Nevertheless, ultramarine is not always entitled to the whole of this commendation. Frequently it is coarse in texture, in which case it is apparently more deep and valuable; yet such blue cannot be used with effect, nor ground fine without injuring its colour. Again, it is apt to be separated in an impure state from the lapis lazuli, which is an exceedingly varying and compound mineral, abounding with earthy and metallic parts in different states of oxidation and composition: hence ultramarine sometimes contains iron as a red oxide, when it has a purple cast; and sometimes the same metal as a yellow oxide, when it is of a green tone; while often it retains a portion of black sulphuret of iron, which imparts a dark and dusky hue. Occasionally, it is true, artists have preferred ultramarine for each of these tones; still are they imperfections which may account for various effects and defects of this pigment in painting. Growing deeper by age has been attributed to ultramarine; but it is only such specimens as would acquire depth in the fire that could be subject to the change; and it has been reasonably supposed that in pictures wherein other colours have faded, it may have taken this appearance by contrast. Ultramarine, prepared from calcined lapis, is not liable to so deepen; but this advantage may be purchased at some sacrifice of the vivid, warm, and pure azure colour of the blue produced from unburnt stone. We have frequently found ultramarine to be darkened, dimmed, and somewhat purpled by ignition; and the same results ensue, in many instances, when the lazulite is calcined. In burning the stone, the sulphur of the pyrites is in a great measure expelled, and during its expulsion has probably a deteriorating influence on the beauty of the colour: our belief in this being so is strengthened by the fact that certain samples of ultramarine, ignited with sulphur, were not improved thereby. Similar effects are likewise caused by a careless or improper mode of treatment, for the finest lapis may yield dingy blues, containing particles of mica, metal, &c., and possessing a dull green, black, or purple hue. Of course the perfection of the pigment is dependant to a large extent upon the quality of the stone itself.