In Bengal, mixing is performed by shutting up the ingredients in
barrels, which are turned either by hand or machinery; each containing
50 lbs. weight, or more, of small brass balls. They have ledges on the
inside, which occasion the balls and composition to tumble about and
mingle together, so that the intermixture of the ingredients, after the
process has been gone through, cannot fail to be complete. The operation
is continued two or three hours; and I think it would be an improvement
in Her Majesty’s system of manufacture if this method of mixing were
adopted.
In England two or three pints of water are used for a 42 lb. charge: but
the quantity is variable; both the temperature and the humidity of the
atmosphere influence it.
Bramah’s hydrostatic press, or a very strong wooden press working with a
powerful screw, lever, and windlass, constitutes the description of
mechanism by which density is imparted to gunpowder. The incorporated or
mill-cake powder is laid on the bed or follower of the press, and
separated, at equal distances, by sheets of copper, so that when the
operation is over, it comes out in large thin solid cakes, or strata,
distinguished by the term press-cake. The mill-cake powder at Waltham
Abbey, is submitted to a mean theoretic pressure of 70 to 75 tons per
superficial foot.
Gunpowder should be thoroughly dried, but not by too high a degree of
heat; that of 140° or 150° of Fahrenheit’s thermometer is sufficient. It
appears to be of no consequence whether it be dried by solar heat; by
radiation from red-hot iron, as in the gloom stove; or by a temperature
raised by means of steam. Her Majesty’s gunpowder is dried by the last
two methods. The grain should not be suddenly exposed to the highest
degree of heat, but gradually.
The method of trial best adapted to shew the real inherent strength and
goodness of gunpowder, appears to be an eight or ten-inch iron or brass
mortar, with a truly spherical solid shot, having not more than
one-tenth of an inch windage, and fired with a low charge. The
eight-inch mortar, fired with two ounces of powder, is one of the
established methods of proof at Her Majesty’s works. Gunpowders that
range equally in this mode of trial, may be depended on as being equally
strong.
Another proof is by four drachms of powder laid in a small neat heap, on
a clean, polished, copper plate; which heap is fired at the apex, by a
red-hot iron. The explosion should be sharp and quick; not tardy, nor
lingering; it should produce a sudden concussion in the air, and the
force and power of that concussion ought to be judged of by comparison
with that produced by powder of known good quality. No sparks should fly
off, nor should beads, or globules of alkaline residuum, be left on the
copper. If the copper be left clean, i. e. without gross foulness, and
no lights, i. e. sparks, be seen, the ingredients may be considered to
have been carefully prepared, and the powder to have been well
manipulated, particularly if pressed and glazed; but if the contrary be
the result, there has been a want of skill or of carefulness manifested
in the manufacture.
“Gunpowder,” says Captain Bishop “explodes exactly at the 600° of heat
by Fahrenheit’s thermometer; when gunpowder is exposed to 500° it alters
its nature altogether; not only the whole of the moisture is driven off,
but the saltpetre and sulphur are actually reduced to fusion, both of
which liquefy under the above degree. The powder on cooling, is found to
have changed its colour from a gray to a deep black; the grain has
become extremely indurated, and by exposure even to very moist air, it
then suffers no alteration by imbibing moisture.”
[Illustration: 531]
The mill for grinding the gunpowder cake may be understood from the
following representation: (_fig._ 531.) _p_, is the water wheel, which
may drive several pairs of stones; _q_, _q_, two vertical bevel wheels,
fixed upon the axis of the great wheel; _r_, _r_, two horizontal bevel
wheels working in _q_, _q_, and turning the shafts _s_, _s_; _t_, _t_,
two horizontal spur wheels fixed to the upper part of the vertical
shafts, and driving the large wheels _u_, _u_. To the shafts of these
latter wheels are fixed the runners _v_, _v_, which traverse upon the
bed stone _w_, _w_; _x_, _x_, are the curbs surrounding the bed stone to
prevent the powder from falling off; _o_ is the scraper. Mill A
represents a view, and mill B a section of the bed stone and curb.
GYPSUM, _Sulphate of Lime_, _Alabaster_, _or Paris Plaster_. This
substance is found in three geological positions in the crust of the
earth; among transition rocks; in the red marl formation; and above the
chalk, in the tertiary beds.
1. The alpine gypsums are ranged by M. Brochant among the transition
class, and are characterized by the presence of anthracite or stone
coal; some of them are white and pure, others gray or yellowish, and
mixed with mica, talc, steatite, black oxide of iron, pyrites, compact
carbonate of lime, sulphur, and common salt. Examples of such localities
are found in the gypsum of _Val-Canaria_ at the foot of Saint Gothard,
that of Brigg in the upper Valais; of the Grilla in the valley of
Chamouni, and of Saint Gervais-les-Bains, near Sallenches in Savoy.
2. The secondary gypsum, or that of the salt mine districts, belongs to
the _red ground_, immediately beneath the lias in the order of
stratification, and therefore a rock relatively antient. Near Northwick,
the red marl beds above the great deposit of rock salt, are irregularly
intersected with gypsum, in numerous laminæ or plates. At Newbiggin in
Cumberland, the gypsum lies in red argillaceous marl, between two strata
of sandstone; and a mile south of Whitehaven, the subterraneous workings
for the alabaster extend 30 yards in a direct line; with two or three
lateral branches extending about 10 yards, at whose extremities are
large spaces where the gypsum is blasted with gunpowder. It is generally
compact, forming a regular and conformable bed, with crystals of
selenite (crystallized gypsum) in drusy cavities. Gypsum occurs in the
red marl in the isle of Axholme, and various other places in
Nottinghamshire. In Derbyshire some considerable deposits have been
found in the same red sandstone, several of which are mined, as at
Chellaston hill, which would exhibit a naked and water-worn rock of
gypsum, were it not for a covering of alluvial clay. It appears in
general to present itself chiefly in particular patches, occasioning a
sudden rise, or an insulated hill, by the additional thickness which it
gives to the stratum of the _red ground_ in these places. The principal
demand for the pure white gypsum, or that faintly streaked with red, is
by the potters in Staffordshire, who form their moulds with the calcined
powder which it affords; only particularly fine blocks are selected for
making alabaster ornaments on the turning lathe. In one of the salt pits
near Droitwich, the strata sunk through, were, vegetable mould, 3 feet;
red marl, 35 feet; gypsum, 40 feet; a river of brine, 22 inches; gypsum,
75 feet; a rock of salt, bored into only 5 feet, but probably extending
much deeper. On the Welsh side of the Bristol channel, gypsum occurs in
the red marl cliffs of _Glamorganshire_, from Pennarth to Lavernock. No
organic remains or metallic minerals have hitherto been found in the
gypsum of this formation.
3. The most interesting gypsums in a general point of view, are
certainly the tertiary, or those of the plains, or hills of
comparatively modern formation. They are characterized, by the presence
of fossil bones of extinct animals, both _mammifera_ and birds, by
shells, and a large proportion of carbonate of lime, which gives them
the property of effervescing with acids, and the title of limestone
gypsums. Such are the gypsums of the environs of Paris, as at the
heights of Montmartre, which contain crystallized sulphate of lime in
many forms, but most commonly the lenticular and lance-shaped.
Sulphate of lime occurs either as a dense compound without water, and is
called _anhydrite_ from that circumstance; or with combined water, which
is its most ordinary state. Of the latter there are 6 sub-species;
sparry gypsum or selenite in a variety of crystalline forms; the
foliated granular; the compact; the fibrous; the scaly foliated; the
earthy.
The prevailing colour is white, with various shades of gray, blue, red,
and yellow. More or less translucent. Soft, sectile, yielding to the
nail. Specific gravity 2·2. Water dissolves about one five-hundredth
part of its weight of gypsum, and acquires the quality of hardness, with
the characteristic selenitic taste. When exposed on red hot coals, it
decrepitates, becomes white, and splits into a great many brittle
plates. At the heat of a baker’s oven, or about 400° Fahr., the combined
water of gypsum escapes with a species of ebullition. At a higher
temperature the particles get indurated. When rightly calcined and
pulverized, gypsum is mixed with water to the consistence of cream, and
poured into moulds by the manufacturers of stucco ornaments and statues.
A species of rapid crystallization ensues, and the thin paste soon
acquires a solid consistence, which is increased by drying the figure in
proper stoves. During the consolidation of the plaster, its volume
expands into the finest lines of the mould, so as to give a sharp and
faithful impression.
The plaster stone of the Paris basin contains about 12 _per cent._ of
carbonate of lime. This body, ground and mixed with water, forms an
adhesive mortar much used in building, as it fixes very speedily. Works
executed with pure gypsum never become so hard as those made with the
calcareous kind; and hence it might be proper to add a certain portion
of white slaked lime to our calcined gypsum, in order to give the stucco
this valuable property. Coloured stuccos of great solidity are made by
adding to a clear solution of glue, any desired colouring tincture, and
mixing-in the proper quantity of the calcined calcareous gypsum.
The compact, fine-grained gypseous alabaster is often cut into various
ornamental figures, such as vases, statuary groups, &c., which take a
high polish and look beautiful, but from their softness are easily
injured, and require to be kept enclosed within a glass shade.
In America and France, the virtues of gypsum in fertilizing land have
been highly extolled, but they have not been realized in the trials made
in this kingdom.
Pure gypsum consists of lime 28; sulphuric acid 40; water 18; which are
the respective weights of its prime equivalent parts.
M. Gay Lussac, in a short notice, in the _Annales de Chimie_ for April
1829, on the setting of gypsum, says that the purest plasters are those
that harden least, and that the addition of lime is of no use towards
promoting their solidity, nor can the heat proper for boiling gypsum
ever expel the carbonic acid gas from the calcareous carbonate present
in the gypsum of Montmartre. He conceives that a _hard_ plaster-stone
having lost its water, will resume more solidity in returning to its
first state, than a plaster-stone naturally tender or soft; and that it
is the primitive molecular arrangement which is regenerated. See
ALABASTER.