Fig. 7.
The exact condition in which the glass is most suitable for bending can only be learned by making a few trials. If it is too soft in consequence of being overheated, the sides will collapse. If, in the endeavour to heat the side A of [Fig. 7] a little more than B, B is insufficiently heated, the tube will be likely to break on the convex side B. If the bent tube be likely to become flattened, and this cannot always be prevented in bending very thin tubes, the fault may be avoided by blowing gently into one end of the tube whilst bending it, for which purpose the other end should be closed beforehand. A tube already flattened may, to some extent, be blown into shape after closing one end and re-heating the bent portion, but it is not easy to give it a really good shape.
When making a bend like that in [Fig. 7], to secure that the arms of the tube C and D, and the curve at B, shall be in one plane, the tube should be held in a position perpendicular to the body, and brought into the position shown in the figure during bending, by which means it will be found easy to secure a good result. Lead glass tubes must be removed from the flame before they become hot enough to undergo reduction. If they should become blackened, however, the stain may be removed by re-heating in the oxidising flame (see [p. 18]).
When a very sharp bend is to be made, it is sometimes best to heat a narrow zone of the glass rather highly in the blow-pipe flame, and to blow the bend into shape at the moment of bending it, as previously described, one end having been closed for that purpose beforehand. Lead glass should be heated for this purpose in the oxidising flame ([pp. 17] to [22]).
The processes of bending large tubes, making U-tubes and spiral tubes, are more difficult operations, and will be explained in [Chap. IV].
Rounding and Bordering the Ends of Tubes.—After cutting a piece of glass tube in two pieces, the sharp edges left at its ends should be rounded by holding them in a flame for a few moments till the glass begins to melt. The oxidising point of a pointed flame may be used for both kinds of glass. The flame will be coloured yellow by soda glass at the moment of melting. This indication of the condition of soda glass should be noted, for it serves as a criterion of the condition of the glass. The ends of soda glass tubes may also be rounded in the flame of a common Bunsen’s burner.
When the end of a tube is to be closed with a cork or stopper, its mouth should be expanded a little, or bordered. To do this, heat the end of the tube by rotating it in the flame till it softens, then remove it from the flame, at once introduce the charcoal cone ([Fig. 5], [p. 11]), and rotate it with gentle pressure against the softened glass till the desired effect is produced. In doing this it is very important that the end of the tube shall be uniformly heated, in order that the enlargement shall be of regular form. If the tube cannot be sufficiently expanded at one operation, it should be re-heated and the process repeated.
Borders, such as are seen on test-tubes, are made by pressing the softened edge of the tube against a small iron rod. The end of the rod should project over the softened edge of the tube at a slight angle, and be pressed against it, passing the rod round the tube, or rotating the tube under the rod.
Sealing, that is closing the ends of tubes, or other openings, in glass apparatus.