MODIFIED METHODS AND SPECIAL OPERATIONS
CAPILLARY TUBING
This is commonly used in many forms of apparatus for gas analysis, and
one is often called upon to join two pieces or to make a tee on it. The
methods are nearly the same as with other tubing, except that more care
and patience are required. The work must be done much more slowly on
account of the thickness of the walls, and open ends of the tube must
always be enlarged before joining them to anything. This is best done by
carefully sealing the end and then blowing, with several suitable
reheatings, to form a pear-shaped bulb as in _a_, Fig. 14. The end of
this is then heated and blown off, and the piece is ready to be joined
to another similar end, or to a piece of ordinary tubing if desired. The
joints are best not blown too much, as thick walls shrink very slowly.
Much may be done by gently pushing the tube together or pulling it apart
in the flame, to remove lumps and irregularities. It is necessary that
the bore of the joint be approximately that of the main tube, and care
must be taken that the latter is not constricted at the point where the
joint begins.
[Illustration: FIG. 14.--Capillary tubing.]
Especial care must be taken to warm the tube slowly when starting and
cool it slowly when through, as the thick walls frequently crack if not
carefully handled. For this reason the whole neighborhood of the joint
must be heated somewhat so that there may not be stresses set up between
the heated and unheated portions.
In making the tee (_b_, Fig. 14) the inability to blow the joint makes
itself decidedly felt, but if the side tube is properly enlarged as
previously described, a good joint can be made by alternately pulling
and pushing on the end of the side tube, and shrinking well.
Very fine capillary tubing should be blown with a rubber bulb instead of
the mouth, so as not to get moisture into the tube. The rubber bulb may
also be used to advantage on some of the coarser capillary tubing.
When a bulb is to be joined to a piece of capillary tubing, the joint is
preferably made before blowing the bulb, and will then be taken up a
little way on the bulb during the process. Care must of course be taken
not to constrict the capillary; the pear-shaped bulb blown on the end
(_a_, Fig. 14) may well extend back a little further than usual into the
tube so as to prevent this. If a bulb is required in the middle of a
capillary tube, the latter is usually best cut and a piece of ordinary
tubing of suitable size sealed in to provide material for the bulb.
GLASS ROD
Joints, tees, etc., in glass rod are made on the same principle as in
tubing, except that of course they cannot be blown, and regularity must
be obtained by accumulating a small mass of uniformly heated glass, and
then drawing it to a suitable rod, on the same principle as Exercise No.
1.
Great care must be taken in heating and cooling this, as in the case of
the capillary tubing, and for the same reasons.
By joining pieces side by side, pressing with carbon plates or a plate
and a rod, and other suitable manipulations, stirrers, spatulas, and
other objects may easily be made from rod, and its manipulation is
relatively easy on account of the fact that one does not have to worry
about the bore of the tube. But the same general rule about not having
thick and thin spots in contact, and making all changes in diameter on a
taper if possible instead of abruptly, applies here. Thick pieces will
cool and contract at different rates from thin ones, and cracks are
likely to develop where they join. Work which has been formed with any
tool must always be heated to the softening point afterward before
allowing it to cool in order to remove the stresses caused by the
contact of the tool with the hot glass.
When it is necessary to join a piece of rod to the side of a piece of
tubing, the end of the rod is made very hot while the wall of the tube
at the spot desired is heated to just below the softening temperature.
The rod can then be pressed into firm union with the tube and drawn a
little to remove the excess of glass without deforming the tube.
MENDING STOPCOCKS
=Mending the Plug.=--The plug of the stopcock occasionally falls out and
is broken. If the break is in the main part of the plug, nothing can be
done except to search for a spare plug of suitable size and grind it to
fit, as described below. If only the little cross-piece at the end is
broken off, it can easily be replaced. In most ordinary stopcocks the
plug is solid, but the little handle is hollow. What has been said above
regarding care in heating and cooling glass rod applies with especial
force here. It is usually best to wind the whole of the plug with
several thicknesses of asbestos cord, leaving bare only the end where
the handle is to be joined. This diminishes the danger of cracking the
plug by too rapid heating, and also makes it more comfortable to hold. A
piece of rather thick-walled tubing of suitable diameter is chosen,
drawn out so as to have a suitable taper (taking care to heat enough of
the tube so that the capillary tail has good wall-thickness and
strength), and then a corresponding taper is drawn to form the other
side of the handle. The result is shown in Fig. 15, _a_. The capillary
tail is now heated and bent back to form a handle which will be in the
same straight line as the axis of the plug (_b_, Fig. 15) and the main
part of the tube drawn off at the dotted line, making a neat seal at
that point. The broken end of the plug is now slowly warmed in the smoky
flame, the heat gradually increased by a gentle stream of air from the
bellows, and the point at which this handle is to be attached finally
brought to the temperature at which the glass flows freely. In the mean
time, the little handle has been warmed almost to the softening point.
It is now quickly pushed into place (_c_, Fig. 15), taking care that its
axis is parallel to the hole in the plug, and then drawn away from the
plug just enough to make a graceful neck instead of the bulging one
indicated by the arrow in the figure. With a fine pointed flame the
little tail is now drawn off at the point indicated by the dotted line
(_c_, Fig. 15) and the whole carefully annealed. If necessary, the
handle can be blown a little before the tail is removed. Local heating
and blowing at the point where the handle joins the plug is often
necessary in order to make a smooth job.
[Illustration: FIG. 15.--Stopcock plug.]
=Regrinding.=--This is sometimes necessary to make stopcocks tight, when
the grinding has not been properly done in the factory. For this, a very
little fine flour of emery or carborundum is the best and quickest. If
this is not at hand, some clean sand may be ground in an agate mortar,
and if possible sieved. Only material which passes the 100-mesh sieve
should be used. It will be ground still finer in the process. For the
final polishing, a little infusorial earth or even kaolin will do.
The surface to be ground is moistened with water and dusted over with a
little of the abrasive. The plug is now inserted in the stopcock, and
turned with a gentle pressure. This turning should be in the same
direction for several revolutions, then in the opposite direction for
several more revolutions, etc. As the abrasive becomes finer during the
grinding, a little more may be added if necessary. In general, only a
little grinding will be required, and one small pinch of carborundum or
emery will be ample. The beginner usually grinds too much, and with too
coarse material. As the grinding surface becomes dry, water is added
drop by drop, and the grinding continued until the abrasive seems to be
reduced to an impalpable powder, most of which has been squeezed out of
the stopcock. The two surfaces in the stopcock are usually grinding upon
each other at this stage, and inspection will show whether the contact
between them is uniformly good. If not, the grinding must be continued
with a little fresh abrasive. If contact appears to be good, the
surfaces are ground together for a little with practically no abrasive,
so as to polish them, and the joint is then washed out and tested.
In grinding in a new plug to replace a broken one, the plug selected
should have practically the same taper as the seat into which it is to
be ground, and should be a very little too large. Care must be taken to
so distribute the abrasive material as to grind mostly on the places
where the plug fits tightly.
=Sealing on a New Tube.=--It frequently happens that one of the tubes of
the stopcock is broken off close to the cock itself, and a new one must
be joined to the stub of the old one. With care, this may often be
successfully done even where the break is within 1/4 inch of the
stopcock. The first step is to clean and dry the stopcock, remove the
plug, cork the open ends of the stopcock sleeve and the other tube, and
wind a couple of layers of asbestos cord carefully over the sleeve and
the most of the corks which close it. A suitable tube, having as near as
possible the same diameter and wall strength as the one broken off, is
selected and a piece the desired length cut off. The broken end of the
tube on the stopcock is now squared off as well as possible, by cutting
or by heating and drawing off the projections, and the new tube sealed
on, usually with the first method (Exercise No. 1). If the break is very
close to the stopcock, very little reheating and blowing can be done, on
account of the danger of getting the stopcock sleeve out of shape, and
the work must be heated very slowly to prevent cracking. The main
reliance is then placed on making a good joint when the tubes are
brought together, and then drawing out this joint a little, at once, to
get an even wall.
CLOSED CIRCUITS OF TUBING.
In some pieces of apparatus closed circuits of circular or rectangular
shape are required. A similar problem is involved in apparatus like the
ordinary Soxhlet extractor, where a small tube is joined to the side of
a large one, bent to form a siphon, and attached again to a continuation
of the original large tube. The difficulty in all such cases is to
provide for the contraction taking place as the last joint cools. If
part of the circuit has the shape of the letter S, or is a spiral, the
natural springiness of the glass will take care of this. If not, the
side of the circuit opposite to the joint and parallel to it must be
heated also, the two being finally heated together to the softening
point after the joint is completed, and then allowed to cool together.
To make the last joint, the rest of the tube is made in approximately
the desired form, the two pieces which are to be joined to make the last
joint being just enough out of the desired position to allow them to
pass one another. The final joint is preferably made in the middle of a
straight piece of tube, not at a tee. The two pieces which are to be
joined are bent so as to just pass each other, marked at the right point
with the glass-knife, and cut there, preferably with a small bead of hot
glass. One or both of these tubes are now warmed to the softening point
in such a place that the tubes can be made to meet properly, and the two
cut ends pressed together. They are now warmed in the flame, and joined
together, either by simultaneously warming the opposite side of the
circuit or some other suitable part, so as to allow the two ends to be
pushed together again after they are softened, or by gently touching the
places that do not unite with a hot bead of glass, and using the glass
to fill up the crack where the ends do not quite meet. Care must be
taken not to leave knots or lumps of glass in the finished joint, and
the latter should be well reblown, and if necessary left as a small bulb
or enlargement, rather than have it have too thick walls.
SPIRALS
Spirals of glass tubing are probably best made free-hand before the
blow-pipe, unless one has a great many of them to make, and extreme
accuracy is desired. To begin with, a piece of tubing of the desired
size (say 3/16 inch in diameter) and a convenient length (about two
feet) is selected, one end closed, and a right-angle bend made about six
inches from the closed end. Holding the closed end in the left hand and
the long open one in the right, the spiral is begun. The short closed
end is to be parallel to the axis of the spiral, and preferably in that
axis. Using a moderate-sized flame, of somewhat yellow color, and taking
care to heat the whole circumference of the tube, the long open end is
wound little by little into a spiral having the short end _a_ (Fig. 16)
as an axis. The bend at _b_, where the tube changes from the radius to
the circumference of the circle, must be rather short, but the tube must
not be flattened or constricted here. Especial pains is to be taken
with the first turn of the spiral (_b_ to _c_, Fig. 16), as the shape of
this determines the diameter of the whole spiral, and serves as a guide
for the rest of the turns. The winding of the tube is best accomplished,
after a portion has been softened, by slowly turning the short end _a_ a
little about its own axis, while the long open end remains where it was.
This winds the tube into a spiral, just as if there were a solid
cylinder in the center of it, and this cylinder was being turned about
its axis, and was winding up the soft glass upon its circumference. As
the cylinder is not actually there, the curve of the turns must be
carefully estimated by the eye, so that the spiral may be uniform and
moderately smooth. When the original piece of tube has been used up,
another piece is sealed on to the open end, and the operation continued
as far as may be required.
[Illustration: FIG. 16.--Making a spiral.]
GROUND JOINTS
It is sometimes required to join two pieces of tubing end to end, by
means of a ground joint. Whenever possible, a regular sealed joint
should be used instead of this ground joint, as it is quicker to make,
and more certain to be tight. Where a ground joint is necessary,
however, it is best made in the conical form shown in _c_, Fig. 17. If
the wall of the tube to be used is not very thick, it is thickened by
collecting glass as for a bulb on the ends of two tubes (Exercise No.
6), and drawing to form cones of suitable shape (_a_ and _b_, Fig. 17)
and of such relative sizes that a will slip about half way into _b_. In
order to make _a_ straight and give it the proper angle, it may be
rolled when hot, upon a hot plate of carbon. Blowing during this rolling
is often helpful to remove depressions. After _b_ has been drawn to
nearly the proper size and shape, it may be smoothed by the use of a
small carbon rod, held inside it at a slight angle, or better by the use
of a truncated hexagonal pyramid of carbon, whose edges have the proper
slant to make the inside of the cone right. The proper taper for both
these cones is the same as that used in stopcocks of similar size. The
hexagonal carbon can easily be made by carefully filing down an electric
light carbon, and finally impregnating it with paraffin or beeswax, and
is extremely useful wherever a conical surface has to be formed from the
inside of a tube.
[Illustration: FIG. 17.--Ground joint.]
The tail is allowed to remain on piece _a_, as a sort of guide in
grinding, and should therefore be in the axis of the tube and have
rather thick walls. Grind with emery or carborundum, as described under
a previous head. (Regrinding plug for stopcock.) If many such joints are
to be made, it will pay to have a little sleeve of brass made with the
proper taper, and rough down the plug _a_ in it to about the proper
size, while _b_ is roughed down by means of a brass or iron plug having
the same taper. This prevents excessive grinding of one-half of the
joint in order to remove a defect in the other half, and is the method
commercially used in making stopcocks.
SEALING IN PLATINUM WIRE
Very often it is necessary to seal platinum wire into the wall of a
tube. Professional glass-blowers usually use a special sort of glass
("Einschmelzglas") which is usually a lead glass, and is made of such
composition that it has the same or practically the same coefficient of
expansion as platinum. A little globule of this glass is sealed into the
tube in such a way that it joins the platinum to the glass of the tube.
To do this, the small globule of special glass is fused on the platinum
wire at the proper point and the tube into which the wire is to be
sealed is heated and a small tail drawn out at the point where the wire
is to be inserted. The lump of the special glass should be from 3/32 to
1/8 inch in diameter, and the tail drawn on the tube should have a
slightly less diameter at the point (about 1/8 inch or less from the
tube) where it is cut off. There are now two ways of sealing in the
wire. (1) The wire with the globule of glass is placed inside the tube
and the latter revolved until the end of the wire sticks out of the cut
tail (_a_, Fig. 18). The latter is now gently heated, and the two glass
surfaces fused together, taking care to use only the end of the hissing
flame, if the special glass contains lead. (See Chapter I, page 1.) The
whole circumference of the tube is then heated and annealed carefully.
(2) The end of the wire which is to be outside the tube is attached to
the end of a thin scrap of glass, by heating the glass and thrusting
the wire into it a very little way. Using this piece of glass as a
handle, the wire is inserted in the cut tail (_b_, Fig. 18) and the
globule brought near to the end of the tail. (If the main tube is cold,
it must of course first be warmed.) With the end of the hissing flame,
as in the first method, the globule of glass is melted and the end of
the tail softened. The wire is now pushed into place, the handle removed
by heating the end and withdrawing it, and the tail reheated a little if
necessary to make it shrink back into line with the walls of the tube.
The whole circumference of the tube is heated at that point and annealed
as usual.
[Illustration: FIG. 18.]
The use of this special glass is not absolutely necessary if the
platinum wire is small (1/4 millimeter or less in diameter), and in fact
it is often better in such cases not to use it, unless the apparatus is
to be subjected to a very high vacuum. On small tubes, especially, it is
undesirable to use the special glass, as a lump of it will usually cause
the tube to crack on cooling. When such glass is not at hand or is not
to be used, the procedure is altered somewhat. The tail which is drawn
out is very fine, having only a sufficient diameter so that when it is
cut off the wire can be inserted in it. Such a fine tail is readily made
by heating a small spot on the tube, touching it with a warm platinum
wire, removing from the flame and drawing out the tail with the wire.
After cutting off the tail the wire is inserted in it, being held on a
scrap of glass as in the previous case, and the wire and tail heated
until the latter shrinks back into line with the walls of the tube. If
too great shrinkage occurs, the place may be blown out gently after
reheating. Thus the wire is sealed through the wall of the tube without
changing the thickness of the latter, and consequently without
developing undue stresses at that point. Such a joint must of course be
carefully reheated and annealed. With fine platinum wire there is very
little risk of the tube cracking if care is taken to avoid formation of
any lump and to reheat the whole circumference of the tube at that
point.
Any glass adhering to the end of the platinum wire, where the scrap of
glass was sealed on for a handle, may be removed when the glass has
cooled by crushing it carefully with a pair of pliers.
SEALING VACUUM TUBES
Tubes which have been evacuated usually are sealed off while they are
still connected to the vacuum pump. The connection should be through a
small, rather thick-walled tube. When this is to be sealed, it is slowly
heated toward the softening point. As the glass just begins to soften,
the air-pressure will force it in, and care must be taken that the
softening is uniform over the whole circumference of the tube. As the
shrinking goes on, the tube is gently drawn out to make a thick-walled
cone at that place, and the end is drawn off as soon as the tube is
sealed. The principal point to be guarded is the thickness of the walls
of the cone, and uniform heating. A thin place or a hot place will give
way under the air-pressure and be sucked into the tube.
CLOSED TUBES FOR HEATING UNDER PRESSURE
(_Carius method for determination of the halogens and sulphur._) In this
case the tubing used must have thick walls (usually about 3/32 inch) to
withstand the pressure. Its external diameter is usually about 3/4 inch.
One length will usually make two tubes of standard length for the cannon
furnace. Especial care must be taken in heating and cooling it on
account of the thick walls. A length is gradually warmed in the center,
finally heated at that point until soft, drawn out, cut apart and
annealed. Taking one of the pieces, the cone is carefully heated and
shrunk, as in Exercise 4, until its walls are as thick as those of the
main tube. A flame with a little tinge of yellow should be used for this
operation to prevent devitrification (page 2), as the thick glass
shrinks slowly. The tail is now drawn off and the whole end heated and
gently blown several times to make a rounded end, like a test-tube, with
walls as thick as those of the main tube. This must be carefully
annealed. It is more important that the walls be thick than that the end
be nicely rounded: it may indeed be left somewhat conical in shape.
At a point about two inches from the open end of the tube, it is slowly
warmed and finally heated to the softening point. Grasping the open end
with a pair of crucible tongs, it is cautiously pulled out, a little at
a time, usually during rotation in the flame, to make a constriction of
moderate wall-thickness, but of sufficient internal diameter to admit
the tube containing the substance. After annealing this, cooling and
cleaning the tube, the acid and salt are introduced (the former by means
of a long-stemmed funnel) and the tube is inclined and rotated about its
axis so that the acid wets its surface about half way up from the
bottom. The substance is now weighed out in a piece of thin-walled glass
tubing, closed at one end, and about two inches long. Inclining the
large tube at a suitable angle, the small one is introduced, closed end
first, and allowed to slide down the walls of the large tube until it
reaches the place where the acid has wet the tube. Here it will stop,
and if the tube is kept inclined during the rest of the operation it
will roll around inside the tube at this point and thus not get down
where any acid is likely to get into it and produce any pressure by
decomposing it before the open end of the tube is sealed. Now the tube
is held in an inclined position, taking care that the acid does not
reach up to the substance, the constricted portion cautiously warmed and
shrunk. It is finally shrunk and drawn out into a somewhat elongated
cone, with walls as thick as the rest of the tube, and when this is
accomplished the end of the cone is sealed and the waste piece drawn
off. Anneal with great care, and cool in such a position that the acid
cannot reach the hot glass. The shrinking of this cone takes a good deal
of patience, and is one of the most important parts of the process. If
the walls are left too thin, the tube may burst when heated, and the
whole labor is lost. If care is taken, the same tube can be used for a
number of determinations, until it becomes quite short.
INDEX
Annealing glass, 4, 24
Bellows, 4
Bending glass, 8
Blowing glass, 13, 19, 20, 21, 24, 29, 31
with a rubber tube, 22
Blowpipe, 4
Bulb at end of tube, 28
in middle of tube, 32
very large, 32
Bulbs, string of, 33
Capillary tube, drawing on larger tube, 9, 54
tubing, working, 43
Carius method, tubes for, 55
Closed circuits of tubing, 48
tubes, for heating under pressure, 55
Collecting glass for bulb, 29, 31, 32
Constricting a tube, 10
Crystallization of glass, see Devitrification.
Cutting glass, 7, 25
Devitrification, 1, 2
Drawing out a tube, 9, 18, 19, 27
Flanging a tube, 11, 14
tool, 11
Gas-washing tube, 35
Glass, annealing, 4, 24
Glass, bending, 8
blowing, 13, 19, 20, 21, 24, 29, 31
collecting for bulb, 29, 31, 32
cutting, 7
defects, 2
grinding, 47
hard, 1
knife, 7
lead, 1
qualities desired, 1
rod and tube, joining, 45
rod, working, 44
shrinking, 18, 19, 22, 26
soft, 1
working temperature, 1, 13, 19, 27
Grinding stopcock or joint, 47
Ground joints, 51
Handle on stopcock, mending, 45
Hard glass, 1
Holding tube, 13, 14
Insertion of tube through another, see Sealing a tube through
another tube.
Joints, ground, 51
Joining rod and tube, 45
tubing end to end: first method, 16
second method, 20
Joining tubes of different diameters, 25
a new tube to a stopcock, 48
Kjeldahl trap, 41
Lead glass, 1
Lump of glass, removed, 18, 19, 20, 21, 24, 26, 30, 38
Platinum wires, sealed into glass, 1, 52
Position for glass-working, 5
Pressure, tubes for heating under, 55
Quality of glass, 1
Rod, glass, working, 44
Rotation of the tube, 13, 19
Rounded end of tube, 35, 38
Rubber tube used for blowing, 22
Sealing a tube through another tube, 35, 39
Sealing vacuum tubes, 55
Shrinking glass, 18, 19, 22, 26, 31
Side tube, blowing, 22, 25
Soda glass, 1
Soft glass, 1
Spirals, making, 50
Stopcocks, mending, 45
Suction pump, 39, 42
Sulphur dioxide tube, 28
"Tail" of glass, drawing out, 9, 54
removed, 30, 35
Tubes, closed, for heating under pressure, 55
"Tee" tube, 22
on capillary tubing, 43
small side tube on a large tube, 24
Vacuum tubes, sealing, 55
Working temperature of glass, 1, 13, 19, 27
+----------------------------------------------------------------------+
| |
| |
| Transcriber's note:- |
| |
| Words in italics are indicated by the use of _underscores_ and words |
| |
| in =bold= by the use of equals signs as shown. |
| |
| |
| |
+----------------------------------------------------------------------+