Popular Technology Volume II Part 15

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.

[Ill.u.s.tration: GLa.s.s BLOWER.]

THE GLa.s.s-BLOWER.

1. Gla.s.s 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 gla.s.s 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 gla.s.s 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 gla.s.s, 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 gla.s.s, the deutoxyde of lead, or, as it is usually denominated, red lead, is the most common. This substance is employed in making flint gla.s.s, 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 quant.i.ties, with the view of rendering the gla.s.s more colorless and transparent. Common nitre produces the same effect. White a.r.s.enic is also added to the materials of this kind of gla.s.s, 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 gla.s.s 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 gla.s.s-houses in the United States, is imported from Germany. The materials, having been sifted, and mixed with a suitable quant.i.ty of water, the h.o.m.ogeneous ma.s.s 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 gla.s.s are so exceedingly extensive, that it is inconvenient, if not impossible, to manufacture every species of it at one gla.s.s-house or at one establishment. Some, therefore, confine their attention to the production of window gla.s.s, 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 gla.s.s, or to that of plate gla.s.s for mirrors.

11. The princ.i.p.al operations connected with the manufacture of different species of gla.s.s, 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 gla.s.s ware, in which it is employed. The manipulations, however, connected with making different articles, are considerably varied, to suit their particular conformation. This circ.u.mstance renders it impossible for us to give more than a general outline of the process of this manufacture.

13. In the formation of window gla.s.s, 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 gla.s.s to the form of an inflated bladder. The inflation is a.s.sisted by the heat, which causes the air and moisture of the breath to expand with great power.

14. Whenever the gla.s.s 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 gla.s.s, 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 gla.s.s then expands into a sheet somewhat resembling a fan. This is supposed to be the oldest method of manufacturing window or plate gla.s.s.

16. The window gla.s.s produced in the manner first described, is called _crown gla.s.s_; and the other, _broad gla.s.s_. 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 acc.u.mulated, the end to which the blowing iron had been attached, is _capped off_ by drawing round it a circle of melted gla.s.s, 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 gla.s.s 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 gla.s.s blown by other methods.

17. _Casting._--Plate gla.s.s formed by the method last mentioned, is denominated _cylinder gla.s.s_; 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 gla.s.s perfectly, and to render the two surfaces parallel, a heavy roller of polished copper, resting upon the rim at the edges, is pa.s.sed 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 quant.i.ty of wet sand, emery, or pulverized flints, is spread upon it, and another gla.s.s 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 gla.s.s-blower's chair. The ornamental moulds are made of cast iron, bra.s.s, 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 ma.s.s 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 gla.s.s. 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 gla.s.s, 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.

23. _Cutting._--The name of _cut gla.s.s_ is given to the kind which is ground and polished in figures, appearing as if cut with a sharp instrument. This operation is confined chiefly to flint gla.s.s, which, being more tough and soft than the other kinds, is more easily wrought. In addition to this, it is considerably more brilliant, producing specimens of greater l.u.s.tre.

24. An establishment for grinding gla.s.s contains a great number of wheels of cast iron, stone, and wood, of different sizes; and the process consists entirely in holding the gla.s.s against these, while they are revolving with rapidity. When a considerable portion of the material is to be removed, the grinding is commonly commenced on the iron wheel, on which is constantly pouring water and sharp sand, from a vessel above, which, from its shape, is called a _hopper_.

25. The period of the invention of gla.s.s is quite unknown; but the following is the usual story of its origin. Some merchants, driven by a storm upon the coasts of Phoenicia, near the River Belus, kindled a fire on the sand to cook their victuals, using as fuel some weeds which grew near. The ashes produced by the incineration of these plants, coming in contact with the sand, united with its particles, and, by the influence of the heat, produced gla.s.s.

26. This production was accidentally picked up by a Tyrian merchant, who, from its beauty and probable utility, was led to investigate the causes of its formation, and who, after many attempts, succeeded in the manufacture of gla.s.s. The legend probably originated in the fact, that gla.s.s was very anciently made at Tyre; and that the sand on the seash.o.r.e in the immediate neighbourhood of the Belus, was well adapted to gla.s.s-making.

27. It is certainly probable, that an accidental vitrification might have given rise to the discovery; but the circ.u.mstance would have been more likely to take place in some operation requiring greater heat than that necessary for dressing food in the open air. The invention of gla.s.s must have been effected as early as fifteen hundred years before our era. It was manufactured very anciently in Egypt; but whether that country or Phoenicia is ent.i.tled to the preference, as regards priority in the practice of this art, cannot be determined.

28. Gla.s.s was made in considerable perfection at Alexandria, and was thence supplied to the Romans as late as the first quarter of the second century. Before this time, however, Rome had her gla.s.s manufactories, to which a particular street was a.s.signed. The attention of the workmen was directed chiefly to the production of bottles and ornamental vases, specimens of which still remain, as monuments of their extraordinary skill.

29. In modern times, the manufacture of gla.s.s was confined princ.i.p.ally to Italy and Germany. Venice became particularly celebrated for the beauty of the material, and the skill of its workmen; and as early as the thirteenth century, it supplied the greatest part of the gla.s.s used in Europe. The artists of Bohemia, also, came to be held in considerable reputation.

30. The art was first practised in England, in the year 1557, when a manufactory was erected at Crutched Friars, in the city of London, and shortly afterwards, another at the Savoy, in the Strand. In these establishments, however, were made little else than common window gla.s.s, and coa.r.s.e bottles, all the finer articles being still imported from Venice. In 1673, the celebrated Duke of Buckingham brought workmen from Italy, and established a manufactory for casting plate gla.s.s for mirrors and coach windows. The art, in all its branches, is now extensively practised in great perfection, not only in Great Britain, but in many of the other kingdoms of Europe.

31. Before the commencement of the late war with England, very little, if any, gla.s.s was manufactured in the United States, except the most common window gla.s.s, and the most ordinary kinds of hollow ware.

Apothecaries' vials and bottles, as well as every other variety of the better kinds of gla.s.s wares, had been imported from Europe, and chiefly from England.

32. Our necessities, created by the event just mentioned, produced several manufactories, which, however, did not soon become flourishing, owing, at first, to inexperience, and, after the peace, to excessive importations. But adequate protection having been extended to this branch of our national industry, by the tariff of 1828, it is now in a highly prosperous condition--so much so, that importations of gla.s.s ware have nearly ceased.

[Ill.u.s.tration: OPTICIAN.]

THE OPTICIAN.

1. The word optician is applicable to persons who are particularly skilled in the science of vision, but especially to those who devote their attention to the manufacture of optical instruments, such as the spectacles, the camera obscura, the magic lantern, the telescope, the microscope, and the quadrant.

2. Light is an emanation from the sun and other luminous bodies, and is that substance which renders opaque bodies visible to the eye. It diverges in a direct line, unless interrupted by some obstacle, and its motion has been estimated at _two hundred thousand miles_ in a second.

3. A _ray of light_ is the motion of a single particle: and a parcel of rays pa.s.sing from a single point, is called _a pencil of rays_.

_Parallel rays_ are such as always move at the same distance from each other. Rays which continually approach each other, are said to _converge_; and when they continually recede from each other, they are said to _diverge_. The point at which converging rays meet is called the _focus_.

4. Any pellucid or transparent body, as air, water, and gla.s.s, which admits the free pa.s.sage of light, is called a _medium_. When rays, after having pa.s.sed through one medium, are bent out of their original course by entering another of different density, they are said to be _refracted_; and when they strike against a surface, and are sent back from it, they are said to be _reflected_.

5. A _lens_ is gla.s.s ground in such a form as to collect or disperse the rays of light which pa.s.s through it. These are of different shapes; and they have, therefore, received different appellations. A _plano-convex_ lens has one side flat, and the other convex; a _plano-concave_ lens is flat on one side, and concave on the other; a _double convex_ lens is convex on both sides; a _double concave_ lens is concave on both sides; a _meniscus_ is convex on one side, and concave on the other. By the following cut, the lenses are exhibited in the order in which they have been mentioned.