Nitre, the fourth ingredient, is also used as a flux, and serves to correct the imperfections that arise from the lead being insufficiently calcined. A fifth article, viz. arsenic, is sometimes used to aid the fluxing ingredients ; but the quantity must be very small, lest the transparency of the glass be hurt by the opake white colour which it imparts when used largely. A sixth, and very important ingredient, is the black oxide of manganese, which is used to destroy any accidental foulness of colour that may arise in the glass, and particularly any tinge given by particles of iron amongst the sand ; but while it destroys the green yellow, or olive colours in glass, by imparting to them a purple tinge, the mixture of colours produces a blacker glass, and of course greatly injures its transparency.
When too much manganese is added, the purple colour may be destroyed by charcoal ; but this can only be done partially, as the purple tinge is not visible until a perfect glass is formed, after which the charcoal cannot be mixed with the glass.
When a batch of flint glass is prepared, it is taken from the mixing-house to the glass-house, and then put into the pots in small quantities of ten or a dozen shovelfuls at a time. When this is melted, which it does in two or three hours, more is added till the pot is full. The mouth of the pot is then completely closed, by putting soft clay round the stopper, except a small aperture, which is left to allow the sandover or glass-gall to escape. This sub stance consists of those salts that are contained in alkalies purified in the common mode, and which have no affinity for silex, and are thrown up to the top, from the glass, or metal as it is termed, being hotter at the back of the pot than it is at the mouth or front. A small declivity takes place on the surface of the glass, and if the pot is full to the brim at the mouth, it will be a little higher at the back part, consequently the liquid sandover runs off. Flint glass requires about 48 hours to its complete fusion, although the furnace (Plate CCLXXV. Fig. 3. See Description of Plates) is carried to as intense a heat as possible. After it is fine, that is melted into liquid glass, and freed from all air-bubbles, preparation is made for the working of the glass. For this purpose the blower, in order to make a common wine glass, takes a hollow tube of iron about four feet and a half long, which he heats red hot at the one end. lie then dips it into the liquid metal contained in the pot, and takes up a quantity of glass, and forms a hol low ball, as described in Sect. III. He then sits down on
a chair with two long arms, to one of which there is a plate of iron fastened, to prevent the burning of the chair by the hot iron pipe and glass ; for the operation must he done with the greatest quickness while the metal is hot. He rests the pipe across the chair arms, and, while he rolls it back and forward with his left hand, he with an instrument similar to a small pair of tongs, catches the solid metal at the end of the hollow ball and draws it out, at the same tune giving it the shape required for the stalk of a wine n-lass • another blower is going on with the same process, and blowing a smaller ball ; and after giving it a sharp cut at the end of the blowpipe, he quickly presses it against the point of the stalk of the glass in the other man's hand, to which it readily adheres as firmly as if there had never been a joint ; and, by giving the pipe a smart stroke with a small piece of iron, it is separated from the small ball now attached to the stalk of the glass, which is instantly given to the finisher or workman, the others being called blowers or footmakers. This workman then makes the glass just hot enough to keep it from breaking, and sits down on his chair, and with a similar pair of tools, while rolling the pipe rapidly on the arms of the chair, he opens the ball at the stalk of the glass, and forms a foot. A boy then takes a small rod of iron, called a panty, and dips it into the metal in the pot, takes out on the extremity of the rod a small portion of glass, thrusts it immediately against the centre of the foot, to which it instantly unites. The workman then with a piece of iron, which he wets with his mouth, touches the globe intended for the bowl of the glass with the wet part, which is still very hot, although so much chilled as to retain its shape, and this in a second or two cracks it round ; and, by giving the pipe a gentle knock, it separates from it, and leaves an open uneven mouth, which the workman instantly heats, and with a pair of shears, clips the heated glass smooth and even in the mouth ; but as the shears have put the glass off that circular form, he heats it again, and by a dexterous twirl and swing round his head, he, if an expert workman, gives it the desired shape to a mathematical exactness, almost without the aid of any tools, The wine glass now finished and chilled a little, by giving the polity a smart blow, the glass separates from the iron, and is carried by a boy with a long forked iron to the seer, where it is placed in a pan already heated for the purpose of annealing.