If the spot of reduced metal produced in the first experiment be next brought into the oxidising flame, it also may gradually be removed. On occasion, therefore, apparatus which has become stained with lead during its production, may be rendered presentable by suitable treatment in the oxidising flame. The process of re-oxidising a considerable surface in this way after it has cooled down is apt to be very tedious, however, and, especially in the case of thin tubes or bulbs, often is not practicable. In working with lead glass, therefore, any reduction that occurs should be removed by transferring the glass to the oxidising flame at once.
Small tubes, and small areas on larger tubes of English glass, may be softened without reduction by means of the pointed oxidising flame; but it is not easy to heat any considerable area of glass sufficiently with a pointed flame. And though it is possible, with care, to employ the hot space immediately in front of the visible end of an ordinary brush flame, which is rich in air, yet, in practice, it will not be found convenient to heat large masses of lead glass nor tubes of large size, to a sufficiently high temperature to get the glass into good condition for blowing, by presenting them to the common brush flame.
It may seem that as glass which has become stained with reduced lead can be subsequently re-oxidised by heating it with the tip of the pointed flame, the difficulty might be overcome by heating it for working in the brush flame, and subsequently oxidising the reduced lead. It is, however, difficult, as previously stated, to re-oxidise a large surface of glass which has been seriously reduced by the action of the reducing gases of the flame, after it has cooled. Moreover, there is this very serious objection, that if, as may be necessary, the action of the reducing flame be prolonged, the extensive reduction that takes place diminishes the tendency of the glass to acquire the proper degree of viscosity for working it, the glass becomes difficult to expand by blowing, seriously roughened on its surface, and often assumes a very brittle or rotten condition.
When it is only required to bend or draw out tubes of lead glass, they may be softened sufficiently by a smoky flame, which, probably owing to its having a comparatively low temperature, does not so readily reduce the lead as flames of higher temperature. But for making joints, collecting masses of glass for making bulbs, and in all cases where it is required that the glass shall be thoroughly softened, the smoky flame does not give good results.
In the glass-works, where large quantities of ornamental and other glass goods are made of lead or flint glass, the pots in which the glass is melted are so constructed that the gases of the furnace do not come into contact with the glass;[4] and as the intensely-heated sides of the melting-pot maintain a very high temperature within it by radiation, the workman has a very convenient source of heat to his hand,—he has, in fact, only to introduce the object, or that part of it which is to be softened, into the mouth of the melting-pot, and it is quickly heated sufficiently for his purpose, not only without contact of reducing gases, but in air. He can therefore easily work upon very large masses of glass. In a special case, such a source of heat might be devised by the amateur. Usually, however, the difficulty may be overcome without special apparatus. It is, in fact, only necessary to carry out the instructions given below to obtain a considerable brush flame rich in air, in which the lead glass can be worked, not only without discoloration, but with the greatest facility.
To Produce an Oxidising Brush Flame.—The blower used must be powerful, the air-tube of the blow-pipe must be about half as great in diameter as the outer tube which supplies the gas. The operator must work his bellows so as to supply a strong and steady blast of air, and the supply of gas must be regulated so that the brush flame produced is free from every sign of incomplete combustion,[5] which may be known by its outer zone being only faintly visible in daylight, and quite free from luminous streaks (see [Fig. 4], [p. 9]). When a suitable flame has been produced, try it by rotating a piece of lead glass at or near the end of the inner blue part of the flame (A [Fig. 4]); the appearance of the glass will quickly indicate reduction. When this occurs move the glass forward to the end of the outer zone B, but keep it sufficiently within the flame to maintain it at a high temperature. If all is right the metallic reduction will quickly disappear, the glass will become perfectly transparent once more, and will present the appearance previously observed in the experiments with the pointed flame, or, if very hot, assume a brownish-red appearance. If this does not occur, the supply of air must be increased or the supply of gas diminished until the proper effects are secured.
In working upon lead glass with the highly oxidising brush flame, it is a good plan to heat it in the reducing part of the flame A for thoroughly softening the glass, and to remove it to the oxidising flame B to burn away the reduced metal. In prolonged operations, in order that reduction may never go too far, hold the glass alternately in the hot reducing flame and in the oxidising flame. The inferiority of the outer oxidising flame to those portions nearer the inner blue zone for softening the glass, may perhaps be accounted for by the presence of a larger proportion of unconsumed air in the former, which being heated at the expense of the hot gases produced by combustion, thereby lowers the temperature of the flame. At or near A ([Fig. 4]) where the combustion is nearly complete, but no excess of air exists, the temperature will naturally be highest.
If a very large tube be rotated in the oxidising flame at B ([Fig. 4]) it may happen that the flame is not large enough to surround the tube, and that as it is rotated those parts of it which are most remote from the flame will cool down too considerably to allow all parts of the tube to be simultaneously brought into the desired condition. This difficulty may be overcome by placing two blow-pipes exactly opposite to each other, at such a distance that there is an interval of about an inch between the extremities of their flames, and rotating the tube between the two flames. It may be necessary to provide two blowers for the blow-pipes if they are large.
Again, if a very narrow zone of a tube of moderate size is to be heated, two pointed flames may be similarly arranged with advantage. Occasionally more than two flames are made to converge upon one tube in this manner.
Another method of preventing one side of a tube from cooling down whilst the other is presented to the flame, is to place a brick at a short distance from the extremity of the flame. The brick checks the loss of heat considerably. A block of beech wood may be used for the same purpose, the wood ignites and thereby itself becomes a source of heat, and is even more effective than a brick.