STATE GEOLOGIST. ay 
stencils which keep out all colors except the one being filled at the time. 
Fach color requires its own stencil. When all the cells are charged, the 
comb is withdrawn gently, leaving the cement powder in little piles whose 
edges overlap one on the other. Without disturbing their positions, a mix- 
ture of coarse sand and cement, properly dampened, is now filled in on top 
of them until the die is full and stroked off level, when it is brought 
under a powerful press which compacts the loose material into a solid 
dense tile. The moisture from the damp backing penetrates and hydrates 
the dry cement composing the face so that in a day or so it has fully set. 
The tiles are placed on slabs of cement mixed with sawdust instead of 
sand; these are very porous and are kept damp so that they may furnish 
water as needed for the chemical process of setting the tile without soft- 
ening its surface or causing the different colors to mix. In order to keep 
these colors clear and bright, and avoid the scum or whitewash, commonly 
found on cement surfaces, some chemical is generally used in the surface 
colors, or in the whole mass. This is generally kept secret, or patented, 
and is sold with the right to use the process. Magnesium fluoride is one 
of the best and commonest anti-scum chemicals in use for cements. 
The tiles, when made and hardened, are used in laying floors exactly 
as clay tiles, being laid in a cement matrix or bed. 
Imitation marble may be obtained by kneading and rolling together 
different colored mortars and pressing them into slabs which, after setting, 
can be ground and given a high degree of polish. 
Figures 10 and 11 show what can be done with cement in ornamental 
lines. 
FACINGS. 
Cement mortar is used in facing concrete work. It is not put on as 
a plaster but the forms in which the concrete body is molded are so made. 
that the outer surface, one to three inches in thickness, can be put in place 
and rammed at the same time as the body of concrete. This gives a 
smooth, finished appearance to the exterior and at the same time provides 
an impervious, durable surface. The mortar is usually made of one part 
cement to two or three parts of clean sand. 
At first, the work was made of uniform texture throughout and the 
surface was coated with a plaster of neat cement, or of one part cement 
and one part fine sand. It was found, however, to be almost impossible 
to make a plaster coat permanently adhere to the body of the concrete 
mass which had become set before the plaster could be applied. The 
method of facing with mortar was then adopted. The latest practice, 
however, is to uSe a wet concrete in which all the voids are filled and 
which contains a slight excess of mortar. When the concrete is rammed, 
if a close tined fork with flat blades or a flat square backed shovel be 
forced down between the concrete mass and the form, it will force back 
