A correspondent thus writes: ‘In your article on “Shot-firing in
Coal-mines” you speak of the dangerous operation of tamping or plugging
the shot-hole with brick or coal dust rammed hard. It must indeed be
a dangerous operation; but cannot the hole be as effectually plugged
without any danger at all? It is usual, after charging a rocket, to
drive in dry clay upon the top of the fuse, to prevent its blowing
through; but a layer of wet plaster of Paris poured in and allowed
to set, dries harder than the clay, and obviates all danger from
concussion or grit. Cannot the shot-hole be in like manner plugged?
Plaster of Paris (gypsum or sulphate of lime) expands, not shrinks,
when combined with water, so that it fills accurately every part of the
bore. If the hole were slightly conical, the smaller end outwards, or
made with an internal flange, the plaster would offer more resistance
than the clay.’