CaO + H{2}O = Ca(OH){2} Lime Water Calcium hydroxide. When the amount of water added to burnt lime is no more than sufficient to effect its transformation into hydroxide, this latter, as already stated, forms a delicate white powder. The addition of more water results in the formation of a homogeneous pulp, of a peculiar fatty character. Since this fatty appearance is only possessed by pure lime, it is a criterion of high quality in burnt lime, and contrasts strongly with that of the less valued poor (or lean) lime. Calcium hydroxide acts as an extremely powerful base, and therefore must not be mixed with colours that are sensitive to the action of strong bases. As a matter of fact, its direct use in painting is very small. Of course, a thin milk of lime is used for whitewashing walls, etc.; and if any colouring ingredients are added they must be such--_e. g._ ochres--as are not affected by the lime. Nevertheless, quick and slaked lime are very important in colour-making, as forming the originating material for the preparation of a number of colours. When slaked lime is mixed with sufficient water to form a stiff pulp, and is left exposed to the air for some time, a change will be observed to take place, the mass solidifying gradually (commencing on the outside) and finally crumbling to a soft white powder. This change is due to chemical action, the lime having a great affinity for carbon dioxide, which it readily takes up from the atmosphere--a fact which explains the solidification mentioned. It would be erroneous to assume that the lime is again completely converted into calcium carbonate in this way; for, though such conversion does ultimately take place, it requires a very long time for completion. The resulting compound is, actually, a double compound of calcium oxide and carbonate. Although this compound has fairly strong basic properties, they are, nevertheless, far weaker than those of caustic lime, being partly neutralised by the carbon dioxide absorbed. If the superficial area of the slaked lime be increased by spreading it out thinly, so as to offer greater opportunity for the action of carbon dioxide, the formation of the double compound in question will be greatly accelerated. This double compound is prepared artificially in special works, and the resulting colours are put on the market under various names. They, too, must not be mixed with colours that are sensitive to alkali, and on this account they cannot be used in fine paints. If applied as a white priming to the walls of rooms, care must be taken to cover the coating with some substance that will protect the topping colour from the action of the lime. For this purpose, painters use a wash of milk, soap and water, etc.