_14.--Small Lead Blocks._
Powder and test numbers, weight of charge, and compression produced in
blocks.
_15.--Calories Developed._
Number of large calories developed per kilogramme of explosive, for
comparison with 1,000 grammes of 40% dynamite, which develop, on an
average, 1,229 large calories.
Blasting Powder Separator.
The grains of black blasting powder are graded by a separator, similar
to those used in powder mills, but of reduced size. It consists of an
inclined wooden box, with slots on the sides to carry a series of
screens, and a vertical conduit at the end for carrying off the grains
as they are screened into separate small bins (Fig. 1, Plate X). At the
upper end of the screens is a small 12 by 16-in. hopper, with a sliding
brass apron to regulate the feed. The screens are shaken laterally by an
eccentric rod operated by hand. The top of the hopper is about 6½ ft.
above the floor. The box is 6 ft. 10 in. long, from tip to tip, and
inclines at an angle of 9 degrees.
After separation the grains fall through a vertical conduit, and thence
to the bins through zinc chutes, 1 by 2 in. in section. Care is taken to
have no steel or iron exposed to the powder.
The screens are held by light wooden frames which slip into the inclined
box from the upper end. In this way, any or all of the screens may be
used at once, thus separating all grades, or making only such
separations as are desired. The screens with the largest meshes are
diagonally-perforated zinc plates. Table 2 gives the number of holes per
square foot in zinc plates perforated with circular holes of the
diameters stated.
TABLE 2.--Number of Holes per Square Foot in Zinc Plates with Circular
Perforations.
-------------+------------
Diameter, | Number
in inches | of holes.
-------------+------------
1/2 | 353
4/10 | 518
1/3 | 782
1/4 | 1,392
1/6 | 1,680
1/8 | 3,456
1/10 | 6,636
1/16 | 12,800
-------------+------------
The finer meshes are obtained by using linen screens with holes of two
sizes, namely, 1/20 in. square and 1/28 in. square.
Until a few years ago, black blasting powder was manufactured in the
sizes given in Table 3.
TABLE 3.--Gradation of Black Blasting Powder.
---------+-----------
Grade. | Mesh.
---------+-----------
CC | 2 - 2½
C | 2½ - 3
F | 3 - 5
FF | 5 - 8
FFF | 8 - 16
FFFF | 16 - 28
---------+-----------
In late years there has been considerable demand for special sizes and
mixed grains for individual mines, especially in Illinois. As no
material change has been made in the brands, the letters now used are
not indicative of the size of the grains, which they are supposed to
represent. Of 29 samples of black blasting powder recently received from
the Illinois Powder Commission, only 10 were found to contain 95% of the
size of grains they were supposed to represent; 4 contained 90%; 7
varied from 80 to 90%; several others were mixtures of small and large
grains, and were branded FF black blasting powder; and one sample
contained only 8.5% of the size of grains it was supposed to represent.
The remaining samples showed many variations, even when sold under the
same name. The practice of thus mixing grades is exceedingly dangerous,
because a miner, after becoming accustomed to one brand of FF powder of
uniform separation, may receive another make of similar brand but of
mixed grains, and, consequently, he cannot gauge the quantity of powder
to be used. The result is often an over-load or a blown-out shot. The
smaller grains will burn first, and the larger ones may be thrown out
before combustion is complete, and thus ignite any fire-damp present.
Lamp Testing Gallery.
At the Pittsburg testing station, there is a gallery for testing safety
lamps in the presence of various percentages of inflammable gas. In this
gallery the safety of the lamps in these gaseous mixtures may be tested,
and it is also possible for mine inspectors and fire bosses to bring
their safety lamps to this station, and test their measurements of
percentage of gas, by noting the length and the appearance of the flame
in the presence of mixtures containing known percentages of methane and
air.
[Illustration: PLATE XI.
Fig. 1.--Impact Machine.