A handbook of laboratory glass-blowing

by Bernard D. Bolas

Easy Examples of Laboratory Glass-Blowing—Cutting and Sealing Tubes for Various Purposes; Test-Tubes, Pressure-Tubes, Tubes for High Temperature Experiments—Thermometer-Bulbs, Bulbs of Special Glass, Pipettes, Absorption-Bulbs or Washing-Bulbs—Joining Tubes; Branches, Exhaustion-Branches, Branches of Dissimilar Glass—Blowing Bulbs; A Thistle Funnel; Cracking and Breaking Glass; Leading and Direction of Cracks—Use of Glass Rod or Strips of Window-Glass; Joining Rod, Feet and Supports—Gripping Devices for use in Corrosive Solutions—The Building Up of Special Forms from Solid Glass.

Perhaps the most common need of the glass-blower whose work is connected with that of the laboratory is for a sealed tube; and the sealing of a tube is an excellent preliminary exercise in glass-blowing.

We will assume that the student has adjusted the blowpipe to give a flame similar to that shown in d, Fig. 1, and that he has learned to maintain a steady blast of air with the bellows; further, we will assume that the tube he wishes to seal is of moderate size, say not more than half an inch in diameter and with walls of from one-tenth to one-fifth of an inch thick.

Fig. 4

A convenient length of tube for the first trial is about one foot; this should be cut off from the longer piece, in which it is usually supplied, as follows:—lay the tube on a flat surface and make a deep cut with the edge of a file. Do not "saw" the file to and fro over the glass. If the file edge has been ground as shown in a, Fig. 3, such a procedure will be quite unnecessary and only involve undue wear; one movement with sufficient pressure to make the file "bite" will give a deep cut. Now rotate the tube through about one-eighth of a turn and make another cut in continuation of the first. Take the tube in the hands, as shown in a, Fig. 4, and apply pressure with the thumbs, at the same time straining at the ends. The tube should break easily. If it does not, do not strain too hard, as it may shatter and cause serious injuries to the hands, but repeat the operation with the file and so deepen the original cuts. In holding a tube for breaking, it is important to place the hands as shown in sketch, as this method is least likely to cause shattering and also minimises the risk of injury even if the tube should shatter. To cut a large tube, or one having very thick walls, it is better to avoid straining altogether and to break by applying a small bead of intensely heated glass to the file cut. If the walls are very thin, a glass-blower's knife should be used instead of a file. The tube and glass-blower's knife should be held in the hand, and the tube rotated against the edge of the knife; this will not produce a deep cut, but is less likely to break the tube. A bead of hot glass should be used to complete the work.

The next operation is to heat the glass tube in the middle; this must be done gradually and evenly; that is to say the tube must be rotated during heating and held some considerable distance in front of the flame at first; otherwise the outer surface of the glass will expand before the interior is affected and the tube will break. From two to five minutes, heating at a distance of about eight inches in front of the flame will be found sufficient in most cases, and another minute should be taken in bringing the tube into the flame. Gradual heating is important, but even heating is still more important and this can only be obtained by uniform and steady rotation. Until the student can rotate a tube steadily without thinking about it, real progress in glass-blowing is impossible.

When the tube is in the flame it must be held just in front of the blue cone and rotated until the glass is soft enough to permit the ends to be drawn apart. Continue to separate the ends and, at the same time, move the tube very slightly along its own axis, so that the flame tends to play a little more on the thicker part than on the drawn-out portion. If this is done carefully, the drawn-out portion can be separated off, leaving only a slight "bleb" on the portion it is desired to seal. This is illustrated by b, Fig. 4.

To convert the seal at b, Fig. 4., into the ordinary form of test-tube seal, it is only necessary to heat the "bleb" a little more strongly, blow gently into the tube until the thick portion is slightly expanded, re-heat the whole of the rounded end until it is beginning to collapse, and give a final shaping by careful blowing after it has commenced to cool. In each case the glass must be removed from the flame before blowing. The finished seal is shown by c, Fig. 4. If desired, the open end may now be finished by heating and rotating the soft glass against the large turn-pin, as illustrated in d, but the turn-pin must not be allowed to become too hot, as if this happens it will stick to the glass. After turning out the end, the lip of glass must be heated to redness and allowed to cool without coming in contact with anything; otherwise it will be in a condition of strain and liable to crack spontaneously. The finished test-tube is shown by e.

When it is necessary to seal a substance inside a glass tube, the bottom of the tube is first closed, as explained above, and allowed to cool; the substance, if a solid, is now introduced, but should not come to within less than two inches of the point where the second seal is to be made. If the substance is a liquid it can more conveniently be introduced at a later stage.