60 MISCELLANEOUS FARM SUBJECTS 



quire shallow trenches, and, up to the ground line, usually the 

 earth walls are firm enough to act as a form. 



Forms should be well planned, so that there may be no diffi- 

 cult measurements to understand. As few pieces of lumber should 

 be used for the work as possible, and these should be fastened to- 

 gether by as few nails as will serve the purpose; otherwise it will 

 be difficult to take the forms apart without splitting them. 



Forms must be strong enough to hold the weight of the con- 

 crete without bulging out of shape. When they bulge, cracks may 

 open between the planks, and the water in the concrete, with some 

 cement and sand, may leak out. This weakens the concrete and 

 causes hollows in the surface, which have a bad appearance after the 

 forms are removed. 



Forms which lose their shape after being used once can hardly 

 be used a second time. A part of the erection cost of forms is saved 

 if the forms are built in as large sections as it is convenient to 

 handle. This saving applies to their removal as well as their erec- 

 tion. Consequently the lightest forms possible with the largest 

 surface area are the most economical. (F. B. 461.) 



Notes. A dry mixture, from which water can be brought to 

 the surface only by vigorous tamping, is probably the strongest, 

 but for the sake of economy, and to insure a dense concrete well 

 filling the molds, a moderately soft mixture is recommended for 

 ordinary purposes. Where the pieces to be molded are thin, and 

 where small reinforcing metal rods are placed close together or 

 near the surface, a rather wet mixture may be necessary to insure 

 the molds being well filled. 



In the manufacture of such articles as pipe, fence posts, and 

 hollow blocks, a rather large proportion of quick-setting cement is 

 sometimes used, the object being to reduce the weight and conse- 

 quent freight charges by means of a strong mixture, as well as to 

 make the concrete impervious to water. The use of a quick-setting 

 cement permits the molds to be removed sooner than would be pos- 

 sible with a slow-setting cement, thus reducing the number of 

 molds necessary for a given output. Quick-setting cements are not 

 recommended for such purposes, however, as they are usually in- 

 ferior to those which set slowly. 



In coloring cement work the best results are obtained by the 

 use of mineral pigments. The coloring matter, in proportions de- 

 pending upon the desired shade, should be thoroughly mixed with 

 the dry cement before making the mortar. By preparing small 

 specimens of the mortar and noting the color after drying, the 

 proper proportions may be determined. For gray or black, use 

 lampblack. For yellow or buff, use yellow ocher. For brown, use 

 umber. For red, use Venetian red. For blue, use ultramarine. 



The time required for concrete to set depends upon the char- 

 acter of the cement, upon the amount and temperature of the 

 water used in mixing, and upon the temperature of the air. Con- 

 crete mixed dry sets more quickly than if mixed wet, and the time 



