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,