Enamels.

Manufacturer and builder 9, 1871

In Vol. II., page 194, we have given the composition of enamels used for coating the inside of iron vessels, and the mode of operation now generally in use for affixing the mass so that the different expansion of iron and enamel in the heat will not affect the stability of the coating.

In this article we intend to discuss the enamels employed for the less useful purposes of ornamentation of gold, copper, fancy wood work, etc. In this connection colored enamels are most frequently used, though white is also employed.

The beauty of the enamels is altogether dependent on the purity of their component parts, and great care is therefore required in the selection.

The presence of iron in the white ground mass is especially obnoxious, and for this reason crude soda is never used in its composition; but always a purified ankle. White enamel, prepared similarly to those given in our former article, is always the ground mass of enamels, and a coloring is produced by the addition of various oxides of metals. To affix enamels to metals, a ground mass of white is burned in under a mutilaurnace first, and the colored enamel is afterward put on with a brush, and burned in at a second and less intense heating. The ground mass is required to be less fusible, and contains, therefore, less fluxing material than the colored enamels afterward added. Besides carbonate of soda, borax, and saltpetre, the silicate of lead is the principal flux used; but the latter must never be employed when a purple color is to be produced by oxide of chloride of gold, as the color would be destroyed during the heating.

The following composition, given by Dr. F. Stohmann, answers excellently for the ground moos, when purple, blue, and other tender colors are to be added subsequently 3 parts of sand, 1 part of chalk, 3 parts of calcined borax, or 3 parts of glass free from lead, 1 part of calcined borax, ¼ part of nitrate of soda, and 1 part of antimoniate of potash. The latter composition is especially adapted for blue colors. The quantity of metal oxides depends entirely on the depth of the color which it is intended to produce.

The quantity of the above component parts can be varied within certain limits; but the principal quality requisite in a good enamel must never he lost sight of; that is, it must, when exposed to a low temperature, become sufficiently soft to show a glassy surface after cooling, and, at the same time, it must not become thinly fluid when smelted. This shows that it is by no means easy to hit always the exact proportions, and that great care is required in the business.

When a white opaque enamel is to be produced, it is especially important to use very pure materials. But if the mass, after smelting, shows a slight tinge only, the clear white can, in most cases, be restored by the addition of a little pyrolueite, which, however, must contain at least 90 to 95 per cent of black oxide of manganese. If the common kinds, which often contain from 30 to 40 per cent of peroxide of iron, are used, they would spoil the color still more. An excess of pure black oxide of manganese is also injurious, as it would produce a violet color. The action of pyrolushes in producing a clear white is simply based on the fact that it gives off oxygen in the heat to organic matter which may be in the paste, and destroys it; and if small quantities of protoxide of iron are present they are oxydized to peroxide. Being itself reduced to the colorless suboxide, it is incorporated in the masts without imparting the violet color, so characteristic to its higher oxides. If a bluish white is desired, it can be easily imparted to the paste by adding it small excess of pyrolusite. A yellowish tinge may thus be completely covered.

To the original basis of white enamels, which has very nearly or entirely the composition of clear crystal glass, a certain proportion of a mixture oxide of lead and tin, and often also sand, is added. The metal oxides are prepared by calcining the metals in the air. This is easily done, as at a certain temperature a proper mixture of lead and tin burns like tinder. It is then only necessary to continually take of the skin of the oxides in order to expose now metal surface to the air. The whole mass is thus finally changed into a yellowish powder, which is best heated a second time until no more sparks tissue from it. The second heating is performed in order to oxydize the small globules of metal which may have been scraped off the surface of the metal together with the oxides. The proportion of lead and tin taken varies greatly, from 15 to 40 parts being used to 100 parts of lead. Oxidation takes place the more readily, the more lead there is in the alloy.

A very fine white enamel is obtained by smelting one part of the oxides (made by calcining 2 parts of lead and 1 of tin) with 2 parts of finely pulverised crystal glees, and a small quantity of pyrolusite. When the many has become entirely fluid, it is poured into cold water, and subsequently powdered to repeat the operation of smelting. This is often done three or four times, in order to produce an entirely homogeneous mass.

The purity of the white is principally dependent on the proportion of tin in the composition, a larger addition producing a more prominent white. The less sand there is used, the snore fusible is the enamel, and vice versa.

Sometimes the antimoniate of potash or soda is used in the composition, but it must never be brought together with a glass containing lead, as a very inferior enamel would result. The usual composition of enamels, in which the antimoniate is present, is 3 parts of crystal glass to 1 part of antimoniate. It is mixed and smelted in the usual way.

For fine white enamels pure sand is usually not taken. A mixture of 1 part of sand, amd ¼ part salt, which has been calcined, is smelted with ¼ part oxide of lead; this mass is powdered and used instead of sand.

According to Clonet, a mixture of 60 parts of sand, 30 parts alum, 35 parts salt, and 100 parts of litharge produces a good white enamel, which resists a high temperature. It is desirable that the sand contain a small proportion of talc.

The most important manufacture in which enameling is used is that of the dials for clocks and watches. The proceeding is as follows: if the dials are large, both sides are covered with a coarsely pulverized mass of enamel, which has been previously prepared, and is only made into a pasty mass with a little water. This is allowed to dry gradually at a low heat, and then the whole is exposed to the heat of a mufflefurnace, until the enamel has become soft and glassy. It is taken out, and the small blisters which are usually found on the surface are ground down with fine sand. The dials are then brought back into the furnace, and once more heated until the enamel just commences to smelt on the surface. On taking them out and cooling them, it is found that the surfaces have the peculiar beautiful lustre which we see on dials. The circles are then drawn and divided into hours and minutes, by means of a machine; the marks are made with black enamel, which is mixed with a little lavender oil; and to burn in the black marks the dials are exposed to another slight heat. Small dials are enameled on one side only.

When jewelry or other ornaments are to be enameled on parts of their surfaces only, the respective spots are first roughened, and an indenture is made either with is chisel or a stamp. These places are then filled with enamel, which is burned in.

Colored enamels are mostly spread on a white ground mass, which has been previously burned in, but sometimes they are put directly on the metal. In the latter case a comparatively low heat is employed in burning in the enamel.

The colors in enamels are produced by oxides of metals, as mentioned above. Their stability in the heat depends on the degree of the affinity which the metal has for the oxygen.

According to Dr. F. Stolimann, the different colors are prepared as follows:

Purple.

Oxide or chloride of gold is used to produce this color. Very little gold is sufficient to color a large quantity of enamel. As before mentioned, a paste must not be used for this enamel which contains lead or tin; it is customary to use one consisting of crystal glass, borax, and saltpetre, or of 3 parts quartz-sand, 1 part chalk, and 3 parts calcined borax. The purple color can not stand high temperatures.

Red.

We have no metal the oxide of which gives us directly a red color. It is therefore produced by adding to the paste of the enamel a mixture of peroxide of iron and clay, which is hard to fuse. But even in burning this in too high a heat must he avoided, in order to prevent the formation of silicate of peroxide of iron, which would destroy the color. The mixture is made by heating 2 to 2¼ parts of copperas and 1 part of sulphate of alumina, first slowly, to drive out the water, then stronger, until the whole has assumed a red color. The right moment is seen by samples, which are from time to time taken out of the furnace, and allowed to cool, when the true color appears. The depth of the red color can be widely varied by altering the proportion of copperas and sulphate of alumina. Thus the proportion above given would produce a deep red, while 3 parts of sulphate of alumina and 1 part of copperas would make a flesh-color.

Yellow.

A good yellow color is made by mixing 1 part of oxide of antimony, 1 part of white-lead, 1 part of alum, and 1 of sal-ammonia. These substances are heated over a fire until the sal-ammonia has sublimated, and the whole has assumed a yellow color. With a small quantity of peroxide of iron, a yellow color can also be produced, if a high heat is used in burning the paste. This may consist of sand, oxide of lead, and borax for the antimony yellow, and of alum, oxide of lead, salt, and talcose sand, for the oxide of iron.

A good yellow may also be produced with the salts of silver. For this purpose a small quatitity of oxide of silver, for instance, is put on the surface of the enamel, which has already been burned before, and a low heat is given. After cooling, metallic silver will be found coating the spot. On scraping this off, the enamel below will be found yellow.

Green.

This is produced by burning 2 ounces of oxide of copper or chrome with 4 ounces of paste. A small quantity of peroxide of iron is sometimes added.

Blue.

Blue is always produced by oxide of cobalt. It is mixed with a paste which contains little or no lead, as this would deteriorate the color.

Black.

Black is produced by adding large quantities of metal oxides with the paste, so that their cutler be comes so dark as to appear black. It can be done, for instance, by mixing pyrolusite, oxide of copper, and oxide of cobalt, or, according to Clouet, by mixing an easily fusible clay with a third of its weight of protoxide of iron.

Violet.

It has already been mentioned that this color is produced by black oxide of manganese. Care must be taken that no organic substances — as, for instance, coal-dust, smoke, etc. — come in contact with it in burning, as these destroy the color.

 

According to Armengaud's Génie Industiel 1865, an enamel which is applicable to fine stonework, metals, and fancy wood-work, is produced without the employment of heat, in the following manner:

1. By mixing mineral salts or oxides, which are capable of forming silicates with silica, in a solution of soda water-glass of syrup consistency, with a certain amount of silica.

2. By precipitating these silicates by means of a fixing medium, which consists of a salt acting by double decomposition.

The artificially produced silicate of soda is dissolved in water, and the solution concentrated to the consistency of syrup; then the finely-pulverized mineral, mixed with finely-ground quartz, is added to the solution of the water-glass, and by careful mixing and rubbing the whole, it is made into a homogeneous paste, or thin dough.

This is spread on the object which it is to adorn, or to guard against the influence of the atmosphere. As soon as the mans has become dry, the fixing-medium, or reagent, which is a more or less clear solution of a salt by which the silicates are to be precipitated, is spread over it with a fine brush.

The precipitation takes place after some time; and when the covering substances have become entirely hard, the covered object attains a much greater durability. This is the case, because it has received an entirely new structure for a thickness of several millimeters from its surface, which now is much more perfectly coherent.

The coherence and different appearance of the surface can be varied in intensity, by
1. Giving a larger addition of silica, from [?] to [?] of the mass, if an enamel is desired which can resist a high temperature.
2. By augmenting the quantity of soda, or potash water-glass. This must be done to the higher a degree the easier fusible the product is intended to be.