Prussian blue is made by mixing prussiate of potash with a salt of iron. The prussiate of potash is obtained by calcining and digesting old leather, blood, hoofs or other animal matter with carbonate of potash and iron filings. This color is much used, especially for dark blues, making purples and intensifying black. It dries well with oil. Slight differences in the manufacture cause considerable variation in tint and color, which leads to the material being known by- different names, such as Antwerp blue, Berlin blue, Harlem blue and Chinese blue. Indigo is produced by steeping certain plants in water and allowing them to ferment. It is a transparent color, works well in oil or water, but is not durable, especially when mixed with white lead.

Ultramarine was originally made by grinding the valuable mineral lapis lazuli. Genuine ultramarine so made is very expensive, but artificial French or German ultramarines are made of better color, and cheaply, by fusing and washing and reheating a mixture of soda, silica, alum and sulphur. This blue is chiefly used for coloring wall papers.

Cobalt blue is an oxide of cobalt made by roasting cobalt ore. It makes a beautiful color and works well in water or oil.

Smalt, Saxon blue and royal blue are colored by oxides of cobalt.

There are a few other blues, such as celestial or Brunswick blue, damp blue and verditer, that are chemical compounds, compounds of alum, copper, lime and other substances.

Brunswick blue is essentially a mixture of Prussian blue and barytes. It is prepared by thoroughly mixing barytes with water, adding a solution of copperas, then a solution of red or yellow prussiate of potash, stirring constantly so as to ensure the thorough incorporation of the barytes with the blue. After filtering, washing and drying, the blue is ready for use.

As a pigment it is quite permanent and resists exposure to the air, light and most of the other influences which act on pigments. It has the curious property of fading a little on exposure to light and of recovering its original intensity of color in the absence of light.

Prussian blue can be mixed with nearly all other pigments without being affected or changed by them or affecting them in any way,

Indigo, or Indian blue, is a pigment manufactured in the East and West Indies from several plants, but principally from the Anil, or Indigofera. It is of various qualities, and has been long known and of great use in dyeing.

In painting it is not so bright as Prussian blue, but is extremely powerful and transparent, hence it may be substituted for some of the uses of Prussian blue, as the latter now is for indigo.

It is of great body and works well both in water and oil. Its relative permanence as a dye has obtained it a false character of extreme durability in painting, a quality in which it is very inferior even to Prussian blue.

Roofing Brush.

Fig. 15. Roofing Brush.

Indigo is injured by impure air, and, in glazing, some specimens are firmer than others, but not durable, in tint with white lead they are all fugitive; when used, however, in considerable body in shadow it is more permanent, but in all respects inferior to Prussian blue in painting. Intense blue is indigo refined by solution and precipitation, in which state it is equal in color to Antwerp blue. By this process indigo becomes more durable and much more powerful, transparent and deep. It washes and works admirably in water; in other respects it has the common properties of indigo.

The indigo plant, in its general appearance, is not unlike the lucerne of our fields. The seed is sown in drills, about 18 inches apart, and soon makes its appearance above the ground, when it requires incessant care to keep the weeds down, which would otherwise soon choke so tender a crop. In about two months the plants begin to flower, and are then cut down, but shoot up again and give two or three more crops in the same year. Formerly indigo was carefully dried after being cut, and even fire heat was sometimes used for the purpose; but now, at least in India, the practice is abandoned, and it is found in every respect better to use the plant whilst fresh and green. The first process is to place in a shallow wooden vat as much as will loosely cover the bottom of it; water is then let in so as to cover the plants about three inches, and heavy wooden frames are put on the top to prevent them from floating. Being left in this state for from fifteen to twenty hours, fermentation is set up, and much gas is disengaged, the water becoming a light green color.

The green liquor is then run off into the second vat, which is placed below the level of the first, in which, whilst the fermentation process is being repeated upon a fresh supply in the first vat, it is violently agitated by being beaten with poles; this causes the grain, as it is called, to separate, and the green matter suspended in the liquor becomes blue and granular, and this change is promoted by the addition of a little lime-water from time to time. When this operation is sufficiently advanced the contents of the vat are allowed to settle, and in a short time the now intensely blue granular matter has sunk to the bottom, leaving the supernatant liquor almost as clear as water; this is then run off nearly to the bottom, and the sediment is run into the third vat, which is below the level of the second; here it awaits several other additions from successive operations, and, a sufficient quantity being accumulated in the third vat, it is suffered to subside and when thoroughly settled the clear liquor is drawn off, and the granular matter is removed and filled into coarse bags, which are hung up to drain.

When sufficiently drained the blue paste is filled into very small boxes, about three inches square, and set to dry in the sun, which soon renders it fit for packing.

There are, of course, other blues, but the above will be sufficient for all purposes, and the painter is urged not to adopt others until he knows their qualities from actual trial, and from having watched the effect which time and exposure to atmospheric action have had upon them.