White, black, yellow, red, or gray are the usual colors. On one of these grounds, before it is dry, about one eighth of an inch of cement of one of the other colors is laid, the pattern is pounced on, and the parts outside the pouncing are scraped off with a modeling-tool, a knife, or a bit of stick. When the whole has set, you have a picture or a pattern in two colors. This sort of work has stood in England for over twenty years when executed in the country, and in Italy the whole fronts of many large palaces have been adorned in this way, and have stood for centuries.
Public buildings built of polished marble, granite, porphyry, jasper, agate, or onyx, or faced with these, are sometimes ornamented by inlaying pictures or patterns with colored marble or precious stones; but I do not know of any external example in England. This work is called pietra dura. The Taj Mahal in India is a celebrated example. There are plenty of slabs, basins, vases, paper-weights, and jewelry imported from India and Italy of pietra dura work.
All external work in calcareous marbles soon perishes in the atmosphere of London, whether plain or inlaid, and all incised work filled with mastic so soon gets blackened that to execute it is merely labor lost. The only other work that can be used externally is in metal. Iron rusts unless constantly painted, and almost all other metals turn black. Real block-tin, not tinned iron, is said to stand the climate of London, but of course does not lack its pall of soot. Iron plates tinned are much used in Switzerland for the covering of steeples, but even there they get rusty. Lead takes its own blackish gray, but, as it otherwise stands the climate well, I wonder it is not more used for ornamental purposes, as vases, statues, roof-crestings, and the like. When I was a boy, some plumbers' shops were ornamented with leaden statues, vases, and ornamental cistern fronts. Lead is still used for ornamental roof-crestings in France, often heightened by gold, black varnish, and color. Lead is still much used for ornamental accessories in Holland—or perhaps I ought to say, was once used. Up to a short time ago there were leaden statues and vases in the gardens of the stately mansions in Mark Lane, near the Tower of London; there are still some at Hampton Court, and they would do very well in the niches or on the pedestals of our red brick fronts, if we could not afford bronze.
It is unnecessary to speak of the ordinary freestones that weather in London, the sandstones, the brick, both cut and moulded, the red, yellow, or gray terra-cotta, for all these have more or less granulated surfaces that can only be cleaned by tooling or rubbing, but plaster has never of late, as far as I know, been even tried—I mean plaster of common sand and lime, or, what is still better, of lime and marble-dust. Vitruvius tellS