then washed, first with dilute muriatic acid, and afterwards with clean water.

M. Lançon, who has made several experiments on this subject, recommends the following proportions of the materials: litharge, one hundred grains; white sand, seventy-five grains; and pure carbonate of potash, ten grains. According to M. Doualt Wieland, an excellent strass may be obtained by either of the following recipes:

M. Wieland observes, that both the lustre and density of the glass are increased by a tranquil and prolonged fusion, and hence recommends that the materials should remain in a melted state for twentyfour hours. The colouring metallic oxides proper for combining with this glass will be noticed in another section of the present article.*

* The vitreous basis preferred by Fontanieu, in his treatise on coloured glasses and artificial gems, is prepared in the following manner : Eight ounces of pure rock crystal or flint, in powder, mixed with twentyfour ounces of carbonate of potash, are calcined and left to cool. This mixture is afterwards transferred to a basin of hot water, and treated

2. CROWN-GLASS.

Crown-glass, which is a silicate of soda and lime, as made in this country, is chiefly used for making windows. Being much more difficult of fusion, and therefore less easily worked, than flint-glass, it is not commonly fashioned, like the latter, into vessels of capacity, whether for use or ornament. Crown-glass is also much harder than flint-glass, and consequently not so well adapted as the latter for cutting and grinding.

The general arrangements of a crown-glass house are quite similar to those of a flint-glass house (fig. 1, page 36). The furnace containing the glass-pots, which is built in a rectangular form, is situated in the centre of the cone, with various smaller furnaces for secondary operations on each side; and the ashpit, called the “cave," runs across the building, as in the flint-glass house. The centre furnace contains

with dilute nitric acid till it ceases to effervesce; when the frit is to be washed until the water comes off tasteless. The frit is now dried and mixed with twelve ounces of fine white lead, and the mixture is levigated and elutriated with a little distilled water. An ounce of calcined borax is to be added to about twelve ounces of the preceding mixture in a dry state, the whole rubbed together in a porcelain mortar, then melted in a clean crucible and poured into cold water. This vitreous matter must be dried, and melted a second and a third time, always in a new crucible; and after each melting, poured into cold water as at first, taking care to separate the reduced lead. To the last glass, ground to powder, five drams of nitre are added; and the mixture being melted for the last time, a mass of crystal will be found in the crucible of a brilliant lustre. To the strass thus prepared M. Fontanieu applied the name of Mayence base. Another mixture, recommended by the same authority, is, eight ounces of white lead, two ounces of powdered borax, three ounces of rock crystal, and half a grain of manganese. (Dr. Ure's Dictionary of Arts, &c., art. Glass-making.)

four or six pots, each sufficiently large to hold half a ton or more of melted glass. The best stone for constructing those parts of the crown-glass furnace which are exposed immediately to the fire, is a firestone of very close grit, procured from the neighbourhood of Newcastle. Extreme nicety is required in the proper adjustment of the blocks, as no cement whatever is employed for the interior of the furnace. The duration of a crown-glass furnace is from eighteen months to three years.

A crown-glass house requires several subsidiary furnaces, adjoining or around the inside of the cone, besides the oven necessary for annealing the glasspots previous to their being set. Of these furnaces the first which comes into use is that called the "colcar,"* for calcining or "fritting" the materials previous to their being completely fused and vitrified. The form of the colcar is generally similar to that of an oven, the fire-place being on one side, so that the flame is enabled to reverberate from the crown of the colcar back to the materials on the sole. The area of this furnace may be from six to eight feet long, and from five to six feet wide, and the height in the middle about two feet. Four of the other subsidiary furnaces are for the purpose of softening the glass while it is being fashioned into a circular plate the first is called the "blowing furnace," to facilitate the blowing of the glass into a large globe; the second, the "bottoming hole;" the third, the "nose hole;" and the fourth, the "flashing furnace;" the use of each of which will be presently described. Another fire-place is that attached to the annealing

* A name corrupted from the French calquaise.

arch. The flashing furnace is represented in elevation in fig. 13, page 53, and the annealing arch in fig. 15.

The materials at present employed for making crown-glass in this country are fine sand, chalk, and either soda-ash (crude carbonate of soda), or salt-cake (dry sulphate of soda), mixed with a little charcoal, with small quantities of black oxide of manganese, arsenious acid, and occasionally oxide of cobalt, to correct any defects in the colour of the glass owing to the presence of oxide of iron. On some parts of the Continent, where carbonate of potash is as available as carbonate of soda, the alkaline constituent is either a mixture of potash and soda or potash only. The analysis, by M. Dumas, of a specimen of crown-glass of German manufacture, afforded results which correspond to one equivalent of neutral silicate of potash with one equivalent of neutral silicate of lime; or K O, Si O2+ Ca O, Si O2. For making crown-glass, either a portion or the whole of the alkali is generally introduced in the state of sulphate, a small proportion of charcoal being added to decompose the sulphuric acid.

2

In consequence of the fluctuating quality of sodaash, crown-glass manufacturers do not constantly adhere to any fixed proportions of the materials, and for the same reason are also under the necessity of determining, by analysis, the amount of real alkali in every fresh supply of ash before it is used on the large scale. The usual proportions employed when all the alkali is introduced in the form of soda-ash, are, 100 parts of quartz sand, 35 to 40 parts of chalk, a quantity

of soda-ash containing about 8 parts of soda, and from 150 to 200 parts of old broken glass or cullet. The proper proportions of arsenious acid and black oxide of manganese can be learned only by trial. The sand must first be calcined at a dull red heat in the colcar, and then be carefully sifted: the chalk is also calcined, but at a more moderate heat; the object of this operation being merely the thorough desiccation of the chalk, and not the expulsion of any portion of its carbonic acid. The sand and chalk having been thus dried and finely sifted, are thoroughly intermixed with the other materials, also dry and in a pulverulent state, and the mixture is ready to be shovelled at once into the glass-pots at a white heat.

Such is the method at present pursued in nearly all the crown-glass houses in this country. But formerly it was an universal practice to subject the mixture of the raw materials (neither of them having been previously heated) to a calcination in the colcar, or fritting furnace, where it was well stirred about with iron paddles and rakes. This practice is still followed, I believe, in some crown-glass houses.

The objects of this preliminary calcination, called the fritting, are, 1°, the expulsion of the water contained in the mixture, whether free or in chemical combination; 2°, the combustion and dissipation of the carbonaceous matters which may be accidentally present in the materials; 3°, the expulsion of carbonic acid from the chalk; and 4o, the union of the base of the alkaline carbonate with silicic acid, the carbonic acid being expelled. The last of these objects of the fritting process seems to be the most

VOL. II.

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