CUTTING AND BENDING GLASS—FORMING GLASS
APPARATUS BEFORE THE BLOW-PIPE—MAKING
AND GRINDING STOPPERS TO APPARATUS, ETC.
In the later pages of this Chapter it will be assumed that the
operations first described have been mastered. The beginner
should therefore practise each operation until he finds himself
able to perform it with some degree of certainty. Generally
speaking, however, after the failure of two or three attempts
to perform any operation, it is best to give up for a few
hours, and proceed to the work next described, returning to
that upon which you have failed subsequently. If, unfortunately,
it should happen that the work next in order involves
the performance of the operation in which the failure has
occurred, it is best to pass on to some later work which does
not demand this particular accomplishment, or to rest a while,
and re-attack the difficulty when refreshed.
Cutting Glass Tubes.—The simplest method of cutting
a glass tube is to make a sharp scratch with a file across the
glass at the point where it is desired to cut it, and on pulling
apart the two ends, it will break clean off. It is important
that the file be sharp. In pulling apart the ends the
scratch should be held upwards, and the pull should have
a downward direction, which will tend to open out the
scratch. In the case of a large tube, a scratch will not
ensure its breaking clean across. The tube must be filed
to some depth, half-way, or even all round it. A good
way of breaking a tube is to place the file in the table
after scratching the glass, to hold the glass tube above
its edge with one hand on each side of the scratch, and
to strike the under side of the tube a sharp blow upon
the edge of the file, directly beneath the scratch. In this
way very even fractures of large and moderately thin tubes
may be made. It answers particularly well for removing short
ends of tube, not long enough to hold; the tube is held
firmly upon the file, and a sharp blow given to the short end
with a piece of large tube or a key.
A file whose faces have been ground till they are nearly
smooth, so as to leave very finely-serrated edges, will be
found useful for cutting glass tubes. Such a file should
be used almost as a knife is used for cutting a pencil in
halves.
The simple methods just described are too violent to be
applied to delicate apparatus, too tedious when employed upon
the largest tubes, and very difficult to apply when the tube to
be cut is very thin, or too short to permit the operator to get
a good grip of it on either side of the file mark. In such
cases, one or other of the following methods will be useful:—
1. Make a scratch with a file, and touch it with the end of
a very small piece of glass drawn out and heated at the tip
to its melting point. It is important that the heated point of
glass be very small, or the fracture is likely to be uneven, or to
spread in several directions. Also, it is best to use hot soda
glass for starting cracks in tubes of soda glass, and lead glass for
doing so in lead glass tubes. If the crack does not pass quite
round the tube, you may pull it asunder, as previously
described, or you may bring the heated piece of glass with
which the crack was started to one end of the crack, and
slowly move it (nearly touching the glass) in the required
direction; the crack will extend, following the movements of
the hot glass. Instead of hot glass, pastils of charcoal are
sometimes employed for this purpose. They continue to burn
when once lighted, and there is no need to re-heat them from
time to time. They should be brought as close to the glass as
is possible without touching it, and, when no longer needed,
should be extinguished by placing the lighted end under
sand, or some other incombustible powder, for they must not
be wetted.
2. A method much practised by the makers of sheet glass,
and suitable for large objects, is to wrap a thread of hot
glass round the tube, at once removing it, and touching any
point of the glass which the thread covered with water or a
cold iron, when a crack will be started and will pass round
the glass where it was heated by the thread.
3. Tubes which are large and slightly conical may have a
ring of red-hot iron passed over them till it comes into contact
with the glass, then, the iron being removed, and a point
on the heated glass being at once touched with cold iron as
before, it will break as desired. Or a string, moistened with
turpentine, may be loosely twisted round the tube, and the
turpentine ignited, afterwards the application of sudden cold
to any point on the zone of hot glass will usually start a crack,
which, if necessary, may be continued in the usual manner.
The last three methods are chiefly useful in dealing with the
largest and thickest tubes, and with bottles.
A fairly stout copper wire, bent into the form of a bow so
that it can be applied when hot to a considerable surface of a
glass tube, will be found superior to the point of hot glass or
metal usually employed, for leading cracks in glass tubes.
With such a wire a tube can be cut so that the cross section
of the end is at any desired angle to the axis of the tube, with
considerable precision. I am indebted for this suggestion to
Mr. Vernon Boys and Dr. Ebert.
Fig. 6.
Bending Glass Tubes.—The blow-pipe flame is not a
suitable source of heat for bending tubes, except in certain
cases which will be mentioned in a subsequent paragraph.
For small tubes, and those of moderate size, a fish-tail burner,
such as is used for purposes of illumination, will answer best.
Use a flame from one to two inches in breadth—from A to A
(Fig. 6), according to the size of the tube which is to be bent.
If the length of tube that is heated be less than this, the bend
will probably buckle on its concave side.
The tube to be heated should be held in the position
shown in Fig. 6, supported by the hands on each side. It
should be constantly rotated in the flame, that it may be
equally heated on all sides. In the figure the hands are
represented above the tube, with their backs upwards. A
tube can be held equally well from below, the backs of the
hands being then directed downwards, and this, I think, is
the more frequent habit. It is difficult to say which position
of the hands is to be preferred. I lately observed how a
tube was held by three skilful amateurs and by a professional
glass-blower. All the former held the tube with the hands
below it. The latter, however, held it from above, as in
Fig. 6. He, however, was working with a rather heavy piece
of tube, and I am inclined myself to recommend that position
in such cases. During a long spell of work, the wrist may be
rested from time to time by changing the position of the
hands.
When the tube has softened, remove it from the flame, and
gently bend it to the desired angle. The side of the tube
last exposed to the flame will be slightly hotter, and therefore
softer, than that which is opposite to it. This hotter side
should form the concave side of the bent tube.
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.
In performing this and all the other operations of glass
blowing, the following points must be constantly kept in
mind:—
(a.) That it is rarely safe to blow glass whilst it is still in the
flame, except in certain special cases that will be mentioned
subsequently. Therefore always remove apparatus from the
flame before blowing.
(b.) That when heating glass tubes, unless it is specially
desired to heat one portion only, the tube must be constantly
rotated in the flame to ensure that it shall be uniformly
heated, and to prevent the tube or mass of glass from assuming
an irregular form.
(c.) Always blow gently at first, and slowly increase the
force applied till you feel or see the glass giving way. It is
a good plan to force the air forward in successive short blasts
rather than in one continued stream.
Fig. 8.
(d.) When it is necessary to force air into tubes of fine bore,
such as thermometer tubes, the mouth must not be used, for
moisture is thereby introduced into the tube, which it is
very difficult to remove again in many cases. All tubes of
very small bore should be blown with the aid of an india-rubber
blowing-bottle, such as are used for spray-producers,
Galton’s whistles, etc. The tube to be blown must be
securely fixed to the neck of the bottle, which is then held
in one hand, and air is forced from it into the tube as it is
required. These bottles are frequently of service to the
glass-blower—e.g., when tubes of very fine bore have to be
united, it is necessary to maintain an internal pressure
slightly exceeding that of the air throughout the operation,
in order to prevent the viscous glass from running together
and closing the tube. An india-rubber blowing-ball is very
convenient for this purpose.
To seal the end of a glass tube (Fig. 8), adjust the flame
so that it will heat a zone of glass about as broad as the
diameter of the tube to be sealed (see A, Fig. 8). Hold
the tube on each side of the point where it is to be sealed in
the manner described in the description of bending glass
tubes (p. 28). Bring the tube gradually into the flame,
and heat it with constant rotation, till the glass softens (for
lead glass the oxidising flame must be used, as has been
already explained).[6] When the glass begins to thicken,
gently pull asunder the two ends, taking care not to pull out
the softened glass too much, but to allow the sides to fall
together, as shown at A. When this has occurred, heat
the glass at the narrow part till it melts, and pull asunder
the two ends. The closed end should present the appearance
shown at D. If the glass be drawn out too quickly its
thickness will be unduly reduced,
and it will present the
appearance shown at B. In
that case apply a pointed
flame at b, and repeat the
previous operation so as to
contract the tube as at c,
taking care not to allow the
glass to become much increased
nor decreased in thickness.
If a considerable mass of
glass be left at d, it may be
removed by heating it to redness,
touching it with the
pointed end of a cold glass
tube, to which it will adhere,
and by which it may be pulled
away.
When the end of the tube
presents the appearance shown
in the diagram D, and the
mass of glass at d is small, the
small lump that remains must be removed by heating it till
it softens, and gently blowing with the mouth, so as to round
the end and distribute the glass more regularly, as shown in E.
The whole end, from the dotted line e, must then be heated
with constant rotation in the flame. If this final heating of
the end e be done skilfully, the glass will probably collapse
and flatten, as at F. The end must then be gently blown
into the form shown at G.
If a flat end to the tube be desired, the tube may be left
in the condition shown by F, or a thin rounded end may be
flattened by pressure on a plate of iron.
If a concave end be wished for, it is only necessary to gently
suck air from the tube before the flattened end has become
solid.
Fig. 9.
In each case, immediately after the tube is completed, it
must be closely wrapped in cotton wool and left to cool.
With good lead glass this last process, though advantageous,
is not absolutely necessary; and as glass cools slowly when
enveloped in cotton wool, this precaution may frequently
be neglected in the case of apparatus made from lead glass.
In order to draw out tubes
for sealing, close to one end,
and thus to avoid waste of
material, it is a good plan to
heat simultaneously the end
of the glass tube A which is
to be sealed, and one end of a piece of waste tube E of about
the same diameter, and when they are fused to bring them
together as at DD (Fig. 9). E will then serve as a handle in
the subsequent operations on A. Such a rough joint as that
at D must not be allowed to cool too much during the work in
hand, or E and A may separate at an inconvenient moment.
Or the glass at the end of the tube may be pressed together
to close the tube, and the mass of glass may be seized with a
pair of tongs and drawn away.
Choking, or Contracting the Bore of a Glass
Tube.—If it be not desired to maintain the uniformity of
external dimensions of the tube whilst decreasing the diameter
of the bore, the tube may be heated and drawn out as described
in the description of sealing tubes on pp. 32-35. This may
be done as shown at A or B in Fig. 8, according to the use
to which the contracted tube is to be put.
Fig. 10.
Greater strength and elegance will be secured by preserving
the external diameter of the tube unchanged
throughout, as shown in Fig. 10. For this purpose heat the
tube with the pointed flame, if it be small, or in the brush
flame if it be of large size, constantly rotating it till the
glass softens and the sides show an inclination to fall together,
when this occurs, push the
two ends gently towards A.
If the tube should become
too much thickened at A,
the fault may be corrected by
removing it from the flame and gently pulling the two ends
apart till it is of the proper size. If the bore at the contracted
part of the tube should become too much reduced, it
may be enlarged by closing one end of the tube with a small
cork, and blowing gently into the open end after sufficiently
heating the contracted part. The tube should be rotated
during blowing or the enlargement produced may be
irregular.
Fig. 11.
When the external diameter of the tube is to be increased
as well as its bore diminished, press together the ends of a
tube heated at the part to be contracted, as already described,
and regulate the size of the bore by blowing into
the tube if at any time it threatens to become too much
contracted.
Widening Tubes.—Tubes may be moderately expanded
at their extremities by means of the charcoal cone (see Bordering,
p. 31). They may be slightly expanded at any other part
by closing one end and gently blowing into the open end of
the tube, after softening the glass at the part to be widened
before the blow-pipe. But the best method of obtaining a
wide tube with narrow extremities (Fig. 11) is to join pieces
of narrow tube AA to the ends of a piece of wider tube B of
the desired dimensions. The method of performing this
operation is described under welding, on pp. 39-47.
Fig. 12.
Piercing Tubes.—The glass-blower very frequently
requires to make a large or small opening in some part of a
tube or other piece of apparatus. This is known as piercing.
Suppose it is desired to make a small hole at the point a in
A (Fig. 12). When the tube has been brought to the flame
with the usual precautions, allow the end of the pointed flame
to touch it at a till an area corresponding to the desired size
of the opening is thoroughly softened. Then expand the
softened glass by blowing to the form shown at B. Re-heat
a, blow a small globe as at C, and carefully break the thin
glass, then smooth the rough edges by rotating them in
the flame till they form a mouth like that of D. Instead of
leaving the bulb to be broken at the third stage C, it is a good
plan to blow more strongly, so that the bulb becomes very
thin and bursts, the removal of the thin glass is then accompanied
by less risk of producing a crack in the thicker parts
of the glass. Openings may be made in a similar manner in
the sides of tubes or in globes, in fact, in almost any position on
glass apparatus. If another tube is to be attached at the
opening, it is a good plan to proceed to this operation before
the tube has cooled down.
Fig. 13.
The openings obtained by the method above described are
too large when platinum wires are to be sealed into them.
Suppose that it is necessary to pierce the tube A of Fig. 13
in order to insert a platinum wire at a; direct the smallest
pointed flame that will heat a spot of glass to redness on the
point a. When the glass is viscous, touch it with the end of
a platinum wire w, to which the glass will adhere; withdraw
the wire and the viscous glass will be drawn out into a
small tube, as shown at B; by breaking the end of this
tube a small opening will be made. Introduce a platinum
wire into the opening, and again allow the flame to play on
the glass at that point; it will melt and close round the wire.
Before the hot glass has time to cool, blow gently into the
mouth of the tube to produce a slightly curved surface,
then heat the neighbouring parts of the tube till the glass
is about to soften, and let it cool in cotton wool. Unless
this is done, I find that glass tubes into which platinum
wires have been sealed are very apt to break during or after
cooling.
To ensure that the tube shall be perfectly air-tight, a small
piece of white enamel should be attached to the glass at a
before sealing in the wire.
Uniting Pieces of Glass to Each Other, known
as Welding, or Soldering.—The larger and more complicated
pieces of glass apparatus are usually made in separate
sections, and completed by joining together the several
parts. This is therefore a very important operation, and
should be thoroughly mastered before proceeding to further
work.
In order to produce secure joints, the use of tubes made of
different kinds of glass must be avoided. Soda glass may be
joined securely to soda glass, especially if the tubes belong to
the same batch, and lead glass to lead glass. But, though by
special care a joint between lead glass and soda glass, if well
made, will often hold together, yet it is never certain that it
will do so.
To join two Tubes of Equal Diameters.—Close one end of
one of the tubes with a small cork. Heat the open end of
the closed tube, and either end of the other tube in a small
flame until they are almost melted, taking care that only the
ends of the tubes are heated, and not to let the glass be
thickened; bring the two ends together with sufficient pressure
to make them adhere, but not sufficient to compress the
glass to a thickened ring. Before the joint has time to cool too
much, adjust your blow-pipe for a pointed flame, if you are
not already working with that kind of flame, and allow the
point of the flame to play on any spot on the joint till it is
heated to redness; rotate the tube a little so as to heat the
glass adjacent to that which is already red-hot, and repeat this
till the whole circumference of the rough joint has been
heated.[7] Repeat the operation last described, but, when each
spot is red-hot, blow gently into the open end of the tube so
as to slightly expand the viscous glass. Finally, rotate the
whole joint in the flame till the glass is softened, and blow
gently as before into the open end of the tube, still rotating it,
in order that the joint may be as symmetrical as possible.
If in the last operation the diameter of the joint becomes
greater than that of the rest of the tube, it may be cautiously
re-heated and reduced by pulling it out, or this may be secured
by gently pulling apart the two ends, whilst the operator
blows it into its final shape.
Fig. 14.
When small tubes, or tubes of fine bore, are to be
joined, in order to prevent
the fused glass from
running together and
closing the tube, it is a
good plan to border and
enlarge the ends that are
to be united, as at A (Fig. 14). Some glass-blowers prefer
to border all tubes before uniting them.
When a narrow tube is to be joined to one that is only
slightly wider, expand the end of the narrow tube till it
corresponds in size to the larger tube. If the tube be too
narrow to be enlarged by inserting a charcoal cone, seal one
end and pierce it as directed (on p. 37).
For joining small thin-walled tubes Mr. Crookes recommends
the use of a small Bunsen flame.
Fig. 15.
In welding pieces of lead glass tube, take care that the
heated glass is perfectly free from reduced lead at the
moment when the two ends of viscous glass are brought into
contact.
To join Tubes of Unequal Sizes End to End (Fig. 15).—Draw
out the larger tube and cut off the drawn-out end
at the part where its diameter is equal to that of the smaller
tube, then seal the smaller tube to the contracted end of
the larger according to the directions given for joining tubes
of equal size. When a good joint has been made, the tube
presents the appearance of A, Fig. 15, the union being at
about bb. Next heat the whole tube between the dotted
lines aa, and blow it into the shape of B in which the dotted
line dd should correspond to the actual line of junction of
the two tubes.
In making all joints it is important to leave no thick
masses of glass about them. If the glass be fairly thin and
uniformly distributed, it is less likely to break during or after
annealing under any circumstances, and especially if it has to
bear alternations of temperature.
Joining a Tube to the Side of another Tube (Fig. 16).—One of
the tubes must be pierced as at A in Fig. 16 (for the method,
see p. 37), and its two ends closed with small pieces of cork.
The edges of the opening, and one end of the other tube,
must then be heated till they melt, and united by pressing
them together. The joint may then be finished as before.
Fig. 16.
A properly blown joint will not present the appearance of
B (Fig. 16), but rather that of C. This is secured by directing
the pointed flame upon the glass at aa (B) spot by spot, and
blowing out each spot when it is sufficiently softened. If the
tubes are large, the whole joint should subsequently be heated
and blown, but in the case of small tubes this is of less importance.
Finally it is to be wrapped whilst hot in cotton
wool for the annealing process.
If a second tube has to be joined near to the first one,
say at b, it is well to proceed with it before the joint first
made cools down, and the joint first made, especially if soda
glass be used, must be held in the flame from time to time
during the process of making the second joint to keep it hot;
if this be not done the first joint is very likely to break. A
joint previously made may, however, be re-heated, if well
made and well annealed.
A three-way tube, like that in Fig. 17, is made by bending
A (Fig. 16) to an angle, and joining B to an opening blown
on the convex side of the angle; or, A of Fig. 16 may be
bent as desired after attaching B in the ordinary
way.
Fig. 17.
Tubes may also be joined to openings made
in the sides of globes or flasks; great care
must be taken, however, especially if the walls
of the globe be thin, to secure that the tube
is well attached to the mouth of the opening
when the melted ends are first brought into
contact, for, with thin glass, any hole that may
be left will probably increase whilst the joint is being blown
into shape, owing to cohesion causing the glass to gather in a
thickened ring round an enlargement of the original opening.[8]
In order to unite a tube of soda glass to a tube of lead
glass, the end of the soda glass tube must be carefully covered
with a layer of soft arsenic glass.[9] This must be done so
perfectly that when the ends to be united are brought
together the lead and soda glass are separated by the enamel
at every point.
To Seal a Tube inside a Larger Tube or Bulb.—Suppose that
an air-trap (3 of Fig. 18) is to be constructed from a small
bulb (A) blown on a glass tube (1).
Fig. 18
Either cut off the tube close to the bulb at B, or better,
remove the end by melting the glass and pulling it away
from B, and then pierce A at B, No. 2, by heating the glass
there and blowing out a small bulb as described under Piercing.
Prepare a tube (4) drawn out at E with a bulb blown
at D. Insert E into the opening B, press D well against
the mouth B and slowly rotate before the blow-pipe till D
adheres to B. Then heat and blow the joint spot by spot
as in other cases, taking care that the glass is blown out on
each side of the joint; lastly, heat the whole joint between aa,
and blow it into its final shape.
These joints are very apt to break after a few minutes or
hours if the glass of D be much thicker than that of the bulb
A. They should be wrapped in cotton wool for annealing
as soon as possible, as the rate at which the tube E cools is
likely to be less rapid than that of the parts of the apparatus
which are more freely exposed to the air; therefore all such
internal joints require very careful annealing, and they should
always be made as thin as is consistent with the use to which
they are to be put.
Tubes may also be sealed into the ends or sides of larger
tubes by piercing them at the point at which the inserted
tube is to be introduced, and proceeding as in the case of the
air-trap just described.
Ozone generators of the form shown on next page (Fig. 19),
afford an interesting example of the insertion of smaller tubes
into larger.
On account of the small space that may be left between
the inner and outer tubes of an ozone generator, and of
the length of the inner tube, its construction needs great
care. I find the following mode of procedure gives good
results. Select the pieces of tube for this instrument as free
from curvature as possible. For the inner tube, a tube
12 mm., or rather more, in external diameter, and of rather
thin glass, is drawn out, as for closing, until only a very
narrow tube remains at C, the end of C is closed the area
round C is carefully blown into shape, so that by melting off
C the tube A will be left with a well-rounded end. A small
bulb of glass is next blown on A at B. This bulb must be of
slightly greater diameter than the contracted end E of the
larger tube (II.), so that B will just fail to pass through E.
The length from B to C must not be made greater than
from E to G on the outside tube. The end at C is then to
be cut off so as to leave a pin-hole in the end of A.
Fig. 19.
The outer tube (II.), whose diameter may be 5 or 6 mm.
greater than that of A, is prepared by sealing a side tube on
it at F, after previously contracting the end E. For this
purpose the end E should be closed and rounded, and then
re-heated and blown out till the bulb bursts. To ensure that
the diameter of the opening is less than that of the tube, care
must be taken not to re-heat too large an area of the end
before blowing it out. It is very important that the cross
section at E shall be in a plane at right angles to the axis of
the tube.
Wrap a strip of writing paper, one inch in breadth, closely
round the end of A at C till the tube and paper will only just
pass easily into the mouth D of the outer tube, push the inner
tube A, with the paper upon it, into D, and when the paper is
entirely within D, withdraw A, and cautiously push the paper a
little further into the outer tube. Insert A into DE through
E, so that the bulb B is embraced by E. Close D with a
cork. Ascertain that the paper does not fit sufficiently
tightly between the two tubes to prevent the free passage of
air, by blowing into the mouth K of A. Air should escape
freely from E when this is done. Gradually bring the line of
contact of B and E and the surrounding parts of the tube
before a pointed flame, after previously warming them by holding
near a larger flame, and rotate them before the flame so
that the glass may soften and adhere. Then heat the joint
spot by spot as usual. In blowing this joint, take care that
the glass on each side of the actual joint is slightly expanded.
It should present the form shown by the dotted lines
in III. (these are purposely exaggerated, however). Finally,
heat the whole joint between the lines JI till it softens, and
simultaneously blow and draw it into its final shape as seen
at III.
The side tube F should not be too near the end E. If,
however, it is necessary to have them close together, the joint
F must be very carefully annealed when it is made; it must
also be very cautiously warmed up before the construction of
the joint at H is begun, and must be kept warm by letting
the flame play over it from time to time during the process
of making the latter joint.
A good joint may be recognised by its freedom from lumps
of glass, its regularity of curve, and by a sensibly circular
line at H, where the two tubes are united.
When the joint after annealing has become quite cold, the
pin-hole at C on the inner tube may be closed, after removing
the paper support, by warming the outer tube, and then
directing a fine pointed flame through D on to C. And the
end D of the outer tube may be closed in the ordinary
manner, or a narrow tube may be sealed to it. As the end of
glass at D will be too short to be held by the fingers when
hot, another piece of tube of similar diameter must be
attached to it to serve as a handle (see p. 35, Fig. 9).
Blowing a Bulb or Globe of Glass.—For this purpose
it is very important that the glass tube employed shall be of
uniform substance. The size and thickness of the tube to be
employed depends partly on the dimensions of the bulb
desired, and partly on the size of neck that is required for
the bulb. It is easier to blow large bulbs on large-sized
tubes than on those of smaller size. When it is necessary to
make a large globe on a small tube, it can be done, however,
if great care be taken to avoid overheating that part of the
small tube which is nearest to the mass of viscous glass from
which the bulb is to be formed. For the purpose of blowing
a very large bulb on a small tube, it is best to unite a wide
tube to that which is to serve as the neck, as it will save
some time in collecting the necessary mass of glass from
which to form the globe.
Fig. 20.
To blow a Bulb at the End of a Tube.—Select a good piece of
tube, say 1·5 cm. in diameter, and about 30 cm. long; draw
out one end to a light tail (a, Fig. 20) about 3 inches in
length. Then heat up a short length of the tube at b, with
a small brush flame, by rotating the glass in the flame, and
gently press it together when soft to thicken it; blow into it
if necessary to preserve the regularity of its figure. Repeat
this process on the portion of tube nearest to that which
has been first thickened, and so on, till as much glass has
been heated and thickened as you judge will serve to make a
bulb of the size desired. You should have a mass of glass
somewhat resembling that shown at B (Fig. 20), but probably
consisting of the results of more successive operations
than are suggested in that diagram. Apply the flame as
before to the narrower parts cc of B, gently compress and
blow until all the small bulbs first made are brought together
into a mass still somewhat resembling the enlarged end of B,
but more nearly cylindrical, with the glass as regularly distributed
as possible, and of such length from d to the contracted
part that the whole of it may easily be heated simultaneously
with the large brush flame of your blow-pipe. Take
great care in the foregoing operations not to allow the sides
of the mass of glass to fall in and run together, and, on the
other hand, do not reduce the thickness of the glass needlessly
by blowing it more than is necessary to give the glass as
regular a form as possible. When you are satisfied with the
mass of glass you have collected, melt off the tail a, and
remove the pointed end of glass that remains, as directed on
page 33. Turn on as large a brush flame as is necessary to
envelop the whole mass of glass that you have collected, and
heat it with constant rotation, so that it may gradually run
together to the form seen at C (Fig. 20), taking care that it
does not get overheated near d, or the tube which is to form
the neck will soften and give way.
The position in which the mass of heated glass is to be
held will depend upon circumstances; if the mass of glass
be not too great, it is best to keep it in a nearly horizontal
position. If the mass of glass be very large, it may be necessary
to incline the end B downwards; but as that is apt
to result in an excess of glass accumulating towards d, avoid
doing so if possible by rotating the glass steadily and rapidly.
If at any time the glass shows indications of collapsing, it
must be removed from the flame and gently blown into shape,
during which operation it may be rotated in the perpendicular
position; indeed, to promote a regular distribution of
the glass by allowing it plenty of time to collect, it is well
from time to time to remove the heated mass of glass from
the flame, and slightly expand it by blowing. Finally, when
a regular mass of glass, such as is shown at C (Fig. 20) has
been obtained, remove it from the flame, and blow it to its
final dimensions. A succession of gentle puffs quickly succeeding
each other should be employed, in order that the progress
of the bulb may be more easily watched and arrested at the
right moment. During the process of blowing, the hot glass
must be steadily rotated.
To collect the glass for blowing a bulb of lead glass, employ
the flame described on pp. 17-22 for heating lead glass.
If the tube be held horizontally whilst the globe is blown,
its form will most nearly approach that of a true globe. If it
be held in the perpendicular position, with the mass of glass
depending from it, the form of the bulb will usually be
somewhat elongated. If it be held perpendicularly, with the
mass of glass upwards, the resulting bulb will be flattened.
When a bulb is not of a sufficiently regular form, it may
sometimes be re-made by re-collecting the glass, and re-blowing
it. The greatest care is needed at the earlier stages of re-heating
to prevent the glass from collapsing into a formless
and unworkable mass. This is to be prevented in all such
cases by gently blowing it into shape from time to time
whilst gathering the glass.
Fig. 21.
To blow a Bulb between two Points (Fig 21).—Select a piece of
suitable tube, seal or cork one end, gather together a mass of
glass at the desired part, as directed for blowing a bulb at the
end of a tube; when a mass of glass has been collected of
sufficient thickness, blow it
into shape from the open
end of the tube by a rapid
succession of short blasts of
air, till the expanding glass
attains the desired dimensions.
The tube must be
held horizontally, and must be rotated steadily during the
process. By slightly pressing together the glass while
blowing, the bulb will be flattened; by slightly drawing apart
the two ends of the tube, it will be elongated.
A pear-shaped bulb may be obtained by gently re-heating
an elongated bulb, say from a to a, and drawing it out. It
is easiest to perform this operation on a bulb which is rather
thick in the glass.
If the tubes bb are to be small, and a globe of considerable
size is wanted, contract a tube as shown in Fig. 22, taking
care that the narrow portions of the tube are about the
same axis as the wider portions, for if this be not the case, the
mouths of the bulb will not be symmetrically placed; seal at
C, cut off the wider tube at B, and make the bulb, as
previously described, from the glass between AA. If, as probably
will be the case, the contracted portions of the tube
be not very regular, they may be cut off, one at a time, near
the bulb, and replaced by pieces of tube of the size desired.
Fig. 22.
When a bulb has to be blown upon a very fine tube, for example
upon thermometer tubing, the mouth should not be employed,
for the moisture introduced by the breath is extremely
difficult to remove afterwards. A small india-rubber bottle
or reservoir, such as those which are used in spray-producers,
Galton’s whistles, etc., securely attached to the open end of
the tube, should be used. With the help of these bottles bulbs
can be blown at the closed ends of fine tubes with ease, though
some care is necessary to produce them of good shape, as it
is difficult to rotate the hot glass properly when working in
this way.
Making and Grinding Stoppers.—Apparatus which
is to contain chemicals that are likely to be affected by the
free admission of air, needs to have stoppers fitted to it.
Making a good stopper is a much less tedious process than is
commonly supposed.
Fig. 23.
Suppose that the tube I. of Fig. 23 is to be stoppered at A,
it must be slightly enlarged by softening the end and opening
it with a pointed cone of charcoal; or a conical mouth for
the stopper may be made by slightly contracting the tube
near one end, as at B, cutting off the cylindrical end of the
tube at the dotted line C, and then very slightly expanding
the end at C with a charcoal cone after its edges have been
softened by heat. In either case the conical mouth should
be as long and regular as possible.
For the stopper take a piece of rather thick tube, of
such size that it will pass easily, but not too easily, into A
or B. Expand this tube at D, as shown in II., by softening
the glass and gently compressing it. The configuration of
the enlarged tube as shown at D may be obtained by heating
and compressing two or more zones of the tube that are
adjacent, one zone being less expanded than the other, so as
to give the sides of the imperfect stopper as nearly as possible
the form shown at D, which, however, is much less regular
than may easily be obtained. Seal off the head of the tube
at H, and heat the glass till it runs together into a nearly
solid mass; compress this with a pair of iron tongs to the
flattened head E. In making D, aim at giving it a form
which will as nearly as possible correspond to that of the
tube into which it is to be ground, and make it slightly
too large, so that only the lower part at D can be introduced
into the mouth of A or B. Before it is ground, the
stopper must be heated nearly to its softening-point and
annealed.
Moisten D with a solution of camphor in recently distilled
turpentine, and dust the wet surface with finely-ground
emery, then gently grind it into its place till it fits properly.
In this operation the tail G, which should fit loosely into
the tube A, will be of assistance by preventing D from
unduly pressing in any direction on A in consequence
of irregular movements. The stopper should be completely
rotated in grinding it. It must not be worked backwards
and forwards, or a well-fitting stopper will not be
produced. Renew the emery and camphorated turpentine
frequently during the earlier part of the grinding; when
the stopper almost fits, avoid using fresh emery, but
continue to remove the stopper frequently at all stages of
the operation. That added at the earlier stages will be reduced
to a state of very fine division, and will therefore leave
the stopper and mouth of A with smoother surfaces than
fresh emery.[10]
Note.—The addition of camphor to the turpentine used
for grinding glass is very important. Notwithstanding its
brittle nature, glass will work under a file moistened with
this solution almost as well as the metals. Small quantities
should be made at a time, and the solution should be kept
in a well-closed vessel, for after long exposure to the air
it is not equally valuable.
If the stopper is to fit a tube contracted like B, it must be
constructed from a piece of tube that will pass through the
contraction at B. The tail GF will not do such good
service as it does in the case of a tube which has been opened
out to receive its stopper, but it will help to guide the
stopper, and should be retained.
When the stopper has been ground into its place, melt off
the tail at F. The flame must be applied very cautiously, as
glass which has been ground is particularly apt to crack on
heating. To avoid all risk of this, the tail may simply be cut
off, and its edges filed smooth with a file moistened freely
with camphorated turpentine.
The stoppers of bottles are not made exactly in the
manner described above, though, on occasion, a new stopper
may be made for a bottle by following those directions. Ill-fitting
stoppers, which are very common, can be very easily
re-ground with emery and camphorated turpentine.
[55]
