14. _Deadening._ This consists in passing a thin coat of glue slightly
warmed, over the parts that are not to be burnished.
15. _Mending_; that is moistening any broken points with a brush, and
applying bits of gold leaf to them.
16. The _vermeil_ coat. Vermeil is a liquid which gives lustre and fire
to the gold; and makes it resemble _or moulu_. It is composed as
follows: 2 ounces of annotto, 1 ounce of gamboge, 1 ounce of vermillion,
half an ounce of dragon’s blood, 2 ounces of salt of tartar, and 18
grains of saffron, are boiled in a litre (2 pints English) of water,
over a slow fire, till the liquid be reduced to a fourth. The whole is
then passed through a silk or muslin sieve. A little of this is made to
glide lightly over the gold, with a very soft brush.
17. _Repassage_; is passing over the dead surfaces a second coat of
deadening glue, which must be hotter than the first. This finishes the
work, and gives it strength.
_Leaf gilding_, on paper or vellum, is done by giving them a coat of gum
water or fine size, applying the gold leaf ere the surfaces be hard dry,
and burnishing with agate.
_Gold lettering_, on bound books, is given without size, by laying the
gold leaf on the leather, and imprinting it with hot brass types.
The _edges of the leaves of books are gilded_, while they are in the
press, where they have been cut smooth, by applying a solution of
isinglass in spirits, and laying-on the gold when the edges are in a
proper state of dryness. The French workmen employ a ground of Armenian
bole, mixed with powdered sugar-candy, by means of white of egg. This
ground is laid very thin upon the edges, after fine size or gum water
has been applied; and when the ground is dry it is rubbed smooth with a
wet rag, which moistens it sufficiently to take the gold.
_Japanners’ gilding_ is done by sprinkling or daubing with wash leather,
some gold powder, over an oil sized surface, mixed with oil of
turpentine. This gives the appearance of frosted gold. The gold powder
may be obtained, either by precipitating gold from its solution in _aqua
regia_ by a solution of pure sulphate of iron, or by evaporating away
the mercury from some gold amalgam.
II. _Chemical Gilding_, or the application of gold by chemical affinity
to metallic surfaces.
A compound of copper with one seventh of brass is the best metal for
gilding on; copper by itself being too soft and dark coloured. Ordinary
brass, however, answers very well. We shall describe the process of wash
gilding, with M. D’Arcet’s late improvements, now generally adopted in
Paris.
_Wash gilding_, consists in applying evenly an amalgam of gold to the
surface of a copper alloy, and dissipating the mercury with heat, so as
to leave the gold film fixed. The surface is afterwards burnished or
deadened at pleasure. The gold ought to be quite pure, and laminated to
facilitate its combination with the mercury; which should also be pure.
_Preparation of the amalgam._ After weighing the fine gold, the workman
puts it in a crucible, and as soon as this becomes faintly red, he pours
in the requisite quantity of mercury; which is about 8 to 1 of gold. He
stirs up the mixture with an iron rod, bent hookwise at the end, leaving
the crucible on the fire till he perceives that all the gold is
dissolved. He then pours the amalgam into a small earthen dish
containing water, washes it with care, and squeezes out of it with his
fingers all the running mercury that he can. The amalgam that now
remains on the sloping sides of the vessel is so pasty as to preserve
the impression of the fingers. When this is squeezed in a shamoy leather
bag, it gives up much mercury; and remains an amalgam, consisting of
about 33 of mercury, and 57 of gold, in 100 parts. The mercury which
passes through the bag, under the pressure of the fingers, holds a good
deal of gold in solution; and is employed in making fresh amalgam.
_Preparation of the mercurial solution._ The amalgam of gold is applied
to brass, through the intervention of pure nitric acid, holding in
solution a little mercury.
100 parts of mercury, and 110 parts by weight of pure nitric acid,
specific gravity 1·33, are to be put into a glass matrass. On the
application of a gentle heat the mercury dissolves with the
disengagement of fumes of nitrous gas, which must be allowed to escape
into the chimney. This solution is to be diluted with about 25 times its
weight of pure water, and bottled up for use.
1. _Annealing._--The workman anneals the piece of bronze after it has
come out of the bands of the turner and engraver. He sets it among
burning charcoal, or rather peats, which have a more equal and lively
flame; covering it quite up, so that it may be oxidized as little as
possible, and taking care that the thin parts of the piece do not become
hotter than the thicker. This operation is done in a dark room, and when
he sees the piece of a cherry red colour, he removes the fuel from about
it, lifts it out with long tongs, and sets it to cool slowly in the air.
2. The _decapage_.--The object of this process is to clear the surface
from the coat of oxide which may have formed upon it. The piece is
plunged into a bucket filled with extremely dilute sulphuric acid; it is
left there long enough to allow the coat of oxide to be dissolved, or at
least loosened; and it is then rubbed with a hard brush. When the piece
becomes perfectly bright, it is washed and dried. Its surface may
however be still a little variegated; and the piece is therefore dipped
in nitric acid, specific gravity 1·33, and afterwards rubbed with a
long-haired brush. The addition of a little common salt to the dilute
sulphuric acid would probably save the use of nitric acid, which is so
apt to produce a new coat of oxide. It is finally made quite dry, (after
washing in pure water) by being rubbed well with tanners’ dry bark,
saw-dust, or bran. The surface should now appear somewhat de-polished;
for when it is very smooth, the gold does not adhere so well.
3. _Application of the amalgam._--The gilder’s _scratch-brush_ or
pencil, made with fine brass wire is to be dipped into the solution of
nitrate of mercury, and is then to be drawn over a lump of gold amalgam,
laid on the sloping side of an earthen vessel, after which it is to be
applied to the surface of the brass. This process is to be repeated,
dipping the brush into the solution, and drawing it over the amalgam,
till the whole surface to be gilded is coated with its just proportion
of gold. The piece is then washed in a body of water, dried, and put to
the fire to volatilize the mercury. If one coat of gilding be
insufficient, the piece is washed over anew with amalgam, and the
operation recommenced till the work prove satisfactory.
4. _Volatilization of the mercury._--Whenever the piece is well coated
with amalgam, the gilder exposes it to glowing charcoal, turning it
about, and heating it by degrees to the proper point; he then withdraws
it from the fire, lifts it with long pincers, and, seizing it in his
left hand, protected by a stuffed glove, he turns it over in every
direction, rubbing and striking it all the while with a long-haired
brush, in order to equalize the amalgam. He now restores the piece to
the fire, and treats it in the same way till the mercury be entirely
volatilized, which he recognises by the hissing sound of a drop of water
let fall on it. During this time he repairs the defective spots, taking
care to volatilize the mercury very slowly. The piece, when thoroughly
coated with gold, is washed, and scrubbed well with a brush in water
acidulated with vinegar.
If the piece is to have some parts burnished, and others dead, the parts
to be burnished are covered with a mixture of Spanish white, bruised
sugar-candy, and gum dissolved in water. This operation is called in
French _epargner_ (_protecting_). When the gilder has _protected_ the
burnished points, he dries the piece, and carries the heat high enough
to expel the little mercury which might still remain on it. He then
plunges it, while still a little hot, in water acidulated with sulphuric
acid, washes it, dries it, and gives it the burnish.
5. The _burnish_ is given by rubbing the piece with burnishers of
hematite (bloodstone). The workman dips his burnisher in water sharpened
with vinegar, and rubs the piece always in the same direction backwards
and forwards, till it exhibits a fine polish, and a complete metallic
lustre. He then washes it in cold water, dries it with fine linen cloth,
and concludes the operation by drying it slowly on a grating placed
above a chafing dish of burning charcoal.
6. The _deadening_ is given as follows. The piece, covered with the
_protection_ on those parts that are to be burnished, is attached with
an iron wire to the end of an iron rod, and is heated strongly so as to
give a brown hue to the _epargne_ by its partial carbonization. The
gilded piece assumes thus a fine tint of gold; and is next coated over
with a mixture of sea salt, nitre, and alum, fused in the water of
crystallization of the latter salt. The piece is now restored to the
fire, and heated till the saline crust which covers it becomes
homogeneous, nearly transparent, and enters into true fusion. It is then
taken from the fire and suddenly plunged into cold water, which
separates the saline crust, carrying away even the coat of _epargne_.
The piece is lastly passed through very weak nitric acid, washed in a
great body of water, and dried by exposure either to the air, over a
drying stove, or with clean linen cloths.
7. _Of or-moulu colour._--When it is desired to put a piece of gilded
bronze into _or-moulu_ colour, it must be less scrubbed with the
scratch-brush than usual, and made to _come back again_ by heating it
more strongly than if it were to be deadened, and allowing it then to
cool a little. The _or-moulu_ colouring is a mixture of hematite, alum,
and sea salt. This mixture is to be thinned with vinegar, and applied
with a brush so as to cover the gilded brass, with reserve of the
burnished parts. The piece is then put on glowing coals, urged a little
by the bellows, and allowed to heat till the colour begins to blacken.
The piece ought to be so hot that water sprinkled on it may cause a
hissing noise. It is then taken from the fire, plunged into cold water,
washed, and next rubbed with a brush dipped in vinegar, if the piece be
smooth, but if it be chased, weak nitric acid must be used. In either
case, it must be finally washed in a body of pure water, and dried over
a gentle fire.
8. _Of red gold colour._--To give this hue, the piece after being coated
with amalgam, and heated, is in this hot state to be suspended by an
iron wire, and tempered with the composition known under the name of
gilder’s wax; made with yellow wax, red ochre, verdigris, and alum. In
this state it is presented to the flame of a wood fire, is heated
strongly, and the combustion of its coating is favoured by throwing some
drops of the wax mixture into the burning fuel. It is now turned round
and round over the fire, so that the flame may act equally. When all the
wax of the colouring is burned away, and when the flame is extinguished,
the piece is to be plunged in water, washed, and scrubbed with the
scratch-brush and pure vinegar. If the colour is not beautiful, and
quite equal in shade, the piece is coated with verdigris dissolved in
vinegar, dried over a gentle fire, plunged in water, and scrubbed with
pure vinegar, or even with a little weak nitric acid if the piece
exhibit too dark a hue. It is now washed, burnished, washed anew, wiped
with linen cloth, and finally dried over a gentle fire.
The following is the outline of a complete, gilding factory, as now
fitted up at Paris.
[Illustration: 529]
_Fig._ 529. Front elevation and plan of a complete gilding workshop.
P. Furnace of _appel_, or draught, serving at the same time to heat the
deadening pan (_poêlon au mat_).
F. Ashpit of this furnace.
N. Chimney of this furnace constructed of bricks, as far as the
contraction of the great chimney S of the forge, and which is terminated
by a summit pipe rising 2 or 3 yards above this contraction.
B. Forge for annealing the pieces of bronze; for drying the gilded
pieces, &c.
C. Chimney of communication between the annealing forge B, and the space
D below the forge. This chimney serves to carry the noxious fumes into
the great vent of the factory.
U. Bucket for the brightening operation.
A. Forge for passing the amalgam over the piece.
R. Shelf for the brushing operations.
E E. Coal cellarets.
O. Forge for the deadening process.
G. Furnace for the same.
M. An opening into the furnace of _appel_, by which vapours may be let
off from any operation by taking out the plug at M.
I. Cask in which the pieces of gilded brass are plunged for the
deadening process. The vapours rising thence are carried up the general
chimney.
J J. Casement with glass panes, which serves to contract the opening of
the hearths, without obstructing the view. The casement may be rendered
movable to admit larger objects.
H H. Curtains of coarse cotton cloth, for closing at pleasure, in whole
or part, one or several of the forges or hearths, and for quickening the
current of air in the places where the curtains are not drawn.
Q. Opening above the draught furnace, which serves for the heating of
the _poêlon au mat_ (deadening pan).
_Gilding on polished iron and steel._--If a nearly neutral solution of
gold in muriatic acid, be mixed with sulphuric ether, and agitated, the
ether will take up the gold, and float above the denser acid. When this
auriferous ether is applied by a hair pencil to brightly polished iron
or steel, the ether flies off, and the gold adheres. It must be fixed by
polishing with the burnisher. This gilding is not very rich or durable.
In fact the affinity between gold and iron is feeble, compared to that
between gold and copper or silver. But polished iron, steel, and copper,
may be gilded with heat, by gold leaf. They are first heated till the
iron takes a bluish tint, and till the copper has attained to a like
temperature; a first coat of gold leaf is now applied, which is pressed
gently down with a burnisher, and then exposed to a gentle heat.
Several leaves either single or double are thus applied in succession,
and the last is burnished down cold.
_Cold gilding._--Sixty grains of fine gold and 12 of rose copper are to
be dissolved in two ounces of aqua regia. When the solution is
completed, it is to be dropped on clean linen rags, of such bulk as to
absorb all the liquid. They are then dried, and burned into ashes. These
ashes contain the gold in powder.
When a piece is to be gilded, after subjecting it to the preliminary
operations of softening or annealing and brightening, it is rubbed with
a moistened cork, dipped in the above powder, till the surface seems to
be sufficiently gilded. Large works are thereafter burnished with pieces
of hematite, and small ones with steel burnishers, along with soap
water.
In gilding small articles, as buttons, with amalgam, a portion of this
is taken equivalent to the work to be done, and some nitrate of mercury
solution is added to it in a wooden trough; the whole articles are now
put in, and well worked about with a hard brush, till their surfaces are
equably coated. They are then washed, dried, and put altogether into an
iron frying-pan, and heated till the mercury begins to fly off, when
they are turned out into a cap, in which they are tossed and well
stirred about with a painter’s brush. The operation must be repeated
several times for a strong gilding. The surfaces are finally brightened
by brushing them along with small beer or ale grounds.
_Gold wire_, is formed by drawing a cylindrical rod of the metal as pure
as may be, through a series of holes punched in an iron plate,
diminishing progressively in size. The gold as it is drawn through,
becomes hardened by the operation, and requires frequent annealing.
_Gold thread_, or _spun gold_, is a flatted silver-gilt wire, wrapped or
laid over a thread of yellow silk, by twisting with a wheel and iron
bobbins. By the aid of a mechanism like the Braiding Machine, a number
of threads may thus be twisted at once by one master wheel. The
principal nicety consists in so regulating the movements that the
successive volutions of the flatted wire on each thread may just touch
one another, and form a continuous covering. The French silver for
gilding is said to be alloyed with 5 or 6 pennyweights, and ours with 12
pennyweights of copper in the pound troy. The gold is applied in leaves
of greater or less thickness, according to the quality of the gilt wire.
The smallest proportion formerly allowed in this country by act of
parliament, was 100 grains of gold to one pound, or 5760 grains of
silver; but more or less may now be used. The silver rod is encased in
the gold leaf, and the compound cylinder is then drawn into round wire
down to a certain size, which is afterwards flatted in a rolling mill
such as is described under MINT.
The liquor employed by goldsmiths to bring out a rich colour upon the
surface of their trinkets, is made by dissolving 1 part of sea-salt, 1
part of alum, 2 parts of nitre, in 3 or 4 of water. This pickle or
sauce, as it is called, takes up not only the copper alloy, but a
notable quantity of gold; the total amount of which in the Austrian
empire, has been estimated annually at 47,000 francs. To recover this
gold, the liquor is diluted with at least twice its bulk of boiling
water; and a solution of very pure green sulphate of iron is poured into
it. The precipitate of gold is washed upon a filter, dried, and purified
by melting in a crucible along with a mixture of equal parts of nitre
and borax.
GONG-GONG; or _tam-tam_ of the Chinese; a kind of cymbal made of a
copper alloy, described towards the end of the article COPPER.
GONIOMETER, is the name of a little instrument made either on mechanical
or optical principles, for measuring the angles of crystals. It is
indispensable to the mineralogist.
[Illustration: 530]
GRADUATOR, called by its contriver M. Wagenmann, _Essigbilder_, which
means in German, vinegar-maker, is represented _fig._ 530. It is an
oaken tub, 5-1/2 feet high, 3-1/2 feet wide at top, and 3 at bottom, set
upon wooden beams, which raise its bottom about 14 inches from the
floor. At a distance of 15 inches above the bottom, the tub is pierced
with a horizontal row of 8 equidistant round holes, of an inch in
diameter. At 5 inches beneath the mouth of the tub, a thick beech-wood
hoop is made fast to the inner surface, which supports a circular oaken
shelf, leaving a space round its edge of 1-1/4 inches, which is stuffed
water tight with hemp or tow. In this shelf, 400 holes at least must be
bored, about 1/8 of an inch in diameter, and 1-1/2 inches apart; and
each of these must be loosely filled with a piece of packthread, or
cotton wick, which serves to filter the liquid slowly downwards. In the
same shelf there are likewise four larger holes of 1-1/2 inches
diameter, and 18 inches apart, each of which receives air-tight a glass
tube 3 or 4 inches long, having its ends projecting above and below the
shelf. These tubes serve to allow the air that enters by the 8
circumferential holes, to circulate freely through the graduator. The
mouth of the tube is covered with a wooden lid, in whose middle is a
hole for the insertion of a funnel, when the liquor of acetification
requires to be introduced. One inch above the bottom, a hole is bored
for receiving a syphon-formed discharge pipe, whose upper curvature
stands one inch below the level of the holes in the side of the tub, to
prevent the liquor from rising so high as to overflow through them. The
syphon is so bent as to retain a body of liquor 12 inches deep above the
bottom of the tub, and to allow the excess only to escape into the
subjacent receiver. In the upper part of the graduator, but under the
shelf, the bulb of a thermometer is inserted through the side, some way
into the interior, having a scale exteriorly. The whole capacity of the
cask from the bottom up to within one inch of the perforated shelf, is
to be filled with thin shavings of beech wood, grape stalks or birch
twigs, previously imbued with vinegar. The manner of using this simple
apparatus, is described under ACETIC ACID.
GRANITE, is a compound rock, essentially composed of quartz, felspar,
and mica, each in granular crystals. It constitutes the lowest of the
geological formations, and therefore has been supposed to serve as a
base to all the rest. It is the most durable material for building, as
many of the ancient Egyptian monuments testify.
The obelisk in the place of Saint Jean de Lateran at Rome, which was
quarried at Syene, under the reign of Zetus, king of Thebes, 1300 years
before the Christian era; and the one in the place of Saint Pierre, also
at Rome, consecrated to the Sun by a son of Sesostris, have resisted the
weather for fully 3000 years. On the other hand there are many granites,
especially those in which felspar predominates, which crack and crumble
down in the course of a few years. In the same mountain, or even in the
same quarry, granites of very different qualities as to soundness and
durability occur. Some of the granites of Cornwall and Limousin readily
resolve themselves into a white kaolin or argillaceous matter, from
which pottery and porcelain are made.
Granite when some time dug out of the quarry, becomes refractory, and
difficult to cut. When this rock is intended to be worked it should be
kept under water; and that variety ought to be selected which contains
least felspar, and in which the quartz or gray crystals predominate.
GRANULATION, is the process by which metals are reduced to minute
grains. It is effected by pouring them in a melted state, through an
iron cullender pierced with small holes, into a body of water; or
directly upon a bundle of twigs immersed in water. In this way copper is
granulated into bean shot, and silver alloys are granulated preparatory
to PARTING; which see.
GRAPHITE; (_Plombagine_, Fr.; _Reissblei_, Germ.) is a mineral substance
of a lead or iron gray colour, a metallic lustre, soft to the touch, and
staining the fingers with a lead gray hue. Spec. grav. 2·08 to 2·45. It
is easily scratched, or cut with a steel edge, and displays the metallic
lustre in its interior. Burns with great difficulty in the outward flame
of the blow-pipe. It consists of carbon in a peculiar state of
aggregation, with an extremely minute and apparently accidental
impregnation of iron. Graphite, called also plumbago and black lead,
occurs in gneiss, mica slate, and their subordinate clay slates and lime
stones; in the form of masses, veins, and kidney-shaped disseminated
pieces; as also in the transition slate, as at Borrodale in Cumberland,
where the most precious deposit exists, both in reference to extent and
quality for making pencils. It has been found also among the coal
strata, as near Cumnock in Ayrshire. This substance is employed for
counteracting friction between rubbing surfaces of wood or metal, for
making crucibles and portable furnaces, for giving a gloss to the
surface of cast iron, &c. See PLUMBAGO, for some remarks concerning the
Cumberland mine.
GRAUWACKE or GREYWACKE, is a rock formation, composed of pieces of
quartz, flinty slate, felspar and clay slate, cemented by a clay-slate
basis; the pieces varying in size from small grains to a hen’s egg.
GRAY DYE. (_Teinture grise_, Fr.; _Graufarbe_, Germ.) The gray dyes in
their numerous shades, are merely various tints of black, in a more or
less diluted state, from the deepest to the lightest hue.
The dyeing materials are essentially the tannic and gallic acid of galls
or other astringents, along with the sulphate or acetate of iron, and
occasionally wine stone. Ash gray is given for 30 pounds of woollen
stuff, by one pound of gall-nuts, 1/2 lib. of wine stone (crude tartar),
and 2-1/2 libs. of sulphate of iron. The galls and the wine stone being
boiled with from 70 to 80 pounds of water, the stuff is to be turned
through the decoction at a boiling heat for half an hour, then taken
out, when the bath being refreshed with cold water, the copperas is to
be added, and, as soon as it is dissolved, the stuff is to be put in and
fully dyed. Or, for 36 pounds of wool; 2 pounds of tartar, 1/2 pound of
galls, 3 pounds of sumach, and 2 pounds of sulphate of iron are to be
taken. The tartar being dissolved in 80 pounds of boiling water, the
wool is to be turned through the solution for half an hour, and then
taken out. The copper being filled up to its former level with fresh
water, the decoction of the galls and sumach is to be poured in, and
the wool boiled for half an hour in the bath. The wool is then taken
out, while the copperas is being added and dissolved; after which it is
replaced in the bath, and dyed gray with a gentle heat.
If the gray is to have a yellow cast, instead of the tartar, its own
weight of alum is to be taken; instead of the galls, one pound of old
fustic; instead of the copperas, 3/4 of a pound of Saltzburg vitriol,
which consists, in 22-3/8 parts, of 17 of sulphate of iron, and 5-3/8 of
sulphate of copper; then proceed as above directed. Or the stuff may be
first stained in a bath of fustic, next in a weak bath of galls with a
little alum; then the wool being taken out, a little vitriol, (common or
Saltzburg) is to be put in, previously dissolved in a decoction of
logwood; and in this bath the dye is completed.
_Pearl-gray_ is produced by passing the stuff first through a decoction
of sumach and logwood (2 libs. of the former to one of the latter),
afterwards through a dilute solution of sulphate or acetate of iron; and
finishing it in a weak bath of weld containing a little alum.
_Mouse-gray_ is obtained, when with the same proportions as for
ash-gray, a small quantity of alum is introduced.
For several other shades, as tawny-gray, iron-gray, and slate-gray, the
stuff must receive a previous blue ground by dipping it in the indigo
vat; then it is passed first through a boiling bath of sumach with
galls, and lastly through the same bath at a lower temperature after it
has received the proper quantity of solution of iron.
For dyeing silk gray, fustet, logwood, sumach, and elder-tree bark, are
employed instead of galls. Archil and annotto are frequently used to
soften and beautify the tint.
The mode of producing gray dyes upon cotton has been sufficiently
explained in the articles CALICO PRINTING and DYEING.
GREEN DYE is produced by the mixture of a blue and yellow dye, the blue
being first applied. See DYEING; as also BLUE and YELLOW DYES, and
CALICO PRINTING.
GREEN PAINTS. (_Couleurs vertes_, Fr.; _Grüne pigmente_, Germ.) Green,
which is so common a colour in the vegetable kingdom, is very rare in
the mineral. There is only one metal, copper, which affords in its
combinations the various shades of green in general use. The other
metals capable of producing this colour are, chromium in its protoxide,
nickel in its hydrated oxide, as well as its salts, the seleniate,
arseniate, and sulphate; and titanium in its prussiate.
Green pigments are prepared also by the mixture of yellows and blues;
as, for example, the green of Rinman and of Gellert, obtained by the
mixture of cobalt blue, and flowers of zinc; that of Barth made with
yellow lake, prussian blue, and clay; but these paints seldom appear in
the market, because the greens are generally extemporaneous preparations
of the artists.
_Mountain green_ consists of the hydrate, oxide, or carbonate of copper,
either factitious, or as found in nature.
_Bremen or Brunswick green_ is a mixture of carbonate of copper with
chalk or lime, and sometimes a little magnesia or ammonia. It is
improved by an admixture of white lead. It may be prepared by adding
ammonia to a mixed solution of sulphate of copper and alum.
_Frise green_ is prepared with sulphate of copper and sal ammoniac.
_Mittis green_ is an arseniate of copper; made by mixing a solution of
acetate or sulphate of copper with arsenite of potash. It is in fact
Scheele’s green.
_Sap green_ is the inspissated juice of buckthorn berries. These are
allowed to ferment for 8 days in a tub, then put in a press, adding a
little alum to the juice, and concentrated by gentle evaporation. It is
lastly put up in pigs’ bladders, where it becomes dry and hard.
_Schweinfurt green_; see SCHWEINFURT.
_Verona green_ is merely a variety of the mineral called green earth.
GREEN VITRIOL is sulphate of iron in green crystals.
GUAIAC; (_Gaiac_, Fr.; _Guajaharz_, Germ.) is a resin which exudes from
the trunk of the _Guaiacum officinale_, a tree which grows in the West
India islands. It comes to us in large greenish-brown, semi-transparent
lumps, having a conchoidal or splintery fracture, brittle and easy to
pulverize. It has an aromatic smell, a bitterish, acrid taste, melts
with heat, and has a spec. grav. of from 1·20 to 1·22. It consists of
67·88 carbon; 7·05 hydrogen; and 25·07 oxygen; and contains two
different resins, the one of which is soluble in all proportions in
ammonia, and the other forms, with water of ammonia, a tarry
consistenced mixture. It is soluble in alkaline lyes, in alcohol,
incompletely in ether, still less so in oil of turpentine, and not at
all in fat oils. Its chief use is in medicine.
GUANO; is a substance of a dark yellow colour; of a strong ambrosial
smell; which blackens in the fire, with the exhalation of an ammoniacal
odour; soluble with effervescence in hot nitric acid. When this solution
is evaporated to dryness, it assumes a fine red colour, evincing the
presence of uric acid. Guano is found upon the coasts of Peru, in the
islands of Chinche, near Pisco, and several other places more to the
south. It forms a deposit 50 or 60 feet thick, and of considerable
extent; and appears to be the accumulation of the excrements of
innumerable flocks of birds, especially herons and flamands, which
inhabit these islands. It is an excellent manure, and forms the object
of a most extensive and profitable trade.
GUM; (_Gomme_, Fr.; _Gummi_, _Pflanzenschleim_, Germ.) is the name of a
proximate vegetable product, which forms with water a slimy solution,
but is insoluble in alcohol, ether, and oils; it is converted by strong
sulphuric acid into oxalic and mucic acids.
There are six varieties of gum: 1. gum arabic; 2. gum senegal; 3. gum of
the cherry and other stone fruit trees; 4. gum tragacanth; 5. gum of
Bassora; 6. the gum of seeds and roots. The first five spontaneously
flow from the branches and trunks of their trees, and sometimes from the
fruits, in the form of a mucilage which dries and hardens in the air.
The sixth kind is extracted by boiling water.
Gum arabic and gum senegal consist almost wholly of the purest gum
called _arabine_ by the French chemists; our native fruit trees contain
some _cerasine_, along with arabine; the gum of Bassora and gum
tragacanth consist of arabine and bassorine.
_Gum arabic_, flows from the _acacia arabica_, and the _acacia vera_,
which grow upon the banks of the Nile and in Arabia. It occurs in
commerce in the form of small pieces, rounded upon one side and hollow
upon the other. It is transparent, without smell, brittle, easy to
pulverize, sometimes colourless, sometimes with a yellow or brownish
tint. It may be bleached by exposure to the air and the sun-beams, at
the temperature of boiling water. Its specific gravity is 1·355,
Moistened gum arabic reddens litmus paper, owing to the presence of a
little supermalate of lime, which may be removed by boiling alcohol; it
shows also traces of the chlorides of potassium and calcium, and the
acetate of potash. 100 parts of good gum, contain 70·40 of arabine,
17·60 of water, with a few per cents. of saline and earthy matters. Gum
arabic is used in medicine, as also to give lustre to crapes and other
silk stuffs.
_Gum senegal_, is collected by the negroes during the month of November,
from the _acacia senegal_, a tree 18 or 20 feet high. It comes to us in
pieces about the size of a partridge egg, but sometimes larger, with a
hollow centre. Its specific gravity is 1·436. It consists of 81·10
arabine; 16·10 water; and from 2 to 3 of saline matters. The chemical
properties and uses of this gum are the same as those of gum arabic. It
is much employed in calico-printing.
_Cherry-tree gum_, consists of 52·10 arabine; 54·90 cerasine; 12 water;
and 1 saline matter.
_Gum tragacanth_, is gathered about the end of June, from the
_astragalus tragacantha_ of Crete and the surrounding islands. It has
the appearance of twisted ribands; is white or reddish; nearly opaque,
and a little ductile. It is difficult to pulverize, without heating the
mortar. Its specific gravity is 1·384. When plunged in water, it
dissolves in part, swells considerably, and forms a very thick mucilage.
100 parts of it consist of 53·30 arabine; 33·30 bassorine and starch;
11·0 water; and from 2 to 3 parts of saline matters. It is employed in
calico printing, and by shoemakers.
_Gum of Bassora_; see BASSORINE.
_Gum of seeds_, as linseed, consists of 52·70 arabine; 28·9 of an
insoluble matter; 10·3 water; and 7·11 saline matter. Neither bassorine
nor cerasine seems to be present in seeds and roots. For _British Gum_,
see STARCH.
GUM RESINS. (_Gomme-résines_, Fr.; _Schleimharze_, Germ.) When incisions
are made in the stems, branches and roots of certain plants, a milky
juice exudes, which gradually hardens in the air; and appears to be
formed of resin and essential oil, held suspended in water charged with
gum, and sometimes with other vegetable matters, such as caoutchouc,
bassorine, starch, wax, and several saline matters. The said concrete
juice is called a gum-resin; an improper name, as it gives a false idea
of the nature of the substance. They are all solid; heavier than water;
in general opaque and brittle; many have an acrid taste, and a strong
smell; their colour is very variable. They are partially soluble in
water, and also in alcohol; and the solution in the former liquid seldom
becomes transparent. Almost all the gum resins are medicinal substances,
and little employed in the arts and manufactures. The following is a
list of them: Asa-fœtida; gum ammoniac; bdellium; euphorbium; galbanum;
gamboge; myrrh; olibanum or frankincense; opoponax; and scammony. Some
of these are described in this work under their peculiar names.
GUNPOWDER. The following memoir upon this subject was published by me in
the Journal of the Royal Institution for October, 1830. It contains the
results of several careful analytical experiments, as also of
observations made at the Royal Gunpowder Works at Waltham Abbey, and at
some similar establishments in the neighbourhood of London.
GUNPOWDER is a mechanical combination of nitre, sulphur, and charcoal;
deriving the intensity of its explosiveness from the purity of its
constituents, the proportion in which they are mixed, and the intimacy
of the admixture.
1. _On the nitre._--Nitre may be readily purified, by solution in water
and crystallization, from the muddy particles and foreign salts with
which it is usually contaminated. In a saturated aqueous solution of
nitre, boiling hot, the temperature is 240° F.; and the relation of the
salt to its solvent is in weight as three to one, by my experiments: not
five to one, as MM. Bottée and Riffault have stated. We must not,
however, adopt the general language of chemists, and say that three
parts of nitre are soluble in one of boiling water, since the liquid has
a much higher heat and greater solvent power than this expression
implies.
Water at 60° dissolves only one-fourth of its weight of nitre; or, more
exactly, this saturated solution contains 21 per cent. of salt. Its
specific gravity is 1·1415; 100 parts in volume of the two constituents
occupy now 97·91 parts. From these data we may perceive that little
advantage could be gained in refining crude nitre, by making a
boiling-hot saturated solution of it; since on cooling, the whole would
concrete into a moist saline mass, consisting by weight of 2-3/4 parts
of salts, mixed with 1 part of water, holding 1/4 of salt in solution,
and in bulk of 1-7/8 of salt, with about 1 of liquid; for the specific
gravity of nitre is 2·005, or very nearly the double of water. It is
better, therefore, to use equal weights of saltpetre and water in making
the boiling-hot solution. When the filtered liquid is allowed to cool
slowly, somewhat less than three-fourths of the nitre will separate in
regular crystals; while the foreign salts that were present will remain
with fully one-fourth of nitre in the mother liquor. On redissolving
these crystals with heat, in about two-thirds of their weight of water,
a solution will result, from which crystalline nitre, fit for every
purpose, will concrete on cooling.
As the principal saline impurity of saltpetre is muriate of soda (a
substance scarcely more soluble in hot than in cold water), a ready mode
thence arises of separating that salt from the nitre in mother waters
that contain them in nearly equal proportion. Place an iron ladle or
basin, perforated with small holes, on the bottom of the boiler in which
the solution is concentrating. The muriate, as it separates by the
evaporation of the water, will fall down and fill the basin, and may be
removed from time to time. When small needles of nitre begin to appear,
the solution must be run off into the crystallizing cooler, in which
moderately pure nitre will be obtained, to be refined by another similar
operation.
At the Waltham Abbey gunpowder works the nitre is rendered so pure by
successive solutions and crystallizations, that it causes no opalescence
in a solution of nitrate of silver. Such crystals are dried, fused in an
iron pot at a temperature of from 500° to 600° F., and cast into moulds.
The cakes are preserved in casks.
About the period of 1794 and 1795, under the pressure of the first wars
of their revolution, the French chemists employed by the government
contrived an expeditious, economical, and sufficiently effective mode of
purifying their nitre. It must be observed that this salt, as brought to
the gunpowder-works in France, is in general a much cruder article than
that imported into this country from India. It is extracted from the
nitrous salts contained in the mortar-rubbish of old buildings,
especially those of the lowest and filthiest descriptions. By their
former methods, the French could not refine their nitre in less time
than eight or ten days; and the salt was obtained in great lumps, very
difficult to dry and divide; whereas the new process was so easy and so
quick, that in less than twenty-four hours, at one period of pressure,
the crude saltpetre was converted into a pure salt, brought to perfect
dryness, and in such a state of extreme division as to supersede the
operations of grinding and sifting, whence also considerable waste was
avoided.
The following is a brief outline of this method, with certain
improvements, as now practised in the establishment of the
_Administration des poudres et salpêtres_, in France.
The refining boiler is charged over night with 600 kilogrammes of water,
and 1200 kilogrammes of saltpetre, as delivered by the salpêtriers. No
more fire is applied than is adequate to effect the solution of this
first charge of saltpetre. It may here be observed, that such an article
contains several deliquescent salts, and is much more soluble than pure
nitre. On the morrow morning the fire is increased, and the boiler is
charged at different intervals with fresh doses of saltpetre, till the
whole amounts to 3000 kilogrammes. During these additions, care is taken
to stir the liquid very diligently, and to skim off the froth as it
rises. When it has been for some time in ebullition, and when it may be
presumed that the solution of the nitrous salts is effected, the muriate
of soda is scooped out from the bottom of the boiler, and certain
affusions or inspersions of cold water are made into the pot, to quicken
the precipitation of that portion which the boiling motion may have kept
afloat. When no more is found to fall, one kilogramme of Flanders glue,
dissolved in a sufficient quantity of hot water, is poured into the
boiler; the mixture is thoroughly worked together, the froth being
skimmed off, with several successive inspersions of cold water, till 400
additional kilogrammes have been introduced, constituting altogether
1000 kilogrammes.
When the refining liquor affords no more froth, and is grown perfectly
clear, all manipulation must cease. The fire is withdrawn, with the
exception of a mere kindling, so as to maintain the temperature till the
next morning at about 88° C. = 190·4 F.
This liquor is now transferred by hand-basins into the crystallizing
reservoirs, taking care to disturb the solution as little as possible,
and to leave untouched the impure matter at the bottom. The contents of
the long crystallizing cisterns are stirred backwards and forwards with
wooden paddles, in order to quicken the cooling, and the consequent
precipitation of the nitre in minute crystals. These are raked as soon
as they fall, to the upper end of the doubly-inclined bottom of the
crystallizer, and thence removed to the washing chests or boxes. By the
incessant agitation of the liquor, no large crystals of nitre can
possibly form. When the temperature has fallen to within 7° or 8° F., of
the apartment, that is, after seven or eight hours, all the saltpetre
that it can yield will have been obtained. By means of the double inward
slope given to the crystallizer, the supernatant liquid is collected in
the middle of the breadth, and may be easily laded out.
The saltpetre is shovelled out of the crystallizer into the washing
chests, and heaped up in them so as to stand about six or seven inches
above their upper edges, in order to allow for the subsidence which it
must experience in the washing process. Each of these chests being thus
filled, and their bottom holes being closed with plugs, the salt is
besprinkled from the rose of a watering-can, with successive quantities
of water saturated with saltpetre, and also with pure water, till the
liquor, when allowed to run off, indicates by the hydrometer, a
saturated solution. The water of each sprinkling ought to remain on the
salt for two or three hours; and then it may be suffered to drain off
through the plug-holes below, for about an hour.
All the liquor of drainage from the first watering, as well as a portion
of the second, is set aside, as being considerably loaded with the
foreign salts of the nitre, in order to be evaporated in the sequel with
the mother waters. The last portions are preserved, because they contain
almost nothing but nitre, and may therefore serve to wash another dose
of that salt. It has been proved by experience, that the quantity of
water employed in washing need never exceed thirty-six sprinklings in
the whole, composed of three waterings, of which the first two consist
of fifteen, and the last of six pots = 3 gallons E.; or in other words,
of fifteen sprinklings of water saturated with saltpetre, and twenty-one
of pure water.
The saltpetre, after remaining five or six days in the washing chests,
is transported into the drying reservoirs, heated by the flue of the
nearest boiler; here it is stirred up from time to time with wooden
shovels, to prevent its adhering to the bottom, or running into lumps,
as well as to quicken the drying process. In the course of about four
hours, it gets completely dry, in which state it no longer sticks to the
shovel, but falls down into a soft powder by pressure in the hand, and
is perfectly white and pulverulent. It is now passed through a brass
sieve, to separate any small lumps or foreign particles accidentally
present, and is then packed up in bags or barrels. Even in the shortest
winter days, the drying basin may be twice charged, so as to dry 700 or
800 kilogrammes. By this operation, the nett produce of 3000 kilogrammes
(3 tons) thus refined, amounts to from 1750 to 1800 kilogrammes of very
pure nitre, quite ready for the manufacture of gunpowder.
The mother waters are next concentrated; but into their management it is
needless to enter in this memoir.
On reviewing the above process as practised at present, it is obvious
that, to meet the revolutionary crisis, its conductors must have
shortened it greatly, and have been content with a brief period of
drainage.
2. _On the sulphur._--The sulphur now imported into this country, from
the volcanic districts of Sicily and Italy, for our manufactories of
sulphuric acid, is much purer than the sulphur obtained by artificial
heat from any varieties of pyrites, and may, therefore, by simple
processes, be rendered a fit constituent of the best gunpowder. As it
not my purpose here to repeat what may be found in common chemical
compilations, I shall say nothing of the sublimation of sulphur; a
process, moreover, much too wasteful for the gunpowder-maker.
Sulphur may be most easily analyzed, even by the manufacturer himself;
for I find it to be soluble in one tenth of its weight of boiling oil of
turpentine, at 316° Fahrenheit, forming a solution which remains clear
at 180°. As it cools to the atmospheric temperature, beautiful
crystalline needles form, which may be washed sufficiently with cold
alcohol, or even tepid water. The usual impurities of the sulphur, which
are carbonate and sulphate of zinc, oxide and sulphuret of iron,
sulphuret of arsenic and silica, will remain unaffected by the volatile
oil, and may be separately eliminated by the curious, though such
separation is of little practical importance.
Two modes of refining sulphur for the gunpowder works have been
employed; the first is by fusion, the second by distillation. Since the
combustible solid becomes as limpid as water, at the temperature of
about 230° Fahrenheit, a ready mode offers of removing at once its
denser and lighter impurities, by subsidence and skimming. But I may
take the liberty of observing, that the French melting pot, as described
in the elaborate work of MM. Bottée and Riffault, is singularly
ill-contrived, for the fire is kindled right under it, and plays on its
bottom. Now a pot for subsidence ought to be cold set; that is, should
have its bottom part imbedded in clay or mortar for four or six inches
up the side, and be exposed to the circulating flame of the fire only
round its middle zone. This arrangement is adopted in many of our great
chemical works, and is found to be very advantageous. With such a
boiler, judiciously heated, I believe that crude sulphur might be made
remarkably pure; whereas by directing the heat against the bottom of the
vessel, the crudities are tossed up, and incorporated with the mass. See
EVAPORATION.
The sulphur of commerce occurs in three prevailing colours; lemon yellow
verging on green, dark yellow, and brown yellow. As these different
shades result from the different degrees of heat to which it has been
exposed in its original extraction on the great scale, we may thereby
judge to what point it may still be heated anew in the refinery melting.
Whatever be the actual shade of the crude article, the art of the
refiner consists in regulating the heat, so that after the operation it
may possess a brilliant yellow hue, inclining somewhat to green.
In seeking to accomplish this purpose, the sulphur should first be
sorted according to its shades; and if a greenish variety is to be
purified, since this kind has been but little heated in its extraction,
the fusion may be urged pretty smartly, or the fire may be kept up till
every thing is melted but the uppermost layer.
Sulphur of a strong yellow tinge cannot bear so great a heat, and
therefore the fire must be withdrawn whenever three fourths of the whole
mass have been melted.
Brown-coloured brimstone, having been already somewhat scorched, should
be heated as little as possible, and the fire may be removed as soon as
one half of the mass is fused.
Instead of melting, separately, sulphurs of different shades, we shall
obtain a better result by first filling up the pot to half its capacity,
with the greenish-coloured article, putting over this layer one quarter
volume of the deep yellow, and filling it to the brim with the
brown-coloured. The fire must be extinguished as soon as the yellow is
fused. The pot must then be closely covered for some time; after which
the lighter impurities will be found on the surface in a black froth,
which is skimmed off, and the heavier ones sink to the bottom. The
sulphur itself must be left in the pot for ten or twelve hours, after
which it is laded out into the crystallizing boxes or casks.
Distillation affords a more complete and very economical means of
purifying sulphur, which was first introduced into the French gunpowder
establishments, when their importation of the best Italian and Sicilian
sulphur was obstructed by the British navy. Here the sulphur need not
come over slowly in a rare vapour, and be deposited in a pulverulent
form called flowers; for the only object of the refiner is to bring over
the whole of the pure sulphur into his condensing chamber, and to leave
all its crudities in the body of the still. Hence a strong fire is
applied to elevate a denser mass of vapours, of a yellowish colour,
which passing over into the condenser, are deposited in a liquid state
on its bottom, whilst only a few lighter particles attach themselves to
the upper and lateral surfaces. The refiner must therefore give to the
heat in this operation very considerable intensity; and, at some height
above the edge of the boiler, he should provide an inclined plane, which
may let the first ebullition of the sulphur overflow into a safety
recipient. The condensing chamber should be hot enough to maintain the
distilled sulphur in a fluid state,--an object most readily procured by
leading the pipes of several distilling pots into it; while the
continuity of the operations is secured, by charging each of the stills
alternately, or in succession. The heat of the receiver must be never so
high as to bring the sulphur to a syrupy consistence, whereby its colour
is darkened.
In the sublimation of sulphur, a pot containing about four cwt. can be
worked off only once in twenty-four hours, from the requisite moderation
of its temperature, and the precaution of an inclined plane, which
restores to it the accidental ebullitions. But, by distillation, a pot
containing fully ten cwt. may complete one process in nine hours at
most, with a very considerable saving of fuel. In the former plan of
procedure, an interval must elapse between the successive charges; but
in the latter, the operation must be continuous to prevent the apparatus
from getting cooled: in sublimation, moreover, where communication of
atmospheric air to the condensing chamber is indispensable, explosive
combustions of the sulphurous vapours frequently occur, with a copious
production of sulphurous acid, and correspondent waste of the sulphur;
disadvantages from which the distillatory process is in a great measure
exempt.
I shall here describe briefly the form and dimensions of the distilling
apparatus employed at Marseilles in purifying sulphur for the national
gunpowder works, which was found adequate to supply the wants of
Napoleon’s great empire. This apparatus consists of only two still-pots
of cast iron, formed like the large end of an egg, each about three feet
in diameter, two feet deep, and nearly half an inch thick at the bottom,
but much thinner above, with a horizontal ledge four inches broad. A pot
of good cast iron is capable of distilling 1000 tons of sulphur before
it is rendered unserviceable, by the action of the brimstone on its
substance, aided by a strong red heat. The pot is covered in with a
sloping roof of masonry, the upper end of which abuts on the brickwork
of the vaulted dome of condensation. A large door is formed in the
masonry in front of the mouth of the pot, through which it is charged
and cleared out; and between the roof-space over the pot, and the cavity
of the vault, a large passage is opened. At the back of the pot a
stone-step is raised to prevent the sulphur boiling over into the
condenser. The vault is about ten feet wide within, and fourteen feet
from the bottom up to the middle of the dome, which is perforated, and
carries a chimney about twelve feet high, and twelve feet diameter
inside.
As the dome is exposed to the expansive force of a strong heat, and to a
very considerable pressure of gases and vapours, it must possess great
solidity, and be therefore bound with iron straps. Between the still and
the contiguous wall of the condensing chamber, a space must be left for
the circulation of air; a precaution found by experience indispensable;
for the contact of the furnaces would produce on the wall of the chamber
such a heat as to make it crack and form crevices for the liquid sulphur
to escape. The sides of the chamber are constructed of solid masonry,
forty inches thick, surmounted by a brick dome, covered with a layer of
stones. The floor is paved with tiles, and the walls are lined with them
up to the springing of the dome; a square hole being left in one side,
furnished with a strong iron door, at which the liquid sulphur is drawn
off at proper intervals. In the roof of the vault are two valve-holes
covered with light plates of sheet-iron, which turn freely on hinges at
one end, so as to give way readily to any sudden expansion from within,
and thus prevent dangerous explosions.
As the chamber of condensation is an oblong square, terminating upwards
in an oblong vault, it consists of a parallelopiped below, and
semi-cylinder above, having the following dimensions:--
Length of the parallelopiped 16-1/2 feet.
Width 10-4/5
Height 7-1/4
Radius of the cylinder 5-2/5
Height or length of semi-cylinder 16-1/2
Whenever the workman has introduced into each pot its charge of ten or
twelve hundred weight of crude sulphur, he closes the charging doors
carefully with their iron plates and cross-bars, and lutes them tight
with loam. He then kindles his fire, and makes the sulphur boil. One of
his first duties (and the least neglect in its discharge may occasion
serious accidents) is to inspect the roof-valves and to clean them, so
that they may play freely and give way to any expulsive force from
within. By means of a cord and chain, connected with a crank attached to
the valves, he can, from time to time, ascertain their state, without
mounting on the roof. It is found proper to work one of the pots a
certain time before fire is applied to the other. The more steadily
vapours of sulphur are seen to issue from the valves, the less
atmospherical air can exist in the chamber, and therefore the less
danger there is of combustion. But if the air be cold, with a sharp
north wind, and if no vapours be escaping, the operator should stand on
his guard, for in such circumstances a serious explosion may ensue.
As soon as both the boilers are in full work the air is expelled, the
fumes cease, and every hazard is at an end. He should bend his whole
attention to the cutting off all communication with the atmosphere,
securing simply the mobility of the valves, and a steady vigour of
distillation. The conclusion of the process is ascertained by
introducing his sounding-rod into the pot, through a small orifice made
for its passage in the wall. A new charge must then be given.
By the above process, well conducted, sulphurs are brought to the most
perfect state of purity that the arts can require; while not above four
parts in the hundred of the sulphur itself are consumed; the crude,
incombustible residuum varying from five to eight parts, according to
the nature of the raw material. But in the sublimation of sulphur, the
frequent combustions inseparable from this operation carry the loss of
weight in flowers to about twenty per cent. See SULPHUR, for a figure of
the subliming apparatus.
The process by fusion, performed at some of the public works in this
country, does not afford a return at all comparable with that of the
above French process, though a much better article is operated upon in
England. After two meltings of rough sulphur (as imported from Sicily or
Italy), eighty-four per cent. is the maximum amount obtained, the
average being probably under eighty; while the product is certainly
inferior in quality to that by distillation.
3. _On the charcoal._--Tender and light woods, capable of affording a
friable and porous charcoal, which burns rapidly away, leaving the
smallest residuum of ashes, and containing therefore the largest
proportion of carbon, ought to be preferred for charring in
gunpowder-works.
After many trials made long ago, black dogwood came to be preferred to
every plant for this purpose; but modern experiments have proved that
many other woods afford an equally suitable charcoal. The woods of black
alder, poplar, lime-tree, horse-chesnut and chesnut-tree, were
carbonized in exactly similar circumstances, and a similar gunpowder was
made with each, which was proved by the same proof-mortar. The following
results were obtained:--
+------------------+-------+-----+
| |Toises.|Feet.|
| +-------+-----+
|Poplar--mean range| 113 | 2 |
|Black alder | 110 | 4 |
|Lime | 110 | 3 |
|Horse-chesnut | 110 | 3 |
|Chesnut-tree | 109 | |
+------------------+-------+-----+
By subsequent experiments confirmatory of the above, it has been further
found that the willow presents the same advantages as the poplar, and
that several shrubs, such as the hazel-nut, the spindle-tree, the
dogberry, the elder-tree, the common sallow, and some others, may be as
advantageously employed. But whichever wood be used, we should always
cut it when full of sap, and never after it is dead; we should choose
branches not more than five or six years old, and strip them carefully,
because the old branches and the bark contain a larger proportion of
earthy constituents. The branches ought not to exceed three-quarters of
an inch in thickness, and the larger ones should be divided lengthwise
into four, so that their pith may be readily burned away.
Wood is commonly carbonized in this country into gunpowder-charcoal in
cast-iron cylinders, with their axes laid horizontally, and built in
brick-work, so that the flame of a furnace may circulate round them. One
end of the cylinder is furnished with a door, for the introduction of
the wood and the removal of the charcoal; the other end terminates in a
pipe, connected with a worm-tub for condensing the pyrolignous acid, and
giving vent to the carburetted hydrogen gases that are disengaged.
Towards the end of the operation, the connexion of the cylinder with the
pyrolignous acid cistern ought to be cut off, and a very free egress
opened for the volatile matter, otherwise the charcoal is apt to get
coated with a fuliginous varnish, and to be even penetrated with
condensable matter, which materially injure its qualities.
In France, the wood is carbonized for the gunpowder works either in
oblong vaulted ovens, or in pits, lined with brick-work or cylinders of
strong sheet-iron. In either case, the heat is derived from the
imperfect combustion of the wood itself to be charred. In general, the
product in charcoal by the latter method is from 16 to 17 parts by
weight from 100 of wood. The pit-process is supposed to afford a more
productive return, and a better article; since the body of wood is much
greater, and the fuliginous vapours are allowed a freer escape. The
surface of a good charcoal should be smooth, but not glistening. See
CHARCOAL.
The charcoal is considered by the scientific manufacturers to be the
ingredient most influential, by its fluctuating qualities, upon the
composition of gunpowder; and, therefore, it ought always to be prepared
under the vigilant and skilful eye of the director of the powder
establishment. If it has been kept for some time, or quenched at first
with water, it is unsuitable for the present purpose. Charcoal
extinguished in a close vessel by exclusion of air, and afterwards
exposed to the atmosphere, absorbs only from three to four per cent. of
moisture, while red-hot charcoal quenched with water may lose by drying
twenty-nine per cent. When the latter sort of charcoal is used for
gunpowder, a deduction of weight must be made for the water present. But
charcoal which has remained long impregnated with moisture, constitutes
a most detrimental ingredient of gunpowder.