By continuing the heat to as great a degree as possible, in tne course of 12, 15, or 13 hours, these materials be come a perfect glass, fit for making wine bottles. The furnace is then reduced to a working heat, by shutting the cave doors, and excluding the air from the grating. The metal, as it cools in the pots, becomes more dense ; and all the heterogeneous matter that was contained in the ashes, and not melted into glass, floats on the top and is skimmed off. The furnace is then filled with coal, in such a way that it will retain what is termed working heat, for four or five hours, when it is again filled so as to•pre serve that degree of heat till the working journey is finish ed. It is impossible to give any correct idea of the pro cess of blowing to a person that never saw glass manufac tured. There are six people employed in the making of one bottle, independent of all the men employed in pre paring the materials, each performing a distinct part; and by that division of labour, they are enabled to make a very large quantity in a journey ; and although ten men and buys are busily working with long hot irons, and red hot glass metal in a liquid state, in a space not exceeding four square yards, yet such is their regularity in passing one another, and handing back and forward their work, which never fails to strike a stranger with terror, that it is very rarely any of them meet with an accident.
One workman, called a gatherer, dips the end of a tube, heated red hot, and about five feet long, into the pot con taining the metal, to which it readily adheres; and after it is cooled a little, lie again immerses the end of the tube so as to cover the metal ; and, by giving it a turn in his hand, lie is enabled to bring out of the pot as much as is required for a common wine bottle. Ile then hands it to the blower, and prepares another ; while the blower, by rolling the metal on a stone or plate, brings it to the end of the pipe or tube : he then holds it to a brass or cast iron mould, and, by blowing down through the tube, makes the glass, which is now getting cold, retain the shape giv en to it, which is that of a common wine bottle. It is then handed to the finisher, who, by means of a cold piece of iron with which he touches the neck while still red hot, but cold enough to retain its shape, cuts it off from the blow-pipe, as completely as if done by a diamond.
This species of glass consists of different ingredients, and is manufactured in a different manner from crown win dow glass. Its ingredients ale, soap boilet's waste 6 bu shels; kelp 3 do.; sand 4 do.
When these materials have been calcined for from 20 to 30 hours, they are removed with iron shovels, while red hot, to the melting furnace, when the pots are filled with it. Br exposure to the heat for 12 or 15 hours, the
whole is reduced to a fluid state. It is then taken out upon tubes in the manner described under Sect. 1V. and blown into globes of nearly a foot in diameter. These globes being carried to the mouth of the oven, a longitudinal and nearly rectilineal crack is produced, by touching it with a cold iron dipped in water. The globe is then opened on a smooth iron plate at the mouth of the furnace, and then forms a circular sheet of thin transparent window glass, See Parke's Essays.
The furnace for crown glass, represented in Plate CCLXXV. Fig. 2. (see description of Plates,) is general ly constructed for four or six pots of such a size as will contain from 16 to 20 cwts. of glass. There are also seve ral other furnaces required in this manufacture: A rever beratory furnace for calcining the materials; flashing fur nace, and bottoming hole, used for the purpose of heating the glass, in order to continue its flexibility till it acquires from the workman the desired shape, with several others called arches, which are used for the purpose of annealing the glass after it is made, and the pots previous to their being set into the furnace. The materials for crown glass, that is, the best window glass, are two parts of kelp to one of fine white sand ; these are the usual proportions ; but the quality of even the best kelp is extremely various, some vitrifying more and some less sand. From six to eight cut. of these materials, after they are well mixed, are put into a reverberatory furnace, of about six feet square, having an arch thrown over it of about two feet in height. On the one side is a grating to contain the fire, with an ash-pit beneath. The bottom of this furnace is raised about 34 feet high, so as to be more convenient for the workmen to turn the materials. As the neutral salt contained in the kelp when heated is extremely penetrat ing, and readily goes through common or even fire bricks, carrying along with it a considerable quantity of alkali, and thereby very much injuring the quality of the glass, various plans have been tried to prevent it, some by mak ing large bricks of fire clay, and others by placing at plate of iron so far under the floor of the furnace, as not to be much affected by the heat, from an idea that when the iron tub or pan is filled with the neutral salt, no more will he lost. Another plan, is to have flues for admitting a stream of cold air below the floor of the furnace, which cools it, and keeps the salt from running off. But it would un doubtedly be a much better plan to separate the alkali from all heterogeneous matter previous to mixing it with the sand.