This section is from the book "A Treatise On Architecture And Building Construction Vol4: Plumbing And Gas-Fitting, Heating And Ventilation, Painting And Decorating, Estimating And Calculating Quantities", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
19. Of the three primary or fundamental colors-red, yellow, and blue, from combinations of which all other colors can be made-blue alone possesses in its entirety that quality technically known as coldness in coloring, and is therefore nearest in the chromatic scale to black. It bears the same relation to shade that yellow does to light, gradually deepening as it does, through various shades of darker hue, until it merges itself into a blue black-its extreme limit.
20. Prussian blue is made by mixing the yellow prus-siate of potash with some salt of iron. The prussiate of potash is obtained by calcining and digesting old leather, blood, hoofs, and other animal matter, with carbonate of potash and iron filings. This pigment dries well with oil and is much used for mixing dark blues, for making purples, and for intensifying blacks. Slight differences in the manufacture give considerable variation in tint and color, which causes the material to be known under various names, such as Antwerp blue, Berlin blue, Chinese blue, etc.
21. Indigo 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 known as well as used, for many centuries, in dyeing processes. In painting it is not as bright as Prussian blue, but extremely powerful and transparent, and may be substituted for some of the uses of Prussian blue, as the latter is for indigo.
Indigo blue is of great body, and glazes and works well, both in oil and water; but its relative permanence as a dye has obtained for it a false character of extreme durability in painting, a quality in which it is very inferior, even to Prussian blue.
22. The indigo plant is not, in general appearance, unlike the lucerne of our fields. The seed is sown in drills, about 18 inches apart, and, in about two months, the plants begin to flower. They are then cut down, but shooting up again, give two or three crops in the same year. As many of the newly cut, fresh, and green plants as will loosely cover the bottom, are placed in a shallow wooden vat. Water is then let in to submerge the plants about three inches, while heavy wooden frames are put on top to prevent them from floating. Left in this state from fifteen to twenty hours, fermentation sets in, much gas is disengaged, and the water becomes a light green color. The green liquor is then run off into a second vat placed below the level of the first, in which, while the fermentation process is being repeated upon a fresh supply of plants in the first vat, it is violently agitated by being beaten with poles, causing the grain to separate. The green matter suspended in the liquor then becomes blue and granular, the change being hastened by the addition of a little lime water. This operation sufficiently advanced, the contents of the vat are allowed to settle, when the intensely blue granular matter sinks to the bottom, leaving the upper layer of the liquor almost as clear as water. This is then run off almost to the bottom and the sediment drained into a third vat, where it awaits several additions from successive operations. A sufficient quantity having been accumulated in the third vat, it is permitted to subside and then thoroughly settle, the clean liquor being drawn off and the granular matter removed and filled into coarse bags hung up to drain. When sufficiently drained, the blue paste is filled into small boxes about three inches square and set to dry in the sun, which soon renders it fit for packing.
23. Ultramarine blue is a pigment obtained from the precious blue stone known as lapis lazuli, but its costliness places it outside the colors commonly used in house painting. Its beautiful shade, however, causes its name to be applied to several imitations or artificial ultramarines, of which the French and German are the most satisfactory.
Artificial ultramarine is prepared by fusing in closed crucibles a mixture of soda, silica, alum, and sulphur, and reheating the greenish product thus obtained until the blue pigment appears, when it may be removed and ground up for use.
24. Brunswick or celestial blue is made by precipitating the alumine from a solution of alum with carbonate of soda, washing the precipitate, and adding sulphate of baryta, sulphate of iron, yellow prussiate of potash, and some bichromate of potash. When dried, this mixture is known as Brunswick or celestial blue, but when the sulphate of baryta is left out, and the material not dried, it is called damp blue.
25. Cobalt blue is the name now appropriated to the modern improved blue pigment prepared with metallic cobalt, or its oxides, although it properly belongs to a class of pigments, including Saxon blue, Dutch ultramarine, Thenard's blue, Royal blue, Hungary blue, Smalt, Zaffre or enamel blue, and Dumont's blue. These differ principally in their degrees of purity and the nature of the earths with which they are compounded. The first is the finest cobalt blue, and may not improperly be called a blue lake, the color of which, like enamel blues, is brought out by fire. When well prepared, it is of a pure blue color, superior to all other blue pigments. It resists the action of strong light and acids, but its beauty declines by the action of time and impure air.