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Tobacco-pipe clay from Dorsetshire is beaten much in water; by this process the finer parts of the clay remain suspended in the water, while the coarser sand and other impurities fall to the bottom. The thick liquid, consisting of water and the finer parts of clay, is further purified by passing it through hair and lawn sieves of different degrees of fineness. After this the liquor is mixed (in various proportions for various ware) with another liquor of the same density, and consisting of flints calcined, ground and suspended in water. The mixture is then dried in a kiln, and being afterwards beaten to a proper temper, it becomes fit for being formed at the wheel into dishes, plates, bowls, etc. When this ware is to be put into the furnace to be baked, the several pieces of it are placed in the cases made of clay, called seggars, which are piled one upon another, in the dome of the furnace; a fire is then lighted, when the ware is brought to a proper temper, which happens in about 48 hours, it is glazed by common salt. The salt is thrown into the furnace through holes in the upper part of it, by the heat of which it is instantly converted into a thick vapor, which, circulating through the furnace, enters the seggar through holes made in its side (the top being covered to prevent the salt from falling on the ware) and attaching itself to the surface of the ware, it forms that vitreous coat upon the surface which is called its glaze.
This is made of the same materials as the flintware, but the proportion in which the materials are mixed is not the same, nor is the ware glazed in the same way. The flintware is generally made of 4 measures of liquid flint, and 18 of liquid clay; the yellowware has a greater proportion of clay in it. In some manufactories they mix 20, and in others 24 measures of clay with 4 of flint. The proportion for both sorts of ware depends very much upon the nature of the clay, which is very variable even in the same pit. Hence a previous trial must be made of the quality of the clay, by burning a kiln of the ware. If there be too much flint mixed with the clay, the ware, when exposed to the air after burning, is apt to crack, and if there be too little, the ware will not receive the proper glaze from the circulation of the salt vapor.
The iron-stone, which contains a portion of argil and silex, is first roasted in a common biscuit-kiln, to facilitate its trituration, and to expel sulphur and other volatile ingredients which it may contain. A large earthen crucible is constructed after the exact model of an iron forge, a part of the bottom of which is filled with charcoal or cokes; these having been previously strewed with ore and about 1/3 part of lime, are raised to an intense heat by a strong blast of air, introduced under the cokes at the bottom. By this heat the ore is fused, and the fluid iron drops through the fuel to the bottom; then follows the scoria, which floats upon the top of the fluid iron. This latter scoria, or, as the workmen call it, slag, is the material used in the manufacture of china, and is much impregnated with iron, and of a compact and dense structure. The slag is next let off, by a hole through the forge, into a clean earthen vessel, where it cools. This last vessel is then broken, in order to detach the slag from it, with hammers. The scoria is next pounded into small pieces and ground in water to the consistence of a fine paste, at the flint-mills of the country. This paste is then evaporated to dryness on a slip-kiln, well known amongst potters. Thus evaporated to dryness, it is used with the other ingredients in the following proportions, viz.:
Prepared iron-stone, 3 cwt.; ground flint, 4 cwt.; ground Cornwall stone, 4 cwt.; Cornwall clay, 4 cwt.; blue oxide of cobalt, 1 lb.
These having been mixed together with water by the slip-maker, are again evaporated on the slip-kiln to the proper consistency for use. The clay, thus prepared, is of course used in the usual manner in the fabrication of the several kinds of vessels.
Porcelain, or china, is a semi-vitrified earthenware of an intermediate nature between common-ware and glass. Chinese porcelain is composed of two ingredients, one of which is hard-stone, called petunse, which is carefully ground to every fine powder, and the other, called kaolin, is a white earthy substance, which is intimately mixed with the ground stone.
Several compositions of mingled earth may yield a true porcelain by being burnt, and the porcelains of various countries differ in their mixtures. But the principal basis of any true porcelain is that kind of clay which becomes white by baking, and which, either by intermingled heterogeneous earth, or by particular additions undergoes in the fire an incipient vitrification, in which the true nature of porcelain consists. Feldspar and gypsum, if added, may give that property to infusible clay.
When porcelain is to be made, the clay is properly selected, carefully washed from impurities and again dried. It is then finely sifted, and most accurately mingled with quartz, ground very fine, to which then is added some burnt and finely-pulverized gypsum. This mass is worked with water to a paste and duly kneaded; it is usually suffered to lie in this state for years. The vessels and other goods formed of this mass are first moderately burnt in earthen pots, to receive a certain degree of compactness and to be ready for glazing. The glazing consists of an easilymelted mixture of some species of earths, as the petrosilex or chert, fragments of porcelain and gypsum, which, when fused together, produce a crystalline or vitreous mass, which, after cooling, is very finely ground, and suspended in a sufficient quantity of water. Into this fluid the rough ware is dipped, by which the glazing matter is deposited uniformly on every part of its surface. After drying, each article is thoroughly baked or burned in the violent heat of the porcelain furnace. It is usual to decorate porcelain by paintings, for which purpose enamels or pastes, colored by metallic oxides, are used, so easy of fusion as to run in a heat less intense than that in which the glazing of the ware melts.
This is a kind of pottery made of sand and clay, and but slightly baked, so that it resists sudden applications of heat. Articles made of this are glazed with an enamel, composed of common salt, sand ground fine, oxide of lead, and oxide of tin. The use of the latter is to give opacity to the glaze.
The composition of the eastern or proper chinaware, according to accounts that have great marks of authenticity, is from two earths; one of which is, as was before mentioned, called petunse; the other a refractory earth, called kaolin.
The preparation of the petunse, or aluminous earth, is by pounding the stone till it is reduced to a very fine powder, and then washing it over to bring it to the most impalpable state, which is thus performed: After the stone is rendered as fine as it can be by pounding or grinding, the powder must be put into a large tub full of water, and, being stirred about, the upper part of the water must be laded out into another tub, by which means the finest particles of the powder will be carried into it. The water in the second tub must be then suffered to stand at rest till the powder be subsided, and as much as can be laded off clear must be put back into the first tub, and there being again stirred about, and loaded with a fresh quantity of the most subtle part of the powder, must be laded again into the second tub as before, and this must be repeated till none be left in the first tub but the grosser part of the stone, which, not being of a due fineness, must be again pounded, and treated as at first. The fine powder obtained in the second tub, must be then freed from the water, by lading off the clear part, and suffering what remains to exhale, till the matter becomes of the consistence of soft clay, when it will be fit to be commixed with the kaolin for use.
The kaolin is prepared in the same manner by washing over; but some specimens are so fine, that there is no occasion for this or any other purification.
From these two mixed together, the clay or paste is formed; but it is said that the proportion of the respective quantities is made to vary according to the intended goodness of the ware, the best being made from equal quantities, and the worst from two of the kaolin to one of the petunse.
The Saxon composition, of which the chinaware is formed, is greatly similar to that of the eastern. In the place of the petunse, a stone is used, which is improperly called in the German language, bleyspatt, or spar of lead. It is a stone of a very opposite nature, as spars are calcareous, and will, on calcining, become lime; on the other hand, this stone is of a vitreous nature. This spar is of a very hard texture, and of a light flesh-color, or pale whitish red. It is prepared by pounding and washing over, which may be done as above directed and it is then ready for compounding with the mica. The mica is employed in the Saxon composition for the other ingredients; and is likewise prepared by grinding and washing over, when it is not in a perfect and pure state, but when it is entirely clean, it may be tempered with the texture, thoroughly broken, and it will be of the consistence of soft clay.
The two kinds of earth being prepared in the state of a soft paste, they are to be incorporated and blended into one mass, which is done by rolling and stirring them well after they are in the same vessel, and then kneading them with the feet till they are thoroughly united. When the compound mass is formed, it is made into cakes, or square pieces, and put by layers into cases of wood or stone, which must be placed in a moist situation, and left for 2 or 3 months; during which time a kind of ferment enters into the mixture, by which the parts of the different matter combine and form a substance with new qualities, unknown while separate. This change shows itself upon the whole mass by a fetid smell, and a greenish or bluish color, and a tenacity like that of clay, or the argillaceous moistened earths. If the time of keeping the paste in this condition be prolonged to a year or more, it will further improve its qualities, but great care must be taken to prevent its becoming dry; to prevent which, there may be occasion to water it. When, however, the described qualities are found in the matter, it is fit for use, and vessels, etc., may be wrought of it without any other preparation, the case below excepted.
The following composition will produce wares, which will possess the properties of the true china, if judiciously managed.
Mix the best white sand, or calcined flints, finely powdered, 20 lbs.; of very white pearlash, 5 lbs.; of white calcined bones, 2 lbs. Temper the whole with the gum Arabic or senegal, dissolved in water.
This requires a considerable force and continuance of heat to bring it to perfection, but it will be very white and good when it is properly treated. Where mica can be obtained, it is preferable to calcined bones, and as it will form a kind of paste for working, a weaker gum-water will answer the purpose.
The furnace for this purpose may be constructed in the same manner as the potter's kilns usually are. The size of the furnace should be according to the quantity of ware required to be baked; but it must not be too small, lest the body of fire may not be sufficient to produce the requisite heat.
The caffettes, or coffins, to contain the pieces when placed in the furnace, are the most material utensils. They should be of good potter's clay, with a third of sand, and are generally made of a round form, with a flat bottom, the rim forming sides, being adapted to the height of the pieces to be inclosed.
The furnace and caffettes being prepared, the ware to be baked must be sorted in the caffettes in the most advantageous manner as to room, and as many caffettes must be set upon them as the furnace will conveniently contain, leaving space for the free passage of the fire betwixt the piles: take care to cover over the uppermost caffettes in each pile, then close the mouth of the furnace, and raise the fire so as to heat the caffetes red hot in every part, and keep them red hot for 12 or 14 hours. It is then to be extinguished, and the furnace left to cool gradually; and when little or no heat remains, the mouth may be opened, and the pieces taken out of the caffettes; when they will be in a condition to receive the glazing, or to be painted with such colors as are used under the glaze.
These require a very fine, tenacious, and refractory clay, which is either naturally of a perfectly white color, or, if it have somewhat of a gray cast will necessarily burn white. A clay of this kind must contain no calcareous or ferruginous earth, and must also be carefully deprived of any sand it may contain by washing. It ought to possess besides, the property of shrinking but little in the fire. If it should not prove sufficiently ductile, it may be meliorated by the admixture of another sort. Last of all, it is beaten, kneaded, ground, washed, and sifted, till it acquires the requisite degree of fineness and ductility. When, after this preparation, the clay has obtained a due degree of ductility, it is rolled out in small portions to the usual length of a pipe, perforated with the wire, and put, together with the wire, into a brass mould, rubbed over with oil, to give it its external form; after which it is fixed into a vice, and the hollow part of the head formed with a stopper. The pipes, thus brought into form, are cleared of the redundant clay that adheres to the seams, a rim or border is made round the head, they are then marked with an iron stamp upon the heel, and the surfaces smoothed and polished. When they are well dried, they are put into boxes, and baked in a furnace.
Take 26 parts of glass, 7 parts litharge, 3 parts nitre, 1 part arsenic, 1/2 part blue calx; either fritted in a glass oven or not.
Take 8 parts of red-lead, 3 parts of iron filings, 3 parts of calcined copper, and 2 parts of zaffre. This, when fused, will produce a brown-black; but if wanted a truer black color, the proportion of zaffre must be increased.
M. Hardsmith proposes the following in place of the ordinary lead glaze: Take boracic acid, 15 lbs.; calcareous spar, 5 lbs.; wood charcoal, 1 lb. Powder the mixture, and calcine to complete fusion; allow it to cool; powder again and apply it as the common lead glaze is applied.
Take 10 parts of glass, 2 parts lead, and 3 or 3 1/2 parts blue calx, as required.
Take 16 parts of glass, 5 parts lead, 1 part arsenic, 2 1/2 parts nitre.
Take 11 parts white frit to the whole of blue frit, and grind them together. Then take of the mica frit, 8 parts of the above, 5 parts flint, 13 parts Cornish stone, 23 parts lead, and 6 oz. common salt.
Take 60 parts of Cornish stone, 20 parts flint, and 120 parts white-lead. Stained with 1 oz. of smalts, as above.
Take 2 parts of litharge, 2 parts tin-ash, and 1 part antimony.
Take 15 parts of Cornish stone, 10 parts flint glass, 5 parts anica flint, 5 parts nitre, 5 parts borax, 1 part common salt, and 1 part sal soda; fritted in a glass oven. Then add 2 parts frit, a. above, to 1 part white-lead. Send to mill to grind very fine, and stain with 7 oz. blue calx.
Take 20 lbs. of white frit, 10 lbs. flint, 26 lbs. stone, 50 lbs. lead, and 4 oz. of blue.
Take 6 parts of white frit, 5 parts flint, 13 parts stone, 25 parts lead, and 55 parts glass.
Take 100 parts of lead, 18 parts flint, and 40 parts manganese.
Take 1/4 oz. of fluxed blue, 1 oz. manganese, 1 oz. red-lead, and 1 oz. flint.
Take 1 qt. of yellow slip, to 1 oz. zaffre.
Take 8 oz. of glass antimony, 16 oz. litharge, 3 oz. manganese, and 4 drs. blue calx.
Take 42 parts of flint-glass, 3 oz. blue calx. Stain. 16 oz. flint-glass, 1 oz. red-lead, 1 oz. arsenic, and 1 oz. nitre. - White enamel. Run down in glass oven, then send with the above stain to the mill, 8 parts of white enamel, dry it and it will be fit for use. Eight parts of the above mixture (stain and white enamel), 6 parts dry flint, 14 parts Cornish stone, 24 parts white stone, which, when sifted, is fit for use.
Take 27 parts of flint, 15 parts nitre, 4 1/2 parts lime, 3 1/2 parts stain. This run down in a glass oven, and, when sent to the mill, add 75 parts of glass, 15 parts lead, 10 parts white enamel, add 2 pailsful of lime, and when it comes from the mill, add 135 parts of lead. Stain to the above, 10 parts of glass, and 5 oz. of blue.
Take 7 oz. of arsenic, 12 oz. potash, 6 oz. nitre, 5 oz. glass, 2 oz. flint, and 3 oz. white-lead.
Take 56 parts of stone, 46 parts borax, 18 parts glass, 15 parts flint, and 40 parts lead.
Take 20 parts of lead, 60 parts stone, 20 parts flint, and 10 parts ground glass.
Take 25 parts of flint, 60 parts stone, 95 parts lead, and 8 parts frit.
Take 175 parts of lead, 100 parts stone, and 35 parts flint.
Take 5 parts of blue calx, 5 1/2 parts coke stone, 1 1/2 parts glass, and 1 part flint.
Trike 7 1/2 parts of flint-glass, 2 1/2 parts redlead, and 2 parts borax.
Take 5 parts of lead, 1 oz. of borax, and 12 oz. of glass.
Take 1 part of calcined copper, 1 1/4 parts red flux. Passed through the enamel kiln, 1 3/4 of calx, sent to the mill for grinding.
Take 1 lb. of calcined copper, pounded fine, and put into the enamel kiln, and it will come out black. Then 1 1/2 oz. of red flux, put through the enamel kiln, second time; then 1 of the above, and 1 3/4 of flux, ground fine for use.
Take green copperas calcined to a fine powder, wash it well 10 or 12 days, and dry it (colcothar); 1 of the above to 6 of red flux.
Take 5 oz. of umber, 2 oz. borax, 1 oz. blue calx. One of the above to 2 flux, as under; 7 1/2 flint-glass, 2 1/2 red-lead, and 2 borax.
Take 3 oz. of calcined umber, 1 oz. borax; run down together. This will fine with gold.
Take 1/2 pt. of linseed-oil, boiled well until of a proper consistence, to which add a small quantity of Barbadoes tar, prepared the same way.
Another. - Take 1 qt. of linseed-oil, 4 oz. flowers of sulphur, 4 oz. balsam of sulphur, 8 oz. black rosin.
Take 2 parts blue clay, 2 parts china clay, 4 parts composition.
Take blue clay, 30 parts; black marl, 5 parts; calcined car, 25 parts; manganese, 2 parts.
Take of white sand 40 lbs.; red-lead, 20 lbs.; pearlash, 20 lbs.; common salt, 12 lbs. Powder this sand by grinding before it be mixed with the other ingredients, and then grind them together, after which, calcine them for some time with a moderate heat. which must be less than will make them melt and run to glass, and when the mixture is cold, grind it to powder again, and, when wanted, temper it with water, and it will then be fit for use.
The proportions of these ingredients may be varied occasionally, for, where the glazing can be fluxed conveniantly with a very strong fire, the quantity of sand may be increased to 60 or 70 lbs., which not only renders the glazing stronger, but makes a saving in the expense. The proportion of pearl-ashes may likewise be diminished, or they may be wholly omitted where the ware is designed for very coarse purposes, and not for domestic uses, where the lead is very improper, being extremely apt to be corroded by acids, and to produce a very unwholesome substance. On this account where good manufactories are established, the lead ought to be excluded from the composition of the glazings, and other fluxes used in its stead, as in the following:
Take of white sand, 40 lbs.; of pearlash, 21 lbs.; and of common salt, 15 lbs. Calcine, and proceed as above.
Where the expense is no object, this glazing may be improved by adding 1 or 2 lbs. of borax, and diminishing the pearlash, in the proportion of 6 lbs for 1 lb. of borax added, or 10 lbs. for 2; in the latter case, 2 lbs. of salt may be also kept out of the composition. The reason for this change is, that if the composition contain so large a proportion of salt, and the glazing be not fluxed for a long time after it is laid on the ware, it will be apt to be dissolved by boiling water, and peel off, if it be exposed to the action of it for any long time.
Another. - Take of sand, 40 lbs.; of wood-ashes, perfectly burnt, 50 lbs.; of pearlash, 10 lbs.; of common salt, 12 lbs
This will make an admirable glazing, where the ashes arc pure, and a strong fire can be given to flux it when laid on the ware. It will be perfectly free from the imperfection of the above, and will be very hard and glossy, and where the expense can be afforded, it may be made more yielding to the fire by the addition of borax, in which case no alteration need be made in the proportion of the other ingredients.
Take of clean sand, 1 cwt., of soda, 44 lbs., and pearlash, 30 lbs. Calcine the mixture.
Take of masticot, prepared as in the preceding, 100 lbs.; calx of tin, 80 lbs., and of common salt, 10 lbs. Calcine and powder this composition three several times.
The calx of tin is prepared and sold under the name of putty. Its goodness consists in its whiteness and purity; the first of which is easily known by comparing it with a specimen of any that is known to be good.
Take of mastic, 10 lbs.; red lead, 60 lbs.; calcined tin or putty, 20 lbs.; and of common salt, 10 lbs. Mix them, and calcine and powder the mixture several times.
Another. - Take 2 lbs. of lead, and somewhat more than 1 lb. of tin. Calcine the two metals till reduced to a powder, by the means used by potters. Then take 2 parts of these ashes, 1 part of white sand, calcined flints or broken white glass and 1/2 pint of common salt. Mix well together the several ingredients, and set the matter to bake in a proper furnace, and urge it at length to melt.
The trouble of calcining the tin and lead may be saved here, as well as on the occasions above-mentioned, by procuring them already reduced to a proper state.
Another. - Take 1 1/2 lbs. of lead and 1 lb. of tin.
Reduce them to the state of a calx, and then take of the calcined matter, 8 parts, and of calcined flints and common salt, each 4 parts. Bring the mixture, by heat, to a state of fusion.
Another. - Take of lead, 3 parts, and of tin, 1 part. Calcine them, and then take of this matter and of calcined flints and common salt, each, 2 parts. Fuse them as above.
Another. - Take of lead, 4 lbs.; tin, 1 lb. Calcine them, and take of the matter 8 parts; of calcined flints, 7 parts, and of common salt, 4 parts. Fuse them as the others.
Take of lead, 4 lbs.; of tin, 1 lb.; of flints, 4 lbs.; of common salt, 1 lb.; and of flint-glass, 1 lb. Melt the mixture, and it will be fit for use.
Another. - Take of lead, 4 lbs., and of tin, 1 lb Calcine them, and take of the matter, 12 parts; of flints, 14 parts; and of common salt, 8 parts.
Fuse them as the others.
Take of lead, 2 parts, and of tin, 1 part. Calcine them, and take of the matter, 1 part; of flints and common salt, each, 1 part. Fuse the mixture.
Take of tin, any quantity, and enclose it in clay or loam and put it in a crucible. Place the crucible in a fire, that the tin may calcine, and then break it. There will be a pound of calx very white, and when it is used to paint with on a white ground, the color will come forth and be much whiter than that of the ground.
Take of tin and antimony, each 2 lbs.; of lead 3 lbs.; or, according to some, equal quantities of all the three ingredients. Calcine the whole, and put them at last in fusion, that they may be vitrified. This glazing will run very soon, and be of a fine yellow color.
The calcining the tin, lead, and antimony together, as here directed, would be a very tedious operation. The calcined tin and red-lead should therefore be used, and the antimony calcined alone. But it is not to be understood that the antimony is to be calcined for this purpose to whiteness, or the state of a perfect calx; which is not easily practicable without nitre, and, if effected, would render the antimony incapable of producing any other color than white. The operation must therefore be performed with a slow fire, by roasting, as it were, the antimony till it loses its metallic appearance and becomes a greenish powder; as is practiced in making the glass of antimony.
Another. - Take 5 parts of red-lead, 2 parts of powdered bark, 1 part of sand, 1 part of any of the preceding white glazings, and 2 parts of antimony. This mixture must be calcined and then fused, and it will give a fine yellow glazing.
Another. - Take 7 parts of the mixture of the calxes of tin and lead, mentioned before in the recipe for preparing the masticot for a white glazing. Add 1 part of antimony, and fuse them together.
Anther. - Take 4 parts of white glass, 1 part of antimony, 3 parts of red-lead, and 1 part of iron scales. Fuse the mixture.
Another. - Take 16 parts of flints, 1 part of filings of iron, and 24 parts of litharge. Fuse the mixture.
Take of red-lead, 3 parts; of powdered bricks very red, 3 1/2 parts; and of antimony, 1 part. Calcine the mixture day and night, for the space of 4 days, in the ashhole of a glass-house furnace. Urge it at last to fusion, and it will produce a very fine lemon colored glazing.
The success of this operation depends greatly on the fineness of the color of the bricks that are powdered. Those which are of a fine red and very brittle, are the best; but such as are gray will not at all answer the end. The same attention should be paid to this matter wherever bricks are used in this kind of preparations.
Take of red-lead, 4 parts; of antimony, 3 parts; of the mixture of the calxes of lead and tin, before mentioned in the masticot for white glazing, 8 parts; and of glass, 3 parts.
When the red lead and calx of tin are used, the proportion of the ingredients will be of red-lead, 10 parts; of antimony and glass, each, 3 parts; and of calcined tin, 2 parts.
Take of red-lead, 3 parts; of antimony, 2 parts; of colcothar, 1 part. Fuse the mixture; and, having powdered the mass, melt it again, and repeat this operation till the fourth time, and a fine gold-colored yellow will be produced.
Any preparation of the calcined iron may be used in the place of the colcothar; and the repeated fusions and levigations seem unnecessary.
Another. - Take of red lead and white flints, each, 12 parts; and of filings of iron, 1 part. Fuse them twice.
This glazing will be transparent. Care must therefore be taken what ground it be laid upon or it will not answer the end of a yellow, but combine with that of the ground, and, indeed, the body of color is too weak to produce any other than a faint yellowish cast, even on a pure white ground.
Take of calcined copper 1 part, and 2 parts of any of the preceding yellow glazings. Fuse them twice, but when the composition is used it must not be laid on too thick, for that would render the color too deep.
Take of red-lead, 1 lb.; powdered flints, 2 lbs.; common salt, 2 lbs.; tartar, 1 lb. Calcine till it is almost white. White flint-glass, 1/2 lb., and zaffre, 1/2 lb. Fuse the whole mixture, and quench the melted mass in water. Repeat the same operation several times. The same proceeding must be adhered to in all the compositions where the tartar enters, otherwise they would be too much charged with salt, and the color would not prove fine. It is proper, moreover, to calcine the mixture gently, day and night, for 48 hours, in a glass-house furnace.
Another. - Take 1 lb. of tartar, 1/4 lb. of red-lead, 1/2 oz. of zaffre; and 1/4 lb. of powdered flints. Fuse the whole, and proceed in the manner stated above.
Take 12 parts of tartar and an equal quantity of flints and zaffre. Proceed as with the above.
Another. - Take 4 oz. of tartar, 2 oz. of red-lead, 5 oz. of powdered flints, and 1/2 dr. of magnesia. Proceed as with the above.
Take 3 lbs. of antimony, 3 lbs. of red-lead, and 1 lb. of colcothar. Grind the whole as fine as possible, and then paint with it.
Another. - Take 2 lbs. of antimony, 3 lbs. of redlead, and 1 lb. of colcothar. Proceed as with the above.
Melt and keep in fusion for 14 minutes a mixture of 1 oz. of fire-stone and glass, 2 drs. of salt, 1/2 oz. of pipeclay, and 1 1/2 oz. of borax. Varnish the pots over with this matter, after they have been in the fire, and put them again in it for about 18 hours.
This varnish is made of equal parts of white glass and soda, finely pulverized, carefully sifted, and mixed.
They take the finest pieces of the petunse and treat them as before mentioned, by pounding and washing over, but extract, by repeated washings over, the very finest part of the powder, which keeps so moist with the water that the mixture forms a liquid mass, which they call the oil of petunse. With this oil they mix an equal weight of borax; they then slake a quantity of quicklime and form layers of that and dried furze, which they set on fire. When they have raised a large heap, after the first one is burnt to ashes, they collect them and the lime, and form layers of them again, with a fresh quantity-of the furze, which they burn as before, and they repeat this 5 or 6 times. They then put the ashes and lime into a vessel with water, adding some borax in the proportion of 1 lb. to 1 cwt. of the ashes; they next wash over the finer part of this mixture, and pour off at last all fluid from the dregs, which they keep together with the solid part, washed over. They mix this composition of lime, ashes, and salts with the mixture above mentioned, of an equal quantity of the oil of petunse and borax, and this compound forms the matter for glazing the ware.
Instead of the petunse, the quartz used in the Saxon manufacture may be employed for forming a similar glazing by treating it in the same manner, and it is said the glazing of the Dresden China is actually made in this way.
Take of the finest white sand, or calcined flints, 20 lbs.; red-lead, 18 lbs.; pearlash, 10 lbs.; and common salt, decrepitated, 4 lbs. Levigate the sand or calcined flints and red-lead well together and afterwards mix them thoroughly with the pearlash and common salt. Fuse the compound in the manner directed for the treatment of glass, till it be perfectly vitrified; then separate the fragments of the pot carefully from it, and reduce it in a flat agate or porphyry mortar to an impalpable powder. Finally, temper it with water to the proper consistence for painting or glazing.
When this glazing is used for embossed or other fine work it should be mixed with a third of its weight of the spar of lead, or other vitrescent earth, in lieu of the petunse, in the composition of the ware paste. Take care that this earth is formed of the best pieces of spar or other substance used, and that it is rendered to an extreme fineness, by washing it over. The design of this addition is to weaken the fluxing powder of the glaze, which, if used alone, would run the corners and edges of the smaller part and impair the sharpness and spirit of the work. It is necessary to pursue the same method with pieces that are to be painted with more delicate designs; for the glazing, melting otherwise again, in the burning in of the colors would become too fluid, and spread them so as to take away the effect of the fine touches.
Herborizations can be of all colors: but the most agreeable is that called bistre, which is composed in the following manner:
A pound of calcined manganese, 6 oz. of burnt iron straw or 1 lb of iron ore, and 3 oz. of flint powder.
The manganese and straw or iron ore must be pounded separately in a mortar, after which the whole is calcined together in an earthen pot. This mixture, thus prepared, is all pounded together, and then mixed in a small tub of water.
The blue, green, and other colors must be composed of the divers substances known to produce them, and mixed, calcined, and pounded in the same manner as for the bistres.
To make the application of these various colors to the pieces it is necessary, instead of diluting them with water, as is practiced for ordinary painting, to make use of any kind of mordant, The most advantageous, and which are employed with the greatest success, are urine, and the essence of tobacco.
If the essence of tobacco is made use of, infuse 2 oz. of good tobacco, in leaves, during 12 hours, in a bottle of cold water, or very simply infuse the 2 oz. of tobacco in a bottle of hot water.
The pieces of clay, after taking a little consistency, are steeped in white or colored wormseed until the bath puts them in a state of moisture. To produce herborizations it will be sufficient, whilst the wormseed is still fresh, and at the moment when the piece is taken down from the tub to lay on slightly, and with a brush, one or several, drops of other colors. Each drop produces a tree more or less great, according as the workman has charged his brush with colors.
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