Both quick and slaked lime dissolve to a certain extent in water, and impart strongly alkaline properties thereto, lime being one of the strongest of bases. On exposure to the air, the solution of quick lime in water (lime-water) quickly forms an opalescent superficial film of calcium carbonate, and in a short time no more lime is present in solution, the whole having been transformed into calcium carbonate, which settles down to the bottom of the vessel as a very fine powder.
Limestone that consists entirely of calcium oxide and carbon dioxide is of rare occurrence in Nature, foreign substances being nearly always present. Since the nature of these admixtures is of the greatest importance to the colour-maker, owing to the considerable influence they exert on the suitability of the minerals for his purposes, it is necessary that these extraneous substances occurring in limestone should be more closely described.
Nearly all varieties of limestone contain certain proportions of ferrous and ferric oxides. The presence of ferrous oxide, when the relative amount is but small, cannot be detected by mere inspection; and even many limestones containing really appreciable quantities of ferrous oxide are pure white in colour so long as they are in large lumps. If, however, such a limestone be reduced to powder and exposed to the air for a short time, it gradually assumes a yellow tinge, the depth of which increases with the length of exposure.
The cause of this change is due to the fact that ferrous oxide has a great affinity for oxygen, by absorbing which it changes into ferric oxide. (Ferrous oxide consists of FeO, ferric oxide of Fe2O3.) Ferrous oxide and its compounds are of a pale green colour which is not very noticeable, whereas ferric oxide has a very powerful yellow colour, and consequently the limestone, when its superficial area has been greatly increased by reduction to powder, assumes the yellow tinge due to ferric oxide. A limestone exhibiting this property can evidently not be used for making white earth colours, but is, at best, only suitable for mixing with other colours.
Occasionally, limestone contains varying quantities of magnesia, and when this oxide is present in large amount, changes into another mineral known as dolomite. In many places this dolomite forms large masses of rock, which, however, is not employed for making colours, owing to the yellow shade imparted by the fairly large amount of ferric oxide present.
Gypsum (Alabaster)
This mineral occurs native in many places, and is frequently worked for a number of purposes. Gypsum occurs in Nature in a great variety of forms. The purest kind is met with either as water-clear crystals, which cleave readily in two directions, or as transparent tabular masses (selenite) which also cleave easily. Micro-crystalline fine-grained gypsum is milk-white in colour, highly translucent and is largely used, under the name of alabaster, in sculpture. Owing to its low hardness, alabaster can be readily cut with a knife, and on this account is frequently shaped by planing or lathe-turning.
Gypsum is generally met with in dense masses, which may be of any colour, grey, blue and reddish shades being the most common, whilst pure white is rarer. The dark-coloured varieties can only be used for manurial purposes; but the white finds a twofold application as a pigment, and, in the calcined state, for making plaster casts.
In point of chemical composition, gypsum consists of sulphate of lime, or calcium sulphate (CaSO4 + 2H2O). It is soluble in water, but only in such small quantity that over 400 parts of the latter are needed to dissolve one part of gypsum. On being heated to between 120° and 130° C., gypsum parts with its two molecules of combined water and becomes anhydrous calcium sulphate or burnt gypsum. When this latter is stirred with water to a pulp, it takes up the water again, with considerable evolution of heat, swelling up considerably and setting quickly to a solid mass.
The number of substances exhibiting this property being small, burnt gypsum is very frequently used for making casts of statuary, and for stucco work in building. Finely ground white gypsum can also be used as a pigment, but is inferior to calcium carbonate in covering power, and is therefore seldom employed for this purpose, though frequently added to other colours. The mineral known as muriacite or anhydrite consists of anhydrous calcium sulphate; and is therefore similar in composition to burnt gypsum; but it lacks the property of combining with water when brought into contact therewith.