GLASS 655 



finished tables ; 5, the reverboratory oven for annealing the pots prior to their being 

 sot upon the founding siege. 



Tho materials of crown-glass used to be, fine sand, by measure 5 parts, or by 

 weight 10 ; ground kelp, by measure 11 parts, or by weight 16 ; but instead of kelp, 

 soda-ash is now generally employed. From 6 to 8 cwts. of sand, lime, and soda-ash, 

 mixed together in wooden boxes with a shovel are thrown on the solo of a large rever- 

 berator}'. Here the mixture is well worked together with iron paddles, flat shovels, and 

 rakes with long handles ; the area of this furnace, being about 6 feet square, and the 

 height 2 feet. The heat soon brings the materials to a pasty consistence, when they 

 must be diligently turned over, to favour, the dissipation of the carbon, sulphur, and 

 other volatile matters of the kelp or soda-ash, and to incorporate the fixed ingredients 

 uniformly with the sand. Towards the end of 3 hours, the fire is considerably raised, 

 and when the fourth hour has expired, the fritting operation is finished. The mass 

 is now shovelled or raked out into shallow cast-iron square cases, smoothed down, and 

 divided before it hardens by cooling, into square lumps, by cross sections with the 

 spade. These frit-bricks are afterwards piled up in a large apartment for use ; and 

 have been supposed to improve with age, by the efflorescence of their saline constitu- 

 ents into carbonate of soda on their surface. 



The founding-pots are filled up with these blocks of frit, and the furnace is power- 

 fully urged by opening all the subterranean passages to its grate, and closing all the 

 doors and windows of the glass-house itself. After 8 or 10 hours the vitrification has 

 made such progress, and the blocks first introduced are so far melted down, that 

 another charge of frit can be thrown in, and thus the pot is fed with frit till the 

 proper quantity is used. In about 16 hours the vitrification of the frit has taken 

 place, and a considerable quantity, amounting often to the cwt. of liquid saline matter 

 floats over the glass. This salt is carefully skimmed off into iron pots with long 

 ladles. It is called Sandiver, or Glass-gall, and consists usually of chloride of sodium, 

 with a little sulphate. The pot is now ready for receiving the topping of cullet, which 

 is broken pieces of window- glass, to the amount of 3 or 4 cwts. This is shovelled in 

 at short intervals ; and as its pressure forces up the residuary saline matter, this is 

 removed ; for were it allowed to remain, the body of the glass would be materially 

 deteriorated. 



The heat is still continued for several hours till the glass is perfect, and the extri- 

 cation of gas called the boil, which accompanies the fusion of crown-glass, has nearly 

 terminated, when the fire is abated by shutting up the lower vault doors and every 

 avenue to the grate, in order that the glass may settle fine. At the end of about 40 

 hours altogether, the fire being slightly raised by adding some coals, and opening the 

 doors, the glass is carefully skimmed, and the working of the pots commences. 



Before describing it, however, we may 

 state that the marginal figure, 1093, 

 shows the base of the crown-house cone, 

 with the four open pots in two ranges, on 

 opposite sides of the furnace, sitting on 

 their raised sieges at each side of the 

 grate. At one side of the base the door 

 of the vault is shown, and its course is 

 marked by the dotted lines. 



The crown- glass furnace, figs. 1094, 1095, is an oblong square, built in the centre of a 

 brick cone, largo enough to contain within it two or three pots at each side of the 

 grate room, which is either divided as shown in the plan, or runs the whole length of 

 the furnace, as the manufacturer chooses. Fig. 1095 is a ground plan, and fig. 1094 a 

 front elevation of a six-pot furnace. 1, 2, 3, fig. 1094, are the working holes for the 

 purposes of ventilation, of putting in the materials, and taking out the metal to be 

 wrought. 4, 5, 6. 7, are pipe-holes for warming the pipes before beginning to work 

 with them. 8, 9, 10, are foot holes for mending the pots and sieges. 11 is a bar of iron 

 for binding the furnace, and keeping it from swelling. 



The arch is of an elliptic form ; though a barrel arch, that is, an arch shaped like 

 the half of a barrel cut longwise through the centre, is sometimes used. But this soon 

 gives way when used in the manufacture of crown-glass, although it does very well 

 in the clay-furnace used for bottle-houses. 



The best stone for building furnaces is fire-stone; it may be obtained in the 

 neighbourhood of Newcastle from the coal-measures generally, and some of the 

 sandstones of ^the eastern counties are found to answer the purpose admirably. The 

 great danger in building furnaces is, lest the cement at the top should give way with 

 the excessive heat, and by dropping into the pots, spoil the metal. The top should 

 therefore be built with stones only, as loose as they can hold together after the centres 

 are removed, and without any cement whatever. The stones expand and come quite 



