Produced by Jon Richfield
ON LABORATORY ARTS
BY
RICHARD THRELFALL, M.A.
PROFESSOR OF PHYSICS IN THE UNIVERSITY OF SYDNEY;
MEMBER OF THE INSTITUTE OF ELECTRICAL ENGINEERS;
ASSOCIATE-MEMBER OF THE INSTITUTE OF CIVIL ENGINEERS;
MEMBER OF THE PHYSICAL SOCIETY
London
MACMILLAN AND CO, LIMITED
NEW YORK: THE MACMILLAN COMPANY
1898
All rights reserved
PREFACE 5
CHAPTER I 8
HINTS ON THE MANIPULATION OF GLASS AND ON GLASS-BLOWING FOR LABORATORY
PURPOSES 8
§ 4. Soft Soda Glass.
§ 6. Flint Glass.
§ 9. Hard or Bohemian, Glass.
§ 10. On the Choice of Sizes of Glass Tube.
§ 11. Testing Glass.
§ 13. Cleaning Glass Tubes.
§ 14. The Blow-pipe.
§ 18. The Table.
§ 19. Special Operations.
§ 20. Closing and blowing out the End of a Tube.
§ 21. To make a Weld.
§ 22. To weld two Tubes of different Sizes.
§ 24. To weld Tubes of very small Bore.
§ 30. To cut very thick Tubes.
§ 31. To blow a Bulb at the End of a Tube.
§ 32. To blow a bulb in the middle of a tube.
§ 33. To make a side Weld.
§ 34. Inserted Joints.
§ 35. Bending Tubes.
§ 36. Spiral Tubes.
§ 37. On Auxiliary Operations on Glass.
§ 38. Boring small Holes.
§ 39. For boring large holes through thick glass sheets.
§ 41. Operations depending on Grinding: Ground-in Joints.
§ 42. Use of the Lathe in Glass-working.
§ 46. Making Ground Glass.
§ 47. Glass-cutting.
§ 48. Cementing.
§ 49. Fusing Electrodes into Glass.
§ 51. The Art of making Air-light Joints.
APPENDIX TO CHAPTER I.
ON THE PREPARATION OF VACUUM TUBES FOR THE PRODUCTION OF PROFESSOR
ROENTGEN'S RADIATION.
CHAPTER II.
GLASS-GRINDING AND OPTICIANS' WORK.
§ 61. Details of the Process of Fine Grinding.
§ 62. Polishing.
§ 63. Centering.
§ 65. Preparing Small Mirrors for Galvanometers.
§ 66. Preparation of Large Mirrors or Lenses for Telescopes.
§ 69. The Preparation of Flat Surfaces of Rock Salt.
§ 70. Casting Specula for Mirrors.
§ 71. Grinding and polishing Specula.
§ 72. Preparation of Flat Surfaces.
§ 73. Polishing Flat Surfaces on Glass or on Speculum Metal.
CHAPTER III.
MISCELLANEOUS PROCESSES.
§ 74. Coating Glass with Aluminium and Soldering Aluminium.
§ 75. The Use of the Diamond-cutting Wheel.
§ 76. Arming a Wheel.
§ 77. Cutting a Section.
§ 78. Grinding Rock Sections, or Thin Slips of any Hard Material.
§ 79. Cutting Sections of Soft Substances.
§ 80. On the Production of Quartz Threads.
§ 84. Drawing Quartz Threads.
§ 86. Drawing Threads by the Catapult.
§ 87. Drawing Threads by the Flame alone.
§ 88. Properties of Threads.
§ 90. On the Attachment of Quartz Fibres.
§ 91. Other Modes of soldering Quartz.
§ 92. Soldering.
§ 94. Preparing a Soldering Bit.
§ 95. Soft Soldering.
§ 97. Soldering Zinc.
§ 98. Soldering other Metals.
§ 99. Brazing.
§ 100. Silver Soldering.
§ 101. On the Construction of Electrical Apparatus--Insulators.
§ 102. Sulphur.
§ 103. Fused Quartz.
§ 104. Glass.
§ 105. Ebonite or Hard Rubber.
§ 106. Mica.
§ 107. Use of Mica in Condensers.
§ 108. Micanite.
§ 109. Celluloid.
§ 110. Paper.
§ 111. Paraffined Paper.
§ 112. Paraffin.
§ 113. Vaseline, Vaseline Oil, and Kerosene Oil.
§ 114. Imperfect Conductors.
§ 116. Conductors.
§ 117. Platinoid.
§ 119. Platinum Silver.
§ 120. Platinum Iridium.
§ 121. Manganin.
§ 122. Other Alloys.
§ 123. Nickelin.
§ 124. Patent Nickel.
§ 125. Constantin.
126. Nickel Manganese Copper.
CHAPTER IV.
ELECTROPLATING AND ALLIED ARTS.
§ 127. Electroplating.
§ 128. The Dipping Bath.
§ 130. Scratch-brushing.
§ 131. Burnishing.
§ 132. Silver-plating.
§ 133. Cold Silvering.
§ 134. Gilding.
§ 135. Preparing Surfaces for Gilding.
§ 136. Gilding Solutions.
§ 137. Plating with Copper.
§ 138. Coppering Aluminium.
§ 140. Alkaline Coppering Solution.
§ 141. Nickel-plating.
142. Miscellaneous Notes on Electroplating.
§ 143. Blacking Brass Surfaces.
§ 144. Sieves.
§ 145. Pottery making in the Laboratory.
APPENDIX.
PLATINISING GLASS.
PREFACE
EXPERIMENTAL work in physical science rests ultimately upon the
mechanical arts. It is true that in a well-appointed laboratory,
where apparatus is collected together in greater or less profusion,
the appeal is often very indirect, and to a student carrying out a set
experiment with apparatus provided to his hand, the temptation to
ignore the mechanical basis of his work is often irresistible.
It often happens that young physicists are to be found whose
mathematical attainments are adequate, whose observational powers are
perfectly trained, and whose general capacity is unquestioned, but who
are quite unable to design or construct the simplest apparatus with
due regard to the facility with which it ought to be constructed.
That ultimate knowledge of materials and of processes which by long
experience becomes intuitive in the mind of a great inventor of course
cannot be acquired from books or from any set course of instruction.
There are, however, many steps between absolute ignorance and
consummate knowledge of the mechanical arts, and it is the object of
the following pages to assist the young physicist in making his first
steps towards acquiring a working knowledge of "laboratory arts."
However humble the ambition may be, no one can be more keenly alive
than the writer to the inadequacy of his attempt; and it is only from
a profound sense of the necessity which exists for some beginning to
be made, that he has had the courage to air his views on matters about
which there are probably hundreds or thousands of people whose
knowledge is superior to his own.
Moreover, nothing has been further from the writer's mind than any
idea of "instructing" any one; his desire is--if happily it may so
befall--to be of assistance, especially to young physicists or
inventors who wish to attain definite mechanical ends with the minimum
expenditure of time. Most people will agree that one condition
essential to success in such an undertaking is brevity, and it is for
this reason that alternative methods as a rule have not been given,
which, of course, deprives the book of any pretence to being a
"treatise." The writer, therefore, is responsible for exercising a
certain amount of discretion in the selection he has made, and it is
hardly to be hoped that he has in all--or even in the majority of
cases--succeeded in recommending absolutely the best method of
procedure.
This brings another point into view. Before all things the means
indicated must be definite and reliable. It is for this reason that
the writer has practically confined himself to matters lying within
his own immediate experience, and has never recommended any process
(with one or two minor exceptions, which he has noted) which he has
not actually and personally carried through to a successful issue.
This, although it is a matter which he considers of the highest
importance, and which is his only title to a hearing, has
unfortunately led to a very personal tone in the book.
With regard to the arts treated of in the following pages, matters
about which information is easily acquired--such as carpentering,
blacksmithing, turning, and the arts of the watchmaker--have been
left on one side. With regard to the last, which is of immense use in
the laboratory, there happen to be at least two excellent and handy
books, viz. Saunier's Watchmakers' Handbook, Tripplin, London, 1892;
and Britton's Watchmakers' Dictionary and Guide.
With regard to carpentering, turning, and blacksmithing, almost any
one who so desires can obtain a little practical experience in any
village. A short chapter has been devoted to glass-blowing, in spite
of there being an excellent and handy book by Mr. Shenstone (The
Methods of Glass-blowing, Rivington) on the subject already in
existence. The reason for this exception lies in the fact that the
writer's methods differ considerably from those advocated by Mr.
Shenstone.
The chapter on opticians' work has had to be compressed to an extent
which is undesirable in dealing with so complex and delicate an art,
but it is hoped that it will prove a sufficient introduction for
laboratory purposes. In this matter the writer is under great
obligations to his friend and assistant, Mr. James Cook, F.R.A.S, who
gave him his first lessons in lens-making some twenty years ago. To
Mr. John A. Brashear of Allegheny, Pa, thanks are due for much
miscellaneous information on optical work, which is included verbatim
in the text, some of it contained originally in printed papers, and
some most kindly communicated to the writer for the purpose of this
book. In particular, the writer would thank Mr. Brashear for his
generously accorded information as to the production of those "flat"
surfaces for which he is so justly famous.
The writer is also indebted to Mr. A. E. Kennelly for some
information as to American practice in the use of insulating material
for electrical work, and to his friends Mr. J. A. Pollock and Dr. C.
J. Martin for many valuable suggestions. For the illustrations
thanks are due to Mrs. Threlfall and Mr. James Cook. With regard to
matters which have come to the writer's knowledge by his being
specifically instructed in them from time to time, due acknowledgment
is, it is hoped, made in the text.
With regard to the question as to what matters might be included and
what omitted, the general rule has been to include information which
the author has obtained with difficulty, and to leave on one side that
which he has more easily attained. All the "unities" have been
consistently outraged by a deliberate use of the English and metric
systems side by side. So long as all the materials for mechanical
processes have to be purchased to specifications in inches and feet,
it is impossible to use the centimetre consistently without
introducing inconvenience. However, everybody ought to, and probably
does, use either system with equal facility.
No attempt has been made at showing how work can be done without
tools. Though, no doubt, a great deal can be done with inferior
appliances where great economy of money and none of time is an object,
the writer has long felt very strongly that English physical
laboratory practice has gone too far in the direction of starving the
workshop, and he does not wish, even indirectly, 'to give any
countenance to such a mistaken policy. Physical research is too
difficult in itself, and students' time is too valuable, for it to be
remunerative to work with insufficient appliances.
In conclusion, the writer would ask his readers to regard the book to
some extent as tentative, and as a means to the procuring and
organising of information bearing upon laboratory arts. Any
information which can be given will be always thankfully received, and
the author hereby requests any reader who may happen to learn
something of value from the book to communicate any special
information he may possess, so that it may be of use to others should
another edition ever be called for.
CHAPTER I
HINTS ON THE MANIPULATION OF GLASS AND ON GLASS-BLOWING FOR LABORATORY
PURPOSES
§ 1. THE art of glass-blowing has the conspicuous advantage, from the
point of view of literary presentation, of being to a great extent
incommunicable. As in the case of other delightful arts--such as
those treated of in the Badminton Library, for instance--the most
that can be done by writing is to indicate suitable methods and to
point out precautions which experience has shown to be necessary, and
which are not always obvious when the art is first approached. It is
not the object of this work to deal with the art of glass-blowing or
any other art after the manner befitting a complete treatise, in which
every form of practice is rightly included. On the contrary, it is my
wish to avoid the presentation of alternative methods.
I consider that the presentation of alternative methods would, for my
present purpose, be a positive disadvantage, for it would swell this
book to an outrageous size; and to beginners--I speak from
experience--too lavish a treatment acts rather by way of obscuring
the points to be aimed at than as a means of enlightenment. The
student often does not know which particular bit of advice to follow,
and obtains the erroneous idea that great art has to be brought to
bear to enable him to accomplish what is, after all, most likely a
perfectly simple and straightforward operation.
This being understood, it might perhaps be expected that I should
describe nothing but the very best methods for obtaining any proposed
result. Such, of course, has been my aim, but it is not likely that I
have succeeded in every case, or even in the majority of cases, for I
have confined myself to giving such directions as I know from my own
personal experience will, if properly carried out, lead to the result
claimed. In the few cases in which I have to refer to methods of
which I have no personal experience, I have endeavoured to give
references (usually taking the form of an acknowledgment), so that an
idea of their value may be formed. All methods not particularised may
be assumed by the reader to have come within my personal experience.
§ 2. Returning to glass-blowing, we may note that two forms of
glass-blowing are known in the arts, "Pot" blowing and "Table"
blowing. In the former case large quantities of fluid "metal"
(technical term for melted glass) are assumed to be available, and as
this is seldom the case in the laboratory, and as I have not yet felt
the want of such a supply, I shall deal only with "table" blowing.
Fortunately there is a convenient book on this subject, by Dr.
Shenstone (Rivingtons), so that what I have to say will be as brief as
possible, consistent with sufficiency for everyday work. As a matter
of fact there is not very much to say, for if ever there was an art in
which manual dexterity is of the first and last importance, that art
is glass-working.
I do not think that a man can become an accomplished glass-blower from
book instructions merely--at all events, not without much unnecessary
labour,--but he can learn to do a number of simple things which will
make an enormous difference to him both as regards the progress of his
work and the state of his pocket.
§ 3. The first thing is to select the glass. In general, it will
suffice to purchase tubes and rods; in the case where large pieces
(such as the bulbs of Geissler pumps) have to be specially prepared by
pot-blowing, the student will have to observe precautions to be
mentioned later on. There are three kinds of glass most generally
employed in laboratories.
§ 4. Soft Soda Glass, obtained for the most part from factories in
Thuringia, and generally used in assembling chemical apparatus.--This
glass is cheap, and easily obtainable from any large firm of apparatus
dealers or chemists. It should on no account be purchased from small
druggists, for the following reasons:-
(a) It is usually absurdly dear when obtained in this way.
(b) It is generally made up of selections of different age and
different composition, and pieces of different composition, even if
the difference is slight, will not fuse together and remain together
unless joined in a special manner.
(c) It is generally old, and this kind of glass often devitrifies with
age, and is then useless for blowpipe work, though it may be bent
sufficiently for assembling chemical apparatus. Devitrified glass
looks frosty, or, in the earlier stages, appears to be covered by
cobwebs, and is easily picked out and rejected.
§ 5. It might be imagined that the devitrification would disappear
when the glass is heated to the fusing point; and so it does to a
great extent, but for many operations one only requires to soften the
glass, and the devitrification often persists up to this temperature.
My experience is that denitrified glass is also more likely to crack
in the flame than good new glass, though the difference in this
respect is not very strongly marked with narrow tubes.
§ 6. Flint Glass.
Magnificent flint glass is made both in England and France. The
English experimenter will probably prefer to use English glass, and,
if he is wise, will buy a good deal at a time, since it does not
appear to devitrify with age, and uniformity is thereby more likely to
be secured. I have obtained uniformly good results with glass made by
Messrs. Powell of Whitefriars, but I daresay equally good glass may
be obtained elsewhere.
For general purposes flint glass is vastly superior to the soft soda
mentioned above. In the first place, it is very much stronger, and
also less liable to crack when heated--not alone when it is new, but
also, and especially, after it has been partly worked. Apparatus made
of flint glass is less liable to crack and break at places of unequal
thickness than if made of soda glass. This is not of much importance
where small pieces of apparatus only are concerned, because these can
generally be fairly annealed; and if the work is well done, the
thickness will not be uneven. It is a different matter where large
pieces of apparatus, such as connections to Geissler pumps, are
concerned, for the glass has often to be worked partly in situ, and
can only be imperfectly annealed.
Joints made between specimens of different composition are much more
likely to stand than when fashioned in soda glass. Indeed, if it is
necessary to join two bits of soda glass of different kinds, it is
better to separate them by a short length of flint glass; they are
more likely to remain joined to it than to each other. A particular
variety of flint glass, known as white enamel, is particularly
suitable for this purpose, and, indeed, may be used practically as a
cement.
§ 7, It is, however, when the necessity of altering or repairing
apparatus complicated by joints arises that the advantage of flint
glass is most apparent. A crack anywhere near to a side, or inserted
joint, can scarcely ever be repaired in the case of soda glass
apparatus, even when the glass is quite thin and the dimensions small.
It should also be mentioned that flint glass has a much more brilliant
appearance than soda glass. Of course, there is a considerable
difference between different kinds of flint glass as to the melting
point, and this may account for the divergency of the statements
usually met with as to its fusibility compared with that of soda
glass. The kind of flint glass made by Messrs. Powell becomes
distinctly soft soon after it is hot enough to be appreciably luminous
in a darkened room, and at a white heat is very fluid. This fluidity,
though of advantage to the practised worker, is likely to give a
beginner some trouble.
§ 8. As against the advantages enumerated, there are some drawbacks.
The one which will first strike the student is the tendency of the
glass to become reduced in the flame of the blow-pipe. This can be
got over by proper adjustment of the flame, as will be explained later
on. A more serious drawback in exact work is the following. In
making a joint with lead glass it is quite possible to neglect to fuse
the glass completely together at every point; in fact, the joint will
stand perfectly well even if it be left with a hole at one side, a
thing which is quite impossible with soft soda glass, or is at least
exceedingly unusual. An accident of this kind is particularly likely
to happen if the glass be at all reduced. Hence, if a joint does not
crack when cold, the presumption is, in the case of soda glass, that
the joint is perfectly made, and will not allow of any leak; but this
is not the case with flint glass, for which reason all joints between
flint glass tubes require the most minute examination before they are
passed. If there are any air bubbles in the glass, especial care must
be exercised.
§ 9. Hard or Bohemian, Glass.
This is, of course, used where high temperatures are to be employed,
and also in certain cases where its comparative insolubility in water
is of importance. It is very unusual for the investigator to have to
make complicated apparatus from this glass. Fused joints may be made
between hard glass and flint glass without using enamel, and though
they often break in the course of time, still there is no reason
against their employment, provided the work be done properly, and they
are not required to last too long.
§ 10. On the Choice of Sizes of Glass Tube.
It will be found that for general purposes tubes about one-quarter
inch in inside diameter, and from one-twentieth to one-fortieth of an
inch thick, are most in demand. Some very thin soda glass of these
dimensions (so-called "cylinder" tubes) will be found very handy for
many purposes. For physico-chemical work a good supply of tubing,
from one-half to three-quarters of an inch inside diameter, and from
one-twentieth to one-eighth inch thick, is very necessary. A few
tubes up to three inches diameter, and of various thicknesses, will
also be required for special purposes.
Thermometer and "barometer" tubing is occasionally required, the
latter, by the way, making particularly bad barometers. The
thermometer tubing should be of all sizes of bore, from the finest
obtainable up to that which has a bore of about one-sixteenth of an
inch. Glass rods varying from about one-twentieth of an inch in
diameter up to, say, half an inch will be required, also two or three
sticks of white enamel glass for making joints.
To facilitate choice, there is appended a diagram of sizes from the
catalogue of a reliable German firm, Messrs. Desaga of Heidelberg, and
the experimenter will be able to see at a glance what sizes of glass
to order. It is a good plan to stock the largest and smallest size of
each material as well as the most useful working sizes.
Fig. 1.
§ 11. Testing Glass.
"Reject glass which has lumps or knots, is obviously conical, or has
long drawn-out bubbles running through the substance." If a scratch be
made on the surface of a glass tube, and one end of the scratch be
touched by a very fine point of fused glass, say not more than
one-sixteenth inch in diameter, the tube, however large it is (within
reason), ought to crack in the direction of the scratch. If a big
crack forms and does not run straight, but tends to turn
longitudinally, it is a sign that the glass is ill annealed, and
nothing can be done with it. If such glass be hit upon in the course
of blow-pipe work, it is inadvisable to waste time upon it; the best
plan is to reject it at once, and save it for some experiment where it
will not have to be heated.
The shortest way of selecting glass is to go to a good firm, and let
it be understood that if the glass proves to be badly annealed it will
be returned. Though it was stated above that the glass should not be
distinctly conical, of course allowance must be made for the length of
the pieces, and, on the other hand, a few highly conical tubes will be
of immense service in special cases, and a small supply of such should
be included.
The glass, as it is obtained, should be placed in a rack, and covered
by a cloth to reduce the quantity of dust finding its way into the
tubes. It has been stated by Professor Ostwald that tubes when reared
up on end tend to bend permanently. I have not noticed this with lead
glass well supported. Each different supply should be kept by itself
and carefully described on a label pasted on to the rack, and tubes
from different lots should not be used for critical welds. This
remark is more important in the case of soda than of lead glass.
In the case of very fine thermometer tubes it will be advisable to
cover the ends with a little melted shellac, or, in special cases, to
obtain the tubes sealed from the works. Soda glass can generally be
got in rather longer lengths than lead glass; the longer the lengths
are the better, for the waste is less.
It is useful to be able to distinguish the different kinds of glass by
the colour. This is best observed by looking towards a bright surface
along the whole length of the tube and through the glass. Lead glass
is yellow, soda glass is green, and hard glass purple in the samples
in my laboratory, and I expect this is practically true of most
samples. [Footnote: Some new lead glass I have is also almost purple
in hue. If any doubt exists as to the kind of glass, it may be tested
at once in the blow-pipe flame, or by a mixture of oils of different
refractive indices, as will be explained later.]
§ 12. The question of the solubility of glass in reagents is one of
great importance in accurate work, though it does not always meet with
the attention it deserves. It is impossible here to go into the
matter with sufficient detail, and the reader is therefore referred to
the Abstracts of the Chemical Society, particularly for the years 1889
and 1892. The memoir by F. Kohlrausch, Wied. Ann. xliv, should be
consulted in the original. The following points may be noted. A
method of testing the quality of glass is given by Mylius (C. S. J.
Abstracts, 1889, p. 549), and it is stated that the resistance of
glass to the action of water can generally be much increased by
leaving it in contact with cold water for several days, and then
heating it to 300° to 400° C. This improvement seems to be due to the
formation of a layer of moist silica on the surface, and its
subsequent condensation into a resisting layer by the heating. Mylius
(C. S. J. Abstracts, 1892, p. 411), and Weber, and Sauer (C. S.
J. Abstracts, 1892, p. 410) have also shown that the best glass for
general chemical purposes consists of:
Silica, 7 to 8 parts
Lime, 1 part
Alkali, 1.5 to 1.1 parts.
This is practically "Bohemian" tube glass.
The exact results are given in the Berichte of the German Chemical
Society, vol. xxv. An excellent account of the properties of glass
will be found in Grove's edition of Miller's Elements of Chemistry.
§ 13. Cleaning Glass Tubes.
This is one of the most important arts in chemistry. If the tubes are
new, they are generally only soiled by dust, and can be cleaned fairly
easily--first by pushing a bit of cotton waste through with a cane,
or pulling a rag through with string--and then washing with sand and
commercial hydrochloric acid. I have heard of glass becoming
scratched by this process, and breaking in consequence when heated,
but have never myself experienced this inconvenience. In German
laboratories little bits of bibulous paper are sometimes used instead
of sand; they soon break into a pulp, and this pulp has a slightly
scouring action.
As soon as the visible impurities are removed and the tube when washed
looks bright and clean, it may be wiped on the outside and held
perpendicularly so as to allow the water film to drain down. If the
tube be greasy (and perhaps in other cases) it will be observed that
as the film gets thinner the water begins to break away and leave dry
spots. For accurate work this grease, or whatever it is, must be
removed; and after trying many plans for many years, I have come back
to the method I first employed, viz. boiling out with aqua regia.
For this purpose, close one end of the tube by a cork (better than a
rubber bung, because cheaper), and half fill the tube with aqua regia;
then, having noted the greasy places, proceed to boil the liquid in
contact with the glass at these points, and in the case of very
obstinate dirt--such as lingers round a fused joint which has been
made between undusted tubes--leave the whole affair for twelve hours.
If the greasiness is only slight, then simply shaking with hot aqua
regia will often remove it, and the aqua regia is conveniently heated
in this case by the addition of a little strong sulphuric acid.
The spent aqua regia may be put into a bottle. It is generally quite
good enough for the purpose of washing glass vessels with sand, as
above explained.
However carefully a tube is cleaned before being subjected to blowpipe
operations, it will be fouled wherever there is an opening during the
process of heating, unless the extreme tip only of an oxidising flame
be employed. Even this should not be trusted too implicitly unless
an oxygas or hydrogen flame is employed.
When a tube or piece of apparatus has been cleaned by acid, so that on
clamping it vertically, dry spaces do not appear, it may be rinsed
with platinum distilled water and left to drain, the dust being, of
course, kept out by placing a bit of paper round the top. For
accurate work water thus prepared is to be preferred to anything else.
When the glass is very clean interference colours will be noticed as
the water dries away.
Carefully-purified alcohol may in some cases be employed where it is
desired to dry the tube or apparatus quickly. In this case an alcohol
wash bottle should be used, and a little alcohol squirted into the top
of the tube all round the circumference. The water film drags the
alcohol after it, and by waiting a few minutes and then adding a few
more drops of alcohol, the water may be practically entirely removed,
especially if a bit of filter paper be held against the lower end of
the tube. It is customary in some laboratories to use ether for a
final rinse, but unless the ether is freshly distilled and very pure,
it leaves a distinct organic residue.
When no more liquid can be caused to drain away, the tube may be dried
by heating it along its length, beginning at the top (to get the
advantage of the reduction of surface tension), and so on all down.
It will then be possible to mop up a little more of the rinsing
liquid. When the tube is nearly dry a loose plug of cotton wool may
be inserted at the bottom. The wool must be put in so that the fibres
lie on an even surface inside the tube, and the wool must be blown
free from dust. Ordinary cotton wool is useless, from being dusty and
the fibres short, and the same remark applies to wadding. Use nothing
but what is known as "medicated" cotton wool with a good long fibre.
The tube will usually soon dry of itself when the cover is lifted an
inch or so. If water has been used, the air-current may be assisted
by means of the water-pump, the air being sucked from the top, so that
the wool has an opportunity of acting as a dust filter; a very slow
stream of air only must be employed. For connecting the tube to the
pump, a bit of India-rubber tube about an inch in diameter, with a
bore of about one-eighth of an inch, may be employed. The end of the
rubber tube is merely pressed against the edge of the glass.
These remarks apply, with suitable modification, to all kinds of
finished apparatus having two openings. For flasks and so on, it is
convenient to employ a blowing apparatus, dust being avoided by
inserting a permanent plug of cotton wool in one of the leading tubes.
The efficiency of this method is greatly increased by using about one
foot of thin copper tube, bent into a helix, and heated by means of a
Bunsen burner; the hot air (previously filtered) is passed directly
into the flask, bottle, or whatever the apparatus may be. This has
proved so convenient that a copper coil is now permanently fastened to
the wall in one of the rooms of my laboratory.
The above instructions indicate greater refinement than is in general
necessary or proper for tubes that have to be afterwards worked by the
blow-pipe. In the majority of cases all that is necessary is to
remove the dust, and this is preferably done by a wad of cotton waste
(which does not leave shreds like cotton wool), followed by a bit of
bibulous filter paper. I would especially warn a beginner against
neglecting this precaution, for in the process of blowing, the dust
undergoes some change at the heated parts of the apparatus, and forms
a particularly obstinate kind of dirt.
In special cases the methods I have advocated for removing dirt and
drying without covering the damp surfaces with dust are inadequate,
but an experimenter who has got to that stage will have nothing to
learn from such a work as this.
§ 14. The Blow-pipe.
I suppose a small book might easily be written on this subject but
what I have to say--in accordance with the limitation imposed--will
be brief. For working lead glass I never use anything but an oxygas
blow-pipe, except for very large work, and should never dream of using
anything else. Of course, to a student who requires practice in order
to attain dexterity this plan would be a good deal too dear. My
advice to such a one is--procure good soda glass, and work it by
means of a modification of a gas blow-pipe, to be described directly.
The Fletcher's blow-pipes on long stems are generally very
inconvenient. The flame should not be more than 5 or 6 inches from
the working table at most, especially for a beginner, who needs to
rest his arms on the edge of the table to secure steadiness.
The kind of oxygas blow-pipe I find most convenient is indicated in
the sketch. (Fig. 2) I like to have two nozzles, which will slip on
and off, one with a jet of about 0.035 inch in diameter, the other of
about double this dimension. The oxygen is led into the main tube of
the blow-pipe by another tube of much smaller diameter, concentric
with the main tube (Fig. 3, at A). The oxygen is mixed with the gas
during its escape from the inner tube, which is pierced by a number of
fine holes for the purpose, the extreme end being closed up. The
inner tube may run up to within half an inch of the point where the
cap carrying the nozzle joins the larger tube.
Fig. 2.
Fig. 3.
If it is desired to use the blow-pipe for working glass which is
already fixed in position to a support, it will be found very
advantageous to use a hooked nozzle. The nozzle shown in the sketch
is not hooked enough for this work, which requires that the flame be
directed 'backwards towards the worker. With a little practice such a
flame may be used perfectly well for blowing operations on the table,
as well as for getting at the back of fixed tubes.
To warm up the glass, the gas supply is turned full on, and enough
oxygen is allowed to pass in to clear the flame. The work is held in
front of, but not touching, the flame, until it is sufficiently hot to
bear moving into the flame itself. The, work is exposed to this flame
until, in the case of lead glass, traces of reduction begin to appear.
When this point is reached the oxygen tap is thrown wide open. I
generally regulate the pressure on the bags, so that under these
circumstances the flame is rather overfed with oxygen. This condition
is easily recognised, as follows. The flame shrinks down to a very
small compass, and the inner blue cone almost disappears; also
flashes of yellow light begin to show themselves--a thing which does
not occur when the proportions of the gases are adjusted for maximum
heating effect.
For many purposes the small dimensions of the flame render it very
convenient, and the high temperature which can be attained at exact
spots enables glass to be fused together after a certain amount of
mixing, which is an enormous advantage in fusing lead glass on to hard
glass. The lead glass should not be heated hot enough to burn, but,
short of this, the more fluid it is the better for joints between
dissimilar samples.
It will be noticed that the blow-pipe can be rotated about a vertical
axis so as to throw the flame in various directions. This is often
indispensable.
§ 15. In general the oxygen flame does not require to be delivered
under so high a pressure as for the production of a lime light. In
England, I presume, most experimenters will obtain their oxygen ready
prepared in bottles, and will not have to undergo the annoyance of
filling a bag. If, however, a bag is used, and it has some advantages
(the valves of bottles being generally stiff), I find that a pressure
produced by placing about two hundredweight (conveniently divided into
four fifty-six pound weights) on bags measuring 3' x 2'6" x 2' (at the
thicker end) does very well. To fill such a bag with oxygen, about
700 grms of potassium chlorate is required.
If the experimenter desires to keep his bag in good order, he must
purify his oxygen by washing it with a solution of caustic soda, and
then passing it through a "tower" of potash or soda in sticks, and,
finally, through a calcium chloride tower. This purifying apparatus
should be permanently set up on a board, so that it may be carried
about by the attendant to wherever it is required. Oxygen thus
purified does not seem to injure a good bag--at least during the
first six or seven years:
In order to reduce the annoyance of preparing oxygen, the use of the
usual thin copper conical bottle should be avoided. The makers of
steel gas bottles provide retorts of wrought iron or steel for
oxygen-making, and these do very well. They have the incidental
advantage of being strong enough to resist the attacks of a servant
when a spent charge is being removed.
The form of retort referred to is merely a large tube, closed at one
end, and with a screw coupling at the other; the dimensions may be
conveniently about 5 inches by 10. The screw threads should be filled
with fireclay (as recommended by Faraday) before the joint is screwed
up. Before purchasing a bottle the experimenter will do well to
remember that unless it is of sufficiently small diameter to go into
his largest vice, he will be inconvenienced in screwing the top on and
off. Why these affairs are not made with union joints, as they should
be, is a question which will perhaps be answered when we learn why
cork borers are still generally made of brass, though steel tube has
long been available.
Fig. 4.
These little matters may appear very trivial--and so they are--but
the purchaser of apparatus will generally find that unless he looks
after details himself, they will not be attended to for him. Whether a
union joint is provided or not, let it be seen that the end of the
delivery tube is either small enough to fit a large rubber tube
connection going to the wash-bottle, or large enough to allow of a
cork carrying a bit of glass tube for the same purpose to be inserted.
This tube should not be less than half an inch in inside diameter.
Never use a new bottle before it has been heated sufficiently to get
rid of grease and carbonaceous dirt. A convenient oxygen-making
apparatus is shown in Fig. 4, which is drawn from "life."
§ 16. For large blow-pipe work with lead glass I recommend a system
of four simple blow-pipes, in accordance with the sketch annexed. I
first saw this system in operation in the lamp factory of the
Westinghouse Electric Company at Pittsburg in 1889, and since then I
have seen it used by an exceedingly clever "trick" glass-worker at a
show. After trying both this arrangement and the "brush flame"
recommended by Mr. Shenstone, I consider the former the more
convenient; however, I daresay that either can be made to work in
competent hands, but I shall here describe only my own choice.
[Footnote: A brush flame is one which issues from the blow-pipe nozzle
shaped like a brush, i.e. it expands on leaving the jet. It is
produced by using a cylindrical air jet or a conical jet with a large
aperture, say one-eighth of an inch. See Fig. 25.]
As will be seen, the blow-pipe really consists of four simple brass
tube blow-pipes about three-eighths of an inch internal diameter and 3
inches long, each with its gas and air tap and appropriate nozzle.
Each blowpipe can turn about its support (the gas-entry pipe) to some
extent, and this possibility of adjustment is of importance, The air
jets are merely bits of very even three-sixteenths inch glass tubing,
drawn down to conical points, the jets themselves being about 0.035
inch diameter.
Fig. 5.
The flames produced are the long narrow blow-pipe flames used in
blow-pipe analysis, and arranged so as to consist mostly of oxidising
flame. The air-supply does not require to be large, nor the pressure
high--5 to 10 inches of water will do--but it must be very regular.
The "trick" glass-blower I referred to employed a foot bellows in
connection with a small weighted gasometer, the Westinghouse Company
used their ordinary air-blast, and I have generally used a large
gas-holder with which I am provided, which is supplied by a Roots
blower worked by an engine.
I have also used a "velocity pump" blower, which may be purchased
amongst others from Gerhardt of Bonn. The arrangement acts both as a
sucking and blowing apparatus, and is furnished with two manometers
and proper taps, etc. As I have reason to know that arrangements of
this kind work very ill unless really well made, I venture to add that
the Gerhardt arrangement to which I refer is No. 239 in his
catalogue, and costs about three pounds. It hardly gives enough air,
however, to work four blow-pipes, and the blast requires to be
steadied by passing the air through a vessel covered with a rubber
sheet.
In default of any of these means being available, one of Fletcher's
foot-blowers may be employed, but it must be worked very regularly. A
table mounted with one blow-pipe made on this plan, and worked by a
double-acting bellows, is recommended for students' use. For working
flint glass, the air jet may be one-eighth of an inch in diameter and
the pressure higher--this will give a brush flame. See Fig. 25.
It will be seen, on looking at the sketch of the blowpipe system, that
the pair of blow-pipes farther from the observer can be caused to
approach or recede at will by means of a handle working a block on a
slide. It often happens that after using all four blow-pipes at once
it is necessary to have recourse to one blow-pipe only, and to do this
conveniently and quickly is rather an object. Now, in my arrangement
I have to turn off both the gas and air from the farther system, and
then put in a bit of asbestos board to prevent the nozzles being
damaged by the flame or flames kept alight. As I said before, when
some experience is gained, glassblowing, becomes a very simple art,
and work can be done under circumstances so disadvantageous that they
would entirely frustrate the efforts of a beginner. This is not any
excuse, however, for recommending inferior arrangements.
Consequently, I say that the pipes leading in gas and air should be
all branches of one gas and one air pipe, in so far as the two remote
and one proximate blow-pipe are concerned, and these pipes should come
up to the table to the right hand of the operator, and should have
main taps at that point, each with a handle at least 2 inches long.
By this arrangement the operator can instantly turn down all the
blow-pipes but one, while, if the inverse operation is required, all
the three pipes can be started at once. [Footnote: I find, since
writing the above, that I have been anticipated in this recommendation
by Mr. G. S. Ram, The Incandescent Lamp and its Manufacture, p.
114.]
The separate air and gas taps must be left for permanent regulation,
and must not be used to turn the supply on or cut it off. In some
respects this blow-pipe will be found more easy to manage than an
oxygas blow-pipe, for the glass is not so readily brought to the very
fluid state, and this will often enable a beginner who proceeds
cautiously to do more than he could with the more powerful instrument.
Though I have mentioned glass nozzles for the air supply, there is no
difficulty in making nozzles of brass. For this purpose let the end
of a brass tube of about one-eighth of an inch diameter be closed by a
bit of brass wire previously turned to a section as shown (Fig. 6),
and then bored by a drill of the required diameter, say -.035 inch. It
is most convenient to use too small a drill, and to gradually open the
hole by means of that beautiful tool, the watchmaker's "broach." The
edges of the jet should be freed from burr by means of a watchmaker's
chamfering tool (see Saunier's Watchmaker's Hand-book, Tripplin,
1882, p. 232, § 342), or by the alternate use of a slip of Kansas
stone and the broach.
Fig. 6.
The construction of this blow-pipe is so simple, that in case any one
wishes to use a brush flame, he can easily produce one simply by
changing his air jets to bits of the same size (say one-eighth to
one-sixteenth of an inch) tubing, cut off clean. To insure success,
the ends of the tubes must be absolutely plane and regular; the
slightest inequality makes all the difference in the action of the
instrument. If a jet is found to be defective, cut it down a little
and try again; a clean-cut end is better than one which has been
ground flat on a stone. The end of a tube may, however, be turned in
a manner hereafter to be described so as to make an efficient jet.
Several trials by cutting will probably have to be made before success
is attained. For this kind of jet the air-pressure must be greatly
increased, and a large Fletcher's foot-blower or, better still, a
small double-action bellows worked with vigour will be found very
suitable. A fitting for this auxiliary blow-pipe is shown in Fig. 5
at B.
Professor Roentgen's discovery has recently made it necessary to give
more particular attention to the working of soft soda glass, and I
have been obliged to supplement the arrangements described by a table
especially intended for work with glass of this character. The
arrangement has proved so convenient for general work that I give the
following particulars. The table measures 5 feet long, 2 feet 11
inches wide, and is 2 feet 9 inches high.
Fig. 7.
It is provided with a single gas socket, into which either a large or
small gas tube may be screwed. The larger tube is 5.5 inches long and
0.75 of an inch in diameter. The smaller tube is the same length, and
half an inch in diameter. The axis of the larger tube is 3.5 inches
above the table at the point of support, and is inclined to the
horizontal at an angle of 12°. The axis of the smaller tube is 2.5
inches above the surface of the table, and is inclined to the
horizontal at the same angle as the larger one.
The air jets are simply pieces of glass tube held in position by
corks. The gas supply is regulated by a well-bored tap. The air
supply is regulated by treading the bellows--no tap is requisite.
The bellows employed are ordinary smiths' bellows, measuring 22 inches
long by 13 inches wide in the widest part. They are weighted by lead
weights, weighing 26 lbs. The treadle is connected to the bellows by
a small steel chain, for the length requires to be invariable. As the
treadle only acts in forcing air from the lower into the upper chamber
of the bellows, a weight of 13 lbs. is hung on to the lower cover, so
as to open the bellows automatically.
The air jets which have hitherto been found convenient are:
for the small gas tube
(1) a tube 0.12 inch diameter drawn down to a jet of 0.032 inch
diameter for small work;
(2) plain tubes not drawn down of 0.14 inch, 0.127 inch, and -0.245
inch diameter, and for the large gas tube, plain tubes up to 0.3 inch
in diameter.
The table is placed in such a position that the operator sits with his
back to a window and has the black calico screen in front of him and
to his right. The object of the screen is to protect the workman
against draughts. The table is purposely left unscreened to the left
of the workman, so that long tubes may be treated.
§ 17. Other appliances which will be required for glass-blowing are
of the simplest character.
(1) Small corks for closing the ends of tubes.
(2) Soft wax--a mixture of bees' wax and resin softened by linseed
oil to the proper consistency, easily found by trial, also used for
temporarily closing tubes.
(3) A bottle of vaseline for lubricating.
(4) An old biscuit tin filled with asbestos in shreds, and an asbestos
towel or cloth for annealing glass after removal from the flame. As
asbestos absorbs moisture, which would defeat its use as an annealing
material, it must be dried if necessary.
(5) A Glass-Cutter's Knife. This is best made out of a fine
three-cornered file, with the file teeth almost ground out, but not
quite; it should be about 2 inches long. After the surface has been
ground several times, it may be necessary to reharden the steel. This
is best done by heating to a full red and quenching in mercury. The
grindstone employed for sharpening the knife should be "quick," so as
to leave a rough edge. I have tried many so-called glass knives "made
in Germany," but, with one exception, they were nothing like so good
as a small French or Sheffield file. In this matter I have the
support of Mr. Shenstone's experience.
(6) A wire nail, about 2 inches long, mounted very accurately in a
thin cylindrical wooden handle about 5 inches long by one-quarter of
an inch diameter, or, better still, a bit of pinion wire 6 inches
long, of which 1.5 inches are turned down as far as the cylindrical
core, An old dentists' chisel or filling tool is also a very good
form of instrument.
(7) A bit of charcoal about 3.5 inches long and 2 wide, and of any
thickness, will be found very useful in helping to heat a very large
tube. The charcoal block is provided with a stout wire handle, bent
in such a manner that the block can be held close above a large glass
tube on which the flames impinge. In some cases it is conveniently
held by a clip stand. By the use of such a slab of charcoal the
temperature obtainable over a large surface can be considerably
increased.
I have seen a wine-glass (Venetian sherry-glass) worked on a table
with four blow-pipes, such as is here described, with the help of a
block of hard wood held over the heated glass, and helping the
attainment of a high temperature by its own combustion.
(8) Several retort stands with screw clips.
(9) Some blocks of wood about 5" X 2" X 2" with V-shaped notches cut
in from the top.
(10) A strong pair of pliers.
(11) An apparatus for cleaning and drying the breath, when blowing
directly by the mouth is not allowable. The apparatus consists of a
solid and heavy block of wood supporting a calcium-chloride tube
permanently connected with a tube of phosphorus pentoxide divided into
compartments by plugs of glass wool. Care should be taken to arrange
these tubes so as to occupy the smallest space, and to have the stand
particularly stable. The exit tube from the phosphorus pentoxide
should be drawn down to form a nozzle, from, say, half an inch to
one-eighth of an inch in diameter, so as to easily fit almost any bit
of rubber tube. The entry to the calcium chloride should be
permanently fitted to about a yard of fine soft rubber tubing, as
light as possible. The ends of this tube should terminate in a glass
mouthpiece, which should not be too delicate.
As an additional precaution against dust, I sometimes add a tube
containing a long plug of glass wool, between the phosphorus pentoxide
and the delivery tube, and also a tube containing stick potash on the
entry side of the calcium chloride tube, but it may safely be left to
individual judgment to determine when these additions require to be
made. In practice I always keep the affair set up with these
additions. The communication between all the parts should be
perfectly free, and the tubes should be nearly filled with reagents,
so as to avoid having a large volume of air to compress before a
pressure can be got up.
The arrangement will be clear by a reference to Fig. 8, which
illustrates the apparatus in use for joining two long tubes. I have
tried blowing-bags, etc, but, on the whole, prefer the above
arrangement, for, after a time, the skill one acquires in regulating
the pressure by blowing by the mouth and lips is such an advantage
that it is not to be lightly foregone.
Fig. 8.
§ 18. The Table.
The system of four blow-pipes is, of course, a fixture. In this case
the table may be about a yard square, and may be covered with asbestos
mill-board neatly laid down, but this is not essential. The table
should have a rim running round it about a quarter of an inch high.
The tools should be laid to the right of the worker, and for this
purpose the blow-pipes are conveniently fixed rather to the left of
the centre of the table, but not so far as to make the leg of the
table come so close to the operator as to make him uncomfortable, for
a cheerful and contented spirit ought to be part of the glass-worker's
outfit.
The most convenient height for a blow-pipe table--with the blow-pipes
about 2 inches above the table top--is 3 feet 2 inches. Nothing is
so convenient to sit upon as a rough music-stool with a good range of
adjustment. The advantage of an adjustable seat lies in the fact that
for some operations one wants to be well over the work, while in
others the advantage of resting the arms against the table is more
important.
§ 19. Special Operations.
The preliminary to most operations before the blow-pipe, is to draw
down a tube and pull it out to a fine point. This is also the
operation on which a beginner should exercise himself in the first
instance. I will suppose that it is desired to draw out a tube about
one-quarter of an inch in diameter, with the object of closing it,
either permanently or temporarily, and leaving a handle for future
operations in the shape of the point, thin enough to cool quickly and
so not delay further work.
For this simple operation most of the glass-blower's skill is
required. The tube must be grasped between the first finger and thumb
of both hands, and held so that the part to be operated on lies evenly
between the two hands. The distance between the operator's thumbs may
conveniently vary from 2.5 to 4 inches. Releasing the grip of the
left hand, let the operator assure himself of his ability to easily
rotate the tube about its axis--by the right thumb and finger--he
will incidentally observe by the "feel" whether the tube is straight
or not.
A good deal of progress can be made from this point before the tube is
heated at all. The operator can acquire a habit of instinctively
rotating the tube by both hands, however the tube itself be moved
about in space, or however it be pushed or pulled. The habit of
constant and instinctive rotation is literally about one-third of the
whole art of glassblowing. It is unlikely, however, that the beginner
will discover that he has not got this habit, until a few failures
draw his attention to it.
The glass tube being held in position lightly yet firmly, and the
operator being sure that he feels comfortable and at his ease, and
that the blow-pipe flame (a single flame in this instance) is well
under control, the preliminary heating may be commenced. With a tube
of the dimensions given this is a very simple affair. Turn the air
partly off, or blow gently, to get a partly luminous gas flame; hold
the tube about an inch from the end of this flame, and turn it round
and round till it commences to soften.
In the case of soda glass it is usual to employ the gas flame only,
but I find that it is better in most cases to use the hot air of a
gently-blown flame, rather than have the disadvantage of the soot
deposited in the alternative operation. When the glass begins to
soften, or even before, it may be moved right into the blow-pipe
flame, and the latter may be properly urged.
It is not possible to give quite explicit and definite instructions,
applicable to every case, as to when the time is ripe for passing the
work into the flame, but the following notes will indicate the general
rules to be observed:-
(1) A thick tube must be warmed more slowly and raised to a higher
temperature than a thin tube.
(2) The same remark applies to a tube of large diameter, as compared
with one of small diameter, whatever the thickness.
(3) In the case of very large or thick tubes the hot air is
advantageously employed at first, and to complete the preliminary
heating, the luminous flame alone may be used. The object of this is
to enable the operator to judge, by the presence of soot, its
inability to deposit--or its burning off if deposited--of the
temperature of the glass, and of the equality of this temperature all
over the surface, for a large and thick tube might be heated quite
enough to enable it to be safely exposed to the full heat before it is
appreciably yielding to the fingers. In general, when the soot burns
off freely, or lead glass begins to show the faintest sign of
reduction, or soda glass begins to colour the flame, it is more than
safe to proceed.
In order to turn on the full flame the operator will form a habit of
holding the work in the left hand only, and he will also take care not
to let anything his right hand may be doing cause him to stop rotating
the tube with his left thumb and finger.
The preliminary adjustment of air or oxygen supply will enable the
change to a flame of maximum power to be made very quickly. The tube
having been introduced with constant rotation, it will soon soften
sufficiently to be worked. The beginner will find it best to decide
the convenient degree of softness by trial.
With soda glass it does not much matter how soft the glass becomes,
for it remains viscous, but with lead glass the viscosity persists for
a longer time and then suddenly gives place to a much greater degree
of fluidity. [Footnote: This is only drawn from my impressions
acquired in glass-working. I have never explicitly tested the matter
experimentally.]
It is just at this point that a beginner will probably meet with his
first difficulty. As soon as the glass gets soft he will find that he
no longer rotates the glass at the same speed by the right and left
hand, and, moreover, he will probably unconsciously bend the tube, and
even deform it, by pushing or pulling.
The second third of the art of the glass-blower consists in being able
to move both hands about, rotating a tube with each thumb and finger,
and keeping the distance between the hands, and also the speed of
rotation, constant. Nothing but long practice can give this facility,
but it is essential that it be acquired to some extent, or no progress
can be made. Some people acquire a moderate proficiency very quickly,
others, of whom the writer is one, only become reasonably proficient
by months, or even years, of practice.
Supposing that the tube is now ready to be drawn down, the operator
will remove it from the flame, and will gently pull the ends apart,
interrupting his turning as little as possible. If the tube be pulled
too hard, or if the area heated be too small (about three-eighths of
an inch in length in the case given would be proper), it will be found
that the ends of the two portions of the tube will be nearly closed at
a very sharp angle (nearly a right angle to the length of the tube),
that the ends will be thin, and that a long length of very fine tube
will be produced. To heat a short length of tube and pull hard and
suddenly is the proper way to make a very fine capillary tube, but, in
general, this is what we want to avoid.
If the operation be successfully performed, the drawn-down tube will
have the appearance exhibited, which is suitable either for
subsequently closing or handling by means of the drawn-down portion.
The straightness of the point can be obtained by a little practice in
"feeling" the glass when the tube is rotated as it cools just before
it loses its viscous condition.
When the operation is carried out properly the shoulder of the "draw"
should be perfectly symmetrical and of even thickness, and its axis
regarded as that of a cone should lie in the axis of the tube
produced. The operation should be repeated till the student finds
that he can produce this result with certainty, and he should not be
discouraged if this takes several days, or even weeks. Of course, it
is probable that within the first hour he will succeed in making a
tolerable job, but it is his business to learn never to make anything
else.
Fig. 9.
Fig. 10. Diagram of a folded end.
§ 20. Closing and blowing out the End of a Tube.
When it is desired to close the end of a particular bit of tube, this
is easily done by heating the end, and at the same time heating the
end of a waste bit of tube or rod; the ends, when placed in contact,
stick together, and a point can be drawn down as before. [Footnote:
"Point" is here used in the technical sense, i.e. it is a thin tail
of glass produced by drawing down a tube.] Having got a point, it
will be found that the thin glass cools enough to allow of the point
being handled after a few moments.
The most convenient way of reducing the point to a suitable length
(say 1.5 inch) is to fuse it off in the flame, but this must be done
neatly; if a tail is left it may cause inconvenience by catching, or
even piercing the finger and breaking off. The blow-pipe flame being
turned down to a suitable size, and the shoulder of the "draw" having
been kept warm meanwhile, let the tip of the flame impinge on a point
where the diameter is about half that of the undrawn tube, and let the
temperature be very high (Fig. 11). The tube is to be inclined to
the flame so that the latter strikes the shoulder normally, or
nearly so. Then, according to circumstances, little or much of the
glass can be removed at will by drawing off the tail (Fig. 12), till,
finally, a small drop of melted glass only, adheres to the end of the
now closed tube (Fig. 13).
Fig. 11.
Fig. 12.
Fig. 13.
Fig. 14.
When this is satisfactorily accomplished, heat the extreme end of the
tube most carefully and equally, holding it in such a position that
the glass will tend to flow from the bead back on to the tube, i.e.
hold the closed end up to the flame, the tube being, say, at 45
degrees to the horizontal. Then when the temperature is such as to
indicate complete softness lift the tube to the mouth, still holding
the tube pointing with its closed end a little above the horizontal,
and blow gently. A beginner almost always blows too hard.
What is wanted, of course, is a continued pressure, to give the
viscous glass time to yield gradually, if it is uniform; or else
intermittent puffs to enable the thinner parts, if there are any, to
cool more, and hence become more resisting than the thicker ones. In
any case a little practice will enable the operator to blow out a
round and even end--neither thicker nor thinner than the rest of the
tube.
§ 21. To make a Weld.
To begin with, try on two bits of glass of the same size, i.e. cut a
seven-inch length of glass in half by scratching it with the knife,
and pulling the ends apart with a slight inclination away from the
scratch. In other words, combine a small bending moment with a
considerable tensional stress. It is important to learn to do this
properly. If the proportions are not well observed, the tube will
break with difficulty, and the section will not be perpendicular to
the main length. If the knife is in good order it will make a fine
deep scratch--the feel of the glass under the knife will enable the
operator to decide when the scratch is made. The operation of cutting
large tubes will be treated further on. The two halves of the tube
being held one in each hand, and one tube closed at one end, the
extremities to be united will be warmed, and then put in the flame as
before.
Fig. 16.
There are many ways of proceeding--perhaps the easiest is as follows.
As soon as the glass shows signs of melting at the ends--and care
should be taken that much more is not heated--take both bits out of
the flame. Stop rotating for a moment, and resting the arms carefully
on the edge of the table, raise the tubes above the flame and bring
the ends swiftly and accurately together. This is a case of "sudden
death no second attempt at making the ends meet can be allowed; if
the tubes join in any other than a perfectly exact manner a kink more
or less objectionable will result. In practice the operator will
learn to bring the ends together, commencing at one point; i.e. the
axes of the tubes will be inclined at first, so as to cause adherence
at one spot only. If this is not quite "fair", then less damage is
done in moving one tube slightly up or down to get the contact exact.
The tubes will then be closed upon one another as if they were hinged
at the joint. This must be done lightly, yet sufficiently, to ensure
that the glass is actually in contact all round.
Having gone so far, replace the tubes--now one--in the flame, and
carefully rotating the glass, raise the temperature higher than in the
operation just described, in fact the higher the temperature, short of
burning the glass, the better. Take the tube out of the flame and
blow into the open end, turning constantly as before. One puff is
enough. Then turn and pull the glass apart till it is of the same
diameter and thickness throughout, and feel that it is straight as
before.
Though it is in general of high importance that the joint should be
well heated, the beginner will probably find that he "ties up" his
glass as soon as it gets really soft.
If his object is to make one joint--at any cost--then let him be
careful to use two bits of exactly the same kind of glass, and only
get the temperature up to the viscous stage. If the joint be then
pulled out till it is comparatively thin, it will probably stand (if
of soda glass); certainly, if of lead glass, though in this case it
may not be sound. In any case the joint should be annealed in the
asbestos box if practicable, otherwise (unless between narrow tubes)
with the asbestos rag. Care must be taken that the asbestos is dry.
§ 22. To weld two Tubes of different Sizes.
To do this, the diameter of the larger tube must be reduced to that of
the smaller. The general procedure described in drawing down must be
followed, with the following modification. In general, a greater
length of the tube must be heated, and it must be made hotter. The
tube is to be gradually drawn in the flame with constant turning till
the proper diameter and thickness of glass are attained.
Fig. 16.
For this operation time must be allowed if the operator's hands are
steady enough to permit of it; the shoulder should form partly by the
glass sinking in and partly by the process of drawing the hot glass
out. A shoulder properly prepared is shown in the sketch. Beginners
generally make the neck too thin on large tubes, and too thick on
smaller ones. There ought to be no great difference in thickness of
glass between the neck on the larger tube, and the smaller tube. The
diameters should be as nearly as possible alike.
Having drawn down the larger tube to a neck, take it out of the flame,
and as it cools pull and turn till the neck is of the right thickness
and is perfectly straight, i.e. make the final adjustment outside the
flame, and to that end have the neck rather too thick (as to glass)
before it is taken out. It is not necessary to wait till the neck
gets cold before the end can be cut off. Make a scratch as before--this
will probably slightly damage the temper of the file knife, but
that must be put up with. Hold the tube against the edge of the
table, so that the scratch is just above the level of the rim, and
strike the upper part a smart blow with the handle of the glass knife
rather in the direction of its length. [Footnote: A bit of hoop iron
nailed against the side of the table is a very convenient arrangement,
and it need not project appreciably above the general level of the
rim.]
Of course this applies to a tube where economy has been exercised and
the end is short. If the tail is long enough to form a handle, the
tube may be pulled apart as before. As a rule a temporary joint
between a tube and a rod is not strong enough to enable the shoulder
to be broken at the scratch by mere pulling. The ends to be welded
must be broken off very clean and true. Subsequent operations are to
be carried out as already described.
§ 23. The above operations will be easily performed on tubes up to
half an inch in diameter, if they are not too long. It is the length
of tube, and consequent difficulty in giving identity of motion with
the two hands, which make the jointing of long tubes difficult. There
are also difficulties if the tubes are very thin, have a very fine
bore or a very large diameter.
All these difficulties merely amuse a good glass-blower, but to an
experimenter who wants to get on to other things before sufficient
skill is acquired (in the movement of the hands and arms) the
following method is recommended. First, use flint glass. Then,
assuming that any drawing down has to be done, do it as well as
possible, for on this the success of the method to be described
especially depends. Be sure that the tubes to be welded are cut off
clean and are as nearly as may be of the same size at the point of
junction.
To fix the description, suppose it is desired to join two tubes (see
Fig. 8), each about one inch in diameter and a yard long. Get four
clip stands and place them on a level table. Be sure that the stands
are firm and have not warped so as to rock. In each pair of clips
place a tube, so that the two tubes are at the same height from the
table, and, in fact, exactly abut, with axes in the same straight
line. Close one tube by a cork and then fix the blowing apparatus as
shown to the other.
In such an operation as this the drying apparatus may be dispensed
with, and a rubber tube simply connected to one end of the system and
brought to the mouth. Take the oxygen blow-pipe and turn the nozzle
till the flame issues towards you, and see that the flame is in order.
Then turn down the oxygen till it only suffices to clear the smoky
flame, and commence to heat the proposed joint by a current of hot
air, moving the flame round the joint. Finally, bring to bear the
most powerful flame you can get out of the blow-pipe, and carry it
round the joint so quickly that you have the latter all hot at once.
Put down the blow-pipe, and, using both hands, press the tubes
together (which wooden clips will readily allow), and after seeing
that the glass has touched everywhere, pull the tubes a trifle apart.
Apply the blow-pipe again, passing lightly over the thin parts, if
any, and heating thicker ones; having the end of the rubber tube in
his mouth, the operator will be able to blow out thick places. When
all is hot, blow out slightly, and having taken the flame away, pull
the tubes a little apart, and see that they are straight.
Throw an asbestos rag over the joint, loosen one pair of the clamps
slightly, and leave the joint to anneal. It is important that the
least possible amount of glass should be heated, hence the necessity
of having the ends well prepared, and it is also important that the
work should be done quickly; otherwise glass will flow from the upper
side downwards and no strong joint will be obtained.
Fig. 17. Tube being opened at one end.
§ 24. To weld Tubes of very small Bore.
If the bore is not so small as to prevent the entrance of the point of
the iron nail, get the ends of the tubes hot, and open the bore by
inserting the end of the nail previously smeared over with a trace of
vaseline. Work the nail round by holding the handle between the thumb
and first finger of the right hand, the tube being similarly placed in
the left. The tube and nail should be inclined as shown in the
sketch.
Never try to force the operation; the nail soon cools the glass, so
that only a very short time is available after each heat; during this
the tube should be rotated against the nail rather than the nail
against the tube. Be careful not to heat a greater length of tube
than is necessary, or the nail will, by its component of pressure
along the tube, cause the latter to "jump up" or thicken and bulge.
Both ends being prepared, and if possible, kept hot, the weld may be
made as before, and the heating continued till the glass falls in to
about its previous thickness, leaving a bore only slightly greater
than before.
It is in operations such as this that the asbestos box will be found
of great use. As soon as one end of the weld is ready cool it in the
flame till soot deposits, and then plunge it into the asbestos. This
will cause it to cool very slowly, and renders it less likely to crack
when again brought into the flame. Turned-out ends, if the glass is
at all thick, are very liable to crack off on reheating, so that they
must be reintroduced (into the flame) with especial care. This
liability to breakage is reduced, but not eliminated, by the asbestos
annealing.
Figs. 18 and 19.
§ 25. When the bore is very fine, it is best to seal off the tubes,
and blow an incipient bulb near one end of each tube. These bulbs may
be cooled in asbestos, and cut across when cold by means of a scratch
touched at one end (Figs. 18 and 19) by a fine point of highly
incandescent glass. For details of this method see p. 46, Fig. 21.
Time is occasionally saved by blowing off the ends of the bulbs. The
details of this process will be described when the operation of making
thistle-headed tubes is dealt with.
§ 26. When the tubes are both of large diameter, long, and very thin
(cylinder tubes), a considerable amount of difficulty will be
experienced. On the whole, it is best to heat each end separately
till the glass thickens a little, anneal in the flame and in asbestos,
and then proceed as in § 22. If the ends are not quite true, it will
be found that quite a thickness of glass may be "jumped" together at
one side of the tubes, while the edges are still apart at the other.
When this looks likely to happen, incline the tubes as if the joint
were a hinge, and bend back quickly; do not simply continue to push
the tubes together in a straight line, or an unmanageable lump of
glass will be formed on one side.
If in spite of these precautions such a lump does form, proceed as
follows. Take a rod of glass, at least one-eighth of an inch thick,
and warm it in the flame at one end. Heat the imperfect joint till it
softens all round, and then bring the flame right up to the thick
part, and heat that as rapidly and locally as possible. The oxygas
flame does this magnificently. Press the heated end of the glass rod
against the thick part, and pull off as much of the lump as it is
desired to remove, afterwards blowing the dint out by a judicious
puff. Finish off as before.
§ 27. Occasionally, when it is seen that in order to produce a joint
closed all round, one side of the tube would be too much thickened, it
is better to patch the open side. For this purpose take a glass rod
about one-sixteenth inch in diameter, and turn the flame to give its
greatest effect, still keeping rather an excess of air or oxygen. See
that the side of the joint already made is kept fairly hot--it need
not be soft; interrupt any other work often enough to ensure this.
Then, directing the flame chiefly on the thin rod, begin to melt and
pull the glass over the edges of the gap. When the gap is closed get
the lump very hot, so that all the glass is well melted together, and
then, if necessary, pull the excess of glass off, as before described.
It must be remembered that this and the method of the previous section
are emergency methods, and never give such nice joints as a
manipulation which avoids them, i.e. when the ends of the tubes are
perfectly straight and true to begin with. Also note that, as the
tubes cannot be kept in rotation while being patched, it is as well to
work at as low a temperature as possible, consistently with the other
conditions, or the glass will tend to run down and form a drop,
leaving a correspondingly thin place behind.
Fig. 20.
§ 28. A very common fault in cutting a tube of about an inch in
diameter is to leave it with a projecting point, as shown. This can
be slowly chipped off by the pliers, using the jaws to crush and grind
away the edge of the projection; it is fatal to attempt to break off
large pieces of glass all at once.
§ 29. It will be convenient here to mention some methods of cutting
large tubes. With tubes up to an inch and a half in diameter, and
even over this--provided that the glass is not very thick--we may
proceed as follows: Make a good scratch about half an inch long, and
pretty deep, i.e. pass the knife backwards and forwards two or three
times. Press a point of melted glass exactly on one end of the
scratch; the glass point even when pressed out of shape should not be
as large as a button one-twelfth of an inch in diameter. If this
fails at first, repeat the operation two or three times.
Fig. 21.
If a crack does not form, touch the hot place with the cold end of the
nail. If no success is obtained, try the other end of the scratch.
If failure still pursues the operator, let him make another cut on the
opposite side of the tube and try again. In general, the tube will
yield the first or second time the hot drop of glass is applied.
Never apply the drop at the centre of the scratch, or a ragged crack,
which may run in any direction, will result. Very often, with a large
tube, the crack formed by a successful operation will only extend a
short distance. In this case it is desirable to entice the crack
round the tube, and not trust to its running straight when the tube is
pulled apart.
On the whole, the best method in this case is to employ a flame
pencil, which should be kept ready for use. This merely consists of a
bit of glass tube of about the same dimensions as an ordinary lead
pencil, drawn down to a very fine jet at one end. The jet must not be
very long or thin, or the glass will soon fuse up. A few trials will
enable the operator to get the proper proportions, which are such that
the tube has the general appearance of a pencil normally sharpened
(say with a cone of 60'). This tube is best made of hard glass.
Connect it to a gas supply by light flexible tubing, and turn down the
gas till the flame from the end of the jet is not more than one-tenth
of an inch long. Then apply the jet, beginning from the end of the
crack, and gradually draw it (the crack) round the tube. The
operation will be assisted if a rubber ring is slipped on the tube to
begin with, so that the eye has some guide as to whether the flame is
being drawn round properly or not. The ring must, of course, be far
enough away to escape the effect of the flame. The crack will be
found to follow the flame in the most docile manner, unless the tube
is thick or badly annealed. Some operators recommend a pencil of
glowing charcoal, but the flame is undoubtedly better.
§ 30. To cut very thick Tubes.
A large number of methods have been proposed, and nearly everybody has
his favourite. The following has always succeeded with me. First
mark on the tube, by means of a little dead black spirit paint,
exactly where the cut is to be. Then sharpen the glass knife and
scratch a quite deep cut all round: there is no difficulty in making
the cut one-twentieth of an inch deep. It will be proper to lubricate
the knife with kerosene after the first mark is made. [Footnote: The
edge of the knife may be advantageously saved by using an old file
moistened with kerosene for the purpose. I find kerosene is not
worse, but, if anything, better than the solution of camphor in
turpentine recommended by Mr. Shenstone.]
If the glass is about one-eighth of an inch thick, the scratch maybe
conveniently about one-twentieth of an inch deep, but if the glass is
anything like one-quarter of an inch thick, the scratch must be much
deeper, in fact, the glass may be half cut through. To make a very
deep scratch, a wheel armed with diamond dust, which will be described
later on, may be used. However, it is not essential to use a diamond
wheel, though it saves time.
When the cut is made to a sufficient depth proceed thus: Obtain two
strips of bibulous paper or bits of tape and twist them round the tube
on each side of the scratch, allowing not more than one-eighth of an
inch between them. Then add a few drops of water to each, till it is
thoroughly soaked, but not allowing water to run away. Dry out the
scratch by a shred of blotting paper.
Turn down the oxygas flame to the smallest dimensions, and then boldly
apply it with its hottest part playing right into the nick and at a
single point. Probably in about two seconds, or less, the tube will
break. If it does not, rotate the tube, but still so that the flame
plays in the nick. After making the tube very hot all round--if it
has not broken--apply the flame again steadily at one point for a few
seconds and then apply a bit of cold iron. If the tube does not break
at once during these processes, let it cool, and cut the groove
deeper; then try again. [Footnote: This method is continually being
reinvented and published in the various journals. It is of unknown
antiquity.]
Fig. 22.
If the tube breaks after great heating and long efforts, it will
probably leave incipient cracks running away from the break, or may
even break irregularly. A good break is nearly always one that was
easily made. If a number of rings have to be cut, or a number of cuts
made on glass tubes of about the same size, it will be found
economical in the end to mount a glazier's diamond for the purpose. A
simple but suitable apparatus is figured (Fig. 23).
Fig. 23.
The only difficulty is to regulate the position of the diamond so that
it cuts. In order to do this, carefully note its cutting angle by
preliminary trials on sheet glass, and then adjust the diamond by
clamps, or by wriggling it in a fork, as shown. Weight the board very
slightly, so as to give the small necessary pressure, and produce the
cut by rotating the tube by hand. When a cut is nearly completed take
great care that the two ends join, or irregularity will result. This
is not always easy to do unless the tube happens to be straight.
Having got a cut, start a crack by means of a fine light watchmaker's
hammer, or even a bit of fused glass, and entice the crack round the
cut by tapping with the hammer or by means of the flame pencil.
If the cut is a true "cut" the tube will break at once. As a supply
of electrical current for lighting will, in the near future, be as
much a matter of course for laboratory purposes as a gas supply, I add
the following note. To heat a tube round a scratch, nothing--not
even the oxygas blow-pipe--is so good as a bit of platinum or iron
wire electrically heated. If the crack does not start by considerable
heating of the glass, stop the current, unwind the wire, and touch the
glass on the crack either with a bit of cold copper wire or a wet
match stem. I prefer the copper wire, for in my experience the water
will occasionally produce an explosion of cracks. On the other hand,
the cold wire frequently fails to start a crack.
Judging from the appearance of thick tubes as supplied by the dealers,
the factory method of cutting off appears to be to grind a nick almost
through the tube, and right round; and for really thick glass this is
the safest but slowest way; a thin emery wheel kept wet will do this
perfectly. Suitable wheels may be purchased from the "Norton" Emery
Wheel Co. of Bedford, Mass, U.S.A.--in England through Messrs.
Churchill and Co. of London, importers.
§ 31. To blow a Bulb at the End of a Tube.
I must admit at once that this is a difficult operation--at all
events, if a large bulb is required. However, all there is to be said
can be said in few words. In general, when a bulb is required at the
end of a tube it will be necessary to thicken up the glass. A
professional glass-worker will generally accomplish this by "jumping
up" the tube, i.e. by heating it where the bulb is required, and
compressing it little by little until a sufficient amount of glass is
collected. The amateur will probably find that he gets a very
irregular mass in this way, and will be tempted to begin by welding on
a short bit of wide and thick tubing preparatory to blowing out the
bulb.
However, supposing that enough glass is assembled by-either of these
methods, and that it is quite uniform in thickness, let the thickened
part be heated along a circle till it becomes moderately soft, and let
it then be expanded about one-fifth, say by gently blowing. It is
perhaps more important to keep turning the glass during bulb-blowing
than in any other operation, and this both when the glass is in the
flame and while the bulb is being blown. It is also very important to
avoid draughts. In general, a bulb is best blown with the tube in a
nearly horizontal position, but sloping slightly upwards from the
mouth. If it be noticed that a bulb tends to blow out more at one
side than another, let the side of greatest protuberance be turned
down, so that it is at the lowest point, reduce the pressure for an
instant, and then blow again. It will be observed that the bulb will
now expand at the top.
The reason of this is chiefly that the under side cools most rapidly
(according to Faraday, Chemical Manipulation, § 1194), and
consequently can expand no further; but also it is not unlikely that
the glass tends to flow somewhat from the upper side, which remains
hot, and consequently the bulb, when the next puff reaches it, will
tend to yield at this point. By heating several zones the tube will
become gradually expanded.
Fig. 24.
Fig. 25.
Fig. 26.
When the length of the thickened part of the tube only slightly
exceeds its diameter (Fig. 25), let the whole be brought to a
temperature which, with flint glass, should be just short of that of
perfect fluidity; and then, holding the tube horizontally and
constantly turning it, let the bulb be blown out to its full size,
noting the appearances and correcting too great protuberance on any
side by the means above mentioned. If the bulb appears pear-shaped
turn the tube so that the melted mass is directed upwards; if the
bulb have the contrary fault, correct in the corresponding manner.
The bulb when finished may be lightly tapped on the table, when, if
there is any weak place owing to inequality of thickness, the bulb
will break, and the operation may be started afresh. "A good bulb is
round, set truly on the tube, and free from lumps of thick glass or
places of excessive thinness." When the amateur has succeeded in
blowing a bulb two inches in diameter on the end of a strong bit of
thermometer tube--say for an air thermometer--he may well seek the
congratulations of his friends.
In case the bulb is not satisfactory on a first attempt, it may be
melted down again, if the following precautions are taken. Directly
creases begin to appear in the bulb let it be withdrawn from the
flame, and gently blown till the creases come out. By alternate
heating and blowing the glass can be got back to its original form, or
nearly so, but unless the operator shows great skill and judgment, the
probability is that the glass will be uneven. By heating and keeping
the thicker parts in the higher position, and blowing a little now and
again, the glass may be got even, and a new attempt may be made. It
must not be supposed that this process can be carried on indefinitely,
for the glass tends to lose its viscous properties after a time, or,
at all events, it "perishes" in some way, especially if it has been
allowed to get very thin; consequently too frequent attempts on the
same glass are unprofitable. Two or three trials are as many as it
generally pays to make. As a rule the largest possible flame may be
used with advantage in this operation.
§ 32. To blow a bulb in the middle of a tube, the procedure is much
like that already treated, but the manipulation is, if anything,
more difficult, for the further end of the tube must be carried and
turned as well as the end which is held to the lips.
§ 33. To make a side Weld.
This is by no means difficult, but is easier with lead glass than with
soda glass. The tube to which it is desired to make a side connection
having been selected, it is closed at one end by rubber tube stops, or
in any other suitable manner. The zone of the proposed connection is
noted, and the tube is brought to near softness round that circle (if
the tube is made actually soft, inconvenience will arise from the
bending, which is sure to occur). Two courses are then open to the
operator, one suitable to a thick tube, the other to a tube of
moderate thickness.
Taking the former first. Provide a piece of glass rod and warm its
end. Direct a small flame against the spot on the thick tube where
the proposed joint is to be. When the glass becomes almost
incandescent at this spot, put the end of the rod against it and draw
out a thread of glass till sufficient "metal" has been removed. Then
fuse off the thread close to the tube.
Fig. 27.
The subsequent procedure is the same as for thin tubes. In this case
heat the spot by the smallest flame available, and get the spot very
hot. Blow it out gently into a bubble, perhaps extending to a height
equal to its diameter. Then heat the top of the bubble till it is
incandescent and blow violently. This will produce an opening fringed
by glass so thin as to exhibit interference colours. Remove the filmy
part, and heat the frayed edges till they cohere and form an incipient
tube. If the flame has been of a correct size, the tube will now be
of the same diameter as the tube to be welded on, and will project
perhaps one-sixteenth of an inch from the surface of the main tube
(Fig. 28).
Fig. 28.
Fig. 29.
When this stage is reached, again heat the tube all round till it
nearly softens, and by means of the other hand heat the end of the
other tube which it is proposed to weld. Just before the main tube
actually softens, turn it so as to heat the edges of the aperture, and
at the same time get the end of the side tube very hot. Take both out
of the flame for an instant, and press the parts together, instantly
slightly withdrawing the side tube. If the operation is well
performed, it will be found that the point of maximum thickness of
glass is now clear of the main tube. The joint is then to be heated
all round and blown out--a rather awkward operation, and one
requiring some practice, but it can be done.
Fig. 30.
If great strength is wanted, heat the main tube all round the joint
bit by bit, and blow each section slightly outwards. If the operator
is confident in his skill, he should then heat the whole joint to the
softening point, blow it out slightly, and then adjust by pulling and
pushing. Cool first in the gas flame, and then plunge the joint into
the asbestos and cover it up--or if too large, throw the asbestos
cloth round it.
In the case of soda glass this final "general heat" is almost
essential, but it is not so with flint glass, and as the general heat
is the most difficult part of the job, it will be found easier to use
lead glass and omit the general heating. With soda glass a very small
irregularity will cause the joint to break when cold, but flint glass
is much more long-suffering. It is easy to perform the above
operation on small tubes. For large ones it will be found best to
employ flint glass and use the clip stands as in the case of direct
welds, treated above, but, of course, with suitable modifications.
Never let the main tube cool after the hole is made until the work is
done.
§ 34. Inserted Joints.
In many instances the performance of apparatus is much improved by
joints of this kind, even when their use is not absolutely essential.
There are two ways in which inserted joints may be made. The first
method is the easier, and works well with flint glass; but when one
comes to apply it to soda glass there is a danger of the glass
becoming too thick near the joint, and this often leads to a cracking
of the joint as the glass cools.
Fig. 31.