Matters.— Cobalt oxide affords an intense blue color when used in strength and can be reduced in tone in lesser proportions: Aside from its use in pigment cobalt oxide has the physical value of to sheet iron through its coefficient of expansion being the same as sheet iron. It, therefore, forms an ideal ingredient for the enamels used in cooking utensils. Perhaps it is here necessary to the uninitiated to state that one of the first require ments in an efficient enamel body is that it shall adhere to the metal under the stress of changes of temperature. On account of the high cost of cobalt nickel oxide is much used, through its considerably lower price. Limonite is a per oxide of manganese (called also pyrolusite) an is used in enamels to bleach out any impurity of tone in the white enamels. Additional propor tions will produce dark violet (the noted manganese violet of the ceramist) and mix ing a proportion of iron oxide with the limonite gives beautiful brown to black effects. Limo nite in small proportions is sometimes blended with the more costly cobalt, producing a red dish blue of much brilliancy. Ferric oxide may be prepared to produce either a bright red or a reddish violet pigment according to the amount of heat applied in its manufacture. Chromic oxide produces a green enamel and cupric oxide gives a dark-green and a bluish-green; again a bluish-green can be obtained by a combination of copper and cobalt salts or chromium and copper salts, varying from bluish-green to greenish-blue according to their equivalents in the mixture. The salts of chromium, cadmium, uranium and titanium can each be used in ob taining yellow enamels. Ferrous chromate produces a lovely brown ; it is called Brongniart's brown, after the great French ceramic chemist. Nickel is used in obtaining a gray enamel. The salts of gold produce a beautiful rose enamel as well as the loveliest pink. •But a far less expensive pink is produced by chalk, quartz, stannic oxide, borax and bichromate of potash mixed and heated to a frit, the pigment being dissolved in water to separate it from solids. Antimony produces several yellows in cluding the noted Naples yellow (lead anti monate) of the art porcelains; they are, how ever i , poisonous and forbidden in food utensils.
Enauziel Manufacture.— In the mixing oper ations only absolutely trustworthy operators are employed. The working formulae are kept strictly secret from all but the highest officials. From this point our description will be devoted to practices applied in the sheet iron and steel enamel industry. Every ingredient has to be dry and the degree of pulverization cannot be too fine to obtain a homogeneous enamel. The same care and time has to be giyen to the process of mixing the different ingredients. The French machine (milangeur) does very effective work. The various chemicals are kept in separate bins, and, in order to maintain secrecy, each ingredient is known only under a letter or number. Taking the raw material from the bins it is loaded into small cars termed ((dollies,* which are loaded to a height approximating the quantity, then run on to a scale and weighed and the excess shoveled back or fresh added. The precaution is taken to have the scale beam and its graduation marks invisible to all but the person presiding over the work. With the completion of loading all the dif ferent materials in correct proportions in their individual gdollie0 the material is mixed on a hard maple floor of the machine. First comes
the coarser material at bottom, the finest on top. The mixture made, it is hoisted by an electric, elevator and run to its bin and the process is renewed in preparing the next kind of enamel, again to be stowed in its special bin. By means of a traveling bucket holding the cor rect amount for a melt the mixed raw material (about 1,200 pounds) is carried to the blast furnace (rarely a crucible furnace). The kind of furnace common in American use is the same that is used in the manufacture of glass and is heated with natural gas or crude oil as the most economical fuel, though coal is used in the older factories. The furnace temperature has to be very carefully regulated as insufficient heat produces a slow melt liable to create a de composition, whereas too high a degree may create combustion or some chemical reaction injurious to the outcome. Some authorities place 1,000° C. for a glaie heat and about 1,300° C. for a ground coat. Control over the heat is permitted by the installation of pyrome ters. A furnace can afford from seven to eight melts in 24 hours. As the ingredients fuse separately according to their different melting points great care has to be taken that the mixed mass be kept stirred lest they separate. The length of time needed for the smelt differs ac cording to the enamels, a white fusing well at two hours while ground enamels and blues take from two-and-a-half to three hours, and so forth. The enamel is now a liquid glass, in which state it is drawn off by releasing a fire clay plug located in the front of the furnace. The molten body flows into a tank of cold water and, with noisy reaction, the vitreous liquid is torn into shreds and small pieces with explosive violence, leaving minute fissures throughout the substance. Besides toughening the enamel body this so-called ((quenching') as sists in easing the next process, which is grind ing. This grinding cannot be too fine, in fact the finer the resulting impalpable powder the brighter the resulting lustre of the enamel. The suddenly quenched glassy mass is known as a of During the grinding other materials are added, such as stannic acid for creating an opaque white, or pigments for the different colors. About 30 hours is required for grind ing in the large ball mills. The latter are cylindrical, about five feet in length and have a diameter of about six feet, and are lined with porcelain bricks. To the frit, which should re tain about 50 per cent of water, is added a small percentage of white ball-clay. About 2 per cent zinc oxide improves a white. The clay addition is made to help hold the other ingredi ents in suspension, hindering them from sub siding according to their specific gravities: it also creates opacity, increasing at the same time, the needed quality of elasticity of the enamel. Other additions are added in proportions in ac cord with the secret formulae, such as sal-am moniac, ammonium-carbonate, magnesium chloride, burnt magnesia, chloride of sodium (table salt), borax, soda, etc. The ground mass should reach the consistency of a rich cream, when it is poured into tanks and left to mature for a week or more.