earths, originally discovered by Vauquelin, in the beryl and emerald.
It may be extracted from either of these minerals, by treating their
powder successively with potash, with water, and with muriatic acid. The
solution by the latter, being evaporated to dryness, is to be digested
with water, and filtered. On pouring carbonate of ammonia in excess into
the liquid, we form soluble muriate of ammonia, with insoluble
carbonates of lime, chrome, and iron, as also carbonate of glucina,
which may be dissolved out from the rest by an excess of carbonate of
ammonia. When the liquid is filtered anew, the glucina passes through,
and may be precipitated in the state of a carbonate by boiling the
liquid, which expels the excess of ammonia. By washing, drying, and
calcining the carbonate, pure glucina is obtained. It is a white insipid
powder, infusible in the heat of a smith’s forge, insoluble in water,
but soluble in caustic potash and soda; as also, especially when it is a
hydrate, in carbonate of ammonia. It has a metallic base called
glucinum, of which 100 parts combine with 45·252 of oxygen to form the
earth. It is too rare to be susceptible of application in manufactures.
GLUE; (_Colle forte_, Fr.; _Leim_, _Tischlerleim_, Germ.) is the
chemical substance gelatine in a dry state. The preparation and
preservation of the skin and other animal matters employed in the
manufacture of glue, constitute a peculiar branch of industry. Those who
exercise it should study to prevent the fermentation of the substances,
and to diminish the cost of carriage by depriving them of as much water
as can conveniently be done. They may then be put in preparation by
macerating them in milk of lime, renewed three or four times in the
course of a fortnight or three weeks. This process is performed in large
tanks of masonry. They are next taken out with all the adhering lime,
and laid in a layer, 2 or 3 inches thick, to drain and dry, upon a
sloping pavement, where they are turned over by prongs, two or three
times a day. The action of the lime dissolves the blood and certain soft
parts, attacks the epidermis, and disposes the gelatinous matter to
dissolve more readily. When the cleansed matters are dried, they may be
packed in sacks or hogsheads, and transported to the glue manufactory at
any distance. The principal substances of which glue is made are the
parings of ox and other thick hides, which form the strongest article;
the refuse of the leather dresser; both afford from 45 to 55 per cent.
of glue. The tendons, and many other offals of slaughter houses, also
afford materials, though of an inferior quality, for the purpose. The
refuse of tanneries, such as the ears of oxen, calves, sheep, &c., are
better articles; but parings of parchment, old gloves, and, in fact,
animal skin, in every form, uncombined with tannin, may be made into
glue.
The manufacturer who receives these materials, is generally careful to
ensure their purification by subjecting them to a weak lime steep, and
rinsing them by exposure in baskets to a stream of water. They are
lastly drained upon a sloping surface, as above described, and well
turned over till the quicklime gets mild by absorption of carbonic acid;
for, in its caustic state, it would damage the glue at the heat of
boiling water. It is not necessary, however, to dry them before they are
put into the boiler, because they dissolve faster in their soft and
tumefied state.
The boiler is made of copper, rather shallow in proportion to its area,
with a uniform flat bottom, equably exposed all over to the flame of the
fire. Above the true bottom there is a false one of copper or iron,
pierced with holes, and standing upon feet 3 or 4 inches high; which
serves to sustain the animal matters, and prevent them from being
injured by the fire. The copper being filled to two thirds of its height
with soft water, is then heaped up with the bulky animal substances, so
high as to surmount its brim. But soon after the ebullition begins they
sink down, and, in a few hours, get entirely immersed in the liquid.
They should be stirred about from time to time, and well pressed down
towards the false bottom, while a steady but gentle boil is maintained.
The solution must be drawn off in successive portions; a method which
fractions the products, or subdivides them into articles of various
value, gradually decreasing from the first portion drawn off to the
last. It has been ascertained by careful experiments that gelatine gets
altered over the fire very soon after it is dissolved, and it ought
therefore to be drawn off whenever it is sufficiently fluid and strong
for forming a clear gelatinous mass on cooling, capable of being cut
into moderately firm slices by the wire. This point is commonly
determined by filling half an egg-shell with the liquor, and exposing it
to the air to cool. The jelly ought to get very consistent in the course
of a few minutes; if not so, the boiling must be persisted in a little
longer. When this term is attained, the fire is smothered up, and the
contents of the boiler are left to settle for a quarter of an hour. The
stop-cock being partially turned, all the thin gelatinous liquor is run
off into a deep boiler, immersed in a warm-water bath, so that it may
continue hot and fluid for several hours. At the end of this time, the
supernatant clear liquid is to be drawn off into congealing boxes, as
will be presently explained.
The grounds, or undissolved matters in the boiler, are to be again
supplied with a quantity of boiling water from an adjoining copper, and
are to be once more subjected to the action of the fire, till the
contents assume the appearance of dissolved jelly, and afford a fresh
quantity of strong glue liquor, by the stop-cock. The grounds should be
subjected a third time to this operation, after which they may be put
into a bag, and squeezed in a press to leave nothing unextracted. The
latter solutions are usually too weak to form glue directly, but they
may be strengthened by boiling with a portion of fresh skin-parings.
[Illustration: 528]
_Fig._ 528. represents a convenient apparatus for the boiling of skins
into glue, in which there are three coppers upon three different levels;
the uppermost being acted upon by the waste heat of the chimney,
provides warm water in the most economical way; the second contains the
crude materials, with water for dissolving them; and the third receives
the solution to be settled. The last vessel is double, with water
contained between the outer and inner one; and discharges its contents
by a stop-cock into buckets for filling the gelatinizing wooden boxes.
The last made solution has about one five hundredth part of alum in
powder usually added to it, with proper agitation, after which it is
left to settle for several hours.
The three successive boils furnish three different qualities of glue.
Flanders or Dutch glue, long much esteemed on the Continent, was made in
the manner above described, but at two boils, from animal offals well
washed and soaked, so as to need less boiling. The liquor being drawn
off thinner, was therefore less coloured, and being made into thinner
plates was very transparent. The above two boils gave two qualities of
glue.
By the English practice, the whole of the animal matter is brought into
solution at once, and the liquor being drawn off, hot water is poured on
the residuum, and made to boil on it for some time, when the liquor thus
obtained is merely used instead of water upon a fresh quantity of glue
materials. The first drawn off liquor is kept hot in a settling copper
for five hours, and then the clear solution is drawn off into the boxes.
These boxes are made of deal, of a square form, but a little narrower at
bottom than at top. When very regular cakes of glue are wished for,
cross grooves of the desired square form are cut in the bottom of the
box. The liquid glue is poured into the boxes placed very level, through
funnels furnished with filter cloths, till it stands at the brim of
each. The apartment in which this is done ought to be as cool and dry as
possible, to favour the solidification of the glue, and should be
floored with stone flags kept very clean, so that if any glue run
through the seams, it may be recovered. At the end of 12 or 18 hours, or
usually in the morning if the boxes have been filled overnight, the glue
is sufficiently firm for the nets, and they are at this time removed to
an upper story, mounted with ventilating windows to admit the air from
all quarters. Here the boxes are inverted upon a moistened table, so
that the gelatinous cake thus turned out will not adhere to its surface;
usually the moist blade of a long knife is insinuated round the sides of
the boxes beforehand, to loosen the glue. The mass is first divided into
horizontal layers by a brass wire stretched in a frame, like that of a
bow-saw, and guided by rulers which are placed at distances
corresponding to the desired thickness of the cake of glue. The lines
formed by the grooves in the bottom of the box define the superficial
area of each cake, where it is to be cut with a moist knife. The
gelatinous layers thus formed, must be dexterously lifted, and
immediately laid upon nets stretched in wooden frames, till each frame
be filled. These frames are set over each other at distances of about
three inches, being supported by small wooden pegs, stuck into mortise
holes in an upright, fixed round the room; so that the air may have
perfectly free access on every side. The cakes must moreover be turned
upside down upon the nets twice or thrice every day, which is readily
managed, as each frame may be slid out like a drawer, upon the pegs at
its two sides.
The drying of the glue is the most precarious part of the manufacture.
The least disturbance of the weather may injure the glue during the two
or three first days of its exposure; should the temperature of the air
rise considerably, the gelatine may turn so soft as to become unshapely,
and even to run through the meshes upon the pieces below, or it may get
attached to the strings and surround them, so as not to be separable
without plunging the net into boiling water. If frost supervene, the
water may freeze and form numerous cracks in the cakes. Such pieces must
be immediately re-melted and re-formed. A slight fog even produces upon
glue newly exposed a serious deterioration; the damp condensed upon its
surface occasioning a general mouldiness. A thunderstorm sometimes
destroys the coagulating power in the whole laminæ at once; or causes
the glue to _turn_ on the nets, in the language of the manufacturer. A
wind too dry or too hot may cause it to dry so quickly, as to prevent it
from contracting to its proper size without numerous cracks and
fissures. In this predicament, the closing of all the flaps of the
windows is the only means of abating the mischief. On these accounts it
is of importance to select the most temperate season of the year, such
as spring and autumn, for the glue manufacture.
After the glue is dried upon the nets it may still preserve too much
flexibility, or softness at least, to be saleable; in which case it must
be dried in a stove by artificial heat. This aid is peculiarly requisite
in a humid climate, like that of Great Britain.
When sufficiently dry it next receives a gloss, by being dipped cake by
cake in hot water, and then rubbed with a brush also moistened in hot
water; after which the glue is arranged upon a hurdle, and transferred
to the stove room, if the weather be not sufficiently hot. One day of
proper drought will make it ready for being packed up in casks.
The pale-coloured, hard and solid, article, possessing a brilliant
fracture, which is made from the parings of ox-hides by the first
process, is the best and most cohesive, and is most suitable for
joiners, cabinet-makers, painters, &c. But many workmen are influenced
by such ignorant prejudices, that they still prefer a dark-coloured
article, with somewhat of a fetid odour, indicative of its impurity and
bad preparation, the result of bad materials and too long exposure to
the boiling heat.
There is a good deal of glue made in France from bones, freed from the
phosphate of lime by muriatic acid. This is a poor article, possessing
little cohesive force. It dissolves almost entirely in cold water, which
is the best criterion of its imperfection. Glue should merely soften in
cold water, and the more considerably it swells, the better generally
speaking, it is.
Some manufacturers prefer a brass to a copper pan for boiling glue, and
insist much on skimming it as it boils; but the apparatus I have
represented renders skimming of little consequence. For use, glue should
be broken into small pieces, put along with some water into a vessel,
allowed to soak for some hours, and subjected to the heat of a
boiling-water bath, but not boiled itself. The surrounding hot water
keeps it long in a fit state for joiners, cabinet-makers, &c.
Water containing only one hundredth part of good glue, forms a tremulous
solid. When the solution, however, is heated and cooled several times,
it loses the property of gelatinizing, even though it be enclosed in a
vessel hermetically sealed. Isinglass or fish-glue undergoes the same
change. Common glue is not soluble in alcohol, but is precipitated in a
white, coherent, elastic mass, when its watery solution is treated with
that fluid. By transmitting chlorine gas through a warm solution of
glue, a combination is very readily effected, and a viscid mass is
obtained like that thrown down by alcohol. A little chlorine suffices to
precipitate the whole of the glue. Concentrated sulphuric acid makes
glue undergo remarkable changes; during which are produced, sugar of
gelatine, leucine, an animal matter, &c. Nitric acid, with the aid of
heat, converts glue into malic acid, oxalic acid, a fat analogous to
suet, and into tannin; so that, in this way, one piece of skin may be
made to tan another. When the mixture of glue and nitric acid is much
evaporated, a detonation at last takes place. Strong acetic acid renders
glue first soft and transparent, and then dissolves it. Though the
solution does not gelatinize, it preserves the property of gluing
surfaces together when it dries. Liquid glue dissolves a considerable
quantity of lime, and also of the phosphate of lime recently
precipitated. Accordingly glue is sometimes contaminated with that salt.
Tannin both natural and artificial combines with glue; and with such
effect, that one part of glue dissolved in 5000 parts of water affords a
sensible precipitate with the infusion of nutgalls. Tannin unites with
glue in several proportions, which are to each other as the numbers 1,
1-1/2, and 2; one compound consists of 100 glue and 89 tannin; another
of 100 glue and 60 tannin; and a third of 100 glue and 120 tannin. These
two substances cannot be afterwards separated from each other by any
known chemical process.
Glue may be freed from the foreign animal matters generally present in
it, by softening it in cold water, washing it with the same several
times till it no longer gives out any colour, then bruising it with the
hand, and suspending it in a linen bag beneath the surface of a large
quantity of water at 60° F. In this case, the water loaded with the
soluble impurities of the glue gradually sinks to the bottom of the
vessel, while the pure glue remains in the bag surrounded with water. If
this softened glue be heated to 92° without adding water, it will
liquefy; and if we heat it to 122°, and filter it, some albuminous and
other impurities will remain on the filter, while a colourless solution
of glue will pass through.
Experiments have not yet explained how gelatine is formed from skin by
ebullition. It is a change somewhat analogous to that of starch into gum
and sugar, and takes place without any appreciable disengagement of gas,
and even in close vessels. Gelatine, says Berzelius, does not exist in
the living body, but several animal tissues, such as skin, cartilages,
hartshorn, tendons, the serous membranes, and bones, are susceptible of
being converted into it.
GLUTEN; (_Colle Vegetale_, Fr.; _Kleber_, Germ.) was first extracted by
Beccaria from wheat flour, and was long regarded as a proximate
principle of plants, till Einhof, Taddei, and Berzelius, succeeded in
showing that it may be resolved by means of alcohol into three different
substances, one of which resembles closely animal albumine, and has been
called _Zymome_, or vegetable albumine; another has been called
_Gliadine_; and a third _Mucine_. The mode of separating gluten from the
other constituents of wheat flour, has been described towards the end of
the article BREAD.
Gluten when dried in the air or a stove, diminishes greatly in size,
becomes hard, brittle, glistening, and of a deep yellow colour. It is
insoluble in ether, in fat and essential oils, and nearly so in water.
Alcohol and acetic acid cause gluten to swell and make a sort of milky
solution. Dilute acids and alkaline lyes dissolve gluten. Its ultimate
constituents are not determined, but azote is one of them, and
accordingly when moist gluten is left to ferment, it exhales the smell
of old cheese.
GLYCERINE, is a sweet substance which may be extracted from fatty
substances. If we take equal parts of olive oil, and finely-ground
litharge, put them into a basin with a little water, set this on a sand
bath moderately heated, and stir the mixture constantly, with the
occasional addition of hot water to replace what is lost by evaporation,
we shall obtain in a short time, a soap or plaster of lead. After having
added more water to this, we remove the vessel from the fire, decant the
liquor, filter it, pass sulphuretted hydrogen through it to separate the
lead, then filter afresh, and concentrate the liquor as much as is
possible without burning upon the sand bath. What remains must be
finally evaporated within the receiver of the air-pump. Glycerine thus
prepared is a transparent liquid, without colour or smell, and of a
syrupy consistence. It has a very sweet taste. Its specific gravity is
1·27 at the temperature of 60°. When thrown upon burning coals, it takes
fire and burns like an oil. Water combines with it in almost all
proportions; alcohol dissolves it readily; nitric acid converts it into
oxalic acid; and according to Vogel, sulphuric acid transforms it into
sugar, in the same way as it does starch. Ferment or yeast does not
affect it in any degree.
Its constituents are, carbon 40; hydrogen 9; oxygen 51; in 100.
GNEISS, is the name of one of the great mountain formations, being
reckoned the oldest of the stratified rocks. It is composed of the same
substances as granite, viz. quartz, mica, and felspar. In gneiss however
they are not in granular crystals, but in scales, so as to give the mass
a slaty structure. It abounds in metallic treasures.
GOLD. (Eng. and Germ.; _Or_, Fr.) This metal is distinguished by its
splendid yellow colour; its great density = 19·3, compared to water 1·0;
its fusibility at the 32d degree of Wedgewood’s pyrometer; its
pre-eminent ductility and malleability, whence it can be beat into
leaves only one 282,000th of an inch thick; and its insolubility in any
acid menstruum, except the mixture of muriatic and nitric acids, styled
by the alchemists _aqua regia_, because gold was deemed by them to be
the king of metals.
Gold is found only in the metallic state, sometimes crystallized in the
cube, and its derivative forms. It occurs also in threads of various
size, twisted and interlaced into a chain of minute octahedral crystals;
as also in spangles or roundish grains, which when of a certain
magnitude are called _pepitas_. The small grains are not fragments
broken from a greater mass; but they shew by their flattened ovoid
shape, and their rounded outline, that this is their original state. The
spec. grav. of native gold varies from 13·3 to 17·7. Humboldt states
that the largest _pepita_ known was one found in Peru, weighing about 12
kilogrammes (26-1/2 lbs. avoird.); but masses have been quoted in the
province of Quito which weighed nearly four times as much.
Another ore of gold is the alloy with silver, or argental gold, the
electrum of Pliny, so called from its amber shade. It seems to be a
definite compound, containing in 100 parts, 64 of gold, and 36 of
silver.
The mineral formations in which this metal occurs, are the crystalline
primitive rocks, the compact transition rocks, the trachytic and trap
rocks, and alluvial grounds.
It never predominates to such a degree as to constitute veins by itself.
It is either disseminated, and as it were impasted in stony masses, or
spread out in thin plates or grains on their surface, or, lastly,
implanted in their cavities, under the shape of filaments or
crystallized twigs. The minerals composing the veins are either quartz,
calc-spar, or sulphate of baryta. The ores that accompany the gold in
these veins are chiefly iron pyrites, copper pyrites, galena, blende,
and mispickel (arsenical pyrites.)
In the ores called auriferous pyrites, this metal occurs either in a
visible or invisible form, and though invisible in the fresh pyrites,
becomes visible by its decomposition; as the hydrated oxide of iron
allows the native gold particles to shine forth on their reddish-brown
ground, even when the precious metal may constitute only the five
millionth part of its weight, as at Rammelsberg in the Hartz. In that
state it has been extracted with profit; most frequently by amalgamation
with mercury, proving that the gold was in the native state, and not in
that of a sulphuret.
Gold exists among the primitive strata, disseminated in small grains,
spangles, and crystals. Brazil affords a remarkable example of this
species of gold mine. Beds of granular quartz, or micaceous specular
iron, in the Sierra of Cocäes, 12 leagues beyond Villa Rica, which form
a portion of a mica-slate district, include a great quantity of native
gold in spangles, which in this ferruginous rock replace mica.
Gold has never been observed in any secondary formation, but pretty
abundantly in its true and primary locality, among the trap rocks of
igneous origin; implanted on the sides of the fissures, or disseminated
in the veins.
The auriferous ores of Hungary and Transylvania, composed of tellurium,
silver pyrites or sulphuret of silver, and native gold, lie in masses or
powerful veins in a rock of trachyte or in a decomposed felspar
subordinate to it. Such is the locality of the gold ore of Königsberg,
of Telkebanya, between Eperies and Tokay in Hungary, and probably that
of the gold ores of Kapnick, Felsobanya, &c., in Transylvania; an
arrangement nearly the same with what occurs in Equatorial America. The
auriferous veins of Guanaxuato, of Real del Monte, of Villalpando, are
similar to those of Schemnitz in Hungary, as to magnitude, relative
position, the nature of the ores they include, and of the rocks they
traverse. These districts have impressed all mineralogists with the
evidences of the action of volcanic fire. Breislak and Hacquet have
described the gold mines of Transylvania as situated in the crater of an
ancient volcano. It is certain that the trachytes which form the
principal portions of the rocks including gold, are now almost
universally regarded as of igneous or volcanic origin.
It would seem, however, that the primary source of the gold is not in
these rocks, but rather in the sienites and greenstone prophyries below
them, which in Hungary and Transylvania are rich in great auriferous
deposits; for gold has never been found in the trachyte of the Euganean
mountains, of the mountains of the Vicentin, of those of Auvergne; all
of which are superposed upon granite rocks, barren in metal.
Finally, if it be true that the ancients worked mines of gold in the
island of Ischia, it would be another example, and a very remarkable
one, of the presence of this metal in trachytes of an origin evidently
volcanic.
Gold is, however, much more common in the alluvial grounds than among
the primitive and pyrogenous rocks just described. It is found
disseminated under the form of spangles, in the siliceous, argillaceous,
and ferruginous sands of certain plains and rivers, especially in their
re-entering angles, at the season of low water, and after storms and
temporary floods.
It has been supposed that the gold found in the beds of rivers had been
torn out by the waters from the veins and primitive rocks, which they
traverse. Some have even searched, but in vain, at the source of
auriferous streams for the native bed of this precious metal. The gold
in them belongs, however, to the grounds washed by the waters as they
glide along. This opinion, suggested at first by Delius, and supported
by Deborn, Guettard, Robitant, Balbo, &c., is founded upon just
observations. 1. The soil of these plains contains frequently, at a
certain depth, and in several spots, spangles of gold, separable by
washing. 2. The beds of the auriferous rivers and streamlets contain
more gold after storms of rain upon the plains than in any other
circumstances. 3. It happens almost always that gold is found among the
sands of rivers only in a very circumscribed space; on ascending these
rivers their sands cease to afford gold; though did this metal come from
the rocks above, it should be found more abundantly near the source of
the rivers. Thus it is known that the Orco contains no gold except from
Pont to its junction with the Po. The Ticino affords gold only below the
Lago Maggiore, and consequently far from the primitive mountains, after
traversing a lake, where its course is slackened, and into which
whatsoever it carried down from these mountains must have been
deposited. The Rhine gives more gold near Strasburg than near Basle,
though the latter be much closer to the mountains. The sands of the
Danube do not contain a grain of gold, while this river runs in a
mountainous region; that is, from the frontiers of the bishoprick of
Passau to Efferding; but its sands become auriferous in the plains
below. The same thing is true of the Ems; the sands of the upper portion
of this river, as it flows among the mountains of Styria, include no
gold; but from its entrance into the plain at Steyer, till its
embouchure in the Danube, its sands become auriferous, and are even rich
enough to be washed with profit.
The greater part of the auriferous sands, in Europe, Asia, Africa, and
America, are black or red, and consequently ferruginous; a remarkable
circumstance in the geological position of alluvial gold. M. Napione
supposes that the gold of these ferruginous grounds is due to the
decomposition of auriferous pyrites. The auriferous sand occurring in
Hungary almost always in the neighbourhood of the beds of _lignites_,
and the petrified wood covered with gold grains, found buried at a depth
of 55 yards in clay, in the mine of Vorospatak near Abrabanya in
Transylvania, might lead us to presume that the epoch of the formation
of the auriferous alluvia is not remote from that of the lignites. The
same association of gold ore and fossil wood occurs in South America, at
Moco. Near the village of Lloro, have been discovered at a depth of 20
feet, large trunks of petrified trees, surrounded with fragments of trap
rocks interspersed with spangles of gold and platinum. But the alluvial
soil affords likewise all the characters of the basaltic rocks; thus in
France, the Cèze and the Gardon, auriferous rivers, where they afford
most gold, flow over ground apparently derived from the destruction of
the trap rocks, which occur _in situ_ higher up the country. This fact
had struck Reaumur, and this celebrated observer had remarked that the
sand which more immediately accompanies the gold spangles in most
rivers, and particularly in the Rhone and the Rhine, is composed, like
that of Ceylon and Expailly, of black protoxide of iron and small grains
of rubies, corindon, hyacinth, &c. Titanium has been observed more
recently. It has, lastly, been remarked that the gold of alluvial
formations is purer than that extracted from rocks.
_Principal Gold Mines._
Spain anciently possessed mines of gold in regular veins, especially in
the province of Asturias; but the richness of the American mines has
made them be neglected. The Tagus, and some other streams of that
country, were said to roll over golden sands. France contains no
workable gold mines; but it presents in several of its rivers auriferous
sands. There are some gold mines in Piedmont; particularly the veins of
auriferous pyrites of Macugnagna, at the foot of Monte Rosa, lying in a
mountain of gneiss; and although they do not contain 10 or 11 grains of
gold in a hundred weight, they have long defrayed the expense of working
them. On the southern slope of the Pennine Alps, from the Simplon and
Monte Rosa to the valley of Aoste, several auriferous districts and
rivers occur. Such are the torrent Evenson, which has afforded much gold
by washing; the Orco, in its passage from the Pont to the Po; the
reddish grounds over which this little river runs for several miles, and
the hills in the neighbourhood of Chivasso, contain gold spangles in
considerable quantity.
In the county of Wicklow, in Ireland, a quartzose and ferruginous sand
was discovered not long ago, containing many particles of gold, with
_pepitas_ or solid pieces, one of which weighed 22 ounces. No less than
1000 ounces of gold were collected.
There are auriferous sands in some rivers of Switzerland, as the Reuss
and the Aar. In Germany no mine of gold is worked, except in the
territory of Salzburg, amid the chain of mountains which separates the
Tyrol and Carinthia.
The mines of Hungary and Transylvania are the only gold mines of any
importance in Europe; they are remarkable for their position, the
peculiar metals that accompany them, and their product, estimated at
about 1430 pounds avoird. annually. The principal ones are in Hungary.
1. Those of Konigsberg. The native gold is disseminated in ores of
sulphuret of silver, which occur in small masses and in veins in a
decomposing felspar rock, amid a conglomerate of pumice, constituting a
portion of the trachytic formation. 2. Those of Borson, Schemnitz. And,
3 of Felsobanya; ores also of auriferous sulphuret of silver, occur in
veins of sienite and greenstone porphyry. 4. Those of Telkebanya, to the
south of Kaschau, are in a deposit of auriferous pyrites amid trap rocks
of the most recent formation.
In Transylvania the gold mines occur in veins often of great magnitude,
6, 8, and sometimes 40 yards thick. These veins have no side plates or
wall stones, but abut without intermediate gangues at the primitive
rock. They consist of carious quartz, ferriferous limestone, heavy spar,
fluor spar, and sulphuret of silver. The mine of Kapnik deserves notice,
where the gold is associated with orpiment, and that of Vorospatak in
granite rocks; those of Offenbanya, Zalatna, and Nagy-Ag, where it is
associated with Tellurium. The last is in a sienitic rock on the limits
of the trachyte.
In Sweden, the mine of Edelfors in Smoland may be mentioned, where the
gold occurs native and in auriferous pyrites; the veins are a brown
quartz, in a mountain of foliated hornstone.
In Siberia, native gold occurs in a hornstone at Schlangenberg or Zmeof,
and at Zmeino-garsk in the Altai mountains, accompanied with many other
ores.
The gold mine of Berezof in the Oural mountains, has been long known,
consisting of _partially decomposed auriferous pyrites_, disseminated in
a vein of greasy quartz. About 1820, a very rich deposit of native gold
was discovered upon the eastern side of the Oural mountains,
disseminated at some yards depth, in an argillaceous loam, and
accompanied with the _débris_ of rocks which usually compose the
auriferous alluvial soils, as greenstone, serpentine, protoxide of iron,
corundum, &c. The rivers of this district possess auriferous sands. The
annual product of the gold mines of Siberia is 3740 pounds avoirdupois.
In Asia, and especially in its southern districts, there are many mines,
streams, rivers, and wastes, which contain this metal. The Pactolus, a
small river of Lydia, rolled over such golden sands, that it was
supposed to constitute the origin of the wealth of Crœsus. But these
deposits are now poor and forgotten. Japan, Formosa, Ceylon, Java,
Sumatra, Borneo, the Philippines, and some other islands of the Indian
Archipelago, are supposed to be very rich in gold mines. Those of Borneo
are worked by the Chinese in an alluvial soil on the western coast, at
the foot of a chain of volcanic mountains.
Little or no gold comes into Europe from Asia, because its servile
inhabitants place their fortune in treasure, and love to hoard up that
precious metal.
Numerous gold mines occur on the two slopes of the chain of the Cailas
mountains in the Oundès, a province of Little Thibet. The gold lies in
quartz veins which traverse a very crumbling reddish granite.
Africa was, with Spain, the source of the greater portion of the gold
possessed by the antients. The gold which Africa still brings into the
market in abundance is always in dust, showing that the metal is
obtained by washing the alluvial soils. None of it is collected in the
north of that continent; three or four districts only are remarkable for
the quantity of gold they produce.
The first mines are those of Kordofan, between Darfour and Abyssinia.
The negroes transport the gold in quills of the ostrich or vulture.
These mines seem to have been known to the antients, who considered
Ethiopia to abound in gold. Herodotus relates that the king of that
country exhibited to the ambassadors of Cambyses, all their prisoners
bound with golden chains.
The second and chief exploitation of gold dust is to the south of the
great desert of Zaara, in the western part of Africa, from the mouth of
the Senegal to the Cape of Palms. The gold occurs in spangles, chiefly
near the surface of the earth, in the bed of rivulets, and always in a
ferruginous earth. In some places the negroes dig wells in the soil to a
depth of about 40 feet, unsupported by any props. They do not follow any
vein; nor do they construct a gallery. By repeated washings they
separate the gold from the earthy matters.
The same district furnishes also the greater part of what is carried to
Morocco, Fez, and Algiers, by the caravans which go from Timbuctoo on
the Niger, across the great desert of Zaara. The gold which arrives by
Sennaar at Cairo and Alexandria, comes from the same quarter. From Mungo
Park’s description, it appears that the gold spangles are found usually
in a ferruginous small gravel, buried under rolled pebbles.
The third spot in Africa where gold is collected, is on the south-east
coast, between the twenty-fifth and the twenty-second degree of south
latitude, opposite to Madagascar, in the country of Sofala. Some persons
think that this was the kingdom of Ophir, whence Solomon obtained his
gold.
In modern times, the richest gold mines are found in America, from which
there is exported annually, 3700 or 4000 pounds avoirdupois of this
metal. It occurs there principally in spangles among the alluvial
earths, and in the beds of rivers; more rarely in veins.
There is little gold in the northern part of America. The United States
have hitherto produced but a slight quantity of alluvial gold, collected
in the gravel-pits of the creeks of Rockhole, district of Lebanon, in
North Carolina. In 1810, a mass was found there, weighing 28 pounds.
This district has furnished the mint of the United States with about 100
lbs. avoirdupois of gold.
South America, especially Brazil, Choco, and Chili, are the regions
which furnish most gold.
The gold of Mexico is in a great measure contained in the argentiferous
veins, so numerous in that country, whose principal localities are
mentioned under the article SILVER. The silver of the argentiferous ores
of Guanaxato, contains one 360th of its weight of gold; the annual
product of the mines being valued at from 2640 to 3300 pounds
avoirdupois.
Oaxaco contains the only auriferous veins exploited as gold mines in
Mexico; they traverse rocks of gneiss and mica slate.
All the rivers of the province of Caracas, to ten degrees north of the
line, flow over golden sands.
Peru is not rich in gold ores. In the provinces of Huailas and Pataz,
this metal is mined in veins of greasy quartz, variegated with red
ferruginous spots, which traverse primitive rocks. The mines called
_pacos de oro_, consist of ores of iron and copper oxides, containing a
great quantity of gold.
All the gold furnished by New Grenada (New Colombia), is the product of
washings, established in alluvial grounds. The gold exists in spangles
and in grains, disseminated among fragments of greenstone and porphyry.
At Choco, along with the gold and platinum, hyacinths, zircons, and
titanium occur. There has been found, as already stated, in the
auriferous localities, large trunks of petrified trees. The gold of
Antioquia is 20 carats fine, that of Choco 21, and the largest lump or
_pepita_ of gold weighed about 27-1/2 pounds avoirdupois. The gold of
Chili also occurs in alluvial formations.
Brazil furnishes the greatest part of the gold now brought into the
market. Yet there is not in this country any gold mine properly so
called; for the veins containing the metal are seldom worked.
It is in the sands of the Mandi, a branch of the Rio-Dolce, at
Catapreta, that the auriferous ferruginous sands were first discovered
in 1682. Since then, they have been found almost everywhere at the foot
of the immense chain of mountains, which runs nearly parallel with the
coast, from the 5th degree south to the 30th. It is particularly near
Villa Rica, in the environs of the village Cocäes, that the numerous
washings for gold are established. The _pepitas_ occur in different
forms, often adhering to micaceous specular iron. But in the province of
Minas Geräes, the gold occurs also in veins, in beds, and in grains,
disseminated among the alluvial loams. It has been estimated in annual
product, by several authors, at about 2800 pounds avoirdupois of fine
metal; worth nearly a million sterling.
We thus see that almost all the gold brought into the market, comes from
alluvial lands, and is extracted by washing.
The gold coin of the ancients was made chiefly out of alluvial gold, for
in these early times the metallurgic arts were not sufficiently advanced
to enable them to purify it. The gold dust from Bambouk in Africa, is of
22-1/4 carats fine, and some from Morocco is even 23.
The gold of Giron, in New Grenada, is of 23-3/4 carats; being the purest
from America. “For those who traffick in gold,” says Humboldt, “it is
sufficient to learn the place where the metal has been collected, to
know its title.”
_Metallurgic treatment of gold._--The gold found in the sands of rivers,
or in auriferous soils, needs not be subjected to any metallurgic
process, properly speaking. The Orpaillers, separate it from the sands,
by washing them first upon inclined tables, sometimes covered with a
cloth, and then by hand in wooden bowls of a particular form.
Amalgamation is employed to carry off from the sand, the minuter
particles of gold they may contain. The people called Bohemians, Cigans,
or Tehinganes, who wash the auriferous sands in Hungary, employ a plank
with 24 transverse grooves cut in its surface. They hold this plank in
an inclined position, and put the sand to be washed in the first groove;
they then throw water on it, when the gold mixed with a little sand
collects usually towards the lowest furrow. They remove this mixture
into a flat wooden basin, and by a peculiar sleight of hand, separate
the gold entirely from the sand. The richest of the auriferous ores
consist of the native gold quite visible, disseminated in a gangue, but
the veins are seldom continuous for any length. The other ores are
auriferous metallic sulphurets, such as sulphurets of copper, silver,
arsenic, &c., and, particularly iron.
The stony ores are first ground in the stamping mill, and then washed in
hand-basins, or on wooden tables.
The auriferous sulphurets are much more common, but much poorer than the
former ores; some contain only one 200,000th part of gold, and yet they
may be worked with advantage, when treated with skill and economy.
The gold of these ores is separated by two different processes; namely,
by fusion and amalgamation.
The auriferous metallic sulphurets are first roasted; then melted into
_mattes_, which are roasted anew; next fused with lead, whence an
auriferous lead is obtained, which may be refined by the process of
cupellation.
When the gold ores are very rich, they are melted directly with lead,
without preliminary calcination or fusion. These processes are however
little practised, because they are less economical and certain than
amalgamation, especially when the gold ores are very poor.
If these ores consist of copper pyrites, and if their treatment has been
pushed to the point of obtaining auriferous rose copper, or even black
copper including gold, the precious metal cannot be separated by the
process of liquation, because the gold having more affinity for copper
than for lead, can be but partially run off by the latter metal. For
these reasons the process of amalgamation is far preferable.
This process being the same for silver, I shall reserve its description
for this metal. The rich ores in which the native gold is apparent, and
merely disseminated in a stony gangue, are directly triturated with
quicksilver, without any preparatory operation. As to the poor ores, in
which the gold seems lost amid a great mass of iron, sulphuret of
copper, &c., they are subjected to a roasting before being amalgamated.
This process seems requisite to lay bare the gold enveloped in the
sulphurets. The quicksilver with which the ore is now ground, seizes the
whole of its gold, in however small quantity this metal may be present.
The gold procured by the refining process with lead, is free from copper
and lead, but it may contain iron, tin, or silver. It cannot be
separated from iron and tin without great difficulty, and expense, if
the proportion of gold be too small to admit of the employment of
muriatic acid.
By cupellation with lead, gold may be deprived of any antimony united
with it.
Tin gives gold a remarkable hardness and brittleness; a piece of gold,
exposed for some time over a bath of red hot tin, becomes brittle. The
same thing happens more readily over antimony, from the volatility of
this metal. A two thousandth part of antimony, bismuth, or lead,
destroys the ductility of gold. The tin may be got rid of by throwing
some corrosive sublimate or nitre into a crucible, containing the melted
alloy. By the first agent, perchloride of tin is volatilized; by the
second, _stannate_ of potash forms, which is carried off in the
resulting alkaline scoriæ.
Gold treated by the process of amalgamation, contains commonly nothing
but a little silver. The silver is dissolved out by nitric acid, which
leaves the gold untouched; but to make this _parting_ with success and
economy on the great scale, several precautions must be observed.
If the gold do not contain fully two thirds of its weight of silver,
this metal being thoroughly enveloped by the gold, is partially screened
from the action of the acid. Whenever, therefore, it is known by a trial
on a small scale, that the silver is much below this proportion, we must
bring the alloy of gold and silver to that standard by adding the
requisite quantity of the latter metal. This process is called
quartation.
This alloy is then granulated or laminated; and from twice to thrice its
weight of sulphuric or nitric acid is to be boiled upon it; and when it
is judged that the solution has been pushed as far as possible by this
first acid, it is decanted, and new acid is poured on. Lastly, after
having washed the gold, some sulphuric acid is to be boiled over it,
which carries off a two or three thousandth part of silver, which nitric
acid alone could not dissolve. Thus perfectly pure gold is obtained.
The silver held in solution by the sulphuric or nitric acid is
precipitated in the metallic state by copper, or in the state of
chloride by sea-salt. See PARTING.
Not only has the ratio between the value of gold and silver varied much
in different ages of the world; but the ratio between these metals and
the commodities they represent, has undergone variations, owing to the
circumstances in which their mines have been successively placed; since
they have always poured a greater quantity of the metals into the market
than has been absorbed by use. This quantity has greatly increased since
the discovery of America, a period of little more than 300 years. The
mines of that continent, rich, numerous, and easily worked, by
augmenting the mass of gold and silver, necessarily lessened the value
of these metals compared with that of the objects of commerce
represented by them, so that every thing else being equal, there is now
required for purchasing the same quantity of commodities, much more gold
or silver than was necessary in the reign of Henry VII., before the
discovery of America. This productiveness of the American mines has had
an influence on those of the ancient continent; many of whose silver and
gold mines have been abandoned, not because the veins or auriferous
sands are less rich than they were; but because their product no longer
represents the value of human labour, and of the goods to be furnished
in return for their exploitation.
In the 3d. vol. of the Mining Journal, p. 331., we have the following
statement as to the produce of the precious metals.--“In 40 years, from
1790 to 1830, Mexico produced 6,436,453_l._ worth of gold, and
139,818,032_l._ of silver. Chile, 2,768,488_l._ of gold, and
1,822,924_l._ of silver. Buenos Ayres, 4,024,895_l._ of gold, and
27,182,673_l._ of silver. Russia, 3,703,743_l._ of gold, and
1,502,981_l._ of silver. Total, 1880 millions sterling, or 47 millions
per annum.”
The following table shews what proportion the product of the mines of
America bears to that of the mines of the ancient continent.
_Table of the Quantities of Gold which may be considered as having been
brought into the European Market, every Year on an Average, from 1790 to
1802._
+------------------------------+-----------+
| Continents. | Gold. |
+------------------------------+-----------+
| ANCIENT CONTINENT. |lbs. Avoir.|
|Asia: | |
| Siberia | 3740 |
|Africa | 3300 |
| | |
|Europe: | |
| Hungary | 1430 |
| Salzbourg | 165 |
| Austrian States }| |
| Hartz and Hessia }| |
| Saxony }| |
| Norway }| 165 |
| Sweden }| |
| France }| |
| Spain, &c. }| |
| +-----------+
|Total of the Ancient Continent| 8800 |
| +-----------+
| NEW CONTINENT. | |
|North America | 2860 |
|South America: | |
| Spanish dominions | 22,000 |
| Brazil | 15,400 |
| +-----------+
|Total of the New Continent | 40,260 |
+------------------------------+-----------+
The mines of America have sent into Europe three and a half times more
gold, and twelve times more silver, than those of the ancient continent.
The total quantity of silver was to that of gold in the ratio of 55 to
1; a very different ratio from that which holds really in the value of
these two metals, which is in Europe as 1 to 15. This difference depends
upon several causes, which cannot be investigated here at length; but it
may be stated that gold, by its rarity and price, being much less
employed in the arts than silver, the demand for it is also much less;
and this cause is sufficient to lower its price much beneath what it
would have been, if it had followed the ratio of its quantity compared
to that of silver. Thus also bismuth, tin, &c., though much rarer than
silver, are, nevertheless, very inferior in price to it. Before the
discovery of America, the value of gold was not so distant from that of
silver, because since that era silver has been distributed in Europe in
a far greater proportion than gold. In Asia the proportion is now
actually only 1 to 11 or 12; the product of the gold mines in that
quarter, being not so much below that of the silver mines as in the rest
of the world.
The total annual production of Gold at present has been estimated as
follows.
From the ancient Spanish colonies of America 10,400 kilogrammes
Brazil 600
Europe and Asiatic Russia 6,200
The Indian Archipelago 4,700
Africa 14,000?
------
35,900 = 36 tons nearly
without taking into account the quantity of gold now extracted from
silver.
Gold has less affinity for oxygen than any other metal. When alone, it
cannot be oxidized by any degree of heat with contact of air, although
in combination with other oxidized bodies, it may pass into the state of
an oxide, and be even vitrified. The purple smoke into which gold leaf
is converted by an electric discharge is not an oxide, for it is equally
formed when the discharge is made through it in hydrogen gas. There are
two oxides of gold; the first or protoxide is a green powder, which may
be obtained by pouring, in the cold, a solution of potash into a
solution of the metallic chloride. It is not durable, but soon changes
in the menstruum into metallic gold, and peroxide. Its constituents are
96·13 metal, and 3·87 oxygen. The peroxide is best prepared by adding
magnesia to a solution of the metallic chloride; washing the precipitate
with water till this no longer takes a yellow tint from muriatic acid;
then digesting strong nitric acid upon the residuum, which removes the
magnesia, and leaves the peroxide in the form of a black or dark brown
powder, which seems to partake more of the properties of a metallic acid
than a base. It contains 10·77 per cent. of oxygen. For the curious
combination of gold and tin, called the PURPLE PRECIPITATE OF CASSIUS,
see this article, and PIGMENTS VITRIFIABLE.
_Gold beating._--This is the art of reducing gold to extremely thin
leaves, by beating with a hammer. The processes employed for this
purpose may be applied to other metals, as silver, platinum, and copper.
Under tin, zinc, &c., we shall mention such modifications of the
processes as these metals require to reduce them to thin leaves. The
Romans used to gild the ceilings and walls of their apartments; and
Pliny tells us, that from an ounce of gold forming a plate of 4 fingers
square, about 600 leaves of the same area were hammered. At the present
day, a piece of gold is extended so as to cover a space 651,590 times
greater than its primary surface when cast.
The gold employed in this art ought to be of the finest standard. Alloy
hardens gold, and renders it less malleable; so that the fraudulent
tradesman who should attempt to debase the gold, would expose himself to
much greater loss in the operations, than he could derive of profit from
the alloy.
Four principal operations constitute the art of gold beating. 1. The
casting of the gold ingots. 2. The hammering. 3. The lamination; and 4.,
the beating.
1. The gold is melted in a crucible along with a little borax. When it
has become liquid enough, it is poured out into the ingot-moulds
previously heated, and greased on the inside. The ingot is taken out and
annealed in hot ashes, which both soften it and free it from grease. The
moulds are made of cast iron, with a somewhat concave internal surface,
to compensate for the greater contraction of the central parts of the
metal in cooling than the edges. The ingots weigh about 2 ounces each,
and are 3/4 of an inch broad.
2. _The forging._--When the ingot is cold, the French gold-beaters
hammer it out on a mass of steel 4 inches long and 3 broad. The hammer
for this purpose is called the forging hammer. It weighs about 3 pounds,
with a head at one end and a wedge at the other, the head presenting a
square face of 1-1/2 inches. Its handle is 6 inches long. The workman
reduces the ingot to the thickness of 1/6 of an inch at most; and during
this Operation he anneals it whenever its substance becomes hard and apt
to crack. The English gold-beaters omit this process of hammering.
3. _The lamination._--The rollers employed for this purpose should be of
a most perfectly cylindrical figure, a polished surface, and so powerful
as not to bend or yield in the operation. The ultimate excellence of the
gold leaf depends very much on the precision with which the riband is
extended in the rolling press. The laminating machine represented under
the article MINT, is an excellent pattern for this purpose. The
gold-beater desires to have a riband of such thinness that a square inch
of it will weigh 6-1/2 grains. Frequent annealings are requisite during
the lamination.
4. _Beating._--The riband of gold being thus prepared uniform, the
gold-beater cuts it with shears into small squares of an inch each,
having previously divided it with compasses, so that the pieces may be
of as equal weight as possible. These squares are piled over each other
in parcels of 150, with a piece of fine calf-skin vellum interposed
between each, and about 20 extra vellums at the top and bottom. These
vellum leaves are about 4 inches square, on whose centre lie the gold
laminæ of an inch square. This packet is kept together by being thrust
into a case of strong parchment open at the ends, so as to form a belt
or band, whose open sides are covered in by a second case drawn over the
packet at right angles to the first. Thus the packet becomes
sufficiently compact to bear beating with a hammer of 15 or 16 pounds
weight, having a circular face nearly 4 inches diameter, and somewhat
convex, whereby it strikes the centre of the packet most forcibly, and
thus squeezes out the plates laterally.
The beating is performed on a very strong bench or stool framed to
receive a heavy block of marble, about 9 inches square on the surface,
enclosed upon every side by woodwork, except the front where a leather
apron is attached, which the workman lays before him to preserve any
fragments of gold that may fall out of the packet. The hammer is
short-handled, and is managed by the workman with one hand; who strikes
fairly on the middle of the packet, frequently turning it over to beat
both sides alike; a feat dexterously done in the interval of two
strokes, so as not to lose a blow. The packet is occasionally bent or
rolled between the hands, to loosen the leaves and secure the ready
extension of the gold; or it is taken to pieces to examine the gold, and
to shift the central leaves to the outside, and vice versa, that every
thing may be equalized. Whenever the gold plates have extended under
this treatment, to nearly the size of the vellum, they are removed from
the packet, and cut into four equal squares by a knife. They are thus
reduced to nearly the same size as at first, and are again made up into
packets and enclosed as before, with this difference, that skins
prepared from ox-gut are now interposed between each gold leaf, instead
of vellum. The second course of beating is performed with a smaller
hammer, about 10 pounds in weight, and is continued till the leaves are
extended to the size of the skins. During this period, the packet must
be often folded, to render the gold as loose as possible between the
membranes; otherwise the leaves are easily chafed and broken. They are
once more spread on a cushion, and subdivided into four square pieces by
means of two pieces of cane cut to very sharp edges, and fixed down
transversely on a board. This rectangular cross being applied on each
leaf, with slight pressure, divides it into four equal portions. These
are next made up into a third packet of convenient thickness, and
finally hammered out to the area of fine gold leaf, whose average size
is from 3 to 3-1/2 inches square. The leaves will now have obtained an
area 192 times greater than the plates before the hammering begun. As
these were originally an inch square, and 75 of them weighed an ounce (=
6-1/2 × 75 = 487-1/2), the surface of the finished leaves will be 192 ×
75 = 14,400 square inches, or 100 square feet per ounce troy. This is by
no means the ultimate degree of attenuation, for an ounce may be
hammered so as to cover 160 square feet; but the waste incident in this
case, from the number of broken leaves, and the increase and nicety of
the labour, make this an unprofitable refinement; while the gilder finds
such thin leaves to make less durable and satisfactory work.
The finished leaves of gold are put up in small books made of single
leaves of soft paper, rubbed over with red chalk to prevent adhesion
between them. Before putting the leaves in these books, however, they
are lifted one by one with a delicate pair of pincers out of the
finishing packet, and spread out on a leather cushion by blowing them
flat down. They are then cut to one size, by a sharp-edge square
moulding of cane, glued on a flat board. When this square-framed edge is
pressed upon the gold, it cuts it to the desired size and shape. Each
book commonly contains 25 gold leaves.
I shall now describe some peculiarities of the French practice of gold
beating. The workman cuts the laminated ribands of an inch broad into
portions an inch and a half long. These are called _quartiers_. He takes
24 of them, which he places exactly over each other, so as to form a
thickness of about an inch, the riband being 1/2 of a line, or 1/24 of
an inch thick; and he beats them together on the steel slab with the
round face (_panne_) of the hammer, so as to stretch them truly out into
the square form. He begins by extending the substance towards the edges,
thereafter advancing towards the middle; he then does as much on the
other side, and finally hammers the centre. By repeating this mode of
beating as often as necessary, he reduces at once all the _quartiers_
(squares) of the same packet, till none of them is thicker than a leaf
of gray paper, and of the size of a square of 2 inches each side.
When the _quartiers_ are brought to this state, the workman takes 56 of
them, which he piles over each other, and with which he forms the first
packet (_caucher_) in the manner already described; only two leaves of
vellum are interposed between each gold leaf. The empty leaves of vellum
at the top and bottom of the packet are called _emplures_. They are 4
inches square, as well as the parchment pieces.
The packet thus prepared forms a rectangular parallelopiped; it is
enclosed in two sheathes, composed each of several leaves of parchment
applied to each, and glued at the two sides, forming a bag open at
either end.
The block of black marble is a foot square at top, and 18 inches deep,
and is framed as above described. The hammer used for beating the first
packet is called the flat, or the enlarging hammer; its head is round,
about 5 inches in diameter, and very slightly convex. It is 6 inches
high, and tapers gradually from its head to the other extremity, which
gives it the form of a hexagonal truncated pyramid. It weighs 14 or 15
pounds.
The French gold-beaters employ besides this hammer, three others of the
same form; namely, 1. The _commencing hammer_, which weighs 6 or 7
pounds, has a head 4 inches in diameter, and is more convex than the
former. 2. The _spreading hammer_, (_marteau a chasser_); its head is
two inches diameter, more convex than the last, and weighs only 4 or 5
pounds. 3. _The finishing hammer_; it weighs 12 or 13 pounds, has a head
four inches diameter, and is the most convex of all.
The beating processes do not differ essentially from the English
described above. The vellum is rubbed over with fine calcined Paris
plaster, with a hare’s foot. The skin of the gold-beater is a pellicle
separated from the outer surface of ox-gut; but before being employed
for this purpose, it must undergo two preparations. 1. It is sweated, in
order to expel any grease it may contain. With this view, each piece of
membrane is placed between two leaves of white paper; several of these
pairs are piled over each other, and struck strongly with a hammer,
which drives the grease from the gut into the paper.
2. A body is given to the pieces of gut; that is, they are moistened
with an infusion of cinnamon, nutmeg, and other warm and aromatic
ingredients, in order to preserve them; an operation repeated after they
have been dried in the air. When the leaves of skin are dry, they are
put in a press, and are now ready for use. After the parchment, vellum,
and gut membrane have been a good deal hammered, they become unfit for
work, till they are restored to proper flexibility, by being placed leaf
by leaf, between leaves of white paper, moistened sometimes with
vinegar, at others with white wine. They are left in this predicament
for 3 or 4 hours, under compression of a plank loaded with weights. When
they have imbibed the proper humidity, they are put between leaves of
parchment 12 inches square, and beat in that situation for a whole day.
They are then rubbed over with fine calcined gypsum, as the vellum was
originally. The gut-skin is apt to contract damp in standing, and is
therefore dried before being used.
The average thickness of common gold leaf is 1/282000 of an inch.
_The art of Gilding._--This art consists in covering bodies with a thin
coat of gold; which may be done either by mechanical or chemical means.
The mechanical mode is the application of gold leaf or gold powder to
various surfaces, and their fixation by various means. Thus gold may be
applied to wood, plaster, pasteboard, leather; and to metals, such as
silver, copper, iron, tin, and bronze; so that gilding generally
speaking includes several arts, exercised by very different classes of
tradesmen.
I. _Mechanical Gilding._--Oil gilding is the first method under this
head, as oil is the fluid most generally used in the operation of this
mechanical art. The following process has been much extolled at Paris.
1. A coat of _impression_ is to be given first of all, namely, a coat of
white lead paint, made with drying linseed oil, containing very little
oil of turpentine.
2. Calcined ceruse is to be ground very well with unboiled linseed oil,
and tempered with essence of turpentine, in proportion as it is laid on.
Three or four coats of this _hard tint_ are to be applied evenly and
drily on the ornaments, and the parts which are to be most carefully
gilded.
3. The _Gold colour_ is then to be smoothly applied. This is merely the
dregs of the colours, ground and tempered with oil, which remain in the
little dish in which painters clean their brushes. This substance is
extremely rich and gluey; after being ground up, and passed through fine
linen cloth, it forms the ground for gold leaf.
4. When the gold colour is dry enough to catch hold of the leaf gold,
this is spread on the cushion, cut into pieces and carefully applied
with the pallet knife, pressed down with cotton, and in the small
ornaments with a fine brush.
5. If the gildings be for outside exposure, as balconies, gratings,
statues, &c., they must not be varnished, as simple oil gilding stands
better; for when it is varnished, a bright sun-beam acting after heavy
rain, gives the gilding a jagged appearance. When the objects are inside
ones, a coat of spirit varnish may be passed over the gold leaf, then a
glow from the gilder’s chafing dish may be given, and finally a coat of
oil varnish. The workman who causes the chafing dish to glide in front
of the varnished surface, must avoid stopping for an instant opposite
any point, otherwise he would cause the varnish to boil and blister.
This heat brings out the whole transparency of the varnish, and lustre
of the gold.
_Oil Gilding_ is employed with varnish polish, upon equipages,
mirror-frames, and other furniture. The following method is employed by
eminent gilders at Paris.
1. White lead, with half its weight of yellow ochre, and a little
litharge, are separately ground very fine; and the whole is then
tempered with linseed oil, thinned with essence of turpentine, and
applied in an evenly coat, called _impression_.
2. When this coat is quite dry, several coats of the hard tint are
given, even so many as 10 or 12, should the surface require it for
smoothing and filling up the pores. These coats are given daily, leaving
them to dry in the interval in a warm sunny exposure.
3. When the work is perfectly dry, it is first softened down with pumice
stone and water, afterwards with worsted cloth and very finely powdered
pumice, till the _hard tint_ give no reflection, and be smooth as glass.
4. With a camel’s hair brush, there must be given lightly and with a
gentle heat, from 4 to 5 coats at least, and even sometimes double that
number, of fine lac varnish.
5. When these are dry, the grounds of the pannels and the sculptures
must be first polished with shave-grass (_de la prèle_); and next with
putty of tin and tripoli, tempered with water, applied with woollen
cloth; by which the varnish is polished till it shines like a mirror.
6. The work thus polished is carried into a hot place, free from dust,
where it receives very lightly and smoothly, a thin coat of _gold
colour_, much softened down. This coat is passed over it with a clean
soft brush, and the thinner it is the better.
7. Whenever the gold colour is dry enough to take the gold, which is
known by laying the back of the hand on a corner of the frame work, the
gilding is begun and finished as usual.
8. The gold is smoothed off with a very soft brush, one of camel’s hair
for example, of three fingers’ breadth; after which it is left to dry
for several days.
9. It is then varnished with a spirit of wine varnish; which is treated
with the chafing dish as above described.
10. When this varnish is dry, two or three coats of copal, or oil
varnish are applied, at intervals of two days.
11. Finally, the pannels are polished with a worsted cloth, imbued with
tripoli and water, and lustre is given by friction with the palm of the
hand, previously softened with a little olive oil, taking care not to
rub off the gold.
In this country, _Burnished gilding_ is practised by first giving a
ground of size whiting, in several successive coats; next applying
gilding size; and then the gold leaf, which is burnished down with
agate, or a dog’s tooth.
_Gilding in distemper_ of the French, is the same as our burnished
gilding. Their process seems to be very elaborate, and the best consists
of 17 operations; each of them said to be essential.
1. _Encollage_, or the _Glue coat_. To a decoction of wormwood and
garlic in water, strained through a cloth, a little common salt, and
some vinegar are added. This composition, as being destructive of worms
in wood, is mixed with as much good glue; and the mixture is spread in a
hot state, with a brush of boar’s hair. When plaster or marble is to be
gilded, the salt must be left out of the above composition, as it is apt
to attract humidity in damp places, and to come out as a white powder on
the gilding. But the salt is indispensible for wood. The first glue
coating is made thinner than the second.
2. _White preparation._ This consists in covering the above surface,
with 8, 10, or 12 coats of Spanish white, mixed up with strong size,
each well worked on with the brush, and in some measure incorporated
with the preceding coat, to prevent their peeling off in scales.
3. _Stopping up_ the pores, with thick whiting and glue, and smoothing
the surface with dog-skin.