(7) Mottled Enamel

(a) The metal is cleaned in the usual way in an acid bath, then, without employing an alkaline bath, it is, after the usual scouring to remove the scale, placed in clear water and allowed to remain at least 1/2 hour to remove the acid. As soon as it is taken from the water, it is coated with a liquid glaze, placed in the drying - room, and the glaze slowly dried thereon. The appearance and character of the enamel are determined during this drying of the glaze, as, according to the length of the time taken in the drying and to the temperature of the drying - room, so will be the amount of oxidation. No definite rule can be given for either, as the process is affected by many circumstances, as, for instance, the degree of humidity of the atmosphere. The iron ware must be carefully watched during the process, and the temperature and time regulated by its appearance. The glaze is prepared from the ordinary ingredients, taking care to have them mixed and thoroughly smelted together in bulk to expel all carbon and other elements that would impede oxidation, and in order that the acid in the glaze may have free action. They are then ground in water and applied in the usual way.

By reason of the presence of the acid of the glaze, an oxidation of the metallic base takes place freely during the drying process, appearing in and throughout the glaze as reddish spots. This oxidation causes the enamel, when it is finally formed in the usual way by baking the iron in the oven, to assume a mottled appearance. By reason of this oxidation also, the enamel is caused to enter the pores of the iron and become more intimately incorporated with the metal, thus rendering the enamel more durable. Another manner of accomplishing these objects when the use of the ordinary bath is retained, is to increase the borate and reduce the alkaline fluxes in the composition of the glaze, thereby overcoming the alkali and ensuring the oxidation of the metal. This process does not prevent the addition of the usual colouring matter to the glaze. (Niedringhaus.)

(b) The composition of the enamel is as follows:- Silica, 100 lb.; soda - ash, 35 lb.; borax, 75 lb.; plaster of Paris or gypsum, 10 to 20 lb.; and arsenious acid in the proportion of 1 1/8 Per cent. of all the other ingredients. When these are compounded, the resultant mixture is an enamel, vitreous enough to carry a glaze of itself, with an affinity for wrought or sheet iron which causes it to adhere with extreme tenacity when burned upon it, and which will not exfoliate or absorb moisture in quantity sufficient to destroy its polish. The ingredients of the enamel, having been comminuted, are carefully mixed together, and brought to a state of complete vitrification in a reverberatory furnace, with observance of the rules applicable to glass - making generally. Then the enamel is run off as usual into water to granulate, when it is ready for grinding. In making an enamel for, wrought or sheet - iron ware, 100 lb. of the enamel is ground in an ordinary porcelain - mill, adding about 5 lb. clay, preferably white, and having a pronounced soapy feeling, which clay helps to give body to the enamel, and to prevent its crazing when it is finally fixed on the iron in the heat of a muffle.

This grinding requires about 1 working day, and should always be continued until the enamel is thoroughly ground, and the clay thoroughly mixed with it. It is advisable to introduce along with the enamel and clay in the grinding process, calcined magnesia carbonate in the proportion of 8 oz. to 100 lb. of the enamel. This salt of magnesia serves to make the enamel coat as finally applied less transparent, contributes to the flecking or spotting of it with white, and thus in a measure prevents the iron base from imparting to the enamel throughout its own dull and unattractive shade. Prepared as above, the mixture is run off through a strainer into tubs, where it is allowed to remain about 1 day, during which time a sort of ripening is effected, when it is finally prepared as follows:- The mixture being brought to about the consistency of cream by the addition, when necessary, of water, magnesia sulphate is added until the mixture is coagulated and pasty, yet still capable of being shaken out into a thin and uniform coat. 2 oz. magnesia sulphate is usually sufficient for 100 lb. of the mixture. The mixture having been finally prepared, the article is dipped into it, having been first prepared by the well - known processes of annealing, pickling, scouring, and washing.

For washing, clear water is used, the iron remaining therein until it is dipped. Care should be taken that the enamel be not laid on too heavily, and that it be evenly distributed. Having received a thin uniform coat by dipping, the article is dried, for if the enamel coat is burned while still damp it will crack or craze. This drying is accomplished, for convenience and despatch, in an oven constructed for the purpose, the range of heat in which varies from about 100° to 200°; F. (38° to 93° C), but it may be effected at summer heat in an ordinary close room. The higher the temperature, the smaller and less distinct the spots; the lower the temperature, the more pronounced the mottlings. The time required depends upon the thickness of the enamel coat, the quantity of moisture present, and the degree of heat employed. When thoroughly dried, the coat is of a whitish colour, and usually either brown or reddish - brown spotted; and when burned in a muffle, is of greyish colour, dark spotted.

The burning process is effected in an ordinary muffle at a red, cherry - red, or slightly greater heat, the ordinary time required being about 4 minutes. (Quinby and Whiting.)

Lead

A method of coating leaden pipes by incrustation with lead sulphide is as follows:-A hot concentrated solution of sodium sulphide is allowed to flow through the pipes for 10 to 15 minutes. They then appear as if coated within with a grey glaze, and water afterwards passed through them remains, it is said, free from lead.