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ON THE CONSTRUCTION OF FURNACES, ETC.
STABILITY OF FURNACE ESSENTIAL-FRITTING FURNACE, OR
CALCAR. ITS USE. -- WORKING FURNACE, DOUBLE FURNACE. — PROPORTIONATE DIMENSIONS OF FURNACE AND POTS. WOOD-FURNACES. - COMPARATIVE CONSUMPTION OF FUEL IN WOOD AND COAL FURNACES, ANNEALING OVEN. —LIERPANS. — GLASS-POTS. -THEIR FORMATION AND SEASONING.
It is essential to the well conducting of the operations of glasshouses that their furnaces should be well and substantially built of the best materials, and according to the most approved construction.
Monsieur Loysel was so deeply impressed with the necessity of devoting the greatest degree of attention to this branch of the art, that nearly one half of his clever treatise “ Sur l'Art de la Verrerie" is occupied with its details. This author not only gives instructions for the choice of materials proper for constructing crucibles and furnaces; but also points out the forms which will be found most convenient and advantageous.
In this country, and since the appearance of the work of M. Loysel, those operations connected with the useful arts which depend upon the agency of heat have become much better understood than formerly, so that principles for the right attainment of which it was then necessary for individuals to search and enquire, and frequently even to experiment, are now become matters of every day practice. It cannot, therefore, be necessary to imitate that author in all his lengthened initiatory directions for the digging and purifying of clay - in his description of the qualities that should determine our selection of this material, or in the detail of mechanical arrangements that will prove most efficacious in giving
durability to furnaces.* All these particulars may safely be committed to the skilfulness of the professional builder.
Some other points there are, however, connected with this subject, wherein a greater degree of knowledge than is at present possessed is indeed desirable. To persons who have bestowed the smallest attention upon the phenomena of heat and combustion it can scarcely fail to appear that science has yet an ample field for research there open to her investigations, and that the arts have still to look for further and most important benefits at the hand of philosophy.
The whole operations of the glasshouse depend upon the stability of its furnaces. In their original structure the prudent manufacturer will, therefore, 'not hesitate to avail himself of the assistance of the ablest builders, and to employ materials which are best qualified by their density and infusibility for resisting the action of violent and long continued heat. No present saving in the cost of construction can at all compensate for the expense and interruptions occasioned by the necessity for frequent repairs, and still less for the losses of time, labour, and materials, that would accompany casualties, the chances of which would be multiplied by want of proper attention to the matters here pointed out. An equal, and per. haps even a greater degree of judgment is required for the selection of materials proper to form glass-pots or crucibles, since these have to withstand the action of heat in as violent a degree as any part of the furnace, and are additionally exposed to injury from the solvent pro. perty possessed by some of the materials, for the fusion of which they are employed. These pots are therefore made of the most refractory, that is, the least fusible, materials, and are fashioned with every possible attention to their strength,
* In stating the degrees of consistence of the clay most proper for the furnaces in different parts and stages of their construction, Mr. Loysel points out a simple method of ascertaining the attainments of these degrees, which has the advantage of being easily accomplished by any workman. It consists in dropping a leaden ball of a given weight from a determinate height upon the mass, which should be of that density which will allow the ball, by the force of gravitation acquired in its descent, exactly to bury itself in the clay.
Three different kinds of furnaces are employed in the manufacture of glass. One, which is called the calcar, a name corrupted from the French word calquaise, is used for that calcination of the materials to which they must be subjected previously to their complete fusion and vitrification. This process, which is called fritting, a term likewise borrowed from the French, is used for various reasons. In the first place, it expels all moisture from the materials, the presence of which would hazard the destruction of the glass-pots. Next it drives off either wholly or in great part, the carbonic acid gas from the chalk and alkalies employed, by which means the swelling of ingredients in the pots is either prevented, or moderated within safe limits. This calcination has the further advantage of destroying all carbonaceous matters that may be present in the materials. But the principal object of previous calcination is, that a chemical union may be effected, or at least commenced, between the silex, the alkali, and the metallic oxides. Otherwise, at the heat of the working furnace, the alkali would fuse, and its comparative levity would cause it to take its station at the surface, while the other ingredients would subside towards the bottom. The uncombined alkali would, in this case, after acting upon and injuring the substance of the crucibles, . be, in great part, volatilised and lost ; and a portion of the sand would remain unvitrified, while the glass actually produced would contain an excessive quantity of silica.
These observations do not apply to the preparation of materials for making flint glass, the fusibility of which is much greater than that of other descriptions, owing to the presence of its large proportion of lead. For this reason, manufacturers of this kind of glass apply the process of calcination to the sand alone, with the view of separating from it all carbonaceous impurities, previous to its admixture with the remaining materials.
The name of fixed alkalies has been given to soda and potash to denote their property of resisting the destructive influence of fire. As they are volatilised
freely by subjection to a red heat, this property cannot, however, be strictly ascribed to either of these substances, and the title must be understood only as distinguishing them from other alkaline bodies, which are acted upon by comparatively low additions of temperature.
The calcar is in the form of an oven about ten feet long, seven feet wide, and two feet high. The coal used in heating it is placed in a sort of trough on one side, and the flame is made to reverberate from the crown of the oven back to the frit. In this operation, care is required to keep the degree of heat at all times within that which would melt the materials, and at first below the point whereat the yet uncombined alkali would be volatilised.
The process must not be hurried at first; but after two or three hours the temperature may be gradually raised until the mass becomes pasty. Having been kept in this state during three or four hours longer, it is then removed from the calcar and cut quickly, before it has time to harden, into square cakes. These are piled away for future use. It is the opinion of many glass-makers that frit is improved in quality by age ; under which impression some among them so manage their operations as not to bring their store successively to use until it has been prepared for at least twelve months.
The working furnace is that wherein the frit, when placed in the glass-pots or crucibles, is fully melted and converted into glass. These crucibles are deep pots, varying in size according to the objects of the manufacturer, but sometimes large enough for each to contain a ton weight of glass. Twelve of these are usually placed, at regular intervals, in the circumference of each kiln, their only opening being at the side nearest to the wall of the kiln, in which they meet with corresponding openings, so that the pots can be readily charged, and their contents as readily removed, by the workmen who stand in recesses formed by projections of the masonry. The external form of the furnace is circular, rising
conically from its base and terminating in a chimney. The interior is dome-shaped, and supported on arches. Flues are constructed under these for the admission of atmospheric air, which, rising through the fire bars that occupy the centre of the floor of the furnace, the flame and heated air are made to envelope the pots, and thence pass on to the chimney issuing from the centre of the dome.
A very important saving in the article of fuel has lately been effected in one of the London glasshouses, that of Messrs. Pellatt and Co., by substituting for one of the large twelve-pot furnaces they had been accustomed to use two similarly constructed, but having only one half the diameter. The circumference of these two being together equal to that of the large furnace, they are made to contain as great a number of pots, and of the same size, as were formerly employed; while the internal area of the two, being together equal to only one half that of the furnace for which they are substituted, the pots are necessarily brought more than before within the intensest influence of the fire. The same chimney is