be covered with white, but glass and water is obtained. Also, approximately five when only one side is to be white the cover-coat enamel per cent of a plastic clay is added t o aid in the suspen- is applied by spraying, in much the same manner as paint is sprayed. sion of the ground glass in the water. The white coating is dried by evaporating the water Demonstration No. 8. Again referring to this small from the coating. The dried coating has a matte apmill, for a normal charge we will use: pearance, and can be brushed away from places that are 100 grams frit not to be covered. One method of doing this is to brush 5 grams clay through a stencil. 40 grams water Demonstration No. 14. This stencil has "A. C. S."
SEPTEMBER, 1948
for the American Chemical Society cut through it. -We object, you will: brush through it and remove some enamel t o form the 1. Know more about porcelain enamel than you did letters "A. C. S." in black with a white background. After firing, it will look like this (Demonstration No. 15). before. 2. Remember more about this demonstration than Thus far we have shown you the main operations in the preparation and application of porcelain enamel. We if I had only talked. may be accused of over-simplification in the presents3. Realize that an industrial process can be most tion of this story, but if we have been successful in our clearly shovn by a laboratory demonstration.
