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