A handbook of laboratory glass-blowing

by Bernard D. Bolas

Now bring the tube into the blowpipe flame gradually, and rotate it, while heating, at the place where it is to be closed. Allow the glass to soften and commence to run together until the diameter of the tube is reduced to about half its original size. Remove from the flame and draw the ends apart, this should give a long, thick extension as shown by f, Fig. 4. If any liquid is to be introduced, it may now be done by inserting a thin rubber or other tube through the opening and running the liquid in. A glass tube should be used with caution for introducing the liquid, as any hard substance will tend to scratch the inside of the glass and cause cracking. The final closure is made by melting the drawn-out extension in the blowpipe flame; the finished seal being shown by g, Fig. 4.

If the sealed tube has to stand internal pressure, it is desirable to allow the glass to thicken somewhat more before drawing out, and the bottom seal should also be made thicker. For such a tube, and especially when it has to stand heating, as in a Carius determination of chlorine, each seal should be cooled very slowly by rotating it in a gas flame until the surface is covered with a thick layer of soot, and it should then be placed aside in a position where the hot glass will not come in contact with anything, and where it will be screened from all draughts.

Joining Tube.—We will now consider the various forms of join in glass tubing which are met with in the laboratory. First, as being easiest, we will deal with the end-to-end joining of two tubes of similar glass. a, b, and c, Fig. 5, illustrate this. One end of one of the tubes should be closed, a lip should be turned out on each of the ends to be joined, and both lips heated simultaneously until the glass is thoroughly soft. Now bring the lips together gently, until they are in contact at all points and there are no places at which air can escape; remove from the flame, and blow slowly and very cautiously until the joint is expanded as shown in b, Fig. 5. Reheat in the flame until the glass has run down to rather less than the original diameter of the tube, and give a final shaping by re-blowing. The chief factors of success in making such a join are, thorough heating of the glass before bringing the two tubes together, and avoidance of hard or sudden blowing when expanding the joint. The finished work is shown by c, Fig. 5.

Fig. 5

To join a small glass tube to the end of a large one, the large tube should first be sealed, a small spot on the extreme end of the seal heated, and air pressure used to expand the heated spot as shown in d. This expanded spot is then re-heated and blown out until it bursts as shown in e, the thin fragments of glass are removed and the end of the small tube turned out as shown in f. After this the procedure is similar to that used in jointing two tubes of equal size.

When these two forms of joint have been mastered, a T piece will present but little difficulty. It is made in three stages as shown in Fig. 5, and the procedure is similar to that used in joining a large and small tube. Care should be taken to avoid softening the top of the "T" too much, or the glass will bend and distort the finished work; although a slight bend can be rectified by re-heating and bending back. Local re-heating is often useful in giving the joint its final shape.

An exhaustion branch is often made by a totally different method. This method is shown by g, h, and i, Fig. 5; g is the tube on which the branch is to be made. The end of a rod of similar glass should be heated until a mass of thoroughly liquid glass has collected, as shown, and at the same time a spot should be heated on that part of the tube where it is desired to make the branch. The mass of hot glass on the rod is now brought in contact with the heated spot on the tube and expanded by blowing as shown by h. The air pressure in the tube is still maintained while the rod is drawn away as shown by i. This will give a hollow branch which may be cut off at any desired point, and is then ready for connection to the vacuum pump.

If the rod used is of a dissimilar glass, the branch should be blown much thinner. Such a branch will often serve as a useful basis for joining two tubes of different composition, as the ordinary type of branch is more liable to crack when made with two glasses having different coefficients of expansion.

Blowing Bulbs.—A bulb may be blown on a closed tube such as that shown by c, Fig. 5, by rotating it in the blowpipe flame until the end is softened, removing it from the flame and blowing cautiously. It is desirable to continue the rotation during blowing. In the case of a very small tube, it is sufficient to melt the end without previous sealing, rotate it in the flame until enough glass has collected, remove from the flame and blow while keeping the tube in rotation.