Glass, Its Composition and Characteristics. Annealing.
Drilling, Grinding, and Shaping Glass by methods other than
Fusion. Stopcocks. Marking Glass. Calibration and Graduation
of Apparatus. Thermometers. Exhaustion of Apparatus. Joining
Glass and Metal. Silvering Glass.
There are three kinds of glass rod and tubing which are easily
obtainable; these are soda-glass, which is that usually supplied by
chemical apparatus dealers when no particular glass is specified;
combustion-glass, which is supplied for work requiring a glass that does
not so easily soften or fuse as soda-glass; and lead-glass, which is
less common. There are also resistance-glass, made for use where very
slight solubility in water or other solutions is desirable, and a number
of other special glasses; but of these soda-glass, combustion-glass,
lead-glass, and resistance-glass are the most important to the
glass-blower whose work is connected with laboratory needs.
_Soda-Glass._--Consists chiefly of sodium silicate, but contains
smaller quantities of aluminum silicate, and often of calcium silicate;
there may also be traces of several other compounds.
The ordinary soda-glass tubing melts easily in the blowpipe flame, it
has not a long intermediate or viscous stage during fusion, but becomes
highly fluid rather suddenly; it does not blacken in the reducing flame.
Bad soda-glass or that which has been kept for many years, tends to
devitrify when worked. That is to say the glass becomes more or less
crystalline and infusible while it is in the flame; and in this case it
is often impossible to do good work with that particular sample of
glass; although the devitrification may sometimes be remedied by heating
the devitrified glass to a higher temperature. The presence of aluminum
compounds appears to have some influence on the tendency of the glass to
resist devitrification. Soda-glass, as a rule, is more liable to crack
by sudden heating than lead-glass, and articles made from soda-glass
often tend to crack spontaneously if badly made or, in the case of
heavier and thicker articles, if insufficiently annealed.
_Combustion-Glass._--Is usually a glass containing more calcium silicate
and potassium silicate than the ordinary "soft" soda-glass. It is much
less fusible than ordinary soda-glass, and passes through a longer
intermediate or viscous stage when heated. Such a glass is not very
suitable for use with the blowpipe owing to the difficulty experienced
in obtaining a sufficiently high temperature. If, however, a certain
amount of oxygen is mixed with the air used in producing the blowpipe
flame this difficulty is minimised.
_Resistance-Glass._--May contain zinc, magnesium, and other substances.
As a rule it is harder than ordinary soda-glass, and less suitable for
working in the blowpipe flame. It should have very little tendency to
dissolve in water, and hence is used when traces of alkali or silicates
would prove injurious in the solutions for which the glass vessels are
to be used.
_Lead-Glass._--This, or "flint" glass as it is often called from the
fact that silica in the form of crushed and calcined flint was often
used in making the English lead-glasses, contains a considerable
proportion of lead silicate. Such a glass has, usually, a particularly
bright appearance, a high refractive index, and is specially suitable
for the production of the heavy "cut-glass" ware.
Lead-glass tubing is easy to work in the blowpipe flame, melts easily,
but does not become fluid quite so suddenly as most soda-glasses;
articles made from it are remarkably stable and free from tendency to
spontaneous cracking, although, as is essential for all the heavy or
"glass-house" work, the massive articles need annealing in the oven.
The two chief disadvantages of lead-glass for laboratory work are that
it is blackened by the reducing gases if held too near to the blue cone
of the blowpipe flame, and that it is rather easily attacked by chemical
reagents; thus ammonium sulphide will cause blackening.
The effect of the reducing flame on lead is not altogether a
disadvantage, however; because a little care in adjusting the blowpipe
and a little care in holding the glass in the right position will enable
the student to work lead-glass without producing the faintest trace of
blackening. This, in addition to being a valuable exercise in
manipulation, will teach him to keep his blowpipe in good order, and
prove a useful aid in his early efforts to judge as to the condition of
the flame. It prevents discouragement if the student does his
preliminary work with the soda-glass, but he should certainly make
experiments with lead-glass as soon as he has acquired reasonable
dexterity with soda-glass.
_Annealing._--Annealing is a process by which any condition of strain
which has been set up in a glass article, either by rapid cooling of one
part while another part still remains hot, or by the application of
mechanical stress after cooling is relieved. Annealing is carried out by
subjecting the article to a temperature just below the softening point
of the glass, maintaining that temperature until the whole article has
become heated through the thicker part, and then reducing the
temperature very gradually; thus avoiding any marked cooling of the
thinner and outer parts first.
For thin glass apparatus of the lamp-blown or blowpipe-made variety in
which there are no marked difference of thickness, such as joins on
tubes, ordinary seals, bulbs, etc., there is little need for annealing;
and even those having rather marked changes of thickness, such as
filter pumps, can be annealed sufficiently by taking care that the last
step in making is heating to just below visible redness in the blowpipe
flame and then rotating in a sooty gas flame until covered with a
deposit of carbon. The article should then be allowed to cool in a place
free from draughts and where the hot glass will not come in contact with
anything.
A few of the blowpipe-made articles, such, for example, as glass
stopcocks, need more careful annealing, and for this purpose a small
sheet-iron oven which can be heated to dull redness over a collection of
gas burners will serve. Better still, a small clay muffle can be used.
In either case, the article to be annealed should be laid on a clean,
smooth, fireclay surface, the temperature should be maintained at a very
dull red for two or three hours and then reduced steadily until the oven
is cold. This cooling should take anything from three to twelve hours,
according to the nature of the article to be annealed. A thick article,
or one having great irregularities in thickness will need much longer
annealing than one thinner or more regular. As a rule, soda-glass will
need more annealing than lead-glass.
_Drilling Glass._--Small holes may be drilled in glass by means of a rod
of hard steel which has been broken off, thus giving a more or less
irregular and crystalline end.
There are several conditions necessary to enable the drilling of small
holes to be carried out successfully:--the first of these is that the
"drill" should be driven at a high speed. This may be done by means of a
geared hand-drill such as the American pattern drill, although a
somewhat higher speed than this will give is even more satisfactory. The
second condition is that the pressure on the drill is neither too light
nor too heavy; this is conveniently regulated by hand. The third
condition is that the drill be prevented from "straying" over the
surface of the glass; for this purpose a small metal guide is useful.
The fourth condition is that a suitable lubricant be used; a strong
solution of camphor in oil of turpentine is perhaps the most suitable.
For commercial work, a diamond drill is often used, but this is scarcely
necessary for the occasional work of a laboratory.
_Larger Holes in Glass._--The method of drilling with a hard steel rod
is not highly satisfactory for anything but small holes. When a larger
hole, say one of an eighth of an inch or more, is needed it is better to
use a copper or brass tube. This tube may be held in an American
hand-drill, but a mixture of carborundum or emery and water is supplied
to the rotating end. Tube or drill must be lifted at frequent intervals
in order to allow a fresh supply of the grinding material to reach the
end. In this case, also, a guide is quite essential in the early stages
of drilling; otherwise the end of the tube will stray. The speed of
cutting may be increased slightly by making a number of radial slots in
the end of the tube; these serve to hold a supply of the grinding
material.
_Grinding Lenses._--This is scarcely within the scope of a book on
glass-blowing for laboratory purposes, but it may be said that the lens
may be ground by means of a permutating mould of hard lead or
type-metal. The rough shaping is done with coarse carborundum or emery,
and successive stages are carried on with finer and finer material. The
last polishing is by the use of jewellers' rouge on the mould, now
lined with a fine textile.
_Filing Glass._--If a new file, thoroughly lubricated with a solution of
camphor in oil of turpentine, is used, there is but little difficulty in
filing the softer glasses. A slow movement of the file, without
excessive pressure but without allowing the file to slip, is desirable.
After a time the cutting edges of the file teeth will wear down and it
will be necessary to replace the file by another.
_Grinding Stoppers._--This is, perhaps, the most common form of grinding
that the laboratory worker will need to perform, and for that reason,
rather full details of the procedure are desirable.
A very crude form of ground-in stopper may be made by drawing out the
neck and the mass of glass which is intended to form the stopper to
approximately corresponding angles, wetting the surfaces with a mixture
of the abrasive material and water, and grinding the stopper in by hand.
Frequent lifting of the stopper is necessary during grinding, in order
to allow fresh supplies of abrasive material to reach the contacts. When
an approximate fit is obtained, the coarse abrasive should be washed
off, care being taken that the washing is complete, and a finer abrasive
substituted. After a while, this is replaced in its turn by a still
finer grinding material.
Such a method of grinding may give a satisfactory stoppering if the
angles of the plug and socket correspond very closely before grinding is
commenced; but if there is a wide difference in the original angles,
then no amount of grinding by this method will produce a good result.
The reason for this is that the plug will become so worn in the
preliminary grinding as to assume the form of a highly truncated cone;
the socket will assume a reverse form, and the end result will be a
loose-fitting plug and socket.
Satisfactory grinding may be carried out by the use of copper or
type-metal cones for the preliminary shaping. Such cones should be
mounted on a mandrel which will fit into the chuck of the American
hand-drill and turned on the lathe to the desirable angle for
stoppering. A number of these cones will be necessary. A number of
similar moulds, that is to say blocks of type-metal or hard lead in
which is a hole corresponding in size and angle to the plug desired,
should be made also. These must be rotated, either in the lathe or by
other means, and are used for the preliminary shaping of the plug. If
but few plugs are to be ground it is unnecessary to provide a means of
rotating the moulds, as the plug may be held in the hand and ground into
the mould in a manner similar to that used in the first method of
stoppering.
[Illustration: Fig. 14]
When the socket and plug have been ground, by the successive use of
cones and moulds, to the desired angle, so that they correspond almost
exactly, the plug is given its final fitting into the socket by
grinding-in with a fine abrasive, in the manner first described.
_Stopcocks._--Although it would be more strictly in keeping with the
form of this book to divide the making of stopcocks into two parts;
shaping by heat and grinding, we will consider the whole operation here,
and take for our example a simple stopcock such as that illustrated by
Fig. 14.
The "blank," _f_, that is the socket before grinding, is made by drawing
out a piece of fairly thick-walled tubing into the form shown by _a_.
Two zones on this tube are then heated by means of a small, pointed
flame, and the tube is compressed along its axis, thus producing two
raised rings as shown by _b_. Two zones, slightly towards the outer
sides of these two raised rings are heated and the tube is drawn while
air pressure is maintained within. This produces two thin-walled bulbs
or extensions similar to those shown by _c_. One of these extensions is
now broken off by means of a sharp blow with the edge of a file or
other piece of metal, and the edges of the broken glass are rounded in
the flame. The other extension is left to serve as a handle. We have now
a piece of glass like that shown by _d_. Now heat a spot on the side of
this, medially between the raised rings, until the glass is on the point
of becoming deformed, and bring the intensely heated end of a smaller
tube in contact with the heated spot. Without disturbing the relative
positions of the two tubes, press the smaller tube down on a thin steel
wire, so that the wire passes along the tube and enters the soft glass;
thus forming a projection inside the sockets as shown by _e_. The wire
must be withdrawn, again immediately. When the wire has been withdrawn,
heat the place where it entered to dull redness, in order to relieve any
strain; break off the thin extension, which up to the present has served
as a handle, round off the broken edges in the flame, and join on and
indent a similar piece of small tubing to the opposite side of the
socket; the socket at this stage being shown by _f_. The "blank" for the
socket is now completed, but it must be heated to dull redness in order
to relieve strain and be placed in an annealing oven, where it should
be annealed for some hours.
The "blank" for the plug offers no special difficulty; it is made by
heating a glass rod and compressing it axially until a mass having the
form shown by _g_, Fig. 14, is produced; the end of this is heated
intensely and brought in contact with the rather less heated side of a
glass tube which has been drawn to the shape desired for the handle;
when contact is made a slight air pressure is maintained in the glass
tube, thus producing a hollow join. The ends of the tube are sealed and
the bottom of the plug is drawn off, thus giving the finished "blank" as
shown by _h_. This blank is now held in a pair of asbestos-covered
tongs, heated to dull redness all over, and transferred to the annealing
oven.
When cold, the socket is ground out by the second method given under
"Grinding Stoppers"; that is to say, by means of type-metal or copper
cone, and the plug is ground to fit in a corresponding mould. When the
fit is almost perfect, the transverse hole is drilled in the plug, and
the final finishing is made with fine abrasive powder. Great care must
be taken in the final grinding that there is no accumulation of
abrasive material in the transverse hole of the plug; if this is allowed
to occur there will be a ring ground out of the socket where the holes
move, and the tightness of the finished stopcock will be lost.
_Marking Glass._--As a preliminary to a consideration of the methods of
graduating and calibrating glass apparatus, it is convenient to consider
the various methods which are available for marking glass. Among these
are, the writing diamond, the carborundum or abrasive pencil, the
cutting-wheel, and etching by means of hydrofluoric acid. Each produces
a different class of marking and each is worthy of independent
consideration.
_The Writing Diamond._--This is the name given to a small irregular
fragment of "bort" which is usually mounted in a thin brass rod. Such a
diamond, if properly selected, has none of the characteristics of a
cutting diamond; although one occasionally finds so-called "writing
diamonds" which will produce a definite cut. These should be rejected.
The writing diamond is used in much the same way as a pencil, but is
held more perpendicularly to the object, and a certain amount of
pressure is necessary. The mark produced is a thin scratch which,
although fairly definite, lacks breadth, and this is a disadvantage
where the marking has to be read at a distance. This disadvantage may to
some extent be overcome by making a number of parallel scratches.
_The Abrasive Pencil._--A rod of carborundum composition may be ground
or filed to a point, and this forms a very useful pencil for general
work. The marking produced is rather less definite than that produced by
a writing diamond, but has the advantage of being broader.
_The Cutting Wheel._--"Cutting" in this case is scarcely the ideal
expression, it should rather be "grinding," but "cutting" is more
commonly used. Exceedingly good graduations may be made by the edge of a
small, thin, abrasive wheel which is mounted on the end of a small
mandrel and driven by a flexible shaft from an electric motor or any
other convenient source of power. The depth of the mark can be
controlled, and very light pressure will suffice.
_Etching._--This is often the quickest and easiest way of marking glass
apparatus. The object to be marked should first be warmed and coated
very thoroughly with a thin film of paraffin wax. When cold, the marking
is made through the paraffin wax by means of a needle point, and the
object is then exposed to the action of hydrofluoric acid. If a shallow
but clearly visible marking is desired, it is well to use the vapour of
the acid; this may be done by bending up a sheet-lead trough on which
the object can rest with the marked surface downwards. A little of the
commercial hydrofluoric acid, or a mixture of a fluoride and sulphuric
acid, is distributed over the bottom of the trough, and the whole
arrangement is allowed to stand for about an hour. The object is washed
thoroughly and the paraffin wax removed, either by melting and wiping
off or by the use of a solvent, and the marking is finished.
If a deep marking is desired, in order that it may afterwards be filled
with some pigment, a better result is obtained by the use of liquid
commercial hydrofluoric acid, which is a solution of hydrogen fluoride
in water. The acid is mopped on to the object after the markings have
been made on the paraffin wax film, and allowed to remain in contact for
a few minutes. It is advantageous to repeat the mopping-on process at
intervals during the etching.
In all cases where hydrofluoric acid is used, or stored, it is of great
importance to keep it well away from any optical instruments, as the
most minute trace of vapour in the air will produce a highly destructive
corrosion of any glass surfaces.
_Methods of Calibration._--In the case of apparatus for volumetric work,
this is usually carried out by weighing, although some of the smaller
subdivisions are often made by measurement. When the subdivisions are
made in this way it is of importance to see that the walls of the tube
or vessel to be calibrated are parallel. Great errors arise in some of
the commercial apparatus from neglect of this precaution. A convenient
method of testing for parallelism, in the case of a wide tube, is to
close one end and to weigh in successive quantities of mercury. An
observation of the length occupied by each successive quantity will
indicate any change in the bore. In the case of capillary tubes, it is
convenient to introduce an unweighed quantity of mercury, measure its
length accurately, and then to move it along the tube in stages, either
by tilting the tube or by the application of air pressure. A measurement
of the length at each stage will indicate whether the bore is
approximately parallel or not. Neither of these methods is to be relied
on without a careful examination of the tube, as it may happen that
there are local irregularities in the bore which compensate for each
other, and do not, therefore, affect the volume of a given length.
Obviously, the smaller the quantity of mercury with which the test is
carried out and the greater the number of observations made, the less
risk will there be of such an error. A liquid, such as water or alcohol,
which wets the glass is not suitable for such a test, unless special
precautions are taken.
When, however, a pipette or burette has to be calibrated to deliver a
certain volume of water, the final calibration must be made with this
liquid. Thus, the burette would first be calibrated by weighing in
definite quantities of mercury of say 13.54 grammes (1 cc at 15 deg.C.),
each of the 1 cc divisions should be marked by some temporary marking.
The burette is now filled with a solution of potassium bichromate and
sulphuric acid and allowed to soak for some time; the bichromate is
washed out and distilled water is put in. Successive quantities of water
are run out of the jet, a fixed time being allowed for draining, and the
weights of the quantities delivered are noted. This procedure will give
the necessary data for altering the marking so that it may correspond to
1 cc _delivered_. Each 1 cc division is now divided into tenths by the
method described below. A final verification of the markings should be
made when the subdivision is completed.
_Subdivision of Graduations._--Mark out the spaces to be subdivided on a
sheet of paper. Take a reliable ruler on which any convenient length is
divided into the desired number and place it across the lines at such an
angle that the limits noted on the rule exactly bridge the gap. Now draw
parallel lines through the markings.
_Copying a Scale._--When a scale has been prepared on paper and it is
necessary to copy that scale on the waxed-glass surface for etching, a
convenient method is to employ a long wooden bar having a sharp needle
passing through it at either end. The scale and object to be marked are
fastened in line with one another, and the caliper bar is used from step
to step. The mark is made by moving the bar through a minute portion of
a circle, which provided that the bar is two or three feet in length,
will not introduce any perceptible error in a scale of say a quarter of
an inch in width. The arrangement is shown by Fig. 15.
[Illustration: Fig. 15]
_Graduating a Thermometer._--Assuming that the thermometer has been made
of carefully selected tubing in which the bore is parallel and free from
any small irregularities, we have only to fix the freezing point and
boiling point. The intervening space may then be divided into 100 (if
the thermometer is to be Centigrade) or 180 (if Fahrenheit). This
division may be carried out by the method given under "Subdivisions of
Graduations." A thermometer should not be calibrated until some weeks
after making, as the glass bulb tends to contract.
_Joining Glass and Metal._--It sometimes happens that one needs to make
a more permanent and less flexible joint between a glass and metal tube
than can be obtained by means of a rubber tube. To this end, any one of
three slightly different methods may be employed. In the method of
Chatelier one first coats the glass with platinum or silver, which may
be done by moistening the glass with platinum chloride or silver nitrate
and then heating to redness; a layer of copper is then deposited
electrolytically on the treated surface of the glass, and soldering is
carried out in the usual manner.
McKelvy and Taylor call attention to two other methods in the _Journal
of the Chemical Society_ for September, 1920. In one of these methods
the glass is coated with platinum by covering it with a suspension of
platinum chloride in oil of lavender and heating until the oil is burnt
off. The metal tube is then tinned on its inner side and soldered to the
prepared glass, slightly acid zinc chloride being used as a flux.
In the second method, a joint is made by means of the Kraus flux, which
consists of equal weights of zinc oxide, borax, and powdered soda-glass
fused together. This is coated on the inner surface of the metal tube,
and the hot glass tube, which has had the end slightly flanged to give
support, is inserted. Fusion of the flux is completed by heating the
outside of metal tube.
_Silvering Glass._--In all cases where it is intended to deposit a
silver mirror on a glass surface, thorough cleaning is essential.
Prolonged soaking in a hot solution of potassium bichromate which has
been acidified with sulphuric acid will often prove useful. The glass
should then be washed thoroughly, rinsed in distilled water, and the
solution should then be used.
There are many formulae for the silvering solution, but that used in
Martin's method may be given:--
A--Nitrate of Silver 40 grammes
Distilled Water 1000 c. cm.
B--Nitrate of Ammonium 60 grammes
Distilled Water 1000 c. cm.
C--Pure Caustic Potash 100 grammes
Distilled Water 1000 c. cm.
D--Pure Sugar Candy 100 grammes
Distilled Water 1000 c. cm.
Dissolve and add:--
Tartaric Acid 23 grammes
Boil for ten minutes, and when cool add:--
Alcohol 200 c. cm.
Distilled Water to 2000 c. cm.
For use take equal parts of A and B. Mix together also equal parts of C
and D in another vessel. Then mix both liquids together in the silvering
vessel and suspend the glass to be silvered face downwards in the
solution. Or if a vessel has to be silvered on the inside, the solution
is poured in. In this case, the deposition of silver may be hastened by
immersing the vessel to be silvered in warm water.
In working with a silver solution containing ammonia or ammonium salts
there is sometimes the possibility of forming an explosive silver
compound. It is well, therefore, to avoid keeping such solutions longer
than is necessary, and to bear in mind that any deposit formed by
solutions containing both silver and ammonia may have explosive
properties, especially when dry.