In China and Japan. The specimens imported from these countries appear
pretty white; but are more unctuous to the touch, and more micaceous
than the porcelain clays of France.
In Saxony. The Kaolin employed in the porcelain manufactories of that
country has a slight yellow or flesh colour, which disappears in the
kiln, proving as Wallerius observed, that this tint is not owing to any
metallic matter.
In France, at Saint-Yriex-la-Perche, about 10 leagues from Limoges. The
Kaolin occurs there in a bed, or perhaps a vein of beds of granite, or
rather of that felspar rock called Pe-tun-tse, which exists here in
every stage of decomposition. This Kaolin is generally white, but
sometimes a little yellowish with hardly any mica. It is meagre to the
touch, and some beds include large grains of quartz, called pebbly by
the China manufacturers. This variety, when ground, affords, without the
addition of any fusible ingredient, a very transparent porcelain.
Near Bayonne. A Kaolin possessing the lamellated structure of felspar,
in many places. The rock containing it is a graphic granite in every
stage of decomposition.
In England, in the county of Cornwall. This Kaolin or China clay is very
white, and more unctuous to the touch than those upon the continent of
Europe mentioned above. Like them it results from the decomposition of
the felspars and granites, occurring in the middle of these rocks. Mr.
Wedgewood found it to contain 60 of alumina or pure clay, and 40 of
silica, in 100 parts.
Pure clay, the alumina of the chemist, is absolutely infusible; but when
subjected to the fire of a porcelain kiln, it contracts into about one
half of its total bulk. It must, however, be heated very cautiously,
otherwise it will decrepitate and fly in pieces, owing to the sudden
expansion into steam of the water combined with its particles, which is
retained with a considerable attractive force. It possesses little
plasticity, and consequently affords a very short paste, which is apt to
crack when kneaded into a cake.
It is not only infusible by itself, but it will not dissolve in the
fusible glasses; making them merely opaque. If either lime or silica be
added separately to pure clay, in any proportion, the mixture will not
melt in the most violent furnace; but if alumina, lime, and silica be
mixed together, the whole melts, and the more readily, the nearer the
mixture approaches to the following proportions:--1 of alumina, 1 of
lime, and 3 of sand. If the sand be increased to five parts, the
compound becomes infusible. These interesting facts show the reciprocal
action of those earths which are mixed most commonly in nature with
alumina.
Iron in small quantity, but in a state not precisely determined, though
probably of protoxide, does not colour the clays till they are subjected
to a powerful heat. There are very white clays, such as those of
Montereau, which do not become red till calcined in the porcelain kiln;
the oxide of iron contained in them, which colours them in that case,
was previously imperceptible. It appears from this circumstance, that
the clays fit for making fine white stone ware, as also the Kaolins
adapted to the manufacture of porcelain, are very rare.
Iron, in larger proportion, usually colours the clays green or
slate-blue, before they have been heated. Such clays, exposed to the
action of fire, become yellow or red according to the quantity of iron
which they contain. When the iron is very abundant, it renders the clays
fusible; but a little lime and silica must also be present for this
effect. The earthenware made with these ferruginous clays, can bear but
a moderate baking heat; it is thick, porous, and possesses the advantage
merely of cheapness, and of bearing considerable alternations of
temperature without breaking.
Alumina and the very aluminous natural clays which possess most
plasticity, are apt to crack in drying, or to lose their shape. This
very serious defect for the purposes of pottery is rectified, in some
measure, by adding to that earth a certain quantity of sand or silica.
Thus, a compound is formed which possesses less attraction for water,
and dries more equably from the openness of its body. The principal
causes of the distortion of earthenware vessels, are the unequal
thickness of their parts, and quicker desiccation upon one side than
another. Hard burnt stone-ware ground to powder, and incorporated with
clay, answers still better than sand for counteracting the great and
irregular contraction which natural pottery paste is apt to experience.
Such ground biscuit is called _cement_; and its grains interspersed
through the ware, may be regarded as so many solutions of continuity,
which arrest the fissures.
The preceding observations point out the principles of those arts which
employ clay for moulding by the wheel, and baking in a kiln. See
PORCELAIN and POTTERY.
CLOTH, MANUFACTURE OF. See TEXTILE FABRICS, WEAVING, WOOL.
CLOTH-BINDING. Nothing places in so striking a point of view the
superior taste, judgment, and resources of London tradesmen over those
of the rest of the world, than the extensive substitution which they
have recently made of embossed silks and calicoes for leather in the
binding of books. In old libraries, cloth-covered boards indeed may
occasionally be seen, but they have the meanest aspect, and are no more
to be compared with our modern cloth-binding, than the _jupon_ of a
trull, with the ballet dress of Taglioni. The silk or calico may be dyed
of any shade which use or fancy may require, impressed with gold or
silver foil in every form, and variegated by ornaments in relief, copied
from the most beautiful productions in nature. This new style of binding
is distinguished not more for its durability, elegance, and variety,
than for the economy and dispatch with which it ushers the offspring of
intellect into the world. For example, should a house eminent in this
line, such as that of Westleys, Friar-street, Doctors’-commons, receive
5000 volumes from Messrs. Longman & Co. upon Monday morning, they can
have them all ready for publication, within the incredibly short period
of two days; being far sooner than they could have rudely boarded them
upon the former plan. The reduction of price is not the least advantage
incident to the new method, amounting to fully 50 per cent. upon that
with leather.
The dyed cloth being cut by a pattern to the size suited to the volume,
is passed rapidly through a roller press, between engraved cylinders of
hard steel, whereby it receives at once the impress characteristic of
the back, and the sides, along with embossed designs over the surface in
sharp relief. The cover thus rapidly fashioned, is as rapidly applied by
paste to the stitched and pressed volume; no time being lost in mutual
adjustments; since the steel rollers turn off the former, of a shape
precisely adapted to the latter. Hard glazed and varnished calico is
moreover much less an object of depredation to moths, and other insects,
than ordinary leather has been found to be.
COBALT. This metal being difficult to reduce from its ores, is therefore
very little known, and has not hitherto been employed in its simple
state in any of the arts; but its oxide has been extensively used on
account of the rich blue colour which it imparts to glass, and the glaze
of porcelain and stone-ware. The principal ores of cobalt are those
designated by mineralogists under the names of _arsenical cobalt_ and
_gray cobalt_. The first contains, in addition to cobalt, some arsenic,
iron, nickel, and occasionally silver, &c. The other is a compound of
cobalt with iron, arsenic, sulphur, and nickel. Among the gray cobalts,
the ore most esteemed for its purity is that of Tunaberg in Sweden. It
is often in regular crystals which possess the lustre and colour of
polished steel. The specific gravity of cobalt pyrites is 6·36 to 4·66.
The Tunaberg variety afforded to Klaproth, cobalt, 44; arsenic, 55·5;
sulphur, 0·5: so that it is an arseniuret. Others, however, contain much
sulphur as well as iron. It imparts at the blowpipe a blue colour to
borax and other fluxes, and gives out arsenical fumes.
The ore being picked to separate its concomitant stony matters, is
pounded fine and passed through a sieve; and is also occasionally
washed. The powder is then spread on the sole of a reverberatory
furnace, the flue of which leads into a long horizontal chimney. Here it
is exposed to calcination for several hours, to expel the sulphur and
arsenic that may be present; the former burning away in sulphurous acid
gas, the latter being condensed into the white oxide or arsenious acid,
whence chiefly the market is supplied with this article. This calcining
process can never disengage the whole of these volatile ingredients, and
there is therefore a point beyond which it is useless to push it; but
the small quantities that remain are not injurious to the subsequent
operations. The roasted ore is sifted anew; reduced to a very fine
powder, and then mixed with 2 or 3 parts of very pure siliceous sand, to
be converted into what is called _zaffre_. With this product glasses are
generally coloured blue, as well as enamels and pottery glaze. In the
works where cobalt ores are treated, a blue glass is prepared with the
zaffre, which is well known under the name of smalt or azure blue. This
azure is made by adding to the zaffre 2 or 3 parts of potash, according
to its richness in cobalt, and melting the mixture in earthen crucibles.
The fused mass is thrown out while hot into water; and is afterwards
triturated and levigated in mills mounted for the purpose. There remains
at the bottom of the earthen pot a metallic lump, which contains a
little cobalt, much nickel, arsenic, iron, &c. This is called _speiss_.
As it is the _oxide_ of cobalt which has the colouring quality, the
calcination serves the purpose of oxidizement, as well as of expelling
the foreign matters.
A finer cobalt-oxide is procured for painting upon hard porcelain, by
boiling the cobalt ore in nitric acid, which converts the arsenic into
an acid, and combines it with the different metals present in the
mineral. These arseniates being unequally soluble in nitric acid, may be
separated in succession by a cautious addition of carbonate of soda or
potash; and the arseniate of cobalt as the most soluble remains
unaffected. It has a rose colour; and is easily distinguishable, whence
the precipitation may be stopped at the proper point. The above solution
should be much diluted, and the alkali should be cautiously added with
frequent agitation.
[Illustration: 291 292]
The cobalt ores, rich in nickel, are exposed to slow oxidizement in the
air, whereby the iron, cobalt, arsenic, and sulphur get oxygenated by
the atmospheric moisture, but the nickel continues in the metallic
state. This action of the weather must not be extended beyond a year,
otherwise the nickel becomes affected, and injures the cobalt blue. The
ore hereby increases in weight, from 8 to 10 per cent. _Fig._ 291. is a
longitudinal section of the furnace: _fig._ 292., a horizontal section
upon a level with the sole of the hearth. It is constructed for wood
fuel, and the hearth is composed of fire-bricks or tiles. The vapours
and gases disengaged in the roasting, pass off through the flues _a a_,
into the channels _b b_, and thence by _c_ into the common vent, or
poison chamber. See the representation of the poison tower of Altenberg,
under the article ARSENIC. The flues are cleared out by means of
openings left at suitable situations in the brick-work of the chimneys.
The azure manufacture is carried on chiefly in winter, in order that the
external cold may favour the more complete condensation of the acids of
arsenic. From 3 to 5 cwt. of Schlich (pasty ore), are roasted at one
operation, and its bed is laid from 5 to 6 inches thick. After two
hours, it must be turned over; and the stirring must be repeated every
half hour, till no more arsenic is observed to exhale. The process being
then finished, the ore must be raked out of the furnace, and another
charge introduced.
The duration of the roasting is regulated partly by the proportion of
sulphur and arsenic present, and partly by the amount of nickel; which
must not be suffered to become oxidized, lest it should spoil the colour
of the smalt. The latter ores should be but slightly roasted, so as to
convert the nickel into _speiss_. The roasted ore must be sifted in a
safety apparatus. The loss of weight in the roasting amounts, upon the
average, to 36 per cent. The roasted ore has a brownish gray hue, and is
called _safflor_ in German, and is distributed into different sorts. F F
S is the finest _safre_; F S, fine; O S, ordinary; and M S, middling.
These varieties proceed from various mixtures of the calcined ores. The
roasted ore is ground up along with sand, elatriated, and, when dry, is
called _zaffre_. It is then mixed with a sufficient quantity of potash
for converting the mixture into a glass.
[Illustration: 293 294]
_Figs._ 293. and 294. represent a round smalt furnace, in two vertical
sections, at right angles to each other. The fire-place is vaulted or
arched; the flame orifice _a_, is in the middle of the furnace; _b_ is
the feed hole; _c_, a tunnel which serves as an ash-pit, and to supply
air; _d_, openings through which the air arrives at the fuel, the wood
being placed upon the vault; _e_, knee holes for taking out the scoriæ
from the pot bottoms; _f_, working orifices, with cast-iron plates _g_,
in front of them. Under these are the additional outlets _h_. The smoke
and flame pass off through the orifices _i_, which terminate in expanded
flues, where the sand may be calcined or the wood may be baked. Eight
hours are sufficient for one vitrifying operation, during which the
glass is stirred about several times in the earthen melting pots.
The preparation of the different shades of blue glass are considered as
secrets in the smelting works; and marked with the following letters:--F
F F C, the finest; F C, fine; M C, middling; O C, ordinary. A melting
furnace, containing 8 pots of glass; produces in 24 hours, from 24 cwts.
of the mixture, 19 cwts. of blue glass; and from 1/2 to 3/4 cwt. of
scoriæ or speiss (_speise_). The composition _speise_, according to
Berthier, is,--nickel, 49·0; arsenic, 37·8; sulphur, 7·8; copper, 1·6;
cobalt, 3·2 in 100. Nickel, arsenic, and sulphur, are its essential
constituents; the rest are accidental, and often absent. The freer the
cobalt ore is from foreign metals, the finer is the colour, and the
deeper is the shade; paler tints are easily obtained by dilution with
more glass. The presence of nickel gives a violet tone.
The production of smalt in the Prussian states amounted, in 1830, to
7452-1/2 cwts.; and, in Saxony, to 9697 cwts.; in 1825, to 12,310 cwts.
One process for making fine smalt has been given under the title AZURE;
I shall introduce another somewhat different here.
The ore of cobalt is to be reduced to very fine powder, and then roasted
with much care. One part, by weight, is next to be introduced, in
successive small portions, into an iron vessel, in which three parts of
acid sulphate of potassa has been previously fused, at a moderate
temperature. The mixture, at first fluid, soon becomes thick and firm,
when the fire is to be increased, until the mass is in perfect fusion,
and all white vapours have ceased. It is then to be taken out of the
crucible with an iron ladle, the crucible is to be recharged with acid
sulphate of potash, and the operation continued as before, until the
vessel is useless. The fused mass contains sulphate of cobalt, neutral
sulphate of potassa, and arseniate of iron, with a little cobalt. It is
to be pulverized, and boiled in an iron vessel, with water, as long as
the powder continues rough to the touch. The white, or yellowish white
residue, may be allowed to separate from the solution, either by
deposition or filtration. Carbonate of potassa, free from silica, is
then to be added to the solution, and the carbonate of cobalt thrown
down is to be separated and well washed, if possible, with warm water;
the same water may be used to wash other portions of the fused mass. The
filtered liquid which first passes, is a saturated solution of sulphate
of potassa: being evaporated to dryness in an iron vessel, it may be
reconverted into acid sulphate by fusing it with one half its weight of
sulphuric acid: this salt is then as useful as at first.
The oxide of cobalt thus obtained, contains no nickel; so little oxide
of iron is present, that infusion of galls does not show its presence;
it may contain a little copper, if that metal exists in the ore, but it
is easily separated by the known methods. Sometimes sulphuretted
hydrogen will produce a yellow brown precipitate in the solution of the
fused mass; this, however, contains no arsenic, but is either sulphuret
of antimony or bismuth, or a mixture of both.
It has been found advantageous to add to the fused mass, sulphate of
iron, calcined to redness, and one tenth of nitre when the residue is
arseniate of iron, and contains no arseniate of cobalt. There is then no
occasion to act upon the residue a second time for the cobalt in it.
This process is founded on the circumstances that the sulphate of cobalt
is not decomposed by a red heat, and that the arseniates of iron and
cobalt are insoluble in all neutral liquids. It is quite evident that to
obtain a perfect result, the excess of acid in the bisulphate of potassa
must be completely driven off by the red heat applied.
110,646 lbs. of smalts were imported into the United Kingdom in 1835,
and 96,949 were retained for home consumption. In 1834, only 16,223 lbs.
were retained.
In 1835, 322,562 lbs. of zaffres were imported, and 336,824 are stated
to have been retained, which is obviously an error. 284,000 lbs. were
retained in 1834.
COCCULUS INDICUS, or Indian berry, is the fruit of the _Menispermum
Cocculus_, a large tree, which grows upon the coasts of Malabar, Ceylon,
&c. The fruit is blackish, and of the size of a large pea. It owes its
narcotic and poisonous qualities to the vegeto-alkaline chemical
principle called _picrotoxia_, of which it contains about one-fiftieth
part of its weight. It is sometimes thrown into waters to intoxicate or
kill fishes; and it is said to have been employed to increase the
inebriating qualities of ale or beer. Its use for this purpose is
prohibited by act of parliament, under a penalty of 200_l._ upon the
brewer, and 500_l._ upon the seller of the drug.
COCHINEAL was taken in Europe at first for a seed, but was proved by the
observations of Lewenhoeck to be an insect, being the female of that
species of shield-louse, or _coccus_, discovered in Mexico, so long ago
as 1518. It is brought to us from Mexico, where the animal lives upon
the _cactus opuntia_ or _nopal_. Two sorts of cochineal are
gathered--the wild, from the woods, called by the Spanish name _grana
silvestra_; and the cultivated, or the _grana fina_, termed also
_mesteque_, from the name of a Mexican province. The first is smaller,
and covered with a cottony down, which increases its bulk with a matter
useless in dyeing; it yields, therefore, in equal weight, much less
colour, and is of inferior price to that of the fine cochineal. But
these disadvantages are compensated in some measure to the growers by
its being reared more easily, and less expensively; partly by the
effect of its down, which enables it better to resist rains and storms.
The wild cochineal, when it is bred upon the field nopal, loses in part
the tenacity and quantity of its cotton, and acquires a size double of
what it has on the wild opuntias. It may therefore be hoped, that it
will be improved by persevering care in the rearing of it, when it will
approach more and more to fine cochineal.
The fine cochineal, when well dried and well preserved, should have a
gray colour, bordering on purple. The gray is owing to the powder, which
naturally covers it, and of which a little adheres; as also to a waxy
fat. The purple shade arises from the colour extracted by the water in
which they were killed. It is wrinkled with parallel furrows across its
back, which are intersected in the middle by a longitudinal one; hence,
when viewed by a magnifier, or even a sharp naked eye, especially after
being swollen by soaking for a little in water, it is easily
distinguished from the factitious, smooth, glistening, black grains, of
no value, called East India cochineal, with which it is often shamefully
adulterated by certain London merchants. The genuine cochineal has the
shape of an egg, bisected through its long axis, or of a tortoise, being
rounded like a shield upon the back, flat upon the belly, and without
wings.
These female insects are gathered off the leaves of the nopal plant,
after it has ripened its fruit, a few only being left for brood, and are
killed, either by a momentary immersion in boiling water, by drying upon
heated plates, or in ovens: the last become of an ash-gray colour,
constituting the _silver_ cochineal, or _jaspeada_; the second are
blackish, called _negra_, and are most esteemed, being probably driest;
the first are reddish brown, and reckoned inferior to the other two. The
dry cochineal being sifted, the dust, with the imperfect insects and
fragments which pass through, are sold under the name of _granillo_.
Cochineal keeps for a long time in a dry place. Hellot says that he has
tried some 130 years old, which produced the same effect as new
cochineal.
We are indebted to MM. Pelletier and Caventou for a chemical
investigation of cochineal, in which its colouring matter was skilfully
eliminated.
Purified sulphuric ether acquired by digestion with it a golden yellow
colour, amounting by Dr. John to one tenth of the weight of the insect.
This infusion left, on evaporation, a fatty wax of the same colour.
Cochineal, exhausted by ether, was treated with alcohol at 40° B. After
30 infusions in the digester of M. Chevreul, the cochineal continued to
retain colour, although the alcohol had ceased to have any effect on it.
The first alcoholic liquors were of a red verging on yellow. On cooling,
they let fall a granular matter. By spontaneous evaporation, this
matter, of a fine red colour, separated, assuming more of the
crystalline appearance. These species of crystals dissolved entirely in
water, which they tinged of a yellowish-red.
This matter has a very brilliant purple-red colour; it adheres strongly
to the sides of the vessels; it has a granular and somewhat crystalline
aspect, very different, however, from those compound crystals alluded to
above; it is not altered by the air, nor does it sensibly attract
moisture. Exposed to the action of heat, it melts at about the fiftieth
degree centigrade (122° Fahr.). At a higher temperature it swells up,
and is decomposed with the production of carburetted hydrogen, much oil,
and a small quantity of water, very slightly acidulous. No trace of
ammonia was found in these products.
The colouring principle of cochineal is very soluble in water. By
evaporation, the liquid assumes the appearance of syrup, but never
yields crystals. It requires of this matter a portion almost
imponderable to give a perceptible tinge of bright purplish red to a
large body of water. Alcohol dissolves this colouring substance, but, as
we have already stated, the more highly it is rectified the less of it
does it dissolve. Sulphuric ether does not dissolve the colouring
principle of cochineal; but weak acids do, possibly owing to their water
of dilution. No acid precipitates it in its pure state. This colouring
principle, however, appears to be precipitable by all the acids, when it
is accompanied by the animal matter of the cochineal.
The affinity of alumina for the colouring matter is very remarkable.
When that earth, newly precipitated, is put into a watery solution of
the colouring principle, this is immediately seized by the alumina. The
water becomes colourless, and a fine red lake is obtained, if we operate
at the temperature of the atmosphere; but if the liquor has been hot,
the colour passes to crimson, and the shade becomes more and more
violet, according to the elevation of the temperature, and the
continuance of the ebullition.
The salts of tin exercise upon the colouring matter of cochineal a
remarkable action. The muriatic protoxide of tin forms a very abundant
violet precipitate in the liquid. This precipitate verges on crimson, if
the salt contains an excess of acid. The muriatic deutoxide of tin
produces no precipitate, but changes the colour to scarlet-red. If
gelatinous alumina be now added, we obtain a fine red precipitate, which
does not pass to crimson by boiling.
To this colouring principle the name _carminium_ has been given, because
it forms the basis of the pigment called carmine.
The process followed in Germany for making carmine, which consists in
pouring a certain quantity of solution of alum into a decoction of
cochineal, is the most simple of all, and affords an explanation of the
formation of carmine, which is merely the carminium and the animal
matter precipitated by the excess of acid in the salt, which has taken
down with it a small quantity of alumina; though it appears that alumina
ought not to be regarded as essential to the formation of carmine. In
fact, by another process, called by the name of Madame Cenette of
Amsterdam, the carmine is thrown down, by pouring into the decoction of
cochineal a certain quantity of the binoxalate of potash. When carbonate
of soda is added, then carminated lake also falls down. That carmine is
a triple compound of animal matter, carminium, and an acid appears from
the circumstance, that liquors which have afforded their carmine, when a
somewhat strong acid is poured into them, yield a new formation of
carmine by the precipitation of the last portions of the animal matter.
But whenever the whole animal matter is thrown down, the decoctions,
although still much charged with the colouring principle, can afford no
more carmine. Such decoctions may be usefully employed to make
carminated lakes, saturating the acid with a slight excess of alkali,
and adding gelatinous alumina. The precipitates obtained, on adding
acids to the alkaline decoctions of cochineal, are therefore true
carmines, since they do not contain alumina; but the small quantity of
alumina which is thrown down by alum in the manufacture of carmine,
augments its bulk and weight. It gives, besides, a greater lustre to the
colour, even though diluting and weakening it a little.
The carmines found in the shops of Paris were analysed, and yielded the
same products. They are decomposed by the action of heat, with the
diffusion at first of a very strong smell of burning animal matter, and
then of sulphur. A white powder remained, amounting to about one-tenth
of the matter employed, and which was found to be alumina. Other
quantities of carmine were treated with a solution of caustic potash,
which completely dissolved them, with the exception of a beautiful red
powder, not acted on by potash and concentrated acids, and which was
recognized to be red sulphuret of mercury or vermillion. This matter,
evidently foreign to the carmine, appears to have been added, in order
to increase its weight.
The preceding observations and experiments seem calculated to throw some
light on the art of dyeing scarlet and crimson. The former is effected
by employing a cochineal bath, to which there have been added, in
determinate proportions, acidulous tartrate of potash, and
nitro-muriatic deutoxide of tin. The effect of these two salts is now
well known. The former, in consequence of its excess of acid, tends to
redden the colour, and to precipitate it along with the animal matter:
the latter acts in the same manner, at first by its excess of acid, then
by the oxide of tin which falls down also with the carmine and animal
matter, and is fixed on the wool, with which it has of itself a strong
tendency to combine. MM. Pelletier and Caventou remark, that “to obtain
a beautiful shade, the muriate of tin ought to be entirely at the
maximum of oxidizement; and it is in reality in this state that it must
exist in the solution of tin prepared according to the proportions
prescribed in M. Berthollet’s treatise on dyeing.”
We hence see why, in dyeing scarlet, the employment of alum is carefully
avoided, as this salt tends to convert the shade to a crimson. The
presence of an alkali would seem less to be feared. The alkali would
occasion, no doubt, a crimson-coloured bath; but it would be easy in
this case to restore the colour, by using a large quantity of tartar. We
should, therefore, procure the advantage of having a bath better charged
with colouring matter and animal substance. It is for experience on the
large scale to determine this point. As to the earthy salts, they must
be carefully avoided; and if the waters be selenitish, it would be a
reason for adding a little alkali.
To obtain crimson, it is sufficient, as we know, to add alum to the
cochineal bath, or to boil the scarlet cloth in alum water. It is also
proper to diminish the dose of the salt of tin, since it is found to
counteract the action of the alum.
The alkalies ought to be rejected as a means of changing scarlet to
crimson. In fact, crimsons by this process cannot be permanent colours,
as they pass into reds by the action of acids.
According to M. Von Grotthuss, carmine may be deprived of its golden
shade by ammonia, and subsequent treatment with acetic acid and alcohol.
Since this fact was made known, M. Herschel, colour maker at Halle, has
prepared a most beautiful carmine.
The officers of Her Majesty’s Customs have lately detected a system of
adulterating cochineal, which has been practised for many years upon a
prodigious scale by a mercantile house in London. I have analyzed about
100 samples of such cochineal, from which it appears that the genuine
article is moistened with gum-water, agitated in a box or leather bag,
first, with sulphate of baryta in fine powder, afterwards with bone or
ivory black, to give it the appearance of _negra_ cochineal, and then
dried. By this means about 12 per cent. of the worthless heavy spar is
sold at the price of cochineal, to the enrichment of the sophisticators,
and the disgrace and injury of British trade and manufactures.
The specific gravity of genuine cochineal is 1·25; that of the cochineal
loaded with the barytic sulphate 1·35. It was taken in oil of turpentine
and reduced to water as unity, because the waxy fat of the insects
prevents the intimate contact of the latter liquid with them, and the
ready expulsion of air from their wrinkled surface. They are not at all
acted upon by the oil, but are rapidly altered by water, especially when
they have been gummed and barytified.
The quantities of cochineal imported into the United Kingdom in the
following years, were:--
Libs.
1827. 320,722
1828. 258,032
1829. 288,456
1830. 316,589
1831. 244,371
1832. 388,478
1833. 359,381
1834. 410,387
1835. 418,320