42 CEMENT MATERIALS AND INDUSTRY. [bull. 243. 
ton of raw material. A mine can be worked steadily and economically 
in all kinds of weather, while an open cut, or quarry, is commonly in a 
more or less unworkable condition for about three months of the year. 
Material won by mining is, moreover, alwa}^s dry and clean. 
^Dredging. — The term " dredging" will be here used to include all 
methods of excavating soft, wet raw materials. In the United States 
the onty raw material for Portland-cement manufacture extensively 
worked by dredging is marl. Occasionally- the clay used is obtained 
from deposits overlain by more or less water; but this is rarely done 
except where the marl and clay are interbedded or associated. 
A marl deposit, in addition to containing much water diffused 
throughout its mass, is usually covered by water to a considerable 
depth. This will frequently require the partial draining of the basin 
in order to get tracks laid near enough to be of service. 
In dredging marl the excavator is frequently mounted on a barge, 
which floats in a channel resulting from previous excavation. Occa- 
sionally, in deposits which either were originally covered by very little 
water or have been drained, the shovel is mounted on a car which 
runs on tracks laid along the edge of the deposit. 
A deposit worked by dredging always occurs in a basin or depres- 
sion, and at a lower elevation than the mill, thus necessitating uphill 
transportation, which may be effected in two ways, the choice depend- 
ing largely upon the manufacturing processes in use at the plant. At 
plants using dome or chamber kilns, or where the marl is to be dried 
before it is sent to the kiln, the excavated material is usually loaded 
by the shovel on cars and hauled to the mill by horse or steam power. I 
At normal marl plants using a very w r et mixture it is probably more 
economical to dump the marl from the excavator into tanks, add suffi- 
cient water to make it flow readily, and pump the fluid mixture to the 
mill in pipes. 
Cost of raw materials at mill. — The most natural way, perhaps, tc 
express the cost of the raw materials delivered at the mill would be tc 
state it as being so many cents per ton or cubic yard, and this is the 
method followed by quarrymen or miners in general. To the cemenl 
manufacturer, however, such an estimate is not so suitable as one based 
on the cost per ton or barrel of finished cement. 
It may be considered that hard and comparatively dry limestones 01 
shales lose 33^ per cent in weight on burning, or that 600 pounds o' 
dry raw material will make about 400 pounds of clinker. Allowing 
something for other losses in manufacture, it is convenient and sufti 
ciently accurate to estimate that 600 pounds of dry raw materia 
will give one barrel of finished cement. The raw material must bi 
increased if it carries any appreciable amount of water. Clays wil 
frequently contain 15 per cent or more of water; while soft chalky 
limestones, if (married during wet weather, may cany over 20 pe: 
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