A Handbook of Laboratory Glass-Blowing Part 3

_Larger Holes in Gla.s.s._--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 bra.s.s 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 gla.s.s-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 coa.r.s.e 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 Gla.s.s._--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 gla.s.ses. 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 ma.s.s of gla.s.s 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 coa.r.s.e abrasive should be washed off, care being taken that the washing is complete, and a finer abrasive subst.i.tuted. 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 a.s.sume the form of a highly truncated cone; the socket will a.s.sume 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.

[Ill.u.s.tration: 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.

_Stopc.o.c.ks._--Although it would be more strictly in keeping with the form of this book to divide the making of stopc.o.c.ks into two parts; shaping by heat and grinding, we will consider the whole operation here, and take for our example a simple stopc.o.c.k such as that ill.u.s.trated 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 gla.s.s are rounded in the flame. The other extension is left to serve as a handle. We have now a piece of gla.s.s like that shown by _d_. Now heat a spot on the side of this, medially between the raised rings, until the gla.s.s 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 pa.s.ses along the tube and enters the soft gla.s.s; 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 gla.s.s rod and compressing it axially until a ma.s.s 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 gla.s.s tube which has been drawn to the shape desired for the handle; when contact is made a slight air pressure is maintained in the gla.s.s 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 acc.u.mulation 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 stopc.o.c.k will be lost.

_Marking Gla.s.s._--As a preliminary to a consideration of the methods of graduating and calibrating gla.s.s apparatus, it is convenient to consider the various methods which are available for marking gla.s.s. 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 cla.s.s 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 bra.s.s 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 gla.s.s 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 gla.s.s 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 quant.i.ties of mercury. An observation of the length occupied by each successive quant.i.ty will indicate any change in the bore. In the case of capillary tubes, it is convenient to introduce an unweighed quant.i.ty 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 quant.i.ty 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 gla.s.s 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 quant.i.ties of mercury of say 13.54 grammes (1 cc at 15C.), each of the 1 cc divisions should be marked by some temporary marking.

The burette is now filled with a solution of pota.s.sium bichromate and sulphuric acid and allowed to soak for some time; the bichromate is washed out and distilled water is put in. Successive quant.i.ties of water are run out of the jet, a fixed time being allowed for draining, and the weights of the quant.i.ties 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 s.p.a.ces 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-gla.s.s surface for etching, a convenient method is to employ a long wooden bar having a sharp needle pa.s.sing 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.

[Ill.u.s.tration: Fig. 15]

_Graduating a Thermometer._--a.s.suming 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 s.p.a.ce 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 gla.s.s bulb tends to contract.

_Joining Gla.s.s and Metal._--It sometimes happens that one needs to make a more permanent and less flexible joint between a gla.s.s 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 gla.s.s with platinum or silver, which may be done by moistening the gla.s.s 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 gla.s.s, 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 gla.s.s 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 gla.s.s, 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-gla.s.s fused together. This is coated on the inner surface of the metal tube, and the hot gla.s.s tube, which has had the end slightly f.l.a.n.g.ed to give support, is inserted. Fusion of the flux is completed by heating the outside of metal tube.

_Silvering Gla.s.s._--In all cases where it is intended to deposit a silver mirror on a gla.s.s surface, thorough cleaning is essential.

Prolonged soaking in a hot solution of pota.s.sium bichromate which has been acidified with sulphuric acid will often prove useful. The gla.s.s 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 gla.s.s 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.

CHAPTER V

Extemporised Gla.s.s-Blowing Apparatus--The Use of Oil or other Fuels--Making Small Rods and Tubes from Gla.s.s Sc.r.a.p--The Examination of Manufactured Apparatus with the View to Discovering the Methods Used in Manufacture--Summary of Conditions Necessary for Successful Gla.s.s-Blowing.