330 COX, REIBLING, AND REYES. 
nated from up-to-date practise in Germany, where hydrating 
the lime with all the sand in silos or slaking drums is advocated 
by the experts. 
The advisability of slaking some limes with steam under pres- 
sure before mixing with the sand is illustrated by fig. 1, Plate III, 
and fig. 2, Plate V. In both instances the same raw meal was 
employed, but the former was made according to the silo system 
and the latter by pre-hydration. In this instance the pre-hydra- 
tion was accomplished simply by treating the quicklime with 
a slight excess of water in a closed air-tight retort. The heat 
of hydration was sufficient to generate and maintain steam under 
pressure for several hours. 
The degree of compactness affects some mixtures more than 
others, but in all instances a molding pressure of from 10,000 to 
15,000 pounds per square inch (700 to 1,050 kilograms per square 
centimeter) is required for maximum strength. However, the 
efficiency of either the raw materials or the commercial value of 
the finished product does not depend solely on the strength of 
the brick obtained, but also upon appearance, density, absorp- 
tion, fire-resistant properties, etc. Furthermore, these are all 
subject more or less to the conditions of manufacture, such as 
granularimetric composition, thoroughness of mixing, and mold- 
ing pressure. A certain coarseness of grain and porosity often 
improves the fire-resistant properties of a brick, but this is gained 
only at a sacrifice of strength and density. A practically un- 
limited range of colors can be produced by the use of artificial 
coloring matter. The ordinary cheap mineral pigments give 
fairly satisfactory result, but Glasenapp '* advocates the staining 
of sand-lime brick by a new method in which the colors are 
formed by the reaction of calcium hydroxide with salts of various 
metals. If polished surfaces are desired, coarse grains of hard 
durable stone are essential. Marble effects and staining are 
usually accomplished at the expense of the strength. 
Fuel.—Almost any commercial fuel is capable of meeting the 
demands of this industry. The highest temperature required is 
for burning the limestone, and as this should be from about 950° 
to 1,050°, firewood, coal, crude petroleum, natural gas, coke, 
reduced oil or petroleum residuum, water and producer gases, 
and even lignites and peat compressed into briquettes have been 
successfully employed. A long flame of low intensity tends to 
produce a uniform distribution of heat and to prevent over- 
“ Tonind.-Ztg. (1899), 1421. 
