The Methods of Glass Blowing and of Working Silica in the Oxy-Gas Flame / For the use of chemical and physical students

by W. A. Shenstone

Fig. 12.

Piercing Tubes.—The glass-blower very frequently requires to make a large or small opening in some part of a tube or other piece of apparatus. This is known as piercing. Suppose it is desired to make a small hole at the point a in A ([Fig. 12]). When the tube has been brought to the flame with the usual precautions, allow the end of the pointed flame to touch it at a till an area corresponding to the desired size of the opening is thoroughly softened. Then expand the softened glass by blowing to the form shown at B. Re-heat a, blow a small globe as at C, and carefully break the thin glass, then smooth the rough edges by rotating them in the flame till they form a mouth like that of D. Instead of leaving the bulb to be broken at the third stage C, it is a good plan to blow more strongly, so that the bulb becomes very thin and bursts, the removal of the thin glass is then accompanied by less risk of producing a crack in the thicker parts of the glass. Openings may be made in a similar manner in the sides of tubes or in globes, in fact, in almost any position on glass apparatus. If another tube is to be attached at the opening, it is a good plan to proceed to this operation before the tube has cooled down.

Fig. 13.

The openings obtained by the method above described are too large when platinum wires are to be sealed into them. Suppose that it is necessary to pierce the tube A of [Fig. 13] in order to insert a platinum wire at a; direct the smallest pointed flame that will heat a spot of glass to redness on the point a. When the glass is viscous, touch it with the end of a platinum wire w, to which the glass will adhere; withdraw the wire and the viscous glass will be drawn out into a small tube, as shown at B; by breaking the end of this tube a small opening will be made. Introduce a platinum wire into the opening, and again allow the flame to play on the glass at that point; it will melt and close round the wire. Before the hot glass has time to cool, blow gently into the mouth of the tube to produce a slightly curved surface, then heat the neighbouring parts of the tube till the glass is about to soften, and let it cool in cotton wool. Unless this is done, I find that glass tubes into which platinum wires have been sealed are very apt to break during or after cooling.

To ensure that the tube shall be perfectly air-tight, a small piece of white enamel should be attached to the glass at a before sealing in the wire.

Uniting Pieces of Glass to Each Other, known as Welding, or Soldering.—The larger and more complicated pieces of glass apparatus are usually made in separate sections, and completed by joining together the several parts. This is therefore a very important operation, and should be thoroughly mastered before proceeding to further work.

In order to produce secure joints, the use of tubes made of different kinds of glass must be avoided. Soda glass may be joined securely to soda glass, especially if the tubes belong to the same batch, and lead glass to lead glass. But, though by special care a joint between lead glass and soda glass, if well made, will often hold together, yet it is never certain that it will do so.

To join two Tubes of Equal Diameters.—Close one end of one of the tubes with a small cork. Heat the open end of the closed tube, and either end of the other tube in a small flame until they are almost melted, taking care that only the ends of the tubes are heated, and not to let the glass be thickened; bring the two ends together with sufficient pressure to make them adhere, but not sufficient to compress the glass to a thickened ring. Before the joint has time to cool too much, adjust your blow-pipe for a pointed flame, if you are not already working with that kind of flame, and allow the point of the flame to play on any spot on the joint till it is heated to redness; rotate the tube a little so as to heat the glass adjacent to that which is already red-hot, and repeat this till the whole circumference of the rough joint has been heated.[7] Repeat the operation last described, but, when each spot is red-hot, blow gently into the open end of the tube so as to slightly expand the viscous glass. Finally, rotate the whole joint in the flame till the glass is softened, and blow gently as before into the open end of the tube, still rotating it, in order that the joint may be as symmetrical as possible. If in the last operation the diameter of the joint becomes greater than that of the rest of the tube, it may be cautiously re-heated and reduced by pulling it out, or this may be secured by gently pulling apart the two ends, whilst the operator blows it into its final shape.