Εικόνες σελίδας
PDF
Ηλεκτρ. έκδοση

Porcelain.

"Violet is made also with purple oxide of gold. A greater quantity of lead in the flux is what gives it this colour, which is almost the same crude or baked. "These three colours totally disappear when exposed to a great porcelain heat.

"Carmine and purple have given us in glass tints only of a dirty violet. The violet, on the other hand, produces on glass a very beautiful effect, but it is liable to turn blue. I have not yet been able to discover the cause of this singular change, which I saw for the first time a few days ago.

Red, Rose, and Brown Colours, extracted from Iron. "These colours are made from red oxide of iron prepared with nitric acid. These oxides are further calcined by keeping them exposed to the action of reat. If heated too much, they pass to brown.

"Their flux is composed of borax, sand, and minium, in small quantity.

"These oxides give rose and red colours capable of supplying the place of the same colours made with oxide of gold. When properly employed on hard porcelain, they do not change at all. I have caused roses to be painted with these colours, and found no difference between the baked flower and that not baked, except what might be expected to result from the brilliancy given to colours by fusion.

"These colours may be employed indiscriminately, either previously fused or not fused.

"In a great heat they in part disappear, or produce a dull brick red ground, which is not agreeable.

"The composition of them is the same both for soft porcelain and for glass. They do not change on the latter; but on soft porcelain they disappear almost entirely on the first exposure to heat, and to make any thing remain they must be employed very deep.

"This singular effect must be ascribed to the presence of lead in the crust or glazing. I assured myself of this by a very simple experiment. I placed this colour on window glass, and having exposed it to a strong baking, it did not change.

"I covered several parts of it with minium; and again exposing it to the fire, the colour was totally removed in the places where the red oxide of lead had been applied.

"By performing this operation on a larger scale in close vessels, a large quantity of oxygen gas was disengaged.

It appears to me that this observation clearly proves the action of oxidated lead on glass as a destroyer of colour: it is seen that it does not act, as was believed, by burning the combustible bodies, which might tarnish the glass, but by dissolving, discolouring, or volatilizing with it the oxide of iron, which might alter its transparency.

Yellows.

"Yellows are colours which require a great deal of care in the fabrication on account of the lead which they contain, and which, approaching sometimes to the metallic state, produces on them black spots.

"The yellows for hard and soft porcelain are the same: they are composed of the oxide of lead, white oxide of antimony, and sand.

"Oxide of tin is sometimes mixed with them; and

when it is required to have them livelier, and nearer Porcelain. the colour du souci, red oxide of iron is added, the too great redness of which is dissipated in the previous fusion to which they are exposed by the action of the lead contained in this yellow. These colours, when once made, never change: they disappear, however, almost entirely when exposed to a porcelain heat.

"These yellows cannot be applied to glass: they are too opake and dirty. That employed by the old painters on glass has, on the contrary, a beautiful transparency, is exceedingly brilliant, and of a colour which approaches near to that of gold. The processes which they gave clearly showed that silver formed part of their composition; but, when exactly followed, nothing sa tisfactory was obtained. C. Miraud, whom I have already had occasion to mention, has found means to make as beautiful paintings on glass as the ancients, by employing muriate of silver, oxide of zinc, white argil, and yellow oxide of iron. These colours are applied on glass merely pounded, and without a flux. The oxide of iron brings the yellow to that colour which it ought to have after baking, and contributes with the argil and oxide of zinc to decompose the muriate of silver without deoxidating the silver. After the baking, there remains a dust which has not penetrated into the glass, and which is easily removed.

"This yellow, when employed thicker, gives darker shades, and produces a russet.

Blues.

"It is well known that these are obtained from the oxide of cobalt. All chemists are acquainted with the preparation of them. Those of Sevres, which are justly esteemed for their beauty, are indebted for it only to the care employed in manufacturing them, and to the quality of the porcelain, which appears more proper for receiving them in proportion to the degree of heat which it can bear.

"I remarked respecting the oxide of cobalt a fact which is perhaps not known to chemists: it is volatile in a violent heat: it is to this property we must ascribe the blueish tint always assumed by white in the neighbourhood of the blue. I have placed expressly on purpose, in the same case, a white piece close to a blue one, and found that the side of the white piece next the blue became evidently blueish.

"The blue of hard porcelain, destined for what is called the ground for a great heat (les fonds au grand feu) is fused with feldspar; that of soft porcelain has for its flux silex, potash, and lead: it is not volatilized like the preceding; but the heat it experiences is very inferior to that of hard porcelain.

"These colours, when previously fused, do not change at all in the application.

"Blues on glass exhibit the same phenomena as those on soft porcelain.

[blocks in formation]

Porcelain. heat are composed with the oxides of cobalt and nic- differences very sensible to an eye accustomed to paint- Porcelain. kel, but a brownish green only is obtained. ing. A mere knowledge of the composition of the colours does not give the talent of executing them well. "In recapitulating the facts above mentioned, to present them under another general point of view, it is seen, "Ist, That among colours generally employed on Facts relabard porcelain one only is susceptible of changing, viz. tive to cocarmine, and the tints into which it enters: that its lours reca. place may be supplied by the reds of iron, and that no pitulated. colour then changes.

"Blueish greens, called celestial blues, which were formerly colours very much in vogue, can be applied only upon soft porcelain; on hard porcelain they constantly become scaly, because potash enters into their composition.

"These greens cannot be applied on glass: they give a dirty colour. To obtain a green on glass, it is necessary to put yellow on one side, and blue, more or less pale, on the other. This colour may be made also by a mixture of blue with yellow oxide of iron. I hope to obtain from oxide of chrome a direct green colour. The trials I have made give me reason to hope for success. Pure chromate of lead, which I applied to porcelain in a strong heat, gave me a pretty beautiful green of great intensity and very fixed.

Bistres and Russets.

"These are obtained by mixtures in different proportions of manganese, brown oxide of copper, and oxide of iron from ombre earth. They are also previously fused with their flux, so that they do not change in any manner on soft porcelain, as lead has not the same action on oxide of manganese as on that of iron, as I assured myself by an experiment similar to that already mentioned.

"This colour fades very speedily on glass.

"Russet grounds in a great heat, known under the name of tortoise-shell grounds, are made in the some manner. Their flux is feldspar: no titanium enters into their composition, though said so in all printed works. Titanium was not known at the manufactory of Sevres when I arrived there. I treated this singular metal in various ways, and never obtained but grounds of a pale dirty yellow, and very variable in its tone.

Blacks.

“Blacks are the colours most difficult to be obtained very beautiful. No metallic oxide gives alone a beautiful black. Manganese is that which approaches nearest to it. Iron gives an opake, dull, cloudy black, which changes very easily to red: the colour-makers, therefore, to obtain a black which they could not hope for from the best theorist, have united several metallic oxides which separately do not give black, and have obtained a very beautiful colour, which, however, is liable to become scaly and dull.

"These oxides are those of mangenese, the brown oxides of copper, and a little of the oxide of cobalt. The gray is obtained by suppressing the copper, and increasing the dose of the flux.

"The manufactory of Sevres is the only one which has hitherto produced beautiful blacks in a strong heat. This is owing rather to the quality of its paste than to any peculiar processes, since it does not conceal them. It is by darkening the blue by the oxides of manganese and iron that they are able in that manufactory to obtain very brilliant blacks.

"Having here made known the principles of the fabrication of each principal colour, it may be readily conceived that by mixing these colours together all the shades possible may be obtained. It is evident also that care in the preparation, choice in the raw materials, and a just proportion of doses, must produce in the results

[ocr errors]

"I have presented to the Institute a head not baked, executed according to this method: and the painting of two roses, that of the one baked, and that of the other not baked. It has been seen that there was no differ

ence between them.

"2d, That among the colours for soft porcelain and enamel, several change in a considerable degree. These are principally the reds of gold and iron, the yellows, the greens, the browns. They have not been replaced by others, because this kind of painting has been almost abandoned.

"3d, That several of the colours on glass change also by acquiring complete transparency. These in particular are the yellows and greens.

"4th, That it is neither by calcinating the colours in a higher degree, nor previously fusing them, as supposed by some, that they are prevented from changing, since these means really alter the changing colours, and produce no effect on the rest. The change which several colours experience on soft porcelain and on glass does not then depend on the nature of their composition, but rather on that of the body on which they are applied.

"Consequently, by suppressing from the colours of hard porcelain the carmine of gold, which is not indispensably necessary, we shall have a series of colours which do not change."

65

66

als for por

As it must be of no small importance to the chemical Results of manufacturer to be acquainted with the results of ex- experiments periments on the effects of heat, when applied to dif- on materi ferent proportions of the materials employed in making celain imporcelain, or other analogous ware, we shall insert the portant. following tables, exhibiting those results. The first table contains the results of the numerous experiments of Achard and Morveau on the vitrification of earths with saline bodies. The mixture of the earths and salts was made in a clay crucible, and, in the experiments of Morveau, the crucible was exposed for two hours to a heat from 22° to 26° of Wedgwood's pyrometer; but in those of Achard, the crucible was kept for three hours in the heat of a strong wind furnace, in which the temperature was probably higher than the former.

The second table presents a view of the effects of the vitrification of earths by means of metallic oxides. The mixtures were exposed in earthen crucibles to the heat of a porcelain furnace during the whole time required to bake porcelain ware.

In the third table are exhibited the curious results of the effects of vitrifying materials on the crucibles in which the vitrification takes place. It is to be observed, that the effects of the same materials, and in the same proportions, are very different, in different vessels ; and without attending to this circumstance, very erroneous conclusions will be drawn in estimating the action of vitrifiable substances on each other. This diversity of the effects of the same materials in different crucibles,

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

M. Silex

4.

1.

A mass resembling agate-but perfectly fused and scintillant.

2.} A green scintillant glass.

[ocr errors]

A soft green transparent glass.

1.}Scoria—the crucible entirely destroyed.

1.

Phosphate of soda and ammonia 2: A white opake, puffy, vitreous mass, deliquescent, and reddening litmus.

M. LIME

Carbonate of soda

A. Chalk

Carbonate of potash

A. Chalk

Carbonate of potash

A. Chalk

Carbonate of potash

M. Lime

Borax

A. Chalk

Borax

A. Chalk

Borax

A. Chalk

Boracic acid

VOL. XVII. Part I.

2.

I.

2.

S

:}

A white spongy opake mass, crumbling between the fingers.

2. Partly fused-the rest pulverulent-the crucible strongly corroded.

I.

[merged small][merged small][merged small][merged small][merged small][merged small][subsumed][merged small][merged small][merged small][ocr errors][subsumed][merged small][merged small][merged small][merged small][merged small][merged small]
[blocks in formation]

M. Magnesia

'Borax

:

[ocr errors]

}

1.7

2.

}

A yellow scintillant glass—the crucible entirely destroyed.

A white opake crumbly mass.

A gray opake ill fused fritt, not cohering to the crucible and deliquescent.

Remained unmelted and uncohering.

Partially melted, but soft and friable.

A fine transparent clear green glass.

Boracic acid

1.

Remained pulverulent.

I.

4.

1.

Part unfused and remaining pulverulent, the rest partially melted.

1:}A green fritt easily friable.

Phosphate of soda and ammonia 2.

[blocks in formation]

A white opake uncobering mass.

A semi-transparent somewhat milky glass of a gelatinous appearance, but very hard and brilliant ou the surface.

Phosphate of soda and ammonia A white mass a little agglutinated but not adhering to the crucible.

M. Barytes (pure)

Carbonate of soda

M. Barytes

Borax

M. Barytes

I. 2.

[blocks in formation]

Phosphate of soda and ammonia 2: A remarkably fine transparent glass.

2.

TABLE II. Containing the Results of the Vitrification of Earths by Metallic Oxides.

[blocks in formation]
[merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][ocr errors][ocr errors][subsumed][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][subsumed][ocr errors][subsumed][ocr errors][subsumed][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]
« ΠροηγούμενηΣυνέχεια »