[Illustration: 892 893 894]
_Figs._ 892, 893, 894, 895, 896., represent the kiln for baking the
biscuit, and also for running the glaze, in the English potteries.
_a_, _a_, _figs._ 892, 893, and 894, are the furnaces which heat the
kiln; of which _b_, in _fig._ 892., are the upper mouths, and _b´_ the
lower; the former being closed more or less by the fire-tile _z_, shown
in _fig._ 896.
_f_ is one fireplace; for the manner of distributing the fuel in it, see
_fig._ 896.
_g_, _y_, _figs._ 892. and 896., are the horizontal and vertical flues
and chimneys for conducting the flame and smoke. _l_ is the laboratory,
or body of the kiln; having its floor _k_ sloping slightly downwards
from the centre to the circumference. _x_, _y_, is the slit of the
horizontal register, leading to the chimney flue _y_ of the furnace,
being the first regulator; _x_, _u_, is the vertical register conduit,
leading to the furnace or mouth _f_, being the second regulator; _v_ is
the register slit above the furnace, and its vertical flue leading into
the body of the kiln; _v´_, _c_, slit for regulating flue at the
shoulder of the kiln; _i_ is an arch which supports the walls of the
kiln, when the furnace is under repair; _c_, _c_, are small flues in the
vault _s_ of the laboratory. _h_, _fig._ 893., is the central flue,
called _lunette_, of the laboratory.
T, T, is the conical tower or _howell_, strengthened with a series of
iron hoops, O´ is the great chimney or _lunette_ of the tower; _p_ is
the door of the laboratory, bound inside with an iron frame.
A, is the complete kiln and _howell_, with all its appurtenances.
B, _fig._ 893., is the plan at the level _d_, _d_, of the floor, to show
the arrangement and distribution of all the horizontal flues, both
circular and radiating.
C, _fig._ 894., is a plan at the level _e_, _e_, of the upper mouths
_b_, of the furnaces, to show the disposition of the fireplaces of the
vertical flues, and of the horizontal registers, or peep-holes.
D, _fig._ 894., is a bird’s-eye view of the top of the vault or dome
_s_, to show the disposition of the vent-holes _c_, _c_.
[Illustration: 895 896]
E, _fig._ 895., is a detailed plan at the level _c_, _c_, of one furnace
and its dependencies.
F, _fig._ 896., is a transverse section, in detail, of one furnace and
its dependencies.
The same letters in all the figures indicate the same objects.
_Charging of the kiln._--The saggers are piled up first in the space
between each of the upright furnaces, till they rise to the top of the
flues. These contain the smaller articles. Above this level, large fire
tiles are laid, for supporting other saggers, filled with teacups,
sugar-basins, &c. In the bottom part of the pile, within the preceding,
the same sorts of articles are put; but in the upper part all such
articles are placed as require a high heat. Four piles of small saggers,
with a middle one 10 inches in height, complete the charge. As there are
6 piles between each furnace, and as the biscuit kiln has 8 furnaces, a
charge consequently amounts to 48 or 50 _bungs_, each composed of from
18 to 19 saggers. The inclination of the bungs ought always to follow
the form of the kiln, and should therefore tend towards the centre, lest
the strong draught of the furnaces should make the saggers fall against
the walls of the kiln, an accident apt to happen were these piles
perpendicular. The last sagger of each bung is covered with an unbaked
one, three inches deep, in place of a round lid. The watches are small
cups, of the same biscuit as the charge, placed in saggers, four in
number, above the level of the flue-tops. They are taken hastily out of
the saggers, lest they should get smoked, and are thrown into cold
water.
When the charging is completed, the firing is commenced, with coal of
the best quality. The management of the furnaces is a matter of great
consequence to the success of the process. No greater heat should be
employed for some time than may be necessary to agglutinate the
particles which enter into the composition of the paste, by evaporating
all the humidity; and the heat should never be raised so high as to
endanger the fusion of the ware, which would make it very brittle.
When ever the mouth or door of the kiln is built up, a child prepares
several fires in the neighbourhood of the _howell_, while a labourer
transports in a wheelbarrow a supply of coals, and introduces into each
furnace a number of lumps. These lumps divide the furnace into two
parts; those for the upper flues being placed above, and those for the
ground flues below, which must be kept unobstructed.
The fire-mouths being charged, they are kindled to begin the baking, the
regulator tile _z_, _fig._ 896., being now opened; an hour afterwards
the bricks at the bottom of the furnace are stopped up. The fire is
usually kindled at 6 o’clock in the evening, and progressively increased
till 10, when it begins to gain force, and the flame rises half-way up
the chimney. The second charge is put in at 8 o’clock, and the mouths of
the furnaces are then covered with tiles; by which time the flame issues
through the vent of the tower. An hour afterwards a fresh charge is
made; the tiles _z_, which cover the furnaces, are slipped back; the
cinders are drawn to the front, and replaced with small coal. About
half-past 11 o’clock the kiln-man examines his furnaces, to see that
their draught is properly regulated. An hour afterwards a new charge of
coal is applied; a practice repeated hourly till 6 o’clock in the
morning. At this moment he takes out his first _watch_, to see how the
baking goes on. It should be at a very pale-red heat; but the watch of 7
o’clock should be a deeper red. He removes the tiles from those furnaces
which appear to have been burning too strongly, or whose flame issues by
the orifices made in the shoulder of the kiln; and puts tiles upon those
which are not hot enough. The flames glide along briskly in a regular
manner. At this period he draws out the watches every quarter of an
hour, and compares them with those reserved from a previous standard
kiln; and if he observes a similarity of appearance, he allows the
furnaces to burn a little longer; then opens the mouths carefully and by
slow degrees; so as to lower the heat, and finish the round.
The baking usually lasts from 40 to 42 hours; in which time the biscuit
kiln may consume 14 tons of coals; of which four are put in the first
day, seven the next day and following night, and the four last give the
strong finishing heat.
_Emptying the kiln._--The kiln is allowed to cool very slowly. On taking
the ware out of the saggers, the biscuit is not subjected to friction,
as in the foreign potteries, because it is smooth enough; but is
immediately transported to the place where it is to be dipped in the
glaze or enamel tub. A child makes the pieces ring, by striking with the
handle of the brush, as he dusts them, and then immerses them into the
glaze cream; from which tub they are taken out by the enameller, and
shaken in the air. The tub usually contains no more than 4 or 5 inches
depth of the glaze, to enable the workman to pick out the articles more
readily, and to lay them upon a board, whence they are taken by a child
to the glaze kiln.
_Glazing._--A good enamel is an essential element of fine stoneware; it
should experience the same dilatation and contraction by heat and cold
as the biscuit which it covers. The English enamels contain nothing
prejudicial to health, as many of the foreign glazes do; no more lead
being added to the former than is absolutely necessary to convert the
siliceous and aluminous matters with which it is mixed into a perfectly
neutral glass.
Three kinds of glazes are used in Staffordshire; one for the common
pipe-clay or cream-coloured ware; another for the finer pipe-clay ware
to receive impressions, called _printing body_; a third for the ware
which is to be ornamented by painting with the pencil.
The glaze of the first or common ware is composed of 53 parts of white
lead, 16 of Cornish stone, 36 of ground flints, and 4 of flint glass; or
of 40 of white lead, 36 of Cornish stone, 12 of flints, and 4 of flint
or crystal glass. These compositions are not fritted; but are employed
after being simply triturated with water into a thin paste.
The following is the composition of the glaze intended to cover all
kinds of figures printed in metallic colours: 26 parts of white felspar
are fritted with 6 parts of soda, 2 of nitre, and 1 of borax; to 20
pounds of this frit, 26 parts of felspar, 20 of white lead, 6 of ground
flints, 4 of chalk, 1 of oxide of tin, and a small quantity of oxide of
cobalt, to take off the brown cast, and give a faint azure tint, are
added.
The following recipe may also be used. Frit together 20 parts of flint
glass, 6 of flints, 2 of nitre, and 1 of borax; add to 12 parts of that
frit, 40 parts of white lead, 36 of felspar, 8 of flints, and 6 of flint
glass; then grind the whole together into an uniform cream-consistenced
paste.
As to the stoneware which is to be painted, it is covered with a glaze
composed of 13 parts of the printing-colour frit, to which are added 50
parts of red lead, 40 of white lead, and 12 of flint; the whole having
been ground together.
The above compositions produce a very hard glaze, which cannot be
scratched by the knife, is not acted upon by vegetable acids, and does
no injury to potable or edible articles kept in the vessels covered with
it. It preserves for an indefinite time the glassy lustre, and is not
subject to crack and exfoliate, like most of the Continental stoneware,
made from common pipe-clay.
In order that the saggers in which the articles are baked, after
receiving the glaze, may not absorb some of the vitrifying matter, they
are themselves coated, as above mentioned, with a glaze composed of 13
parts of common salt, and 30 parts of potash, simply dissolved in water,
and brushed over them.
_Glaze kiln._--This is usually smaller than the biscuit kiln, and
contains no more than 40 or 45 bungs or columns, each composed of 16 or
17 saggers. Those of the first bung rest upon round tiles, and are well
luted together with a finely ground fire-clay of only moderate cohesion;
those of the second bung are supported by an additional tile. The lower
saggers contain the cream-coloured articles, in which the glaze is
softer than that which covers the blue printed ware; this being always
placed in the intervals between the furnaces, and in the uppermost
saggers of the columns. The bottom of the kiln, where the glazed ware is
not baked, is occupied by printed biscuit ware.
Pyrometric balls of red clay, coated with a very fusible lead enamel,
are employed in the English potteries to ascertain the temperature of
the glaze kilns. This enamel is so rich, and the clay upon which it is
spread, is so fine-grained and compact, that even when exposed for three
hours to the briskest flame, it does not lose its lustre. The colour of
the clay alone changes, whereby the workman is enabled to judge of the
degree of heat within the kiln. At first the balls have a pale red
appearance; but they become browner with the increase of the
temperature. The balls, when of a slightly dark-red colour, indicate the
degree of baking for the hard glaze of pipeclay ware; but if they become
dark brown, the glaze will be much too hard, being that suited for
_ironstone_ ware; lastly, when they acquire an almost black hue, they
show a degree of heat suited to the formation of a glaze upon porcelain.
The _glazer_ provides himself at each round with a stock of these ball
_watches_, reserved from the preceding baking, to serve as objects of
comparison; and he never slackens the firing till he has obtained the
same depth of shade, or even somewhat more; for it may be remarked, that
the more rounds a glaze kiln has made, the browner the balls are apt to
become. A new kiln bakes a round of enamel-ware sooner than an old one;
as also with less fuel, and at a lower temperature. The watch-balls of
these first rounds have generally not so deep a colour as if they were
tried in a furnace three or four months old. After this period, cracks
begin to appear in the furnaces; the horizontal flues get partially
obstructed, the joinings of the brickwork become loose; in consequence
of which there is a loss of heat and waste of fuel; the baking of the
glaze takes a longer time, and the pyrometric balls assume a different
shade from what they had on being taken out of the new kiln, so that the
first watches are of no comparable use after two months. The baking of
enamel is commenced at a low temperature, and the heat is progressively
increased; when it reaches the melting point of the glaze, it must be
maintained steadily, and the furnace mouths be carefully looked after,
lest the heat should be suffered to fall. The firing is continued 14
hours, and then gradually lowered by slight additions of fuel; after
which the kiln is allowed from 5 to 6 hours to cool.
[Illustration: 897 898]
_Muffles._--The paintings and the printed figures applied to the glaze
of stoneware and porcelain are baked in muffles of a peculiar form.
_Fig._ 897. is a lateral elevation of one of these muffles; _fig._ 898.
is a front view. The same letters denote the same parts in the two
figures.
_a_ is the furnace; _b_, the oblong muffle, made of fire-clay,
surmounted with a dome pierced with three apertures _k_, _k_, _k_, for
the escape of the vaporous matters of the colours and volatile oils with
which they are ground up; _c_ is the chimney; _d_, _d_, feed-holes, by
which the fuel is introduced; _e_, the fire-grate; _f_, the ash-pit;
channels are left in the bottom of the furnace to facilitate the passage
of the flame beneath the muffle; _g_ is a lateral hole, which makes a
communication across the furnace in the muffle, enabling the kiln man to
ascertain what is passing within; _k_, _k_, are the lateral chinks for
observing the progress of the firing or flame; _l_, is an opening
scooped out in the front of the chimney to modify its draught.
The articles which are printed or painted upon the glaze are placed in
the muffle without saggers, upon tripods, or movable supports furnished
with feet. The muffle being charged, its mouth is closed with a
fire-tile well luted round its edges. The fuel is then kindled in the
fireplaces _d_, _d_, and the door of the furnace is closed with bricks,
in which a small opening is left for taking out samples, and for
examining the interior of the muffle. These sample or trial pieces,
attached to a strong iron wire, show the progress of the baking
operation. The front of the fireplaces is covered with a sheet-iron
plate, which slides to one side, and may be shut whenever the kiln is
charged. Soon after the fire is lighted, the flame, which communicates
laterally from one furnace to another, envelopes the muffle on all
sides, and thence rises up the chimney.
_Printing of stoneware._--The printing under the stoneware glaze is
generally performed by means of cobalt, and has different shades of blue
according to the quantity of colouring matter employed. After having
subjected this oxide to the processes requisite for its purification, it
is mixed with a certain quantity of ground flints and sulphate of
baryta, proportioned to the dilution of the shade. These materials are
fritted and ground; but before they are used, they must be mixed with a
flux consisting of equal parts by weight of flint glass and ground
flints, which serves to fix the colour upon the biscuit, so that the
immersion in the glaze liquor may not displace the lines printed on, as
also to aid in fluxing the cobalt.
The following are the processes usually practised in Staffordshire for
printing under the glaze.
The cobalt, or whatever colour is employed, should be ground upon a
porphyry slab, with a varnish prepared as follows:--A pint of linseed
oil is to be boiled to the consistence of thick honey, along with 4
ounces of rosin, half a pound of tar, and half a pint of oil of amber.
This is very tenacious, and can be used only when liquefied by heat;
which the printer effects by spreading it upon a hot cast-iron plate.
The printing plates are made of copper, engraved with pretty deep lines
in the common way. The printer, with a leather muller, spreads upon the
engraved plate, previously heated, his colour, mixed up with the above
oil varnish, and removes what is superfluous with a pallet knife; then
cleans the plate with a dossil filled with bran, tapping and wiping as
if he were removing dust from it. This operation being finished, he
takes the paper intended to receive the impression, soaks it with
soap-water, and lays it moist upon the copper-plate. The soap makes the
paper part more readily from the copper, and the thick ink part more
readily from the biscuit. The copper-plate is now passed through the
engraver’s cylinder press, the proof leaf is lifted off and handed to
the women, who cut it into detached pieces, which they apply to the
surface of the biscuit. The paper best fitted for this purpose is made
entirely of linen rags; it is very thin, of a yellow colour, and
unsized, like tissue blotting-paper.
The stoneware biscuit never receives any preparation before being
imprinted, the oil of the colour being of such a nature as to fix the
figures firmly. The printed paper is pressed and rubbed on with a roll
of flannel, about an inch and a half in diameter, and 12 or 15 inches
long, bound round with twine, like a roll of tobacco. This is used as a
burnisher, one end of it being rested against the shoulder, and the
other end being rubbed upon the paper; by which means it transfers all
the engraved traces to the biscuit. The piece of biscuit is laid aside
for a little, in order that the colour may take fast hold; it is then
plunged into water, and the paper is washed away with a sponge.
When the paper is detached, the piece of ware is dipped into a caustic
alkaline lye to saponify the oil, after which it is immersed in the
glaze liquor, with which the printed figures readily adhere. This
process, which is easy to execute, and very economical, is much
preferable to the old plan of passing the biscuit into the muffle after
it had been printed, for the purpose of fixing and volatilizing the
oils. When the paper impression is applied to pieces of porcelain, they
are heated before being dipped in the water, because, being already
semi-vitrified, the paper sticks more closely to them than to the
biscuit, and can be removed only by a hard brush.
The impression above the glaze is done by quite a different process,
which dispenses with the use of the press. A quantity of fine clean glue
is melted and poured hot upon a large flat dish, so as to form a layer
about a quarter of an inch thick, and of the consistence of jelly. When
cold it is divided into cakes of the size of the copper-plates it is
intended to cover.
The operative (a woman) rubs the engraved copper-plate gently over with
linseed oil boiled thick, immediately after which she applies the cake
of glue, which she presses down with a silk dossil filled with bran. The
cake licks up all the oil out of the engraved lines; it is then
cautiously lifted off, and transferred to the surface of the glazed ware
which it is intended to print. The glue cake being removed, the enamel
surface must be rubbed with a little cotton, whereby the metallic
colours are attached only on the lines charged with oil; the piece is
then heated under the muffle. The same cake of glue may serve for
several impressions.
_Ornaments and colouring._--Common stoneware is coloured by means of two
kinds of apparatus; the one called the blowing-pot, the other the
worming-pot. The ornaments made in relief in France, are made hollow
(intaglio) in England, by means of a mould engraved in relief, which is
passed over the article. The impression which it produces is filled with
a thick clay paste, which the workman throws on with the blowing-pot.
This is a vessel like tea-pot, having a spout, but it is hermetically
sealed at top with a clay plug, after being filled with the pasty
liquor. The workman, by blowing in at the spout, causes the liquor to
fly out through a quill pipe which goes down through the clay plug into
the liquor. The jet is made to play upon the piece while it is being
turned upon the lathe; so that the hollows previously made in it by the
mould or stamp are filled with a paste of a colour different from that
of the body. When the piece has acquired sufficient firmness to bear
working, the excess of the paste is removed by an instrument called a
_tournasin_, till the ornamental figure produced by the stamp be laid
bare; in which case merely the colour appears at the bottom of the
impression. By passing in this manner several layers of clay liquor of
different colours over each other with the blowing-pot, net-work and
decorations of different colours and shades are very rapidly produced.
The serpentine or snake pots, established on the same principle, are
made of tin plate in three compartments, each containing a different
colour. These open at the top of the vessel in a common orifice,
terminated by small quill tubes. On inclining the vessel, the three
colours flow out at once in the same proportion at the one orifice, and
are let fall upon the piece while it is being slowly turned upon the
lathe; whereby curious serpent-like ornaments may be readily obtained.
The clay liquor ought to be in keeping with the stoneware paste. The
blues succeed best when the ornaments are made with the finer pottery
mixtures given above.
_Metallic lustres applied to stoneware._--The metallic lustre being
applied only to the outer surface of vessels, can have no bad effect on
health, whatever substances be employed for the purpose; and as the
glaze intended to receive it is sufficiently fusible, from the quantity
of lead it contains, there is no need of adding a flux to the metallic
coating. The glaze is in this case composed of 60 parts of litharge, 36
of felspar, and 15 of flints.
The silver and platina lustres are usually laid upon a white ground,
while those of gold and copper, on account of their transparency,
succeed only upon a coloured ground. The dark-coloured stoneware is,
however, preferable, as it shows off the colours to most advantage; and
thus the shades may be varied by varying the colours of the ornamental
figures applied by the blowing-pot.
The gold and platina lustre is almost always applied to a paste body
made on purpose, and coated with the above-described lead glaze. This
paste is brown, and consists of 4 parts of clay, 4 parts of flints, an
equal quantity of kaolin (china clay), and 6 parts of felspar. To make
brown figures in relief upon a body of white paste, a liquor is mixed up
with this paste, which ought to weigh 26 ounces per pint, in order to
unite well with the other paste, and not to exfoliate after it is baked.
_Preparation of gold lustre._--Dissolve first in the cold, and then with
heat, 48 grains of fine gold in 288 grains of an aqua regia, composed of
1 ounce of nitric acid and 3 ounces of muriatic acid; add to that
solution 4-1/2 grains of grain tin, bit by bit; and then pour some of
that compound solution into 20 grains of balsam of sulphur diluted with
10 grains of oil of turpentine. The balsam of sulphur is prepared by
heating a pint of linseed oil, and 2 ounces of flowers of sulphur,
stirring them continually till the mixture begins to boil; it is then
cooled, by setting the vessel in cold water; after which it is stirred
afresh, and strained-through linen. The above ingredients, after being
well mixed, are to be allowed to settle for a few minutes; then the
remainder of the solution of gold is to be poured in, and the whole is
to be triturated till the mass has assumed such a consistence that the
pestle will stand upright in it; lastly, there must be added to the
mixture 30 grains of oil of turpentine, which being ground in, the gold
lustre is ready to be applied. If the lustre is too light or pale, more
gold must be added, and if it have not a sufficiently violet or purple
tint, more tin must be used.
_Platina lustre._--Of this there are two kinds; one similar to polished
steel, another lighter and of a silver-white hue. To give stoneware the
steel colour with platina, this metal must be dissolved in an aqua regia
composed of 2 parts of muriatic acid, and 1 part of nitric. The solution
being cooled, and poured into a capsule, there must be added to it, drop
by drop, with continual stirring with a glass rod, a _spirit of tar_,
composed of equal parts of tar and sulphur boiled in linseed oil and
filtered. If the platina solution be too strong, more spirit of tar must
be added to it; but if too weak, it must be concentrated by boiling.
Thus being brought to the proper pitch, the mixture may be spread ever
the piece, which being put into the muffle, will take the aspect of
steel.
The oxide of platina, by means of which the silver lustre is given to
stoneware, is prepared as follows:--After having dissolved to saturation
the metal in an aqua regia composed of equal parts of nitric and
muriatic acid, the solution is to be poured into a quantity of boiling
water. At the same time, a capsule, containing solution of sal-ammoniac
is placed upon a sand-bath, and the platina solution being poured into
it, the metal will fall down in the form of the well-known yellow
precipitate, which is to be washed with cold water till it is perfectly
edulcorated, then dried, and put up for use.
This metallic lustre is applied very smoothly by means of a flat camel’s
hair brush. It is then to be passed through the muffle kiln; but it
requires a second application of the platinum to have a sufficient body
of lustre. The articles sometimes come black out of the kiln, but they
get their proper appearance by being rubbed with cotton.
_Platina_ and _gold lustre_; by other recipes.
_Platina lustre._--Dissolve 1 ounce of platinum in aqua regia formed of
2 parts of muriatic acid and 1 part of nitric acid, with heat upon a
sand-bath, till the liquid is reduced to two-thirds of its volume; let
it cool; decant into a clean vessel, and pour into it, drop by drop,
with constant stirring, some distilled tar, until such a mixture is
produced as will give a good result in a trial upon the ware in the
kiln. If the lustre be too intense, more tar must be added; if it be too
weak, the mixture must be concentrated by further evaporation.
_Gold lustre._--Dissolve four shillings’ worth of gold in aqua regia
with a gentle heat. To the solution, when cool, add 2 grains of grain
tin, which will immediately dissolve. Prepare a mixture of half an ounce
of balsam of sulphur with a little essence of turpentine, beating them
together till they assume the appearance of milk. Pour this mixture into
the solution of gold and tin, drop by drop, with continual stirring; and
place the whole in a warm situation for some time.
It is absolutely necessary to apply this lustre only upon an enamel or
glaze which has already passed through the fire, otherwise the sulphur
would tarnish the composition.
These lustres are applied with most advantage upon chocolate and other
dark grounds. Much skill is required in their firing, and a perfect
acquaintance with the quality of the glaze on which they are applied.
_An iron lustre_, is obtained by dissolving a bit of steel or iron in
muriatic acid, mixing this solution with the spirit of tar, and applying
it to the surface of the ware.
_Aventurine glaze._--Mix a certain quantity of silver leaf with the
above-described soft glaze, grind the mixture along with some honey and
boiling water, till the metal assume the appearance of fine particles of
sand. The glaze being naturally of a yellowish hue, gives a golden tint
to the small fragments of silver disseminated through it. Molybdena may
also be applied to produce the aventurine aspect.
_The granite-like gold lustre_, is produced by throwing lightly with a
brush a few drops of oil of turpentine upon the goods already covered
with the preparation for gold lustre. These cause it to separate and
appear in particles resembling the surface of granite. When marbling is
to be given to stoneware, the lustres of gold, platina, and iron are
used at once, which blending in the fusion, form veins like those of
marble.
_Pottery and stoneware of the Wedgewood colour._--This is a kind of
semi-vitrified ware, called _dry bodies_, which is not susceptible of
receiving a superficial glaze. This pottery is composed in two ways: the
first is with barytic earths, which act as fluxes upon the clays, and
form enamels: thus the Wedgewood _jasper_ ware is made.
The white vitrifying pastes, fit for receiving all sorts of metallic
colours, are composed of 47 parts of sulphate of barytes, 15 of felspar,
26 of Devonshire clay, 6 of sulphate of lime, 15 of flints, and 10 of
sulphate of strontites. This composition is capable of receiving the
tints of the metallic oxides and of the ochrous metallic earths.
Manganese produces the dark purple colour; gold precipitated by tin, a
rose colour; antimony, orange; cobalt, different shades of blue; copper
is employed for the browns and the dead-leaf greens; nickel gives, with
potash, greenish colours.
One per cent. of oxide of cobalt is added; but one half, or even one
quarter, of a per cent. would be sufficient, to produce the fine
Wedgewood blue, when the nickel and manganese constitute 3 per cent. as
well as the carbonate of iron. For the blacks of this kind, some English
manufacturers mix black oxide of manganese with the black oxide of iron,
or with ochre. Nickel and umber afford a fine brown. Carbonate of iron,
mixed with bole or _terra di Sienna_, gives a beautiful tint to the
paste; as also manganese with cobalt, or cobalt with nickel. Antimony
produces a very fine colour when combined with the carbonate of iron in
the proportion of 2 per cent., along with the ingredients necessary to
form the above-described vitrifying paste.
The following is another vitrifying paste, of a much softer nature than
the preceding. Felspar, 30 parts; sulphate of lime, 23; silex, 17;
potter’s clay, 15; kaolin of Cornwall (china clay), 15; sulphate of
baryta, 10.
These vitrifying pastes are very plastic, and may be worked with as much
facility as English pipe-clay. The round ware is usually turned upon the
lathe. It may, however, be moulded, as the oval pieces always are. The
more delicate ornaments are cast in hollow moulds of baked clay, by
women and children, and applied with remarkable dexterity upon the
turned and moulded articles. The coloured pastes have such an affinity
for each other, that the detached ornaments may be applied not only with
a little gum water upon the convex and concave forms, but they may be
made to adhere without experiencing the least cracking or chinks. The
coloured pastes receive only one fire, unless the inner surface is to be
glazed; but a gloss is given to the outer surface. The enamel for the
interior of the black Wedgewood ware, is composed of 6 parts of red
lead, 1 of silex, and 2 ounces of manganese, when the mixture is made in
pounds’ weight.
The operation called _smearing_, consists in giving an external lustre
to the unglazed semi-vitrified ware. The articles do not in this way
receive any immersion, nor even the aid of the brush or pencil of the
artist; but they require a second fire. The saggers are coated with the
salt glaze already described. These cases, or saggers, communicate by
reverberation the lustre so remarkable on the surface of the English
stoneware; which one might suppose to be the result of the glaze tub, or
of the brush. Occasionally also a very fusible composition is thrown
upon the inner surface of the muffle, and 5 or 6 pieces called
_refractories_ are set in the middle of it, coated with the same
composition. The intensity of the heat converts the flux into vapour; a
part of this is condensed upon the surfaces of the contiguous articles,
so as to give them the desired brilliancy.
_Mortar body_, is a paste composed of 6 parts of clay, 3 of felspar, 2
of silex, and 1 of china clay.
White and yellow figures upon dark-coloured grounds are a good deal
employed. To produce yellow impressions upon brown stoneware, ochre is
ground up with a small quantity of antimony. The flux consists of flint
glass and flints in equal weights. The composition for white designs is
made by grinding silex up with that flux, and printing it on, as for
blue colours, upon brown or other coloured stoneware, which shows off
the light hues.
_English porcelain or china._--Most of this belongs to the class called
tender or soft porcelain by the French and German manufacturers. It is
not, therefore, composed simply of _kaolin_ and _petuntse_. The English
china is generally baked at a much lower heat than that of Sèvres,
Dresden, and Berlin; and it is covered with a mere glass. Being
manufactured upon a prodigious scale, with great economy and certainty,
and little expenditure of fuel, it is sold at a very moderate price
compared with the foreign porcelain, and in external appearance is now
not much inferior.
Some of the English porcelain has been called ironstone china. This is
composed usually of 60 parts of Cornish stone, 40 of china clay, and 2
of flint glass; or of 42 of the felspar, the same quantity of clay, 10
parts of flints ground, and 8 of flint glass.
The glaze for the first composition is made with 20 parts of felspar, 15
of flints, 6 of red lead, and 5 of soda, which are fritted together;
with 44 parts of the frit, 22 parts of flint glass, and 15 parts of
white lead, are ground.
The glaze for the second composition is formed of 8 parts of flint
glass, 36 of felspar, 40 of white lead, and 20 of silex (ground flints).
The English manufacturers employ three sorts of compositions for the
porcelain biscuit; namely, two compositions not fritted; one of them for
the ordinary table service; another for the dessert service and tea
dishes; the third, which is fritted, corresponds to the paste used in
France for sculpture; and with it all delicate kinds of ornaments are
made.
+--------------+------------+------------+-------------+
| | First | Second | Third |
| |composition.|composition.|composition. |
+--------------+------------+------------+-------------+
|Ground flints | 75 | 66|Lynn sand 150|
|Calcined bones| 180 | 100| 300|
|China clay | 40 | 96| 100|
|Clay | 70 |Granite 80|Potash 10|
+--------------+------------+------------+-------------+
The glaze for the first two of the preceding compositions consists of,
felspar 45, flints 9, borax 21, flint glass 20, nickel 4. After fritting
that mixture, add 12 parts of red lead. For the third composition, which
is the most fusible, the glaze must receive 12 parts of ground flints,
instead of 9; and there should be only 15 parts of borax, instead of 21.