A somewhat similar method is employed in giving to concrete the appearance of cut stone. The materials used in the surfacing mortar are Portland cement and crushed rock, the character of the rock depending upon the color and texture desired in the finish. The molds having been removed after the proper time has elapsed, the mortar covering the face of the particles of crushed rock is removed by brushing or by washing the surface with a weak acid solution, followed by clean water, and finally by an alkaline solution to prevent any further action of traces of the acid which might be left on the face. This last method is said to be patented, "the patent covering the obtaining of a natural stone finish for concrete by mechanical, chemical or other means." 2 It is hoped that such a blanket patent is somewhat less formidable than it appears.

If the sheathing planks of the molds can be removed about twenty-four hours after the concrete is placed, the same effect may be produced without the use of acid. By using plenty of water the cement and finer portions of crusher dust in the face may be washed out with a stiff corn broom, leaving the facets of the crushed rock exposed.

1 Capt. Lansing H. Beach, Corps of Engrs., U. S. A., in charge. Work described by Mr. W. I. Douglas, Engr. of Bridges, D. C, Engineering News, Jan. 22, 1903.

2 Engineering News, May 21, 1903.

534. Pointing And Bush-Hammering

If the molds have been left in place until the concrete is set hard and it is found that the face of the concrete is not what is desired, it may still be improved although it may not be plastered. With this object the face is sometimes tooled with the stone cutter's point to give the appearance of rough pointed or rock face masonry. Grooves may be cut to block off the work into rectangles of the proper size, then a draft of one to two inches may be left along all of these artificial joints, and within the draft line the rough pointing may be done.

A cheaper method, however, is to bush-hammer the entire face, and this tends to mask any lack of uniformity in color or smoothness. Bush-hammering may be done by ordinary laborers at a small Cost, as one man can go over from fifty to one hundred square feet in ten hours, making the Cost from 1 3/4 cents to 3 1/2 cents per square foot, with labor $1.75 per day. Where it is decided beforehand to bush-hammer the work, less pains need be taken in dressing the lagging of the forms.

535. Colors For Concrete Finish

The addition of coloring matter to cement and concrete is not at present widely practiced, and consequently experience has not been sufficient to indicate just what colors may be used without detriment to the work. Lampblack has been most commonly employed, giving different shades of gray according to the amount used. In any large work where the use of coloring matter is desirable and there is not time to institute thorough tests, the advice of a cement chemist should be sought. The dry mineral colors, mixed in proportions of two to ten per cent, of the cement, give shades approaching the color used. Bright colors are difficult to obtain and would not be in keeping with a masonry structure except in architecture.

When mixed with an American Portland cement mortar, containing one part cement to two parts by weight of a yellow river sand, the particles of which are largely quartz, the colors indicated in the following table are obtained.

With no coloring matter added, the mortar was a light greenish slate when dry. Ultra marine green, in amounts up to 8 - per cent, of the cement, had no apparent effect on the color of this mortar. Variations in character of cement and sand will affect the result obtained in using coloring matter. The colors indicated below are for dry mortars ; when the mortar is wet the shades are usually darker. None of the materials mentioned in the table seems to affect the early hardening of the mortar, though very much larger proportions might prove injurious. With red lead, however, even one per cent, is detrimental, and larger proportions are quite inadmissible.

Colored Mortars

Colors Given to Portland Cement Mortars Containing Two Parts River Sand to One Cement.

536. In some cases it may be sufficient to color the surface of the work by painting. Ordinary oil paints are sometimes applied after washing the surface of the wall with very dilute sulphuric acid, one part acid to 100 parts water, but the permanence of such a finish seems very questionable.

The method of obtaining a gray finish by painting with a thin grout of cement and plaster of Paris has already been described (§ 531). Similar methods may be used with the dry mineral colors, and, while their permanency cannot be vouched for, it seems a more reasonable procedure than to paint a concrete surface with oil paints. One pound red iron ore to ten pounds cement mixed dry, and then made into a very thin grout and applied to a well cleaned concrete surface with a whitewash brush, gives a pleasing brick-red color; and a rich dark red is given by one pound red iron ore to three pounds cement. The earlier this is applied after the concrete has set, the more likely is it to remain permanent.