Cobalt Blue is the name now appropriated to the modern improved blue 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, Zuff're 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 colour of which is brought up by fire, in the manner of enamel blues; and it is, when well prepared, of a pure blue colour, neither tending to green nor purple, and approaching in brilliancy to the finest ultramarine. It has not, however, the body, transparency, and depth, nor the natural and modest hue, of the latter; yet it is superior in beauty to all other blue pigments. Cobalt blue works better in water than ultramarine in general does; and is hence an acquisition to those who have not the management of the latter, and also on account of its cheapness. It resists the action of strong light and acids; but its beauty declines by time, and impure air greens and ultimately blackens it.

It dries well in oil, does not injure or suffer injury from pigments in general, and may be used with a proper flux in enamel painting, and perhaps also in fresco.

Various appellations have been given to tin's pigment from its preparers and venders, and it has been called Vienna blue, Paris blue, azure, and, very improperly, ultramarine.

2. Smalt

Smalt, sometimes called Azure, is an impure vitreous cobalt blue, prepared upon a base of silex, and much used by the laundress for iuntralizing the tawny or Isabella colour of linen, etc, under the name of Powder-blue. It is in general of a coarse gritty texture, light blue colour, and little body. It does not work so well as the preceding, but dries quickly, and resembles it in other respects; - it varies, however, exceedingly in its qualities; and the finer sorts, called Dumonl's blue, which is employed in water-colour painting, is remarkably rich and beautiful.

3. Royal Blue

Royal Blue is a deeper coloured and very beautiful smalt, and is also a vitreous pigment, principally used in painting on glass and enamel, in which uses it is very permanent; but in water and oil its beauty soon decays, as is no uncommon case with other vitrified pigments; and it is not in other respects an eligible pigment, being, notwithstanding its beautiful appearance, very inferior to other cobalt blues.