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by rubbing the work with rotten-stone upon a small straight bar of pewter. Some delicacy is here required to avoid scratching or producing flaws in the enamel by pressing too hard. In this way the piece is rendered perfectly even. But the last brilliant polish is given by a piece of deal wood and the same rotten-stone.
This is the general method of applying enamel; but some colours require more precaution in the management of the fire. Opaque colours require less management than the transparent. A variety of circumstances must be attended to in transparent colours; every colour requires gold of a particular fineness.
When different colours are intended to be placed beside one another, they are kept separate by a small edge or prominency, which is left in the gold for that purpose, and is polished along with the enamel.
The enamelling upon silver is effected nearly in the same manner as that of gold; but the changes sustained by the colours upon silver by the action of the fire are much more considerable than when gold is used.
Copper is not much used by enamellers, on account of the difficulty which attends the attempt to fix beautiful colours upon it. When this metal is used, the common practice is to apply a coating of opaque white enamel, and upon this other colours which are more fusible than the white.
A good effect is produced in toys by leaving part of the gold bare. For this purpose its surface is cut into suitable compartments by the engraver. This, however, is an expensive method, and is for this reason occasionally imitated by applying small and very thin pieces of gold upon the surface of the enamel, where they are fixed by the fire, and afterwards covered by a transparent vitreous coating.
A method of taking off the enamel from any toy without injuring the metallic part, is often a desirable object. For this purpose a mixture of common salt, nitre, and alum in powder, is applied upon the enamel, and the piece is put into the furnace. As soon as the fusion has taken place, the piece is to be suddenly thrown into water, which causes the enamel to fly off, either totally or in part. Any part which may still remain is to be removed by repeating the same operation a second time.
To coat vessels of iron or copper for culinary purposes with an enamel capable of defending the metal from the action of any
solvent, and for enduring any heat or transition from heat to cold, appears a desirable object; and many experiments have been made on the subject by Mr. Soen Rinman of the Royal Academy of Stockholm.
The following compositions he found answer very well on copper. 1. The white semitransparent fluor spar and sulphate of lime in equal quantities, powdered, mixed, and calcinated in a white heat; then powdered, made into a thin paste with water, and applied a little warm to the vessel also warmed. Then dried and heated gradually to a certain point, a very strong heat, greater than is generally obtained in an assaying fur- nace, is to be applied as quickly as possible. 2. Sixty parts of lime, one hundred of fluor spar, sixty of gypsum, twenty of quartz, and one of manganese are calcinated, ground, and applied in a similar manner. 3. Four parts of fluor spar, four of gypsum, and one of litharge, melted into a straw-coloured glass, ground and applied in the same way, required a much stronger heat. 4. Five parts of fluor spar, five of gypsum, two of minium, two of flint glass, half a part of borax, the same of oxide of tin, and one twenty-fifth of a part of oxide of cobalt melted together made an enamel ; which, when ground, and applied as the others, fused with a less degree of heat. This, Mr. Rinman imagines, would have been acted upon in length of time by sulphuric acid. The oxide of cobalt was prepared by saturating a solution of cobalt in aqua-fortis with common salt, and evaporating to dryness.
As these would not do for iron, he tried the following: 1. minium, nine parts; flint glass, six; pure potash, two; nitre, two; borax, one; were ground together, put into a covered crucible, which they only half filled and fused into glass. This poured out on a piece of marble, quenched in water, powdered and made into a thin paste, was laid on both sides of an iron vessel. After having been dried and heated gradually, the vessel was put under a muffle, well heated in an assaying furnace, and in half a minute the enamel melted. The vessel being then withdrawn, was found enamelled of a beautiful black colour, which appeared to be owing to a thin layer of oxided iron seen through the transparent glaze. 2 The same, with one hundredth part of oxide of cobalt prepared as above, covered the vessel more perfectly with a blue enamel. 3, The same ground with
potters' white lead, which consists of four parts of lead and one of tin, produced a very smooth grey enamel, more firm and hard than the preceding. A small quantity of red oxide of iron gave it a fine dark red colour. 4. Flint glass, twelve parts; minium, eighteen; potash, four; nitre, four; borax, two; oxide of tin three; oxide of cobalt, one-eighth of a part, gave a smooth pearl coloured enamel, not brittle or subject to crack, and capable of enduring sudden changes of heat and cold, as well as the action of oils, alkalies, and weak acids; but it cannot resist the stronger vegetable acids, and still less the mineral.
These enamels were applied only on hammered iron, cast iron being too thick to be heated with sufficient quickness. But they have been applied to the thin cast vessels in England. It seems unnecessary to add, none of them will bear hard blows; and this is perhaps the reason why they have not been more used with us.
The application of enamel colours to glass or earthenware constitutes a peculiar branch of the art. M. Brougniart, of the porcelain manufactory at Sèvres, has given a good account of them. (Nicholson's Journal, Vol. III. 4to.)
These bodies may be divided into three very distinct classes from the nature of the substances that compose them, the effects produced on them by the colours, and the changes they undergo. These are, 1. enamel; soft porcelain, and all the glazes, enamels, or glasses, which contain lead in any considerable quantity. 2. Hard porcelain, or such as is glazed with feldspar. 3. Glass, in which there is no lead, such as the common window glass. The principles of com position of these colours, and the general phenomena they present on these three grounds or supporters, are regularly treated of.
Colours in enamel painting have been longest known. Enamel is a glass, rendered opaque by oxide of tin, and very fusible by the oxide of lead. It is this last which in particular gives it properties very different from those of the other excipients of metallic colours. Hence all the glasses and glazes which contain lead, have the properties of enamel, and what we may assert of the one will apply to the other with very little difference.
Such are the white and transparent glazes of Dutch or Delft ware, and the glaze of the porcelain called soft ware.
particularly at Sévres, and indeed for a long time almost exclusively at that manufactory, has for its base vitreous frit nearly opaque, capable of being acted upon by marl, and its glaze is a very transparent glass containing much lead.
The colours made use of are the same as those for enamelling, consequently the changes these colours undergo in enamel must take place in this species of porcelain. The causes of the change being the same in both.
The colours for enamel and soft porcelain require less flux than the others, because the glass on which they are placed softens sufficiently to be penetrated by them.
This solvent may be either the mixture of glass of lead and pure silex, called rocaille, or this same glass mixed with that of borax.
Montamy says, that glass of lead ought not to be used in the flux for enamel; he employs borax alone. He then dilutes or makes up his colours in a volatile oil.
On the contrary, the painters of the manufactory at Sévres use only colours without borax, because they dilute them with gum, and borax does not dilute them well this way. M. Broaguiart is convinced that both methods are equally good, and that Montamy is not justified in excluding the fluxes of lead, as they are employed without inconvenience every day, and even render the management of colours more easy.
It is remarked, that in the baking of these colours the glaze is softened so much as to be easily penetrated by them; and this is one great cause of the change they undergo. They become diluted by the mixture with the glaze, and the first fire changes a painting apparently finished, into a very slight sketch.
The oxide of lead contained in the glaze is a more powerful cause of the great changes these colours undergo. Its destructive action is principally exercised on the reds of iron, and is very remarkable.
It has already been shewn that the two principal causes of the change which colours on enamel and tender porcelain undergo, do not relate to the composition of these colours, but entirely to the nature of the glass on which they are placed. The assertion that the colours of porcelain are subject to considerable change, relates to the colours of soft porcelain, a species of ware now almost totally abandoned.
Hence, it follows, that the paintings of porcelain require to be several times reThis porcelain, the first made in France, touched and burned, in order to possess the
necessary strength. Though these paintings have always a certain softness, they are constantly more brilliant, and never subject to the inconvenience of scaling off.
Hard porcelain is the second species of ground or excipient for the metallic colours. It is known that the base of this porcelain is a very white argil, called kaolin, mixed with a siliceous and calcareous solvent, and the glaze is nothing but feldspar fused without an atom of lead.
This porcelain, which is that of Saxony, is of a much later date at Sévres than the soft or tender. The colours employed are of two kinds, the first used for representing different objects are baked with a very inferior fire to that required for baking the porcelain itself. They are very numerous and varied.
The others, which require to be fused at as great a heat as that for baking the porcelain, are laid on the general surface. They are much less numerous.
The colours for painting are made up very nearly of the same materials as those for tender porcelain; they only contain more flux. This flux is composed of the glass of lead (called rocaille) and of borax. M. Brougniart asserts, that he has not met with any work that treats of the composition, use, and effects of these colours. In fact, it has no where been asserted, in print, that all these colours, except one, are unchangeable in fire; whereas it has been often asserted, in books, that paintings in enamel are subject to considerable change.
When the porcelain is put into the fire to bake the colours, the feldspar glaze dilates and opens in pores, but does not become soft. As the colours do not penetrate it, they are not subject to the changes they undergo on tender porcelain. It must, however, be observed, that they lose a little of their intensity by acquiring the transparence given them by the fusion.
When works of little importance are made, they need not be re-touched; but this is necessary when a painting is to be highly finished. This re-touching is not more distinguishable in paintings on porcelain than in that of any other species of painting.
One of the great inconveniences of these colours is, that they scale or fly off when the fire is often applied.
This has been particularly remarked at Sévres, on acconnt of the solidity and infusibility with which porcelain is there manufactured. But these qualities cause it to
resist the alterations of heat and cold for a longer time, and gives its ground a more brilliant colour. On the other hand, the porcelains of Paris being more vitreous, transparent, and of a blueish cast, generally crack if boiling water is frequently poured into them.
In order to remedy this evil, without altering the quality of the body, Brougniart softens the glaze a little, by introducing more siliceous or calcareous flux according to the nature of the feldspar. This method succeeded, and for twelvemonths then past, the colours had past two and three times through the fires without cracking, provided there were not too much flux, and they were not laid on too thick.
It has been remarked, that when soda and potash have been introduced, the colours scaled, so that they cannot be used as fluxes. These alkalies being volatilized, abandon the colours which cannot adhere to the glaze by themselves.
It has been observed, that other colours are likewise prepared, which being laid upon the general surface, are fused by the same fire as bakes the porcelain. These colours are but few, because there are few metallic oxides that can support such a fire without being volatilized or discoloured. Their solvent is the feldspar. As they incorporate with the glaze they never crack, and are more brilliant.
The third receptacle of metallic vitri fiable colours is glass without lead.
The application of these colours constitutes the art of painting upon glass; an art much practised in former ages, but which was, till lately, supposed to be lost, because out of fashion. It, however, too immediately depends on the art of painting on ena mel and porcelain to be lost. Descriptions of the processes may be found in different books.
A book entitled, "L'Origine de l'art de la Peinture sur Verre," published at Paris in the year 1693, and "Le Traité de l'art de la Verriere," by Neri and Kunckel, seem to be the first works containing complete descriptions of this art. Those published since, even the great work of Leviel, which constitutes part of "Les Arts et Metiers," of the French academy, and of the " Encyclopédie Methodique," are only compilations from the two former works.
It is somewhat remarkable, that if we follow the processes exactly as they are described in these works, as our author has done with some of them, the colours of
which they pretend to give the receipt would never be fabricated. They only serve to shew an able practitioner the method, and leave it to him to correct or make additions. This was found to be the case by Citizen Meraud, who was engaged to prepare them for the manufactory of Sévres. He was obliged to make the colours for painting on glass rather from his own experience than from the instructions in the work just mentioned.
The materials and fluxes which enter into the compositions of the colours for painting on glass are, in general, the same as those applied to porcelain. They vary only in their proportions; but a great number of the colours used for enamel and porcelain cannot be applied on glass; many of them, when seen by transmitted light, entirely change their aspect, and exhibit an obscure tint, which can be of no use when deprived of the white ground which throws them out. We shall point out these when we treat of the colours in particular. Those colours which can be used on this body sometimes change in the baking, and acquire a great transparency. They are generally beautiful only when placed between the eye and the light, and they answer the purpose intended in painting glass.
There is more difficulty in baking plates of coloured glass than is commonly thought. The bending of the piece, and the alteration of the colours, are to be avoided. All the treatises we have consulted recommend the use of gypsum. This method sometimes succeeded with Brougniart, but generally the glass became white, and cracked in all directions. It appears, that the glasses which are too alkaline, and which are far the most common in clear white glasses, are attacked by the hot sulphuric acid of the sulphate of lime. He was able with ease to bake much larger glasses than any before painted, by placing them on very smooth plates of earth or unglazed porcelain.
Concerning the several particular Colours. After having collected the several phenomena which each class of vitrifiable colours offer with regard to the bodies on which they are placed, we must shew the particular and most interesting phenomena which every principal species of colours employed on tender porcelain, on glass, and in the fire that bakes the porcelain present.
Concerning the Reds, Purples, and Violets oir tained from Gold.
The carmine-red is obtained from the purple precipitate of Cassius. It is mixed with about six parts of its flux, and this mixture is directly employed without being first fused. It is then of a dirty violet, but acquires the beautiful carmine by baking. It is however very delicate; a little too much heat or carbonated vapours easily spoil it; yet it is more beautiful when baked with charcoal than with wood.
This colour, and the purple which differs little from it, as well as the shades which are obtained from their mixture with other colours, really change in all porcelains, and in the hands of all operators. But this is the only one which changes on hard porcelain. It may be replaced by a substitution of rose colour from iron, which does not change; so that by excluding from the pallet the carmine made from gold, and substituting the rose-coloured oxide of iron here spoken of, we have a pallet composed of colours, none of which are subject to any remarkable change. The rose-coloured oxide of iron has been long known, but was not employed on enamel, because it is then subject to considerable change. Or perhaps, when the painters on enamel became painters on porcelain, they continued to work according to their ancient method.
It might be supposed, that by previously reducing the colour named carmine, already mixed with its solvent into a vitreous matter, the last tint would be obtained; but the fire which must be used to melt this vitreous mass destroys the red colour. Besides, it is found that to obtain this colour in perfection, it is necessary to pass it through the fire as little as possible.
The carmine of tender porcelain is made of fulminating gold, gently decomposed, and muriate of silver; there is no tin in it, which proves it is not necessary for the fabrication of a purple colour that the oxide of this last metal, and that of gold, should be combined.
Violet is likewise obtained from the purple oxide of gold. This colour proceeds from having a greater quantity of lead in the flux, and it is nearly of the same tint,
whether crude or baked.
These three colours totally disappear in the strong fire necessary to bake porcelain.
Carmine and purple afforded, upon glass, only tints of a dirty violet. The violet on the contrary has a beautiful effect, but is subject to change to blue.
Concerning the Red, Rose, and Brown Colours, of lead, white oxide of antimony, and sand.
obtained from Iron.
These colours are made from red oxydated iron, prepared with nitric acid. The oxides are calcined still more by exposing them to the action of fire. If too much heated they change to a brown.
Their flux is composed of borax and minium in small quantity.
These are the oxides which afford the rose and red colours, which may be substituted instead of the same colours made from oxide of gold. If properly applied on hard porcelain they never change. Brougman made roses with these colours, and there was no difference between the flower before and after baking, except the brilliancy which colours naturally receive from fusion. The colours may either be previously fused or not, at pleasure.
In a violent fire, they either partly disappear, or produce a dull and brick-dust red colour, which is not at all agreeable.
Their composition is the same, either for tender porcelain or for glass. They do not change on the latter, but on the former they almost entirely disappear by the first fire; and they must be laid on very heavily in order to have any part visible.
It is to the presence of lead in their glaze that this singular effect must be attributed. Brougman ascertained this by a very simple experiment. He placed this colour on window glass, and fired it very strongly and it did not change. He then covered some parts of it with minium, and again exposed it to the fire. The colours totally disappeared in those places where the red oxide of lead had been applied. When this experiment was performed on a larger scale, in a closed vessel, a large quantity of oxygen gas was disengaged.
This observation seems clearly to prove the effect of oxyded lead as a discolourer of glass. We see that it does not operate as has been supposed, by burning combustible impurities in the glass, but by dissolving, discolouring, and volatilizing the oxide of iron, which may effect its clearness.
Concerning the Yellows.
Yellows are colours which require much precaution in fabricating, on account of the lead they contain; which, sometimes, by approaching to the metallic state produces black spots.
The yellows of hard and tender porcelain are the same. They are composed of oxide
Oxide of tin is sometimes added; and bling the colour of marigold, red oxide of when it is required very lively and resemdisappears during the previous fusion they iron is added, the very deep colour of which undergo, on account of the lead contained in this yellow. When these colours are once made they do not change; they disappear almost entirely in the porcelain fireyellows.
These cannot be applied to glass, they are opaque and muddy. That employed by the ancient painters on glass is, on the contrary, beautifully transparent, very brilliant, and of a colour approaching gold. The processes they give indicate that it contains a mixture of silver; but when exactly followed they afford nothing satifactory. Citizen Meraud succeeded in making it as beautiful as the ancient painters on glass, by employing muriate of silver, oxide of zinc, white clay, and the yellow oxide of iron. These colours are applied to glass simply ground and without flux. The oxide of iron gives the yellow nearly the same tinge as it ought to have after the baking, and contributes, with the clay and oxide of zinc, to decompose the muriate of silver without disoxydating the silver itself. A powder remains after baking which does not penetrate the glass, and may be easily cleared off.
This yellow when employed in greater quantities affords deeper shades, and produces a reddish colour.
Concerning the Blues.
the oxide of cobalt; their preparation is known to every chemist. The superiority at Sévres, so justly reputed for the supe-` riority of its blues, is owing merely to the
These are known to be obtained from
care taken in its fabrication, and to the quality of the porcelain, which appears more proper to receive it on account of the violent fire it can support.
Brougniart observed one fact respecting the oxide of cobalt, which is, perhaps, not known to every chemist. It is volatile in a violent heat; to this property must be attributed the bluish tint which the white (bordering upon blue) always receives. A white piece was purposely put in the same case next to a blue, the side of the white piece which was turned towards the blue became very bluish.
The blue of hard porcelain, prepared for what is called a blue ground by strong fire,