A Handbook of Laboratory Glass-Blowing Part 2

A large tube is sealed at the bottom and the top is lipped, as in making a test-tube. A smaller tube is then joined on by a method similar to that given on page 18, but without making a perforation in the bottom of the large tube. Heating and expanding by air pressure, first through the large tube, then through the smaller tube and then again through the large tube, will give a satisfactory finish to this part of the work.

[Ill.u.s.tration: Fig. 11]

The syphon tube is now joined on to the large tube as shown by _a_, Fig.

11, care being taken to seal the other end of the syphon tube before joining. The details of the final and re-entrant joint of the syphon tube are shown at the lower part of _a_. This join is made by expanding the sealed end of the syphon tube into a small, thick-walled bulb, and the bottom of this bulb is burst out by local heating and blowing; the fragments of gla.s.s are removed and the edges made smooth by melting. A similar operation is carried out on the side of the tube to which the syphon tube is to be joined. This stage is shown by _a_. Now heat the syphon tube at the upper bend until it is flexible, and press the bulb at its end into the opening on the side of the other tube. Hold the gla.s.s thus until the syphon is no longer flexible. The final join is made by heating the two contacting surfaces, if necessary pressing the edges in contact with the end of a turn-pin, fusing together and expanding. The finished apparatus is shown by _c_.

_Electrodes._--A thin platinum wire may be sealed into a capillary tube without any special precautions being necessary. The capillary tube may be drawn out from the side of a larger tube by heating a spot on the gla.s.s, touching with a gla.s.s rod and drawing the rod away; or the exhaustion branch described on page 18 may be used for the introduction of an electrode. It is convenient sometimes to carry out the exhaustion through the same tube that will afterwards serve for the electrode. The electrode wire is laid inside the branch before connecting to the exhaustion pump. When exhaustion is completed the tube is heated until the soft gla.s.s flows round the platinum and makes the seal air-tight.

The branch is now cut off close to the seal on the pump side, a loop is made in the projecting end of the platinum wire, and the seal is finished by melting the cut-off end.

Platinum is usually employed for such work, but if care is taken to avoid oxidation it is not impossible to make fairly satisfactory seals with clean iron or nickel wire. Hard rods of fine graphite, such as are used in some pencils, may also be sealed into gla.s.s, but it seems probable that air would diffuse through the graphite in the course of time.

Another method for the introduction of an electrode is ill.u.s.trated by _d_, _e_, _f_ and _g_, Fig. 11. In this case the bulb or thin-walled tube into which the electrode is to be sealed is perforated by a quick stab with an intensely heated wire--preferably of platinum--which is then withdrawn before the gla.s.s has had time to harden, and thus a minute circular hole is made. The electrode is coated with a layer of similar gla.s.s, or of the specially made enamel which is sold for this purpose, inserted into the bulb or tube by any convenient opening, and adjusted by careful shaking until the platinum wire projects through the small hole. The bulb or tube is then fused to the coating of the electrode and the whole spot expanded slightly by blowing. The appearance of the finished seal is shown by _g_. It is well to anneal slightly by smoking.

_Thermometers._--Apart from the notes on page 20 with respect to the blowing of a suitable bulb on capillary tubing there is little to say in connection with the gla.s.s working needed in making a plain thermometer.

The size desirable for the bulb will be determined by the bore of the capillary tube, the coefficient of expansion of the liquid used for filling, and the range of temperature for which the thermometer is intended.

Filling may be carried out as follows:--Fit a small funnel to the open end of the capillary by means of a rubber tube, and pour into the funnel rather more than enough of the liquid to be used than is required to fill the bulb. Mercury or alcohol will be used in practice, most probably. Warm the bulb until a few air bubbles have escaped through the liquid and then allow to cool. This will suck a certain amount of liquid into the bulb. Now heat the bulb again, and at the same time heat the capillary tube over a second burner. The liquid will boil and sweep out the residual air, but it is necessary to heat the capillary tube as well in order to prevent condensation. Allow the bulb and tube to cool, then repeat the heating once more. By this time the bulb and tube should be free from air, and cooling should give a completely filled thermometer. Remove the funnel and heat the thermometer to a few degrees above the maximum temperature for which it is to be used; the mercury or other filling liquid will overflow from the top, and, as the temperature falls, will recede, thus allowing the end of the capillary to be drawn out. Reheat again until the liquid rises to the top of the tube, then seal by means of the blowpipe flame. The thermometer is now finished except for graduation; this is dealt with on page 75.

_An Alarm Thermometer._--A thermometer which will complete an electric circuit when a certain temperature is reached may be made by sealing an electrode in the bulb and introducing a wire into the top, which in this case is not sealed. Naturally, this thermometer will be filled with mercury. There is considerable difficulty in filling such a bulb without causing it to crack.

Several elaborations of this form are made, in which electrodes are sealed through the walls of the capillary tube, thus making it possible to detect electrically the variation of temperature when it exceeds any given limits.

_An Enclosed or Floating Thermometer._--The construction of this type of thermometer is shown by _h_ and _i_, Fig 11. It is made in the following stages:--A bulb is blown on the drawn-out end of a thin-walled tube as shown by _h_. A small bulb is blown on the end of a capillary tube, burst, and turned out to form a lip which will rest in the drawn-out part of the thin-walled tube but is just too large to enter the bulb.

The capillary tube is introduced and sealed in position, care being taken to expand the joint a little. The thermometer is filled and the top of the capillary tube closed by the use of a small blowpipe flame. A paper scale having the necessary graduations is inserted, and the top of the outer tube is closed as shown by _i_.

_A Maximum and Minimum Thermometer._--If a small dumb-bell-shaped rod of gla.s.s or metal is introduced into the capillary tube of a horizontally placed, mercury-filled thermometer in such a position that the rising mercury column will come in contact with it, the rod will be pushed forward. When the mercury falls again the rod will be left behind and thus indicate the maximum temperature attained. If a similar dumb-bell-shaped rod is introduced into an alcohol-filled thermometer and pushed down until it is within the alcohol column, it will be drawn down by surface tension as the column falls; but the rising column will flow pa.s.sed it without causing any displacement; thus the minimum temperature will be recorded.

Six's combined maximum and minimum thermometer is shown by _b_, Fig. 11.

In this case both maximum and minimum records are obtained from a mercury column, although the thermometer bulb is filled with alcohol. It is an advantage to make the dumb-bell-shaped rods of iron, as the thermometer can then be reset by the use of a small magnet, another advantage consequent on the use of metal being that the rods can be easily adjusted, by slight bending, so as to remain stationary in the tubes when the thermometer is hanging vertically, and yet to move with sufficient freedom to yield to the pressure of the recording column.

The thermometer may be filled by the following method:--When the straight tube has been made the first dumb-bell is introduced and shaken down well towards the lower bulb, the tube is now bent to its final shape and the whole thermometer filled with alcohol as described on page 44. Now heat the thermometer to a little above the maximum temperature that it is intended to record, and pour clean mercury into the open bulb while holding the thermometer vertically. Allow to cool, and the mercury will be sucked down. The second dumb-bell is now introduced, sufficient alcohol being allowed to remain in the open bulb to about half fill it, and the alcohol in this bulb is boiled to expel air. The tube through which the bulb was filled in now sealed.

_Clinical Thermometers._--The clinical thermometer is a maximum thermometer of a different type. In this case there is a constriction of the bore at a point just above the bulb. When the mercury in the bulb commences to contract, the mercury column breaks at the constriction and remains stationary in the tube, thus showing the maximum temperature to which it has risen.

_Vacuum Tubes._--There are so many forms of these that it is scarcely practicable or desirable to give detailed instructions for making them; but an application of the various methods of gla.s.s-working which have already been explained should enable the student to construct most of the simpler varieties. An interesting vacuum tube is made which has no electrodes, but contains a quant.i.ty of mercury. When the tube is rocked so as to cause friction between the mercury and the gla.s.s sufficient charge is produced to cause the tube to glow.

_A Sprengel Pump._--This, in its simplest form, is ill.u.s.trated by _a_, Fig. 12. Such a form, although highly satisfactory in action, needs constant watching while in action, as should the mercury funnel become empty air will enter the exhausted vessel. Obviously, the fall-tube must be made not less than thirty inches long; the measurement being taken from the junction of the exhaustion branch with the fall-tube to the top of the turned-up end.

[Ill.u.s.tration: Fig. 12]

_A Macleod Pump._--One form of this is ill.u.s.trated by _b_, Fig. 12. It has the advantage that the mercury reservoir may be allowed to become empty without affecting the vacuum in the vessel being exhausted.

_"Spinning" Gla.s.s._--By the use of suitable appliances, it is quite possible to draw out a continuous thread of gla.s.s, which is so thin as to have almost the flexibility and apparent softness of woollen fibre; a ma.s.s of such threads const.i.tutes the "gla.s.s wool" of commerce.

The appliances necessary are:--a blowpipe capable of giving a well-formed flame of about six or eight inches in length, a wheel of from eighteen inches to three feet in diameter and having a flat rim of about three inches wide, and a device for rotating the wheel at a speed of about three hundred revolutions per minute.

A very satisfactory arrangement may be made from an old bicycle; the back wheel having the tyre removed and a flat rim of tin fastened on in its place. The chain drive should be retained, but one of the cranks removed and a handle subst.i.tuted for the remaining pedal. The whole device is shown by Fig. 13.

[Ill.u.s.tration: Fig. 13]

The procedure in "spinning" gla.s.s is as follows:--First melt the end of a gla.s.s rod and obtain a large ma.s.s of thoroughly softened gla.s.s, now spin the wheel at such a speed that its own momentum will keep it spinning for several seconds. Touch the end of the melted rod with another piece of gla.s.s and, without withdrawing the original rod from the blowpipe flame, draw out a thread of molten gla.s.s and twist it round the spinning wheel. If this is done properly, the thread of gla.s.s will grip on the flat rim, and by continuing to turn the wheel by hand it is possible to draw out a continuous thread from the melted rod, which must be advanced in the blowpipe flame as it is drawn on the wheel. If the rod is not advanced sufficiently the thread will melt off, if it is advanced too much, so as to heat the thick part and allow the gla.s.s to become too cool at the point of drawing out, then the thread will become too thick, but it is easy after a little practice to obtain the right conditions. Practice is necessary also in order to find the right speed for the wheel.

When sufficient gla.s.s has been "spun," the whole "hank" of thin thread may be removed by drawing the thumb-nail across the wheel at any point on its flat rim, thus breaking the threads, and allowing the "hank" to open.

_Brushes for Use with Strong Acids._--Gla.s.s wool, if of fine enough texture to be highly flexible, can be used to make acid-resisting brushes. A convenient method for mounting the spun gla.s.s is to melt the ends of the threads together into a bead, and then to fuse the bead on to a rod; thus giving a brush. If a pointed brush is necessary, the point may be ground on an ordinary grindstone or carborundum wheel by pressing the loose end of the spun gla.s.s against the grinding wheel with a thin piece of cardboard.

When using brushes of this description, it is well to bear in mind the fact that there is always a liability of a few threads of gla.s.s breaking off during use.

CHAPTER IV

Gla.s.s, Its Composition and Characteristics. Annealing.

Drilling, Grinding, and Shaping Gla.s.s by methods other than Fusion. Stopc.o.c.ks. Marking Gla.s.s. Calibration and Graduation of Apparatus. Thermometers. Exhaustion of Apparatus. Joining Gla.s.s and Metal. Silvering Gla.s.s.

There are three kinds of gla.s.s rod and tubing which are easily obtainable; these are soda-gla.s.s, which is that usually supplied by chemical apparatus dealers when no particular gla.s.s is specified; combustion-gla.s.s, which is supplied for work requiring a gla.s.s that does not so easily soften or fuse as soda-gla.s.s; and lead-gla.s.s, which is less common. There are also resistance-gla.s.s, made for use where very slight solubility in water or other solutions is desirable, and a number of other special gla.s.ses; but of these soda-gla.s.s, combustion-gla.s.s, lead-gla.s.s, and resistance-gla.s.s are the most important to the gla.s.s-blower whose work is connected with laboratory needs.

_Soda-Gla.s.s._--Consists chiefly of sodium silicate, but contains smaller quant.i.ties of aluminum silicate, and often of calcium silicate; there may also be traces of several other compounds.

The ordinary soda-gla.s.s 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-gla.s.s or that which has been kept for many years, tends to devitrify when worked. That is to say the gla.s.s 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 gla.s.s; although the devitrification may sometimes be remedied by heating the devitrified gla.s.s to a higher temperature. The presence of aluminum compounds appears to have some influence on the tendency of the gla.s.s to resist devitrification. Soda-gla.s.s, as a rule, is more liable to crack by sudden heating than lead-gla.s.s, and articles made from soda-gla.s.s often tend to crack spontaneously if badly made or, in the case of heavier and thicker articles, if insufficiently annealed.

_Combustion-Gla.s.s._--Is usually a gla.s.s containing more calcium silicate and pota.s.sium silicate than the ordinary "soft" soda-gla.s.s. It is much less fusible than ordinary soda-gla.s.s, and pa.s.ses through a longer intermediate or viscous stage when heated. Such a gla.s.s 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-Gla.s.s._--May contain zinc, magnesium, and other substances.

As a rule it is harder than ordinary soda-gla.s.s, 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 gla.s.s vessels are to be used.

_Lead-Gla.s.s._--This, or "flint" gla.s.s 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-gla.s.ses, contains a considerable proportion of lead silicate. Such a gla.s.s has, usually, a particularly bright appearance, a high refractive index, and is specially suitable for the production of the heavy "cut-gla.s.s" ware.

Lead-gla.s.s tubing is easy to work in the blowpipe flame, melts easily, but does not become fluid quite so suddenly as most soda-gla.s.ses; articles made from it are remarkably stable and free from tendency to spontaneous cracking, although, as is essential for all the heavy or "gla.s.s-house" work, the ma.s.sive articles need annealing in the oven.

The two chief disadvantages of lead-gla.s.s 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 gla.s.s in the right position will enable the student to work lead-gla.s.s 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-gla.s.s, but he should certainly make experiments with lead-gla.s.s as soon as he has acquired reasonable dexterity with soda-gla.s.s.

_Annealing._--Annealing is a process by which any condition of strain which has been set up in a gla.s.s 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 gla.s.s, 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 gla.s.s 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 gla.s.s will not come in contact with anything.

A few of the blowpipe-made articles, such, for example, as gla.s.s stopc.o.c.ks, 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 m.u.f.fle 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-gla.s.s will need more annealing than lead-gla.s.s.

_Drilling Gla.s.s._--Small holes may be drilled in gla.s.s 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 gla.s.s; 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.