CHAPTER VII.

VITREOUS SILICA

Introductory--Properties of Vitreous Silica--Preparing

non-splintering Silica from Brazil Pebble--Apparatus--The

Method of Making Silica Tubes--Precautions--Making Larger

Tubes and other Apparatus of Silica--Quartz Fibres, 84-95

INDEX, 97

CHAPTER I.

_GLASS-BLOWER'S APPARATUS._

=Introductory.=--I shall endeavour to give such an account of the

operations required in constructing glass apparatus as will be useful to

chemical and other students; and as this book probably will come into

the hands of beginners who are not in a position to secure any further

assistance, I shall include descriptions even of the simple operations

which are usually learned during the first few hours of practical work

in a chemical or physical laboratory. I shall not give any particular

account of the manufacture of such apparatus as thermometers, taps,

etc., because, being in large demand, they can be bought so cheaply that

time is not profitably spent in making them. But it will be found that

what is included will enable any one, who will devote sufficient time to

acquiring the necessary manipulative dexterity, to prepare such

apparatus as test-tubes, distillation flasks, apparatus for washing

gases, ozone generating tubes, etc., when they are required, as they

often are, without delay or for special purposes. The amateur probably

will not succeed in turning out apparatus so finished in appearance as

that of the professional glass-blower until after long practice, but

after a little daily practice for the space of a few weeks, any one who

is fairly skilful in ordinary manipulation, and who perseveres in the

face of failure at first, will find himself able to make almost all the

apparatus he needs for lecture or other experiments, with a considerable

saving in laboratory expenses, and, which very often is more important,

without the delay that occurs when one depends upon the professional

glass-worker. In the case of those who, like myself, work in the

provinces, this latter advantage is a very weighty one.

After the description of the instruments used in glass-blowing, which

immediately follows, the following arrangement of the subject has been

adopted. In the first place, an account of the two chief kinds of glass

is given, and of the peculiarities in the behaviour of each of them

before the blow-pipe, which is followed by a tolerably minute

description of the method of performing each of the fundamental

operations employed in fashioning glass apparatus. These are not very

numerous, and they should be thoroughly mastered in succession,

preferably upon tubes of both soda and lead glass. Then follows, in

Chapter IV., an account of the application of these operations to

setting up complete apparatus, full explanations of the construction of

two or three typical pieces of apparatus being given as examples, and

also descriptions of the modes of making various pieces of apparatus

which in each case present one or more special difficulties in their

construction; together with an account, which, I think, will be found

valuable, of some apparatus that has been introduced, chiefly during

recent years, for experimenting upon gases under reduced pressure,

_e.g._ vacuum taps and joints. Finally, in Chapter V., there is a short

account of the methods of graduating and calibrating glass apparatus for

use in quantitative experiments.

=The Working-place.=--The blow-pipe must be placed in a position

perfectly free from draughts. It should not face a window, nor be in

too strong a light, if that can be avoided, for a strong light will

render the non-luminous flames, which are used in glass-blowing, almost

invisible, and seriously inconvenience the operator, who cannot apply

the various parts of the flames to his glass with the degree of

certainty that is necessary; neither can he perceive the condition of

the glass so correctly in a strong light, for though in many operations

the glass-worker is guided by feeling rather than by seeing, yet sight

plays a very important part in his proceedings.

My own blow-pipe is placed near a window glazed with opaque glass, which

looks southwards, but is faced by buildings at a short distance. In dull

weather the light obtained is good; but on most days I find it

advantageous to shade the lower half of the window with a green baize

screen. Some glass-blowers prefer gaslight to daylight.

The form of the table used is unimportant, provided that it is of a

convenient height, and allows free play to the foot which works the

blower underneath it. The blower should be _fixed_ in a convenient

position, or it will get out of control at critical moments. The table,

or that part of it which surrounds the blow-pipe, should be covered with

sheet-iron to protect it from the action of the fragments of hot glass

that will fall upon it. The tubes that supply air and gas to the

blow-pipe should come from beneath the table, and may pass through holes

cut for the purpose.

Many glass-blowers prefer to work at a rather high table, and sit on a

rather high stool, so that they are well above their work. No doubt this

gives extra command over the work in hand, which is often valuable. On

the other hand, it is somewhat fatiguing. For a long spell of labour at

work which is not of a novel character nor specially difficult, I am

disposed to recommend sitting on a chair or low stool, at a table of

such height as will enable the elbows to rest easily upon it whilst the

glass is held in the flame. The precise heights that are desirable for

the table and stool, and the exact position of the blow-pipe, will

depend upon the height and length of arm of the individual workman, and

it must therefore be left to each person to select that which suits him

best. A moveable rest made of wood, for supporting the remote end of a

long piece of glass tube a few inches above the table, whilst the other

end is being heated in the flame, will be found convenient.

=The Blow-pipe.=--Formerly a lamp, in which sweet oil or tallow was

burnt, was employed for glass-working, and such lamps are still

occasionally used. Thus, lamps burning oil or tallow were used on board

the _Challenger_ for hermetically sealing up flasks of water collected

at various depths to preserve them for subsequent examination. I shall

not, however, give an account of such a lamp, for the gas apparatus is

so much more convenient for most purposes that it has now practically

superseded the oil lamps. Fig. 1 shows a gas blow-pipe of exceedingly

simple construction, which can be easily made, and with which good work

can be done.

[Illustration: FIG. 1.]

The tube _A_ is of brass, and has a side tube _B_ brazed to it, ten to

twelve centimetres from the end _E_, according to the dimensions of the

tube. A tube of glass, _EC_, is fitted into _A_ by a cork at _D_. _B_

is connected to a supply of gas by a flexible tube, _C_ is similarly

connected to the blower. By means of _CE_ a stream of air can be forced

into gas burning at the mouth of the blow-pipe _G_, and various flames,

with the characters described in a later section, can be produced with

this instrument. For producing the pointed flame (Fig. 3, p. 9) the

opening _E_ of the air-tube should be contracted to the size of a large

knitting needle. For producing a flame of large size, rich in air (Fig.

4, p. 9), the internal diameter of _E_ may be nearly half as great as

that of _A_ without disadvantage.

This blow-pipe may be fixed in position by the spike _F_, which will fit

into holes in a block of wood or a large cork. Several of these holes in

various positions should be made in the block, so that the position of

the blow-pipe may be varied easily. Two taps must be provided in

convenient positions near the edge of the table to enable the workman to

regulate the supplies of air and gas. These taps should be fixed to the

table and be connected with the gas and air supplies respectively on one

side, and with the blow-pipe on the other, by flexible tubes. If

blow-pipes of this kind be used, at least two of them should be

provided; one of small dimensions for working on small tubes and joints,

the other of larger size for operations on larger tubes. It will be

convenient to have both of them ready for use at all times, as it is

sometimes necessary to employ large and small flames on the same piece

of work in rapid succession. By having several air-tubes of different

sizes fitted to each blow-pipe, a greater variety of work may be done.

For the larger blow-pipe, the internal diameter of _A_ may be fifteen to

seventeen millimetres.

For the smaller instrument, eleven millimetres for the diameter of _A_

would be a useful size.

When a slightly greater outlay can be afforded it will be most

convenient to purchase the blow-pipe. They can be obtained of compact

form, supported on stands with universal joints giving great freedom of

movement, and with taps for regulating the supplies of gas and air, at

comparatively small cost.

As figures of various blow-pipes can be seen in the price-lists of most

dealers in apparatus, they are not given here. Their introduction would

be of but little service, for the construction of that which is adopted

can be readily ascertained by taking it to pieces. The simplest

blow-pipe usually used for glass-working is that of Herapath. This has

two taps to regulate the air and gas supplies respectively, and will

give a considerable variety of flames, which will be discussed

subsequently.

An excellent blow-pipe, made on the same principle as that shown in Fig.

1, but more substantially and with interchangeable jets, can be obtained

from Messrs. Muller of Holborn for a moderate outlay.

Another very good blow-pipe is the Automaton blow-pipe of Mr. Fletcher

of Warrington. In this, one tap regulates the supply both of air and

gas, which is a great gain when difficult work is in hand. Automaton

blow-pipes are made of two sizes. I have found that the larger size,

with a powerful bellows, heats large pieces of lead glass very

satisfactorily. On the other hand, the fine-pointed oxidising flame of

the Herapath blow-pipe is, perhaps, the most suitable for working joints

of lead glass. Therefore a good equipment would be a small Herapath

blow-pipe and a large-sized Automaton. If only one blow-pipe is

purchased it should be either a medium-sized Herapath, or the smaller

Automaton, as those are most useful for general work.

Mr. Fletcher also makes an ingenious combination of two blow-pipes in

which the gas and air supplies are regulated by a single lever-handle.

This is very convenient, and gives flames that answer well with tubes

made of soft soda glass, and it is very useful for general work. For use

with lead glass the supply of air is rather too small, and does not

enable one to get such good results. This can be easily amended,

however. By slightly increasing the size of the air-tube of the smaller

blow-pipe, and having increased the supply of air to the larger

blow-pipe also, by reducing the external diameter of the end of the

innermost tube, I now get medium-sized brush flames and pointed flames

with this blow-pipe, that are equal to any I have used for heating lead

glass.

For small laboratories the inexpensive No. 5 Bunsen burner of Mr.

Fletcher, which is convertible into a blow-pipe, will be very useful.

Jets of several sizes to fit the air-tubes of blow-pipes may be obtained

with them, and will serve for regulating the supply of air to the flame.

=The Bellows.=--The usual blowing apparatus is some form of foot-blower.

These may be obtained fitted to small tables with sheet-iron tops. But a

much less expensive apparatus is the large foot-blower made by Mr.

Fletcher of Warrington, which can be used at an ordinary table or

laboratory bench. Good foot-blowers can also be obtained from makers of

furnace bellows.

No part of the glass-blower's equipment exceeds the bellows in

importance. The best blower procurable should therefore be adopted. A

bellows which, when used with a large blow-pipe, will not enable you to

heat large pieces of lead glass tube to redness without blackening the

glass when the directions for heating lead glass on pages 17-21 are

followed, should on no account be received. I am told that at some

places, where the water-supply is at very high pressure, it is utilised

for working blow-pipes by means of the apparatus described below, and

that some glass-workers find it advantageous to use such automatic

blowers. But after a little practice, the effort of working the blower

with the foot whilst manipulating the glass is not a source of serious

inconvenience. Indeed, as it gives a certain degree of control over the

flame without the use of the hands, the foot-blower is preferable. It is

worth while to describe an automatic blower, however.

=Automatic Blower= (Fig. 2).--A strong glass tube _A_ is welded into a

somewhat larger tube _B_ so that its end is about 2 mm. from the

contraction at _G_. _B_ has a side tube _C_ joined to it. The narrow end

of _B_ is fixed by an india-rubber cork to a strong bottle _D_ of two or

three litres capacity. The india-rubber cork also carries an exit tube

_E_, and _D_ is pierced near its bottom by a small hole at _F_.

[Illustration: FIG. 2]

In using the apparatus _A_ is connected with the water-supply, and water

passing through _G_, carries air with it into _D_. The water escapes

from _D_ by the opening at _F_, and the air is allowed to pass out by

the tube _E_, its passage being regulated by a tap. Fresh supplies of

air enter _B_ by _C_.

=Blow-pipe Flames=--_The Pointed Flame._--If the gas tap of a Herapath

blow-pipe be adjusted so that comparatively little gas can pass, and if

the foot-blower be then worked cautiously, a long tongue of flame ending

in a fine point will be produced (Fig. 3). This flame will subsequently

be described as the _pointed flame_. It should be quite free from

luminosity, and as the amount of air necessary for securing a pointed

flame is large, in proportion to the gas, there is excess of oxygen

towards the end _C_. By adjusting the proportions of air and gas,

pointed flames of various dimensions can be obtained with the same

blow-pipe. The part of a pointed flame to be used in glass-working is

the tip, or in some cases the space slightly beyond the tip.

[Illustration: FIG. 3.]

[Illustration: FIG. 4.]

_The Brush Flame._--If a large supply of gas be turned on and a

considerable blast of air sent into the flame, a non-luminous flame of

great size will be obtained (Fig. 4). In form it somewhat resembles a

large camel's hair pencil, and may conveniently be described as a

_brush flame_. The chief advantage of a large-sized blow-pipe is, that

with it a large brush flame may be produced, which is often invaluable.

By gradually diminishing the supply of gas and air smaller brush flames

may be produced.

The jet used to supply air to the Herapath blow-pipe is usually too

fine, and consequently does not permit the passage of sufficient air to

produce a brush flame that contains excess of oxygen, even with the aid

of a very powerful blower. My own Herapath blow-pipe only gives a

satisfactory oxidising brush flame when the jet is removed altogether

from the end of the air-tube. For producing pointed flames the finer jet

of the air-tube must be used, but when a highly oxidising flame of large

size is required it must be removed. The internal diameter of the

central air-tube should be nearly half as great as that of the outer or

gas-supply tube. Fletcher's Automaton with the large air jet gives a

very liberal supply of air, and produces an excellent oxidising brush

flame. In the case of the larger-sized Automaton a consequence of this

is, however, that when fitted with the large jet it will not give so

good a pointed flame as the Herapath, which, in its turn, gives an

inferior oxidising brush. By fitting finer jets to the air-tube of

Fletcher's apparatus pointed flames can be secured when necessary.

_The Smoky Flame._--By turning on a very free supply of gas, and only

enough air to give an outward direction to the burning gas, a smoky

flame, chiefly useful for annealing and for some simple operations on

lead glass, is produced.

The Gimmingham blow-pipe and Fletcher's combination blow-pipe, in

addition to the above flames, are also adapted to produce a non-luminous

flame, resembling that of the Bunsen gas-burner, which is very

convenient for the preliminary heating of the glass, and also for

gradually cooling finished apparatus. It is not necessary to describe

the method of using these last-mentioned blow-pipes. With the more

complicated of them directions for its use are supplied.

Mr. Madan has suggested the use of oxygen in place of air for producing

the oxidising flame required for working lead glass, and to produce a

flame of high temperature for softening tubes of hard, or combustion,

glass. For the latter purpose the employment of oxygen may be adopted

with great advantage. For working lead glass, however, it is quite

unnecessary if the directions already given are followed.

The student's subsequent success will so largely depend upon his

acquaintance with the resources of his blow-pipe, and on the facility

with which he can take advantage of them, that no pains should be spared

in the effort to become expert in its management as soon as possible. A

few experiments should now be made, therefore, upon the adjustment of

the flame, until the student is able to produce and modify any form of

flame with promptness and certainty.

[Illustration: FIG. 5.]

The remaining apparatus used in glass-working consists of triangular and

other files, charcoal pastils for cutting glass, pieces of sound

charcoal of various diameters with conical ends; it is convenient to

have one end somewhat less pointed than the other (Fig. 5). Corks of

various sizes; the smallest, which are most frequently needed, should be

carefully cut with sharpened cork borers from larger corks. Besides

these there should be provided some freshly distilled turpentine in

which camphor has been dissolved,[1] fine and coarse emery powder, and

some sheets of cotton-wadding, an india-rubber blowing-bottle, glass

tubes, a little white enamel, and a pair of iron tongs.

[1] Half an ounce of camphor to about six ounces of turpentine will do

very well.

CHAPTER II.

_VARIETIES OF GLASS AND THEIR MANAGEMENT._

All the varieties of glass that are ordinarily met with contain silica

(SiO_{2}) associated with metallic oxides. In a true glass there are at

least two metallic oxides. The unmixed silicates are not suitable for

the purposes of glass. They are not so capable of developing the viscous

condition when heated as mixtures--some of them are easily attacked by

water, and many of those which are insoluble are comparatively

infusible. There is generally excess of silica in glass, that is, more

than is necessary to form normal silicates of the metals present. The

best proportions of the various constituents have been ascertained by

glass-makers, after long experience; but the relation of these

proportions to each other, from a chemical point of view, is not easy to

make out.

The varieties of glass from which tubes for chemical glass-blowing are

made may be placed under three heads, and are known as[2]--

Soft soda glass. Also known as French glass.

Lead glass. Also known as English glass.

Hard glass.

[2] For details of the composition of the various glasses, some work on

glass-making may be consulted.

In purchasing glass tubes, it is well to lay in a considerable stock of

tubes made of each of the two first varieties, and, if possible, to

obtain them from the manufacturer, for it frequently happens that pieces

of glass from the same batch may be much more readily welded together

than pieces of slightly different composition. Yet it is not well to lay

in too large a stock, as sometimes it is found that glass deteriorates

by prolonged keeping.

As it is frequently necessary to make additions, alterations, or repairs

to purchased apparatus, it is best to provide supplies both of soft soda

glass and lead glass, for though purchased glass apparatus is frequently

made of lead glass, yet sometimes it is formed from the soda glass, and

as it is a matter of some difficulty to effect a permanent union between

soda glass and lead glass, it is desirable to be provided with tubes of

both kinds.

Many amateurs find that soda glass is in some respects easier to work

with than lead glass. But, on the other hand, it is somewhat more apt to

crack during cooling, which causes much loss of time and disappointment.

Also, perhaps in consequence of its lower conductivity for heat, it very

often breaks under sudden changes of temperature during work. If,

however, a supply of good soda glass is obtained, and the directions

given in this book in regard to annealing it are thoroughly carried out,

these objections to the use of soda glass will, to a great extent, be

removed. I find, however, that when every precaution has been taken,

apparatus made of soda glass will bear variations of temperature less

well than that made of lead glass. Therefore, although the comparatively

inexpensive soda glass may be employed for most purposes without

distrust, yet I should advise those who propose to confine themselves to

one kind of glass, to take the small extra trouble required in learning

to work lead glass.

In order to secure glass of good quality, a few pieces should be

obtained as a sample, and examined by the directions given below. When

the larger supply arrives, a number of pieces, taken at random, should

be examined before the blow-pipe, to compare their behaviour with that

of the sample pieces, and each piece should be separately examined in

all other respects as described subsequently.

Hard glass is used for apparatus that is required to withstand great

heat. It is difficult to soften, especially in large pieces. It should

only be employed, therefore, when the low melting points of soda or lead

glass would render them unsuitable for the purpose to which the finished

apparatus is to be put. What is sold as Jena combustion tube should be

preferred when this is the case.

=Characters of good Glass.=--Glass tubes for glass-blowing should be as

free as possible from knots, air-bubbles, and stripes. They should be in

straight pieces of uniform thickness, and cylindrical bore. It is not

possible to obtain glass tubes of absolutely the same diameter from one

end to the other in large quantities, but the variations should not be

considerable.

When a sharp transverse scratch is made with a good file on a piece of

tube, and the scratch is touched with a rather fine point of red-hot

glass (this should be lead glass for a lead glass tube, and soda glass

for a tube of soda glass), the crack which is started should pass round

the glass, so that it may be broken into two pieces with regular ends.

If the crack proceeds very irregularly, and especially if it tends to

extend along the tube, the glass has been badly annealed, and should not

be employed for glass-blowing purposes. It is important that the point

of hot glass used shall be very small, however. Even good glass will

frequently give an irregular fracture if touched with a large mass of

molten glass.

Finally, glass tube which is thin and of small diameter should not

crack when suddenly brought into a flame. But larger and thicker tubes

will not often withstand this treatment. They should not crack, however,

when they are brought into a flame gradually, after having been held in

the warm air in front of it for a minute or so.

Good glass does not readily devitrify when held in the blow-pipe flame.

As devitrified glass very often may be restored to its vitreous

condition by fusion, devitrification most frequently shows itself round

the edges of the heated parts, and may be recognised by the production

of a certain degree of roughness there. It is believed to be due to the

separation of certain silicates in the crystallised form. Hard glass,

which contains much calcium, is more apt to devitrify than the more

fusible varieties.[3]

[3] The presence of silicates of calcium and aluminum are considered to

promote a tendency to devitrification in glass; and glasses of complex

composition are more apt to devitrify than the simpler varieties. See

_Glass-making_, by Powell, Chance, and Harris, Chap. IV.

Glass tubes are made of various sizes. When purchasing a supply, it is

necessary to be somewhat precise in indicating to the vendor the sizes

required. I have therefore placed at the end of the book, in an

appendix, a table of numbered diagrams. In ordering tubes it will

usually only be necessary to give the numbers of the sizes wished for,

and to specify the quantity of each size required. In ordering glass

tubes by weight, it must be remembered that a great many lengths of the

smaller sizes, but very few lengths of the larger sizes, go to the

pound. Larger-sized tubes than those on the diagram are also made. In

ordering them the external diameter and thickness of glass preferred

should be stated.

=Cleaning and Preparing a Tube.=--It is frequently much easier to clean

the tube from which a piece of apparatus is to be made than to clean

the finished apparatus. A simple method of cleaning a tube is to draw a

piece of wet rag which has been tied to a string through the tube once

or twice, or, with small tubes, to push a bit of wet paper or cotton

wool through them. If the dirt cannot be removed in this way, the

interior of the tube should be moistened with a little sulphuric acid in

which some bichromate of potassium has been dissolved. In any case, it

must finally be repeatedly rinsed with distilled water, and dried by

cautiously warming it, and sucking or blowing air through it. In order

to avoid heating delicate apparatus which has become damp and needs

drying, the water may be washed out with a few drops of spirit, which is

readily removed at a low temperature.

Before using a glass tube for an operation in which it will be necessary

to blow into it, one end of it must be contracted, unless it is already

of such a size that it can be held between the lips with perfect ease;

in any case, its edges must be rounded. For descriptions of these

operations, see page 35. The other end must be closed. This may be done

by means of a cork.

=Presenting Glass to the Flame.=--Glass tubes must never be brought

suddenly into the flame in which they are to be heated. All glass is

very likely to crack if so treated. It should in all cases be held for a

little while in front of the flame, rotated constantly in the hot air

and moved about, in order that it may be warmed over a considerable

area. When it has become pretty hot by this treatment, it may be

gradually brought nearer to the flame, and, finally, into contact with

it, still with constant rotation and movement, so as to warm a

considerable part of the tube. When the glass has been brought fairly

into contact with the flame, it will be safe to apply the heat at the

required part only. Care must be taken in these preliminary operations

to avoid heating the more fusible glasses sufficiently to soften them.

=Methods of working with Lead and soft Soda Glass respectively.=--When

lead glass is heated in the brush flame of the ordinary Herapath

blow-pipe, or within the point of the pointed flame, it becomes

blackened on its surface, in consequence of a portion of the lead

becoming reduced to the metallic state by the reducing gases in the

flame. The same thing will happen in bending a lead glass tube if it is

made too hot in a luminous flame. A practical acquaintance with this

phenomenon may be acquired by the following experiment:--

Take a piece of lead glass tube, bring it gradually from the point of a

pointed flame to a position well within the flame, and observe what

happens. When the glass reaches the point _A_ (Fig. 3), or thereabouts,

a dark red spot will develop on the glass, the area of the spot will

increase as the glass is brought further in the direction _A_ to _B_. If

the glass be then removed from the flame and examined, it will be found

that a dark metallic stain covers the area of the dark red spot

previously observed. Repeat the experiment, but at the first appearance

of the dark spot slowly move the glass in the direction _A_ to _C_. The

spot will disappear, and, if the operation be properly performed, in its

place there will be a characteristically greenish-yellow luminous spot

of highly heated glass. In this proceeding the reduced lead of the dark

spot has been re-oxidised on passing into the hot gases, rich in oxygen,

which abound at the point of the flame. If one end of the tube has been

previously closed by a piece of cork, and if air be forced into the tube

with the mouth from the open end before the luminous spot has become

cool, the glass will expand. If the experiment be repeated several

times, with pointed flames of various sizes, the operator will quickly

learn how to apply the pointed flame to lead glass so that it may be

heated without becoming stained with reduced lead.

If the spot of reduced metal produced in the first experiment be next

brought into the oxidising flame, it also may gradually be removed. On

occasion, therefore, apparatus which has become stained with lead during

its production, may be rendered presentable by suitable treatment in the

oxidising flame. The process of re-oxidising a considerable surface in

this way after it has cooled down is apt to be very tedious, however,

and, especially in the case of thin tubes or bulbs, often is not

practicable. In working with lead glass, therefore, any reduction that

occurs should be removed by transferring the glass to the oxidising

flame at once.

Small tubes, and small areas on larger tubes of English glass, may be

softened without reduction by means of the pointed oxidising flame; but

it is not easy to heat any considerable area of glass sufficiently with

a pointed flame. And though it is possible, with care, to employ the hot

space immediately in front of the visible end of an ordinary brush

flame, which is rich in air, yet, in practice, it will not be found

convenient to heat large masses of lead glass nor tubes of large size,

to a sufficiently high temperature to get the glass into good condition

for blowing, by presenting them to the common brush flame.

It may seem that as glass which has become stained with reduced lead can

be subsequently re-oxidised by heating it with the tip of the pointed

flame, the difficulty might be overcome by heating it for working in the

brush flame, and subsequently oxidising the reduced lead. It is,

however, difficult, as previously stated, to re-oxidise a large surface

of glass which has been seriously reduced by the action of the reducing

gases of the flame, after it has cooled. Moreover, there is this very

serious objection, that if, as may be necessary, the action of the

reducing flame be prolonged, the extensive reduction that takes place

diminishes the tendency of the glass to acquire the proper degree of

viscosity for working it, the glass becomes difficult to expand by

blowing, seriously roughened on its surface, and often assumes a very

brittle or rotten condition.

When it is only required to bend or draw out tubes of lead glass, they

may be softened sufficiently by a smoky flame, which, probably owing to

its having a comparatively low temperature, does not so readily reduce

the lead as flames of higher temperature. But for making joints,

collecting masses of glass for making bulbs, and in all cases where it

is required that the glass shall be thoroughly softened, the smoky flame

does not give good results.

In the glass-works, where large quantities of ornamental and other glass

goods are made of lead or flint glass, the pots in which the glass is

melted are so constructed that the gases of the furnace do not come into

contact with the glass;[4] and as the intensely-heated sides of the

melting-pot maintain a very high temperature within it by radiation, the

workman has a very convenient source of heat to his hand,--he has, in

fact, only to introduce the object, or that part of it which is to be

softened, into the mouth of the melting-pot, and it is quickly heated

sufficiently for his purpose, not only without contact of reducing

gases, but in air. He can therefore easily work upon very large masses

of glass. In a special case, such a source of heat might be devised by

the amateur. Usually, however, the difficulty may be overcome without

special apparatus. It is, in fact, only necessary to carry out the

instructions given below to obtain a considerable brush flame rich in

air, in which the lead glass can be worked, not only without

discoloration, but with the greatest facility.

[4] See _Principles of Glass-making_, p. 31.

_To Produce an Oxidising Brush Flame._--The blower used must be

powerful, the air-tube of the blow-pipe must be about half as great in

diameter as the outer tube which supplies the gas. The operator must

work his bellows so as to supply a strong and _steady_ blast of air, and

the supply of gas must be regulated so that the brush flame produced is

free from every sign of incomplete combustion,[5] which may be known by

its outer zone being only faintly visible in daylight, and quite free

from luminous streaks (see Fig. 4, p. 9). When a suitable flame has been

produced, try it by rotating a piece of lead glass at or near the end of

the inner blue part of the flame (_A_ Fig. 4); the appearance of the

glass will quickly indicate reduction. When this occurs move the glass

forward to the end of the outer zone _B_, but keep it sufficiently

within the flame to maintain it at a high temperature. If all is right

the metallic reduction will quickly disappear, the glass will become

perfectly transparent once more, and will present the appearance

previously observed in the experiments with the pointed flame, or, if

very hot, assume a brownish-red appearance. If this does not occur, the

supply of air must be increased or the supply of gas diminished until

the proper effects are secured.

[5] Nevertheless the supply of air must not be so excessive as to reduce

the temperature of the flame sufficiently to prevent the thorough

softening of the glass, which will occur if the bellows is worked with

too much zeal.

In working upon lead glass with the highly oxidising brush flame, it is

a good plan to heat it in the reducing part of the flame _A_ for

thoroughly softening the glass, and to remove it to the oxidising flame

_B_ to burn away the reduced metal. In prolonged operations, in order

that reduction may never go too far, hold the glass alternately in the

hot reducing flame and in the oxidising flame. The inferiority of the

outer oxidising flame to those portions nearer the inner blue zone for

softening the glass, may perhaps be accounted for by the presence of a

larger proportion of unconsumed air in the former, which being heated at

the expense of the hot gases produced by combustion, thereby lowers the

temperature of the flame. At or near _A_ (Fig. 4) where the combustion

is nearly complete, but no excess of air exists, the temperature will

naturally be highest.

If a very large tube be rotated in the oxidising flame at _B_ (Fig. 4)

it may happen that the flame is not large enough to surround the tube,

and that as it is rotated those parts of it which are most remote from

the flame will cool down too considerably to allow all parts of the tube

to be simultaneously brought into the desired condition. This difficulty

may be overcome by placing two blow-pipes exactly opposite to each

other, at such a distance that there is an interval of about an inch

between the extremities of their flames, and rotating the tube between

the two flames. It may be necessary to provide two blowers for the

blow-pipes if they are large.

Again, if a very narrow zone of a tube of moderate size is to be heated,

two pointed flames may be similarly arranged with advantage.

Occasionally more than two flames are made to converge upon one tube in

this manner.

Another method of preventing one side of a tube from cooling down whilst

the other is presented to the flame, is to place a brick at a short

distance from the extremity of the flame. The brick checks the loss of

heat considerably. A block of beech wood may be used for the same

purpose, the wood ignites and thereby itself becomes a source of heat,

and is even more effective than a brick.

Fuller details of the management of lead glass under various

circumstances will be found in the subsequent descriptions of operations

before the blow-pipe.

Before proceeding to work with soda glass, the student should not only

verify by experiments what has been already said, but he should

familiarise himself with the action of the blow-pipe flame on lead glass

by trying the glass in every part of the flame, varying the proportions

of gas and air in every way, repeating, and repeating, his experiments

until he can obtain any desired effect with certainty and promptitude.

He should practice some of the simpler operations given in Chapter III.

in order to impress what he has learned well on his mind.

=Management of Soda Glass.=--In working with soda glass the following

points must be constantly kept in mind. That as it is much more apt than

lead glass to crack when suddenly heated, great caution must be

exercised in bringing it into the flame; and that in making large joints

or in making two joints near each other, all parts of the tube adjacent

to that which, for the moment, is being heated, must be kept hot, as it

is very apt to crack when adjacent parts are unequally heated. This may

be effected by stopping work at short intervals and warming the cooler

parts of the tube, or by the use of the brick or block of wood to check

radiation, or even by placing a supplementary blow-pipe or Bunsen burner

in such a position that its flame plays upon the more distant parts of

the work, not coming sufficiently into contact to soften the glass,

however, but near enough to keep it well heated. Lastly, to prevent the

finished work from falling to pieces after or during cooling, the

directions given under the head of annealing must be carefully carried

out.

In very much of his work the glass-blower is guided more by the _feel_

of the glass than by what he sees. The power of feeling glass can only

be acquired by practice, and after a certain amount of preliminary

failure. As a rule I have observed that beginners are apt to raise their

glass to a higher temperature than is necessary, and that they employ

larger flames than are wanted. If glass be made too soft it may fall so

completely out of shape as to become unworkable except in very skilful

hands. The following rules, therefore, should be strictly adhered to.

Always employ in the first instance the smallest flame that is likely to

do the work required. In operations involving _blowing out_ viscous

glass, attempt to blow the glass at low temperatures before higher ones

are tried. After a little experience the adoption of the right-sized

flame for a given purpose, and the perception of the best condition of

glass for blowing it, become almost automatic.

I may add that glass which is to be bent needs to be much less heated

than glass which is to be blown.

=Annealing.=--If apparatus, the glass of which is very thin and of

uniform substance, be heated, on removal from the source of heat it will

cool equally throughout, and therefore may often be heated and cooled

without any special precautions. If the glass be thick, and especially

if it be of unequal thickness in various parts, the thinner portions

will cool more quickly than those which are more massive; this will

result in the production of tension between the thicker and thinner

parts in consequence of inequality in the rates of contraction, and

fractures will occur either spontaneously or upon any sudden shock.

Thus, if a hot tube be touched with cold or wet iron, or slightly

scratched with a cold file, the inequality of the rate of cooling is

great, and it breaks at once. It is therefore necessary to secure that

hot glass shall cool as regularly as possible. And this is particularly

important in the case of articles made of soda glass. Some glass-blowers

content themselves with permitting the glass to cool gradually in a

smoky flame till it is covered with carbon, and then leave it to cool

upon the table. But under this treatment many joints made of soda glass

which are not quite uniform in substance, but otherwise serviceable,

will break down. In glass-works the annealing is done in ovens so

arranged that the glass enters at the hottest end of the oven where it

is uniformly heated to a temperature not much below that at which it

becomes viscous, and slowly passed through the cooler parts of the

chamber so that it emerges cold at the other end. This method of

annealing is not practicable in a small laboratory. But fortunately very

good results can be obtained by the following simple device, viz.:--

By wrapping the hot apparatus that is to be annealed closely in cotton

wool, and leaving it there till quite cold. The glass should be wrapped

up immediately after it is blown into its final shape, as soon as it is

no longer soft enough to give way under slight pressure. And it should

be heated as uniformly as possible, not only at the joint, but also

about the parts adjacent to the joint, at the moment of surrounding it

with the cotton. Lead glass appears to cool more regularly than soda

glass, and these precautions may be more safely neglected with apparatus

made of lead glass; but not always. At the date of writing I have had

several well-blown joints of thick-walled capillary tube to No. 16 (see

diagram, p. 82), break during cooling, in consequence of circumstances

making it dangerous to heat the neighbourhood of the joint so much as

was necessary.

The black carbonaceous coat formed on hot glass when it is placed in

cotton wool may be removed by wiping with methylated spirit, or, if it

be very closely adherent, by gently rubbing with fine emery, moistened

with the spirit.

Cotton wool is rather dangerously inflammable; it should therefore be

kept out of reach of the blow-pipe flame, and care should be taken that

the glass is not placed in contact with it at a sufficiently high

temperature to cause its ignition.

Another method of annealing is to cover the hot glass with hot sand, and

allow it to cool therein.

As in the case of lead glass, so with soda glass. A thorough

acquaintance with the effect of the various parts of the flame upon it

should be gained before further work is entered upon, for which purpose

an hour or more spent in observing its behaviour in the flame will be

fully repaid by increased success subsequently.

=The Use of Combustion Tube.=--It is often necessary to construct

apparatus of what is known as hard glass or combustion tube. It is

almost as easy to work combustion tube as to deal with lead and soda

glass if the oxy-hydrogen flame be employed.

It is not necessary to set up a special apparatus for this purpose; many

of the ordinary blow-pipes can be used with oxygen instead of with air.

It is only necessary to connect the air-tube of the blow-pipe with a

bottle of compressed oxygen instead of with the bellows. The connecting

tube should not be too wide nor too long, in order to avoid the

accumulation in it, by accident, of large quantities of explosive

mixtures.

Two precautions are necessary in manipulating hard glass in the

oxy-hydrogen flame. The glass must _not_ be overheated. At first one is

very apt to go wrong in this direction. The supply of oxygen must _not_

be too great; a small hissing flame is not what is wanted. If either of

these precautions are neglected most glass will devitrify badly. With a

little care and experience, devitrification can be absolutely avoided.

Ordinary combustion tube can be used, but I find that the glass tube

(Verbrennungsroehr) made by Schott & Co. of Jena, which can be obtained

through any firm of dealers in apparatus, is far better than the

ordinary tube.

By following these instructions, any one who has learned how to work

with lead or soda glass will find it easy to manipulate hard glass.

CHAPTER III.

_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.

=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.

[Illustration: FIG. 6.]

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.

[Illustration: 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.

(_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.

[6] Remember that when the lead glass is heated to the proper

temperature it will present an appearance which may be described as a

greenish phosphorescence. At higher temperatures it assumes an

orange-red appearance. If it loses its transparency and assumes a dull

appearance, it must be moved further into the oxidising parts of the

flame.

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.

[Illustration: FIG. 8.]

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.

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.

[Illustration: FIG. 9.]

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.

[Illustration: 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.

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.

[Illustration: FIG. 11.]

[Illustration: 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.

[Illustration: 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.

[7] Some glass-blowers at once work on the glass as next described,

without this preliminary treatment. I find that some glass, usually soda

glass, will not always bear the necessary movements without breaking

unless first heated all round.

[Illustration: 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.

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.

[Illustration: FIG. 15.]

_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.

[Illustration: 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.

[Illustration: 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]

[8] If such an opening be observed, it may usually be closed by touching

its edges with a fused point of glass at the end of a drawn out tube.

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.

[9] This can be obtained from Messrs. Powells, Whitefriars Glassworks.

_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).

[Illustration: 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_.

[Illustration: 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.

[Illustration: 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.