ELEMENTARY EXERCISES
EXERCISE NO. 1
JOINING TWO PIECES OF TUBING, END TO END--FIRST METHOD
This exercise is most easily learned on tubing with an exterior diameter
of 1/4 inch, or a little less, having moderately heavy walls. A piece of
such tubing is heated before the blow-pipe at a point ten or twelve
inches from the end, and there drawn out to a capillary as previously
described (page 9). The capillary is sealed off about two inches from
the main tube, and the latter is cut near the middle. Care should be
taken to get square ends where the cut is made (page 7). The flame is
now so regulated that it is a little broader than the diameter of the
tube, the sealed half of the tube taken in the left hand and the other
half in the right. The open end of the sealed part and one of the ends
of the other part are now held in opposite sides of the flame, inclined
at a slight angle to one another as indicated in Fig. 5, and rotated and
heated until the surfaces of both ends are just softened. The two ends
are then carefully and quickly brought together (_a_, Fig. 6), removed
from the flame and pulled apart a little, to reduce the lump formed at
the joint as much as possible, as indicated in _b_. The joint is then
tested by blowing into the open end of the tube to see if it is tight.
If so, the flame is reduced to half or less than half of its former
size, and the joint heated in it, holding the tube and continually
rotating it as directed in the last chapter (page 13).
[Illustration: FIG. 5.--Softening ends of two pieces of tubing.]
[Illustration: FIG. 6.--Joining two pieces of tubing end to end--first
method.]
As the tube softens and tends to shrink, the two ends are pressed
together a little and the walls allowed to thicken slightly, as in _c_.
It is then quickly removed from the flame and gently blown as indicated
in _d_, continuing the rotation of the tube during the blowing, and at
the same time pressing the ends of the tube together a little so as to
make a _short_ thick-walled bulb. The joint is then returned to the
flame and reheated, rotating as before, shrinking to about the shape of
_e_. When this stage is reached, the glass should be very hot and fluid,
and the mass of hot glass thick enough to remain at its working
temperature for about five seconds after removal from the flame. The
glass is now reblown as indicated in _f_, to form a bulb having walls of
practically the same thickness as the original tube. As soon as the bulb
is blown, the tube is removed from the mouth, held horizontally in front
of the worker, and gently drawn out to form one continuous tube, as
indicated in _g_. During both the blowing and drawing of this bulb the
rotation must be continued, and both blowing and drawing must be
carefully regulated so that the resulting tube may have the same
internal and external diameter at the joint as elsewhere.
=Discussion.=--In making the original joint, (_a_, Fig. 6), care should
be taken that the lump formed is as small as possible so that it may be
entirely removed during the subsequent operations. For this reason, only
the very tip ends of the two pieces of tubing are held in the flame, and
the softening should not extend more than 1/16 inch down the tube. As
soon as the ends are sufficiently soft to stick together, they are made
to do so. The first drawing of the tube (_b_) should take place
immediately, and reduce the lump as much as possible without making the
adjacent walls of the tube thin. The whole purpose of the rest of the
manipulation is to absorb or "iron out" the lump at the joint. For this
reason, care is taken that this lump is always in the center of the
flame while the joint is being heated, and a small flame is used so
that little of the main tube may be softened. During the first shrinking
of the joint (_c_) the walls next the lump, being thinner than it is,
reach the softening temperature first and are thickened by the slight
pushing together of the ends, so that they taper from the lump to the
unchanged wall. Upon blowing this joint, these thickened walls blow out
with the lump, but as they are thinnest next the unchanged tube, they
stiffen there first. Then as the thicker parts are still hot, these blow
out more, and with the lump make a more or less uniform wall. By this
first operation most of the lump will have been removed, provided it was
not too large at first, and the tube was hot enough when it was blown.
Beginners almost invariably have the glass too cool here, and find
difficulty in blowing out a satisfactory bulb. Under such circumstances
the lump will be scarcely affected by the operation.
During the shrinking of this bulb, the thinner parts of course are the
first to reach the softening point, and thus contract more than the
thick parts, so that practically all of the lump can be absorbed, and a
uniformly thickened part of the tube left as in _e_. When this is just
accomplished, the second bulb must be blown during one or two seconds,
and the tube then drawn out as described, so as to change the bulb to a
tube. The drawing must proceed with care: portions nearest the unchanged
tubes are the first to reach the proper diameter, and must be given time
to just set at that point before the center of the bulb is finally drawn
into shape. The drawing is perhaps best done intermittently in a series
of quick pulls, each drawing the tube perhaps 1/16 inch, and each taking
place as the thumbs and first fingers grasp the tube for a new turn in
the rotation. If the tube is not rotated during the blowing, the bulbs
will be lop-sided and it will be impossible to get a joint of uniform
wall-thickness; if rotation is omitted during the drawing, the tube
will almost invariably be quite crooked.
If the lump still shows distinctly after the operations described, the
cross-section of the tube will be as in _h_, and the tube will be likely
to break if ever reheated at this point after it becomes cold. The
operations _d_, _e_, _f_, and _g_ may be repeated upon it, and it may be
possible to get it to come out all right.
Care must be taken not to blow the bulbs _d_ and _f_ too thin as they
then become very difficult to handle, and the joint is usually spoiled.
The wall-thickness of these bulbs must never be much less than that of
the original tube. If the joint as completed has thinner walls than the
rest of the tube, it will be more easily broken. It should be remembered
that the length of the finished tube must be exactly the same as that of
the original piece, if the walls of the joint are to be of their
original thickness. Therefore the pushing together during the two
operations _c_ and _d_ must shorten the tube just as much as the final
drawing (_f_ to _g_) lengthens it.
The interval between the removal of the work from the flame and the
beginning of the blowing must be made as short as possible, or else the
portions next the main parts of the tube will set before they can be
blown out, and cause irregular shrunken areas.
EXERCISE NO. 2
JOINING TWO TUBES END TO END--SECOND METHOD
The method described in Exercise No. 1 is very satisfactory for joining
short lengths of straight tubing, but becomes inconvenient or impossible
when the pieces are long or bent, on account of the difficulty in
uniformly rotating such work. In such cases, this second method is
used. It does not usually give as smooth and pretty a joint as the first
method, and takes a little longer.
The joint is begun exactly as in the first method, and the manipulation
is the same until after the preliminary tight joint (_b_, Fig. 6) is
made. The flame is reduced as usual, but instead of rotating the tube in
the flame, only one part of the circumference is heated, and this is
allowed to shrink thoroughly before blowing. It is then blown gently so
that it becomes a slight swelling on the tube, and the operation
repeated on an adjoining part of the joint. Three or four repetitions of
the operation will usually cover the whole circumference of the joint,
in a small tube, the result being a swelling roughly similar to the
first thick bulb in the first method (_d_, Fig. 6). If all the lumps of
the original joint have not been removed by this operation, it may now
be repeated upon such parts as may require it. The thickness of the wall
in the bulb should be about the same as that in the original tube. The
whole of the expanded joint is now heated as uniformly as may be until
soft enough so that it begins to shrink a little, and the swelling is
gently drawn down to the same diameter as the main tube, as in the first
case. Any irregularities in the finished joint may be corrected by local
reheating, shrinking or blowing as required.
=Discussion.=--In using this method, especially with larger sizes of
tubing, it is very important to keep the whole circumference of the
joint hot enough during the operation so that it does not crack apart at
the part which has not yet been worked. For that reason the first
heating, shrinking and blowing should be performed as quickly as
possible, leaving the resulting irregularities to be corrected later,
rather than attempting to reblow the same part of the joint several
times in succession until it is satisfactory. Care must be taken in this
as in the first method that the blowing follows immediately upon the
completion of the shrinking and removal of the object from the flame:
delay in blowing will cause shrunken places where the joint meets the
original tubes, on account of the cooling and setting of the glass
before it was blown. Most beginners err in being afraid to shrink the
part of the joint enough before blowing it. On small tubing, the
shrinkage may often extend so far that the inner surface of the shrunken
part reaches the center of the tube. Insufficient shrinking results in
failure to remove the lump formed at the original joint. It is often of
advantage, after blowing out part of the joint, to allow that part a few
seconds to set before going on with the rest, keeping the whole joint
warm meanwhile in or near the smoky flame. This helps to prevent the
twisting of the joint, or other distortion incident to the handling of a
piece of work of awkward shape.
In making a joint on a very long or heavy piece by this method, it is
often advantageous to attach a piece of rubber tubing to the open end,
hold the other end of this tubing in the mouth during the process, and
blow through it, rather than attempt to bring the end of the glass up to
the mouth. This enables one to keep closer watch on the joint, and avoid
drawing it out or distorting it in handling. On the other hand, the
rubber tube is an inconvenience on account of its weight and the
consequent pull on the end of the apparatus, and makes rotation
difficult.
EXERCISE NO. 3
THE "TEE" TUBE
The operations involved are two: the blowing of a short side tube on a
piece of tubing, and sealing another piece of tubing on this, by what is
essentially the second method as just described.
[Illustration: FIG. 7.--The "tee" tube.]
The two pieces of tubing to be used each have one end cut square and the
other sealed in the usual manner. The longer of the two is now heated at
the point at which the joint is to be made, until it begins to color the
flame. A small flame is used, and the tube rotated until the flame
begins to be colored, when the rotation is stopped, and only one spot
heated until a spot the diameter of the tube to be sealed on has become
red hot and begun to shrink. This is now gently blown out into a small
bulb, as in _a_, Fig. 7, and it will be noted that this bulb will have
walls tapering from the thick walls of the tube to a very thin wall at
the top. The sides of this bulb, below the dotted line, are to form the
small side tube to which the main side tube is to be sealed. The top of
the bulb is now softened by directing a small flame directly upon it,
and as soon as it shrinks to the level indicated by the dotted line, it
is removed from the flame and quickly blown out to form a thin bulb, as
indicated in _b_, Fig. 7. This will usually be so very thin that a
stroke of the file or glass-knife will break it off at the dotted line,
leaving the side tube, to which the short piece of tubing is now sealed
according to the second method (Exercise No 2). In doing this, care is
taken to direct the flame partly on the main tube in the two crotches,
so that both tubes blow out a little and give space for the gases to
turn in, as indicated in _c_, Fig. 7, and at the same time increase the
mechanical strength of the job. On the other hand, care is taken not to
deform the main tube, and not to produce such a bulge or bulb at the
joint as will prevent the finished tube from lying flat on a table.
=Discussion.=--Most beginners tend to err in the first steps of this
operation, by blowing too hard and too long when blowing out the little
bulb. The result is a large, very thin bulb, which breaks off in such a
way as to leave a hole in the main tube, occupying nearly half the
circumference of the tube at that point, instead of the neat side tube
which they should have. It is not difficult to seal a tube on this side
tube, but it is very difficult to seal a tube into a hole in another
tube. Care should be taken here, as in the two previous exercises, that
the lump obtained at the joint when the two tubes are put together is
made as small as possible, and reduced if possible by gently drawing on
the side tube as soon as the tubes have actually joined. It is much
easier to prevent the formation of a lump at the joint than it is to
remove the lump after it is formed. The remarks previously made about
blowing quickly after removing the work from the flame apply here with
especial force. A "tee" tube, from its very nature, is exposed to a good
many strains, so care must be taken that the walls of the joint are of
uniform thickness with the rest of the tube.
The beginner will find it easiest to make this tube out of two pieces of
the same tube, about 1/4 inch in diameter. Larger or smaller tubing is
usually more difficult. If tubing much more than 1/4 inch is used, the
whole joint, including part of the main tube, must be heated nearly to
the softening point at the close of the operation, and well annealed, as
described in Chapter 1 (page 3) or it will be almost certain to crack.
In the larger sizes of tube it will be necessary to heat the whole
circumference of the main tube frequently during the operation, to
prevent it from cracking.
In sealing a small tube on the side of a large one, it is usually
advisable, after warming the spot where the joint is to be made, to
attach a small drop of glass to the tube at that point, and direct the
flame upon that, thus supplying at the same time both a definite point
to be heated and an extra supply of glass for the little side tube which
is desired. In this way it is also easier to blow out a side tube with a
sufficiently small diameter. If the diameter of this tube should be much
greater than that of the small tube, the latter may be enlarged with a
carbon or a flanging tool.
EXERCISE NO. 4
TO JOIN TWO TUBES OF DIFFERENT DIAMETERS
In this case the first method (Exercise No. 1) is to be used whenever
possible, as it gives a much smoother joint than the second method. The
directions given will describe the adaptation of this method to the
problem: if the second method must be used on account of awkward shape,
etc., of the work, the modifications required will be obvious to any one
who has learned to make the joint by the first method.
After sealing or corking one end of the larger tube, the other end is
drawn out to form a tail as described on page 9, taking care to have the
tube uniformly heated, and to draw the tail rapidly enough so that the
cone is short, as indicated in _a_, Fig. 8. The tube is now rotated, a
small flame directed against the cone at right angles to an element of
it, and it is allowed to shrink a little, as indicated in _b_, Fig. 8,
so that its walls will thicken. When the tail is cut off, at the dotted
line, the diameter of the opening and the thickness of the walls at that
point should correspond with the dimensions of the tube to be sealed on.
As the glass is hot, the scratch for cutting it must be made with a file
(moisten the edge!), and it often will not break square across. Before
proceeding to seal on the small tube, any large projections on the cut
end are best removed, by warming the cut surface a little, directing the
small flame upon each projection in turn and touching it with a warm
scrap of glass. It will adhere to this and may then be removed by
rotating this scrap a little so as to wind up the projection on it, and
then drawing it off, while the flame is still playing on the spot. This
must be done rapidly and care taken not to soften the main part of the
cone.
[Illustration: FIG. 8.--Joining two tubes of different diameters.]
The large tube is now taken in the left hand, the small one in the
right, the ends heated and joined in the usual manner, taking care not
to get any larger lump at the joint than necessary. A small flame is now
directed on the cone at right angles to its elements as before, and the
tube rotated so as to heat the whole circumference. The flame should be
just large enough to heat the whole of the cone. As the latter shrinks,
the lump at the joint is brought into the edge of the flame, and it and
a very little of the small tube allowed to shrink with the cone.
When well shrunk and heated to blowing temperature the joint is removed
from the flame and blown gently with careful rotation, pushing the tubes
together a little when the blowing is about finished, so that the cone
becomes a short thick half-bulb, as shown in _d_, Fig. 8. This
corresponds to the first thick bulb in the first method (_d_, Fig. 6),
and is treated similarly. It is again heated and shrunk, taking care not
to involve either the large tube or the small one in the shrinking,
blown quickly to about the same shape as before, (_d_, Fig. 8), and then
gently drawn out into a smooth cone (_e_), exactly as in the first
exercise. Care should be taken not to draw too rapidly or too far, as
then the resulting cone (_f_) is weaker than it should be, and does not
look well.
=Discussion.=--The beginner will find that this operation is best
learned on two tubes which are not too nearly of the same diameter. A
tube about 5/8 inch in diameter and one a little less than 1/4 inch will
be suitable. Both should have moderately heavy walls (1/16 inch or a
trifle over for the large tube, and a trifle less for the small one) but
the large tube should not be too heavy or else it will be hard to
prevent melting down too much of the small tube, and getting this drawn
out too thin during the process. One of the troublesome features of this
exercise is the difficulty of rotating two tubes of different diameters
with the same angular velocity, so as not to twist the joint. Another
difficulty is found in getting the cone uniformly heated to blowing
temperature without overheating and overshrinking the small tube. The
reason for this is obviously the much greater circumference of the cone,
especially at its large end, so that relatively much less of it is being
heated at any time. The beginner is also inclined to start with too long
a cone, or else heat so much of the large tube that part of its glass is
included in the cone, with the result that in order to get the right
wall-thickness the cone must be made too long (_g_, Fig. 8). This does
not look well, and usually will be irregular in shape.
EXERCISE NO. 5
TUBE FOR CONDENSING SULPHUR DIOXIDE
This is useful as a test of mastery of the preceding exercise. A piece
of 3/16 or 7/32 inch tubing is joined to each end of a piece of tubing
5/8 by about 5 inches, and two constrictions made in the large tube, by
the method described on page 10. The small tubes are then bent in the
same plane, as shown, and their ends fire-polished (Fig. 9).
[Illustration: FIG. 9.--Tube for condensing sulphur dioxide.]
EXERCISE NO. 6
BULB AT THE END OF A TUBE
For this exercise tubing of 1/4 inch diameter and moderately strong
walls is selected. A tail is drawn out on one end of the tube, and a
piece of tubing about nine or ten inches long is cut off. The tail
should be carefully drawn in the axis of the tube, and in the same
straight line with it, as it is to be used as a handle in assembling the
glass for the bulb. This tail must be long enough so that it can be
conveniently held in the left hand, as described on page 13, and rotated
about the same axis as the main tube. Holding the main tube in the right
hand and the tail in the left, the tube is rotated in a large flame so
that a piece of it, beginning where the tail stops and extending about
an inch to the right, may be uniformly heated to the highest temperature
at which it can be kept in shape. As soon as this temperature is
reached, the tube is removed from the flame, continuing the rotation and
taking care not to draw out the heated part, and gently blown. The
rotation is carefully continued during the blowing, holding the tube in
approximately a horizontal position. As soon as the tube has expanded a
little the tail is pushed gently toward the main tube, continuing the
gentle blowing. If this is properly done, the heated piece of tube will
become a short bulb of about double its original diameter, and about the
same wall thickness as the original tube. It will have somewhat the
appearance of _a_, Fig. 10, when properly manipulated.
[Illustration: FIG. 10.--Blowing a bulb on the end of a tube.]
The tube is now reheated as before, taking care this time that the
heating extends over all that part of the bulb to the right of the
dotted line in the figure, as well as part of the main tube adjoining.
If this heating has been properly placed, when the operation of blowing
and pushing together is repeated the result will be to lengthen the bulb
into a uniform cylinder, as shown in _b_, Fig. 10. Otherwise the result
will be a series of bulbs, as in _c_, Fig. 10, separated by thickened
ridges which will be almost impossible of removal later and will
disfigure the final bulb. This operation of heating, blowing and pushing
together is repeated several times, until the cylinder becomes as long
as can be conveniently handled (about 1-1/4 inches to 1-1/2 inches). If
more glass is needed than is then contained in the cylinder, the latter
may now be heated as a whole, and blown and pushed gently into a shorter
cylinder of a slightly greater diameter, and more glass then added as
before.
When enough glass has been collected for the bulb, it is all well heated
and blown gently a couple of times, pushing the mass together as
required, until a thick bulb like _d_, Fig. 10, is obtained. The tail
must now be removed at the point indicated by the dotted line. To do
this, a very fine flame is directed on the point where the tail joins
the bulb, and the tube well rotated as the glass softens at that point.
When sufficiently soft, the work is raised a little, so that the flame
instead of striking the glass squarely at the point indicated passes
below and tangential to it. The tail is now drawn off slowly, continuing
the rotation, raising the work just out of the flame whenever the thread
of glass drawn off becomes too thin, and lowering it again to the point
where the flame just touches it when the glass stiffens a little. By
this means the tail may be drawn off without leaving an appreciable lump
behind, as indicated in _e_ and _f_, Fig. 10. When as much of the extra
glass has been removed as is practicable, the flame is brought to play
squarely upon the little lump left, the last of the tail removed, and
the lump heated and gently blown to a small excrescence on the main
bulb. The whole end of the latter is now heated until it begins to
shrink a little, and gently blown to make it uniform in thickness. The
whole bulb is then heated in a flame of the proper size, so that it all
may shrink to about two-thirds of its diameter. The flame must be very
carefully chosen and directed, so as to shrink all the bulb, right up to
the main tube, but not soften the latter. As soon as this stage is
reached, the bulb is removed from the flame, continuing the even
rotation, and blown to the desired size, preferably by a series of
gentle puffs following one another at very short intervals. During the
blowing, the main tube is held in a horizontal position, and any
tendency of the bulb to fall out of line is corrected by the rotation.
If the shape of the bulb or its size are not satisfactory, it may be
shrunk again and reblown. Such shrinking should begin in a large yellow
flame, with just enough air to give it direction. The amount of air may
be gradually increased as the bulb shrinks and the walls become thick
enough to bear it without collapsing. If the bulb starts to collapse at
any time, it must be immediately blown enough to regain its convex
surface, before the shrinking proceeds further.
=Discussion.=--In collecting the glass for the bulb, enough must be
gathered to give the walls the desired strength. Since the area of a
sphere is proportional to the cube of its diameter, it is evident that
doubling the size of a bulb diminishes the thickness of its walls to a
very large extent. The limit of diameter for a strong bulb on ordinary
1/4-inch tubing, collecting the glass as above, is about 1-1/2 inches,
and the beginner will do well not to blow his bulbs more than an inch in
diameter.
The collection of the glass is one of the most important parts of the
process. If the mass of glass be twisted, furrowed or ridged, or
lop-sided, it is very difficult to get a good, even, spherical bulb, no
matter how many times it is shrunk and blown. The greatest care should
therefore be taken to get a uniform cylinder, on the same axis as the
main tube; and to this end the rotation of the tube must be carried on
very evenly. For method of holding the tube, see page 14.
If a very large bulb is required, it will often be economical to seal on
the end of the tube a short piece of a large tube, provided with the
proper tail, and use the glass in the large tube for the bulb instead of
attempting to collect it from the small tube. In this case part of the
small tube will usually be included in the bulb, so that the joint comes
in the latter, and not where it joins the tube. As the amount of glass
carried on the end of the tube increases in weight and size the
difficulties of heating it uniformly, keeping it in the proper position
and handling it increase rapidly.
In collecting glass, it is usually best not to leave the part of the
cylinder next the tube with too thick walls. This is always the coolest
part during the preparation for blowing the bulb, consequently it does
not get blown out, and causes an ugly thickened appearance on that end
of the bulb.
If the bulb grows too long or pear-shaped, it may be easily shortened by
heating to the blowing temperature, and then blowing gently with the
main tube in a vertical position, and the bulb at the top of it. Gravity
will then shorten the bulb nicely.
The finished bulb should be a nearly perfect sphere, with the axis of
the tube passing through its center, and the portion of the tube
adjoining the bulb must not be distorted, twisted, or blown out. In
order to prevent the distortion of the tube, care must be taken that it
is never heated quite to its softening point during the process.
EXERCISE NO. 7
BLOWING A BULB IN A TUBE
The tube is selected and one end closed as in the previous exercise, but
it should be cut a little longer, say about twelve inches. Beginning at
a point about four inches from the closed end, glass is collected and
blown into a thick-walled bulb, exactly as in the previous exercise.
Greater care must be taken, however, that the cylinder collected and
this thick bulb are of uniform thickness and set squarely in the axis of
the tube. Instead of removing the tail, the bulb must be blown in this
case with both pieces of tubing attached, and care must be taken that
they "line up" properly, _i.e._, are in the same straight line, and that
this line passes as near as may be through the center of the bulb. The
tube is held in approximately horizontal position during the blowing of
the bulb, as in the previous case, and especial care taken with the
rotation. Both pieces of tube must of course be rotated at the same
rate, and their softened ends must be kept at exactly the proper
distance from each other, so that the bulb may be spherical and not
elongated. If the blowing of the bulb be quickly and accurately done, it
may usually be completed before the glass is quite set, and the
alignment of the two tubes may then be rectified while looking straight
through the bore of the tube.
=Discussion.=--The two points of greatest importance are the collection
of the glass, and the uniform rotation of the tube. A larger tube may be
sealed in the middle of a small one when a large amount of glass is
necessary. The piece of tubing used for the exercise must be long enough
so that the fingers may be kept on a cool part of the glass without
getting uncomfortably near the ends of the tube. It should not be any
longer than necessary, however, as the extra weight and length make the
manipulation of the hot glass more difficult.
When a string of bulbs are required on the same tube, a piece of glass
18 inches long may be used at the start, and the first bulb made near
the closed end, as described. Each succeeding bulb will then be in plain
view during the blowing, and when the open end becomes too short for
comfort, it may be dried out, cut off, and another piece joined to it,
starting as in the first method (Exercise No. 1), but instead of drawing
out the thick bulb to a tube, it is made part of the glass collected for
the next bulb. If the string of bulbs becomes awkward to handle on
account of its length and weight, it may be made in several parts and
these later sealed together by the second method, preferably blowing
through a rubber tube attached to the open end, as described on page 22.
Very neat small bulbs may be made on tubing of a diameter of 3/16 inch
or a little less, but the beginner is advised to start with tubing of
about 1/4 inch diameter. The use of tubing with too thick walls usually
produces bulbs which are thick-walled at the point where they leave the
tube, but inclined to be too thin at the point of maximum diameter
(perpendicular to the axis of the tube) where most of the strain comes
and strength is particularly needed.