MENDING STONE-WORK MOSAICS—CERESA-WORK—PORCELAIN OR CROCKERY MOSAIC

Mending or repairing stone, involving its imitations, is a widely extended branch of technical science, and one which has of late years called forth much invention. The most widely spread and ancient means of uniting and repairing this material is mortar, or the mixture of burned and then slacked lime with water. Lime is made most commonly from limestone or marble. It improves in quality when carbonate of lime in organic formation, such as sea-shells, is used; and there are degrees of excellence in these, from common oyster-shells to others of a finer kind, such as those with which the brilliantly white and hard chunam of India is made. In certain places mortar, when well made, becomes with age as hard as flint. In American towns, where anthracite coal is burned, it rots away in chimneys under the influence of sulphurous acid with great rapidity. In the Pacific Islands, where lime is made from delicate small sea-shells or coral, and mortar is like a paint or enamel, a missionary has recorded that, when he taught the natives how to make it, they whitewashed everything, even to the children, who thus became white people.

The misapplied word mastic, which suggests a gum, refers to certain modifications of mortar into which oil enters; also the oxides of lead or zinc. “Oil forms with these an insoluble soap, which includes or binds the other materials, forming, after one month’s drying, a very hard substance,” which some say is as hard as stone, but which depends entirely on the quality and combination; for I have seen so-called mastic applied to coating cheaply built houses, which cracked or crumbled away like mere plaster of Paris.

To thoroughly amalgamate mastics, it is usual to put their ingredients into casks which are two-thirds filled, and then revolved by machinery. The oil is then added. At least two days are required for the process. The following recipes for mastics are among the best, having been approved by Lehner. It may here be remarked, once for all, not only as regards mastics, but all recipes in this work, that unless the materials indicated are of the very best quality, and the processes be most thoroughly carried out, the experimenter cannot expect complete success. More than this, the experimenter must not be satisfied with a single trial. If every recipe could be at once executed by every cook, we should find the most exquisite cookery on every table in Europe. I once published the correct recipe for making objects of a peculiar kind of papier-mâché hardened. It was very easy to make. I had seen specimens of the ware, and I received the recipe from the inventor. Moreover, a great deal of money had been made by it. However, soon after I had published it I received an indignant letter from the head of a large manufacturing house, stating that they had tried my recipe and utterly failed!

French Mastic:—

Quartz or flint sand, parts 300

Powdered quicklime, ” 100

Litharge, ” 50

Linseed-oil, ” 35

Paget’s Mastic:—

Flint sand 315

Washed chalk 105

White lead 25

Minium 10

Sugar of lead in solution 45

Linseed-oil 35

The paste or “dough” thus formed should be ground with horizontal rollers in a mill, such as is used for chocolate, until all the ingredients are very thoroughly amalgamated.

A very good cement for mending, especially where the objects are exposed to water, whether they be of stone or earthenware, is made as follows:—

Powdered glass 40

Washed litharge 40

Linseed-oil varnish 20

The powdered glass is prepared by heating glass red-hot, casting it into water, grinding and sifting it. This powder is saturated with the linseed-oil varnish, and heated in a kettle. This cement sets hard in three days. Lehner observes that glass-powder serves in such recipes to resist the action of acids, &c., since it forms in combination on the surface a glaze of great hardness; that is, the glass and lead form a chemical combination. Pulverised calcined glass therefore acts not as an “indifferent” but as a chemical ingredient.

Caseine, or Cheese, forms the basis of several recipes for mending stone, as when there are holes in a block or the mortar has given way. To prepare it for use (Lehner), we let milk stand in a cool place, skimming away with the utmost care all the cream. Place this on a filter, and pour on it rain-water till it is purified from every trace of lactic acid; then tie it in a cloth, boil it in water, and spread it on blotting-paper in a warm place, when it will be a horn-like substance. This will keep for a long time. To prepare it for use, rub it in a saucer with water.

To mend stone make the following:—

Caseine 12

Slacked lime 50

Fine sand 50

Another recipe:—

Boil new cheese in water till it draws out in threads, stirring in slacked lime and sifted wood-ashes in the following proportions:—

Cheese 100

Water 200

Slacked lime 25

Wood-ashes 20

This may also be used to close cavities in trees or in wood.

A cheese cement for stone, and for many other purposes, is made as follows. It may be kept for a long time, and is very durable (Lehner):—

Caseine 200

Calcined lime 40

Camphor 1

This must be closely incorporated and kept well corked. When it is to be used mix it with water, and apply at once.

The following cement was used by the Romans especially in setting mosaics. It becomes as hard as marble, and sets with great rapidity:—To one quart of milk add the white of five eggs, and stir in powdered quicklime till a paste is formed. This composition may be used to repair or make scagliola, which is fragments of marble or stone embedded in a hard mass. When it sets, polish the surface with rasps, and rub down with a rough stone, and finally polish with marble dust, and then emery or tripoli. Beautiful slabs for tables, columns, floors, and walls can thus be made. It is valuable for repairing.

Ceresa is allied to this. We make a basis of this or any other cement which will hold firmly, and press into the surface powdered glass, which may be fine or of any degree of coarseness. Coarse grains shine most brilliantly; fine powder is best adapted to delicate shading. The effect is best when mosaic stones and gold cubes are sparingly introduced. To make the gold cubes, take two small panes of window glass, cover one side of each with varnish or mastic cement, lay between them gold-leaf, and join them. Very beautiful pictures can be made in this manner. Nor is it at all necessary that they should be finely executed for ordinary decoration. All that is needed for this beautiful and little-known art is the cement, a quantity of glass or stone of different colours, and a mortar and pestle. The mosaic cubes, with those of gold, can be bought in London.

Allied to this is an art which I believe I can claim to have invented. It consists of breaking waste chinaware, crockery, or fictile ware into small squares or triangles, and setting them as mosaic in cement. The advantage of it is the cheapness of the material, and the infinite number of shades of colour which can be selected for it. Its disadvantage is, that it will not wear as a pavement, but it is perfectly adapted to walls.

A strong, coarse cement for brick or stone work in building is made as follows:—

Slacked lime 40

Brick-dust 10

Iron filings 10

Ox-blood 8

Water 8

The blood is stirred as it comes from the slaughtered beast with a broom for ten minutes to break the fibre. It should then be mixed with the water and kneaded with the powder. Glue may be substituted for the blood. This cement, if properly made, sets very hard and adhesively.

For tiles, bricks, or composition:—

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.

For an exhaustive work on the subject of not only mending stone-work, but also of making artificial stone and many cements, as well as combining and adapting to use paper, cellulose, sawdust and shavings, gypsum, chalk, glue, &c., including not only ancient but also the most recent recipes, consult Die Fabrikation künstlicher plastischer Massen, by Johannes Hofer; Leipzig, A. Hartleben, price 4s.