The factory cost of concrete blocks varies according to the cost of materials. Let us assume cement to be $1.50 per barrel of 380 pounds, and sand and gravel 25 cents per ton. With a 1 to 4 mixture, 1 barrel cement will make 1,900 pounds of solid concrete, or at 130 pounds per cubic foot, 14.6 cubic feet. The cost of materials will then be:
| Cement, 380 pounds | $1.50 |
| Sand and gravel, 1,500 pounds | 0.19 |
| Total | $1.69 |
or 11.5 cents per cubic foot solid concrete. Now, blocks 9 inches high and 32 inches long make 2 square feet of face of wall, each. Blocks of this height {699} and length, 8 inches thick, make 1 1/3 cubic feet of wall; and blocks 12 inches thick make 2 cubic feet of wall. From these figures we may calculate the cost of materials for these blocks, with cores or openings equal to 1/3 or 1/2 the total volume, as follows:
| Per cubic foot of block, 1/3 opening | 7.7 cts. |
| Per cubic foot of block, 1/2 opening | 5.8 cts. |
| Block 8 x 9 x 32 inches, 1/3 opening | 10.3 cts. |
| Block 8 x 9 x 32 inches, 1/2 opening | 7.7 cts. |
| Block 12 x 9 x 32 inches, 1/3 opening | 15.4 cts. |
| Block 12 x 9 x 32 inches, 1/2 opening | 11.6 cts. |
If one-third of the cement is replaced by hydrate lime the quality of the blocks will be improved, and the cost of material reduced about 10 per cent. The cost of labor required in manufacturing, handling, and delivering blocks will vary with the locality and the size and equipment of factory. With hand mixing, 3 men at an average of $1.75 each will easily make 75 8-inch or 50 12-inch blocks, with 1/3 openings, per day. The labor cost for these sizes of blocks will therefore be 7 cents and 10 1/2 cents respectively. At a factory equipped with power concrete mixer and cars for transporting blocks, in which a number of machines are kept busy, the labor cost will be considerably less. An extensive industry located in a large city is, however, subject to many expenses which are avoided in a small country plant, such as high wages, management, office rent, advertising, etc., so that the total cost of production is likely to be about the same in both cases. A fair estimate of total factory cost is as follows:
| Material | Labor | Total | |
|---|---|---|---|
| 8 x 32 inch, 1/3 space | 10.3 | 7 | 17.3 cts. |
| 8 x 32 inch, 1/2 space | 7.7 | 6 | 13.7 cts. |
| 12 x 32 inch, 1/3 space | 15.4 | 10.5 | 25.9 cts. |
| 12 x 32 inch, 1/2 space | 11.6 | 9 | 20.6 cts. |
With fair allowance for outside expenses and profit, 8-inch blocks may be sold at 30 cents and 12-inch at 40 cents each. For laying 12-inch blocks in the wall, contractors generally figure about 10 cents each. Adding 5 cents for teaming, the blocks will cost 55 cents each, erected, or 27 1/2 cents per cubic foot of wall. This is less than the cost of common brick, and the above figures show that this price could be shaded somewhat, if necessary, to meet competition.—S. B. Newberry in a monograph issued by the American Association of Portland Cement Manufacturers.
Artificial Marbles.
II.—Imitation Black Marble.—A black marble of similar character to that exported from Belgium—the latter product being simply prepared slate—may be produced in the following manner: The slate suitable for the purpose is first smoothly polished with a sandstone, so that no visible impression is made on it with a chisel—this being rough—after which it is polished finely with artificial pumice stone, and lastly finished with extremely light natural pumice stone, the surface then presenting a soft, velvet-like appearance. After drying and thoroughly heating the finely polished surface is impregnated with a heated mixture of oil and fine lampblack. This is allowed to remain 12 hours; and, according to whether the slate used is more or less gray, the process is repeated until the gray appearance is lost. Polishing thoroughly with emery on a linen rag follows, and the finishing polish is done with tin ashes, to which is added some lampblack. A finish being made thus, wax dissolved in turpentine, with some lampblack, is spread on the polished plate and warmed again, which after a while is rubbed off vigorously with a {700} clean linen rag. Treated thus, the slate has the appearance of black marble.
STONE CEMENTS: See Adhesives.