| Slacked lime | 100 |
| Sifted stone-coal ashes | 50 |
| Stirred ox-blood | 15 |
It may be observed that many of the cheaper cements can be employed to form large bricks by combination with broken stone or rubble, gravel, pebbles, brickbats, &c. Another method, called Concrete, is to make cases of boards, and to form a solid wall by pouring in the mixture, or ramming it down, according to its hardness. Thus a house is made entirely in one piece; but its excellence depends entirely on the quality of the cement employed, and on the care taken in building. Simple lime mortar, if not of a superior quality, hastily formed, as I have seen, is very apt to crack and break off. Where hydraulic cement is cheap and good, houses can be built as firm as granite. A good and strong cement of this kind can be made as follows:—
| Burned lime | 10 |
| Caseine | 12 |
| Hydraulic cement | 30 |
The proportions may be very much varied in such cements according to their price, but generally with a satisfactory result.
Fractures or discolorations in marble, as in statuary, are so perfectly repaired in Florence that the juncture is not perceptible. Even dark spots are drilled out. The process is to drill a round concave hole, and cut the piece to be inserted so as to exactly fit as a convex plug. It is then fastened in with transparent mastic or other clear cement. It will be seen, on due consideration, that this is extremely ingenious, because by it alone can a perfectly tight fit be secured. By turning the plug in the hollow it speedily grinds itself into an accurate plug; so when the cement is applied it can be reduced to a minimum—in fact, by this means the line of junction is reduced to its finest limit.
Where a very strong cement is needed for stone-work, it can be prepared by mixing a fine cement powder—e.g., Portland cement—with liquid silicate of soda. As it dries almost at once, it must be promptly applied. It is particularly well adapted for building under water, since it then becomes extremely hard. Before applying it smear the stone with pure silicate.
The following is highly commended by Lehner:—
Mending statues of gypsum or plaster of Paris is allied to stone-work. The broken edges are washed with water till no more is absorbed and the surface remains wet. Then stir fresh calcined white plaster of Paris with much water to a thin paste, and continue to stir this till it is cold. Then rapidly paint this paste on the broken edges, continuing to press the two together till they set hard.
It is, says Lehner, a peculiarity of gypsum that when mixed with alum dissolved in water it takes a much longer time to harden, but is very much harder in the end. Thus, if we let the powdered gypsum lie for twenty-four hours in alum-water, dry it, and then calcine it again, the powder when mixed with water sets to a stone as hard as marble.
Plaster of Paris and alum, combined with the fine powder of calcined glass, form a very hard and durable cement, of very general utility in all mending of stone-work.