GENERAL OPERATIONS
=Cutting the Glass.=--For this purpose a "glass-knife" is preferred to a
file, if the glass is cold: if it is hot a file must always be used, and
its edge slightly moistened to prevent drawing the temper. The
glass-knife is simply a flat piece of hard steel, with the edges ground
sharp on an emery wheel. The bevel of the edge should be from 30 to 60
degrees. An old flat file can easily be ground into a suitable knife.
The glass-knife makes a narrower scratch than the file but appears more
likely to start the minute crack which is to cause the tube to break at
that point, and the break is more likely to give a good square end. The
scratch should be made by passing part of the knife or file once across
the glass, never by "sawing" the tool back and forth. This latter
procedure dulls the tool very quickly.
In breaking a piece of glass tubing, many persons forget that it is
necessary to _pull_ the ends apart, as well as to bend the tube very
_slightly_ in such a direction as to open up the minute crack started in
the scratch. Care in breaking the tube is essential, as it is impossible
to do as good work with uneven ends as with square ones.
When tubing of large diameter or thin wall is to be cut, it is often
better not to attempt to break it in the usual way, but to heat a very
small globule of glass (1/16 to 1/8 inch diameter) to red heat, and
touch it to the scratch. This will usually start the crack around the
tube; if it has not proceeded far enough, or has not gone in the
desired direction, it may be led along with a hot point of glass. This
is put a little beyond the end of the crack, and as the latter grows out
toward it, moved along the path where the crack is desired. This point
of glass is also very useful in breaking off very short ends of tubes,
where there is not room to get a firm enough hold and sufficient
leverage to break the tube in the ordinary way, and for breaking tubes
attached to large or heavy objects, which would be likely to make
trouble if treated in the ordinary way.
Another way of cutting large tubing, especially if it has rather thick
walls, is to make a scratch in the usual way, and then turn on the
smallest and sharpest possible flame of the blast lamp. The tube is next
taken in both hands and held horizontally above the flame so that the
scratch is exactly over it. The tubing is now rotated rapidly about its
axis, and lowered so that the flame is just tangent to its lower side.
After about ten seconds of heating, it is removed from the flame and the
hot portion quickly breathed upon, when it will generally crack apart
very nicely. Care must be taken to hold the tube at right angles to the
flame during the heating, and to rotate it so that only a narrow strip
of the circumference is heated, and the scratch should be in the center
of this heated strip. By this means tubing as large as two inches in
diameter is readily broken.
Griffin's glass cutter, which contains a hardened steel wheel, like that
on any ordinary window-glass cutter, and a device by which this can be
made to make a true cut clear around the tube, is a very handy article,
especially for large tubing, and may be obtained from any dealers in
chemical apparatus.
=Bending Glass.=--Inasmuch as this is one of the commonest operations in
the laboratory, it is assumed that the reader knows how to perform it.
However, it should be noted that in order to obtain the best results a
broad (fish-tail burner) flame should generally be used, and the tube
rotated on its axis during the heating, and allowed to bend mostly by
its own weight. If large tubing is to be bent, one end must be stoppered
and great care used. Whenever the tube shows signs of collapsing or
becoming deformed, it must be gently blown out into shape, heating the
desired spot locally if necessary. A blast-lamp is likely to be more
useful here than the fish-tail burner.
=Drawing Out a Tube.=--Most students learn this the first day of their
laboratory work in chemistry, but few take pains to do it well. The tube
should be heated in the flame of a Bunsen burner, or blast lamp
(preferably the latter) until it is very soft. During this time it must
be continuously rotated about its axis, and so held that the edges of
the heated zone are sharply defined; _i.e._, it should not be allowed to
move back and forth along its own axis. When so hot that it cannot
longer be held in shape, the tube is removed from the flame, and the
ends slowly and regularly drawn apart, _continuing the rotation of the
tube about its axis_. By regulating the rate of drawing and the length
of tube heated, the desired length and diameter of capillary may be
obtained. The tube should always be rotated and kept in a straight line
until the glass has set, so that the capillary may have the same axis as
the main tube. This capillary or "tail" is often a very necessary handle
in glass-blowing, and if it is not straight and true, will continually
make trouble.
In drawing out very large tubing, say from one to two inches in
diameter, it is often necessary to draw the tube _in the flame_,
proceeding very slowly and at a lower temperature than would be used
with small tubing. This is partly on account of the difficulty of
heating large tubing uniformly to a high temperature, and partly in
order to prevent making the conical part of the tube too thin for
subsequent operations.
=Constricting a Tube.=--Where a constriction is to be made in a tube,
the above method must be modified, as the strength of the tube must be
maintained, and the constricted portion is usually short. Small tubes
are often constricted without materially changing their outside
diameter, by a process of thickening the walls. The tube is heated
before the blast lamp, rotating it about its axis as later described,
and as it softens is gradually pushed together so as to thicken the
walls at the heated point, as in _a_, Fig. 1. When this operation has
proceeded far enough, the tube is removed from the flame, and the ends
cautiously and gently drawn apart, continuing the rotation of the tube
about its axis and taking care not to draw too rapidly at first. The
resulting tube should have a uniform exterior diameter, as shown in _b_,
Fig. 1.
[Illustration: FIG. 1.--Constricting a tube.]
This method of constriction is not suited to tubes much over 1/4 inch in
diameter, since the mass of glass in the constricted part becomes so
thick as to be difficult to handle when hot, and likely to crack on
cooling. Larger tubes are therefore constricted by heating in a narrow
flame, with constant rotation, and when soft, alternately gently pulling
the ends apart and pushing them together, each motion being so regulated
that the diameter of a short section of the tube is gradually reduced,
while the thickness of the wall of the reduced portion remains the same
as that of the rest of the tube, or increases only slightly. This
pulling and pushing of the glass takes place _in the flame_, while the
rotation is being continued regularly. The result may appear as
indicated in _c_, Fig. 1. The strength of the work depends upon the
thickness of the walls of the constricted portion, which should never be
less than that in the main tube, and usually a little greater. This
operation is most successful with tubing having a relatively thin wall.
=Flanging a Tube.=--This operation produces the characteristic flange
seen on test-tubes, necks of flasks, etc., the object being twofold: to
finish the end neatly and to strengthen it so that a cork may be
inserted without breaking it. This flanging may be done in several ways.
In any case the first operation is to cut the tube to a square end, and
then heat this end so that the extreme sixteenth or eighth of an inch of
it is soft and begins to shrink. The tube is of course rotated during
this heating, which should take place in a flame of slightly greater
diameter than the tube, if possible. The flange is now produced by
expanding this softened part with some suitable tool. A cone of charcoal
has been recommended for this purpose, and works fairly well, if made so
its height is about equal to the diameter of its base. The tube is
rotated and the cone, held in the other hand, is pressed into the open
end until the flange is formed. A pyramid with eight or ten sides would
probably be better than the cone.
[Illustration: FIG. 2.--Flanging tool.]
A better flanging tool is made from a triangular piece of copper or
brass, about 1/16 inch thick, and mounted in a suitable handle. Such a
tool is shown in Fig. 2, being cut from a sheet of copper and provided
with a handle made by wrapping asbestos paper moistened with sodium
silicate solution about the shank of the tool. It is well to have
several sizes and shapes of these tools, for different sizes of tubing.
The two sizes most used will be those having about the following
dimensions: (1) _a_ = 2 inches, _b_ = 1 inch; (2) _a_ = 1 inch, _b_ = 1
inch. When the end of the tube is softened, the tool is inserted at an
angle, as indicated in Fig. 3, and pressed against the soft part, while
the tube is quickly rotated about its axis. If the flange is
insufficient the operation may be repeated. The tool should always be
warmed in the flame before use, and occasionally greased by touching it
to a piece of wax or paraffin. After the flange is complete, the end
must be heated again to the softening temperature and cooled slowly, to
prevent it from cracking.
[Illustration: FIG. 3.--Flanging a tube with flanging tool.]
[Illustration: FIG. 4.--Flanging a tube with carbon rod or wire.]
Some glass-blowers use a small carbon rod, about 3/16 inch in diameter,
as a flanging tool for tubes larger than about 3/8 inch diameter, and a
small iron wire or similar piece of metal for smaller tubes. In this
case the tube is heated as above described, and the rod or wire inserted
in the end at an angle and pressed against the softened part, as
indicated in Fig. 4, while the tube is rotated about its axis. For
large heavy tubes a larger carbon would be used.
=Rotation of the Tube.=--This is the fundamental manipulation in
glass-blowing, and upon it more than all else depends the uniformity and
finish of the work, and often the possibility of accomplishing the work
at all. Directions for it will be given on the assumption that the
reader is right-handed; if otherwise, the position of the hands is of
course reversed. The object of rotation is to insure even heating of the
whole circumference of the tube at the point of attack, to equalize the
effect of gravity on the hot glass and prevent it from falling out of
shape when soft, and to keep the parts of the tube on each side of the
heated portion in the same straight line.
In rotating the tube, both hands must be used, so that the two ends may
revolve at the same rate and the glass in the hot part not be twisted.
The rotation is performed by the thumb and first finger of each hand,
the other fingers serving to support the tube. As it is almost always
necessary to follow rotating and heating a tube by blowing it, the hands
should be so placed that it will be easy to bring the right-hand end up
to the mouth without shifting the hold on the glass. For this reason the
left hand grasps the glass with the palm down, and the right hand with
the palm turned toward the left. If there is any choice, the longer and
heavier part of the tube is usually given to the left hand, and it is
planned to blow into the shorter end. This is because it is easier to
support the tube with the hand which has the palm down. This support is
accomplished by bending the hand at the wrist so that it points slightly
downward, and then curling the second, third and little fingers in under
the tube, which is held between them and the palm. This support should
be loose enough so that the thumb and first finger can easily cause the
tube to rotate regularly on its axis, but firm enough to carry all the
weight of the tube, leaving the thumb and first finger nothing to do but
rotate it. The hand must be so turned, and the other fingers so bent,
that the thumb and first finger stretch out nearly to their full length
to grasp the tube comfortably.
The right hand is held with the palm toward the left, the fingers except
the first slightly bent, and the tube held between the first finger and
the thumb while it rests on the second finger and that portion of the
hand between the base of the first finger and the thumb. Rotation of the
tube is accomplished by rolling it between the thumbs and first fingers:
the rotation being continued in the same direction regularly, and not
reversed. It is better to roll slowly and evenly, with a series of light
touches, each of which moves the tube a little, than to attempt to turn
the tube a half a revolution or so with each motion of the hands. The
hands must be held steady, and the tube must be under good control at
all times, so that both ends may be rotated at the same angular
velocity, even though they may be of different diameters, and the tube
be neither drawn apart nor pushed together unless such a motion is
expressly desired, as it sometimes is. The hot part of the glass must be
constantly watched to see that it is uniformly rotated and not twisted,
nor pulled out or pushed together more than is desired. Care must also
be taken to keep the parts of the tube in the same straight line, or as
near it as possible, during the heating and all other manipulations.
When flanging a tube, it is held and rotated with the left hand as above
described, while the right hand holds the flanging tool.
When part of the end of a tube must be heated, as in Exercise 6, and
rotation must be very carefully performed and continued during the
blowing, both hands are used. The right hand is held as above
described, and the left hand close to it and either as above described
or else with the palm toward the right, grasping the tube in the same
way as the right hand does. This puts both hands in a position where the
tube may be blown and rotated uniformly while its axis is kept
horizontal.
Smoothness and exactness are the two things for which the beginner must
constantly strive in glass-blowing, and they are only attained by a
careful attention to the details of manipulation, with a steady hand and
watchful eye. Every move must count, and the exercise must be finished
with a minimum of reheating and retouching, for the best results.