Part 12

8. Before the clay, thus purified from extraneous and coarser particles, is formed into vessels, it is beaten with a stout piece of wood, until the mass has become of an equal consistence throughout, and then repeatedly cut into two pieces with a wire, and slapped together to expel the air. The former of these operations is called _wedging_, and the latter, _slapping_.

9. _White and cream-colored wares_ are made of clays which contain so little oxyde of iron, that it does not turn red in burning, but, on the contrary, improves in whiteness in the furnace. There are several species of white clay, found in many different localities, most of which, however, are known under the denomination of _pipe-clay_; or they are distinguished by the names of the places where they are obtained.

10. In preparing these clays for use, they are reduced to a minute division by machinery, and afterwards dissolved in water, and otherwise treated in a manner similar to that used for the better kinds of common wares, as described in the seventh and eighth paragraphs. For the purpose of diminishing the shrinkage in the fire, and with the view of increasing the whiteness of the ware, pulverized flint-stone is added to the clay, in the proportion of about one part of the former to five of the latter.

11. In reducing the silex to the requisite fineness, it is first brought to a red heat; and, while in this state, it is thrown into cold water, to diminish the cohesion of its parts. It is then pounded by machinery, levigated with water in a mill, sifted, mashed, and otherwise treated like the clay. The materials are mixed while in a state of thin pulp.

12. The several operations performed by the potter, in converting the clay thus prepared into different kinds of vessels, and in completing the whole process of the manufacture of earthen ware, may be included under the following divisions, viz., throwing, turning, pressing, burning, painting and printing, and glazing. They are not, however, all used in producing and finishing vessels of every shape and quality.

13. _Throwing._--This operation is performed on a potter's wheel, which consists of a round table, and some simple means to put it in motion. The clay having been placed on the centre of this machine, the workman communicates to the latter a rotary motion with his foot, and gives the proposed form to the material with his hands, which have been previously wet with water, to prevent them from sticking. This method is used for all vessels and parts of vessels of a circular form; and, in many cases, no other operation is necessary to give them the requisite finish, so far as their conformation is concerned.

14. _Turning._--The vessels are cut from the thrower's wheel with a small wire; and when, by the evaporation of moisture, they have become firm enough to endure the operation, they are turned on a lathe. The objects of this operation are to communicate to them a more exact shape, and to render them more uniform in thickness. The potter's wheel, with the addition of some contrivance to hold the pieces in a proper position, is frequently used for turning. The coarser kinds of common wares are never turned.

15. _Pressing._--Vessels, or parts of vessels, which are of an irregular shape, and which cannot be formed on the wheel, are usually made by a process called _pressing_. This kind of work is executed in moulds made of plaster of Paris, and these are formed on models of clay or wood, which have been made in the exact shape of the proposed vessel. Sometimes individual specimens of the wares of one country or pottery are used as models in another; in such cases, the expense of the moulds is considerably diminished.

16. The moulds frequently consist of several parts, which fit accurately together; for example, the mould for a pitcher is composed of two pieces for the sides, and one for the bottom. In forming a pitcher in such a mould, the material, which has been spread out to a proper and uniform thickness, is laid upon the inside of each portion of it, and the superfluous clay is trimmed off with a knife. The mould is then closed, and thin strips of clay are laid over the seams; the removal of the several pieces of the mould, completes the operation.

17. Handles, spouts, figures in relief, and other additions of this nature, are separately made in moulds, and stuck on the vessel with the same kind of materials, sometimes mingled with a small proportion of plaster of Paris. These appendages are added after the vessels have become partially solid in the air.

18. _Burning._--All vessels, even after they have been dried in the atmosphere, are in a very frangible state; and, to render them sufficiently firm for use, they are submitted to the process of burning in a kiln. To preserve the ware from injury while enduring the fire, the several pieces are enclosed in cylindrical boxes called _saggers_, which are made of baked clay. These boxes are placed one above another around the sides of the kiln, which is of a circular form, and gradually tapering to the top.

19. In burning the coarser wares, every piece is not thus inclosed; but, between every two saggers, a naked piece is placed. A moderate fire is first raised, which is gradually increased, until the contents of the kiln are brought to a red heat. The burning occupies between twenty-four and forty-eight hours. All wares, except the coarsest kinds, are twice, and sometimes thrice, burned; and, after having been once submitted to the process, they are said to be in a state of _biscuit_.

20. _Painting and printing._--When the vessels are to be ornamented with colors, it is necessary, in most cases, that this part of the work be done after the first burning. In China, and at the porcelain manufactory in Philadelphia, the drawings are executed by hand with a pencil. The same method is used in Europe in elaborate pieces of workmanship. But, in the common figured wares, where but one color is used, the designs are first engraved on metallic plates, and impressions are taken from them on thin paper, by means of a copperplate printing-press.

21. In transferring to vessels designs thus produced, the paper, while in a damp state, is applied closely to the surface of the biscuit, and rubbed on with a piece of flannel. The porosity of the earthen material causes the immediate absorption of the coloring matter, which, in all cases, is some metallic oxyde. For a blue color, the oxyde of cobalt is used; and for a black, those of manganese and iron. The paper is washed from the ware with a sponge.

22. _Glazing._--To prevent the penetration of fluids, and to improve the appearance of the ware, a superficial vitreous coating is necessary. This can be produced by the aid of various substances; but, in a majority of cases, red lead is the basis of the mixture employed for this purpose. Equal parts of ground flints and red lead are used for the common cream-colored wares. These materials are mixed with, and suspended in, water, and each piece is dipped in the liquid. The moisture is soon absorbed by the clay, leaving the glazing particles on the surface, which, in the burning that follows, is converted into a uniform and durable vitreous coating.

23. _Stone ware._--The materials of this ware, as well as the mode of preparing them, differ but little from those of the common and better kinds of earthen wares. The clays, however, which contain but little or no oxyde of iron are chosen, since this substance would cause the ware to melt and warp, before a sufficient degree of heat could be applied to give it the requisite hardness.

24. The glazing is formed by a vitrification of the surface of the vessels, caused by the action of common salt thrown into the kiln, when it has been raised to its greatest heat. This glazing is more perfect than that on ordinary earthen wares, being insoluble by most chemical agents. It is hardly necessary to remark that this method of glazing precludes the use of saggers.

25. _Porcelain._--This ware exceeds every other kind in the delicacy of its texture, and is peculiarly distinguished by a beautiful semi-transparency, which is conspicuous when held against the light. In China, it is made chiefly of two kinds of earth; one of which is denominated _petuntze_, and the other _kaolin_; but both are varieties of feldspar, found in the mountains, in different localities. They are brought to the manufactories from a distance in the form of bricks; the materials, as taken from the mines, having been reduced to an impalpable powder in mortars, either by the labor of men or by water-power.

26. These materials are combined in different proportions in the manufacture, according to the quality of the proposed ware. In the best kind, equal quantities are used; but for those of inferior quality, a greater proportion of petuntze is employed. The translucency so much admired in porcelain, or _tseki_, as the Chinese call it, is owing to the petuntze, which, in burning, partially melts, and envelops the infusible kaolin.

27. It is not known who was the inventor of porcelain, as the Chinese annals are silent with regard to this point; nor do we know more of the date at which the manufacture was commenced. It is certain, however, that it must have been before the fifth century of the Christian era. Since this ware has been known to Europeans, it has been manufactured chiefly, and in the greatest perfection, in the large and populous village of King-te-ching.

28. Porcelain was first brought to Europe from Japan and China, and for a long time its materials and mode of manufacture remained a secret, in spite of the efforts of the Jesuit missionaries, who resided in those countries. At length, in 1712, Father Entrecolles sent home to France, specimens of petuntze and kaolin, together with a summary description of the process of the manufacture.

29. Shortly after this important event had transpired, it was discovered that materials nearly of the same kind existed in abundance in various parts of Europe. The manufacture of porcelain was, therefore, soon commenced in several places; and it has since been successfully carried on.

30. The porcelain wares of Europe are superior to those of the Chinese, in the variety and elegance of their forms, as well as in the beauty of the designs executed upon them; but, as some of the processes successfully practised in China, remain still to be learned by the Europeans, the Oriental porcelain has not yet been equalled in the hardness, strength, and durability of its body, and in the permanency of its glaze. The manufacturers of Saxony are said to have been the most successful in their imitations in these respects.

31. The porcelain earths are found in various parts of the United States, but particularly at Wilmington, in the state of Delaware. Nevertheless, there is now but one porcelain manufactory in our country, and this is yet in its infancy. The establishment is located in Philadelphia, and it has been lately incorporated, with the privilege of one hundred thousand dollars capital.

32. The principle of induration by heat, is the same in the manufacture of earthen wares as in making bricks; and, as the latter can be more easily dispensed with than the former in a primitive state of society, it is but reasonable to suppose that earthen ware was first invented; but the art of making bricks must have been practised before the deluge, or the posterity of Noah would not have attempted so soon as about one hundred years after that catastrophe, to build a city and a tower of these materials. It is, therefore, evident, that this art was of antediluvian origin; and it was probably one of the earliest brought to any degree of perfection.

33. The art of the potter was practised more or less by every nation of antiquity, and the degree of perfection to which it was carried in every country corresponded with the state of the arts generally. The Greeks were consequently very celebrated for their earthen wares. The Etruscans have also been particularly noted for their manufacture of the elegant vases which have been dug, in modern times, from the depositories of the dead, in Lower Italy.

34. Until the commencement of the manufacture of porcelain in Europe, this art continued in a very rude condition, although practised to a considerable extent in many places. It was much improved in England about the year 1720, by the addition of flints to the usual material; and, between thirty and forty years after this, it was brought to great perfection, in all its branches, chiefly through the scientific exertions of the celebrated potter, Josiah Wedgewood.

THE GLASS-BLOWER.

1. Glass is a substance produced from a combination of silicious earths with alkalies, and, in many cases, with metallic oxydes. The basis of every species of glass is silex, which is found in a state nearly pure in the sands of many situations. It is also found in the common flints and quartz pebbles.

2. When quartz pebbles or flints are employed, they must be first reduced to powder. This is done by grinding them in a mill, after they have been partially reduced, by heating them in the fire, and plunging them into cold water. Sand has the advantage of being already in a state of division sufficiently minute for the purpose. To prepare it for application, it only requires to be washed and sifted, in order to free it from the argillaceous and other substances unfit for use. A great proportion of the sand employed in the manufacture of the better kinds of glass in the United States, is taken from the banks of the Delaware River.

3. The alkaline substances used are potash and soda. For the finer kinds of glass, pearlash, or soda procured by decomposing sea-salt, is used; but, for the inferior sorts, impure alkalies, such as barilla, Scotch and Irish kelp, and even wood-ashes, as well as the refuse of the soap-boiler's kettle, are made to answer the purpose. Lime, borax, and common salt, are also frequently used as a flux in aid of some of the other substances just mentioned.

4. Of the metallic oxydes which make a part of the materials of some glass, the deutoxyde of lead, or, as it is usually denominated, red lead, is the most common. This substance is employed in making flint glass, which is rendered by it more fusible, heavy and tough, and more easy to be ground or cut, while, at the same time, it increases its brilliancy and refractive power.

5. Black oxyde of manganese is also used in small quantities, with the view of rendering the glass more colorless and transparent. Common nitre produces the same effect. White arsenic is also added to the materials of this kind of glass, to promote its clearness; but, if too much is used, it communicates a milky whiteness. The use of this substance in drinking vessels is not free from danger, when the glass contains so much alkali as to render any part of it soluble in acids.

6. The furnace in which the materials are melted is a large conical stack, such as is represented at the head of this article. In some cases, it is surrounded by a large chimney, which extends above the roof of the building. In the sides are several apertures, near which are placed the crucibles, or melting-pots, containing the materials. The fuel is applied in an arch, which is considerably lower than the surface of the ground on which the operators stand, while at work.

7. The melting-pots are made chiefly of the most refractory clays and sand. Much of the clay used for this purpose, in many of the glass-houses in the United States, is imported from Germany. The materials, having been sifted, and mixed with a suitable quantity of water, the homogeneous mass is formed into crucibles, by spreading it on the inside of vessels which are much in the shape of a common wash-tub. After the clay has become sufficiently solid to sustain itself, the hoops are removed from the vessel, and the several staves taken apart.

8. The crucibles are suffered to dry in the atmosphere for two or three months, after which they are applied to use as they may be needed. Before they are placed in the main furnace, they are gradually raised to an intense heat in one of smaller dimensions, built for this express purpose. The fuel employed in fusing the _metal_ is chiefly pine wood, which, in all cases, is previously dried in a large oven. Four of the five furnaces near Philadelphia, which belonged to Doctor Dyott, were heated with rosin.

9. The materials having been mixed, in the proposed proportions, which are determined by weight, they are thrown into the melting-pots, and, by a gradually increasing heat, reduced to a paste, suitable for application by the blower. This part of the process is commonly performed at night, while the blowers are absent from the works.

10. The applications of glass are so exceedingly extensive, that it is inconvenient, if not impossible, to manufacture every species of it at one glass-house or at one establishment. Some, therefore, confine their attention to the production of window glass, and such articles of hollow ware as may be made, with profit, from the same kind of paste. Others make vials and other species of ware, employed by the druggist, apothecary, and chemist. And again, the efforts, at some factories, are confined entirely to the manufacture of flint glass, or to that of plate glass for mirrors.

11. The principal operations connected with the manufacture of different species of glass, after the paste has been prepared, may be included under the following heads; viz., blowing, casting, moulding, pressing and grinding; although all these are never performed in one and the same establishment.

12. _Blowing._--The operation of blowing is nearly or quite the same in the production of every species of glass ware, in which it is employed. The manipulations, however, connected with making different articles, are considerably varied, to suit their particular conformation. This circumstance renders it impossible for us to give more than a general outline of the process of this manufacture.

13. In the formation of window glass, the workman gathers upon the end of an iron tube a sufficient amount of the metal, which he brings to a cylindrical form by rolling it upon a cast iron or stone table. He then blows through the tube with considerable force, and thus expands the glass to the form of an inflated bladder. The inflation is assisted by the heat, which causes the air and moisture of the breath to expand with great power.

14. Whenever the glass has become too stiff, by cooling, for inflation, it is again softened by holding it in the blaze of the fuel, and the blowing is repeated, until the globe has been expanded to the requisite thinness. Another workman next receives it at the other end, upon an iron rod, called a _punt_, or _punting iron_, when the blowing iron is detached. It is now opened, and spread into a smooth sheet, by the centrifugal force acquired by the rapid whirl given to it, in the manner exhibited in the preceding cut. The sheet thus produced is of a uniform thickness, except at the centre, where the iron rod had been attached.

15. An inferior kind of window glass, the materials of which are sand, kelp, and soap-boilers' waste, is made by blowing the _metal_ into cones, about a foot in diameter at their base; and these, while hot, are touched on one side with a cold iron dipped in water. This produces a crack, which runs through the whole length of the cone. The glass then expands into a sheet somewhat resembling a fan. This is supposed to be the oldest method of manufacturing window or plate glass.

16. The window glass produced in the manner first described, is called _crown glass_; and the other, _broad glass_. But by neither of these methods can the largest panes be produced. The blowing for these differs from the methods just described, in that the material is blown into an irregular cylinder, open at its further end. When a sufficient number of these cylinders have accumulated, the end to which the blowing iron had been attached, is _capped off_ by drawing round it a circle of melted glass, and the cylinder is divided longitudinally by touching it through its whole length with a hot iron. The cylinders, in this state, are put into the annealing oven, where, by aid of a heat which raises the glass to redness, it is expanded into sheets. These sheets are then broken into panes of several sizes by the aid of a diamond and a straight edge, as in the case of glass blown by other methods.

17. _Casting._--Plate glass formed by the method last mentioned, is denominated _cylinder glass_; and it is used not only for windows, but also for mirrors not exceeding four feet in length. Plates of greater dimensions are produced by a process called _casting_. The casting is performed by pouring the material, in a high state of fusion, upon a table of polished copper of large size, and having a rim elevated above its general surface, as high as the proposed plate is to be thick. To spread the glass perfectly, and to render the two surfaces parallel, a heavy roller of polished copper, resting upon the rim at the edges, is passed over it.

18. Plates thus cast are always dull and uneven. To render them good reflectors, it is necessary to grind and polish them. The plate to be polished is first cemented with plaster of Paris to a table of wood or stone. A quantity of wet sand, emery, or pulverized flints, is spread upon it, and another glass plate, similarly cemented to a wooden or stone surface, is placed upon it. The two plates are then rubbed together, until their surfaces have become plane and smooth. The last polish is given by colcothar and putty. Both sides are polished in the same manner.

19. _Moulding._--Ornamental forms and letters are produced on the external surface of vessels, by means of metallic moulds; and the process by which this kind of work is performed is called moulding. In the execution, the workman gathers upon the end of his iron tube, a proper amount of the material, which he extends, and brings to a cylindrical form, by rolling it upon his table. He then expands it a little by a slight blast, and afterwards lets it down into the mould, which is immediately filled by blowing still stronger through the tube.

20. The vessel is then taken from the mould, and disengaged from the tube. The same tube, or a punting iron having been attached to the bottom, the other end is softened in the fire, and brought to the proposed form with appropriate tools, while the iron is rolled up and down upon the long arms of the glass-blower's chair. The ornamental moulds are made of cast iron, brass, or copper, and are composed of two parts, which open and shut upon hinges. The moulds for plain vials, castor oil bottles, small demijohns, &c., are made of the kind of clay used for the crucibles. These consist merely of a mass of the clay, with a cylindrical hole in it of proper diameter and depth.

21. _Pressing._--This process is applied in the production of vessels or articles which are very thick, and which are not contracted at the top. The operation is performed in iron moulds, which consist of two parts, and which have upon their internal surfaces the figures to be impressed upon the glass. The material, while in an elastic condition, is put into the lower part of the mould; and the other part, called the _follower_, is immediately brought upon it with considerable force.

22. Every species of glass, before it can be used with safety, must be _annealed_, to diminish its brittleness. The annealing consists merely in letting down the temperature by degrees. Small boys, therefore, convey the articles, whatever they may be, as fast as they are made, to a moderately heated oven, which, when filled, is suffered to cool by degrees.