The Methods of Glass Blowing and of Working Silica in the Oxy-Gas Flame Part 2

In very much of his work the gla.s.s-blower is guided more by the _feel_ of the gla.s.s than by what he sees. The power of feeling gla.s.s can only be acquired by practice, and after a certain amount of preliminary failure. As a rule I have observed that beginners are apt to raise their gla.s.s to a higher temperature than is necessary, and that they employ larger flames than are wanted. If gla.s.s be made too soft it may fall so completely out of shape as to become unworkable except in very skilful hands. The following rules, therefore, should be strictly adhered to.

Always employ in the first instance the smallest flame that is likely to do the work required. In operations involving _blowing out_ viscous gla.s.s, attempt to blow the gla.s.s at low temperatures before higher ones are tried. After a little experience the adoption of the right-sized flame for a given purpose, and the perception of the best condition of gla.s.s for blowing it, become almost automatic.

I may add that gla.s.s which is to be bent needs to be much less heated than gla.s.s which is to be blown.

=Annealing.=--If apparatus, the gla.s.s of which is very thin and of uniform substance, be heated, on removal from the source of heat it will cool equally throughout, and therefore may often be heated and cooled without any special precautions. If the gla.s.s be thick, and especially if it be of unequal thickness in various parts, the thinner portions will cool more quickly than those which are more ma.s.sive; this will result in the production of tension between the thicker and thinner parts in consequence of inequality in the rates of contraction, and fractures will occur either spontaneously or upon any sudden shock.

Thus, if a hot tube be touched with cold or wet iron, or slightly scratched with a cold file, the inequality of the rate of cooling is great, and it breaks at once. It is therefore necessary to secure that hot gla.s.s shall cool as regularly as possible. And this is particularly important in the case of articles made of soda gla.s.s. Some gla.s.s-blowers content themselves with permitting the gla.s.s to cool gradually in a smoky flame till it is covered with carbon, and then leave it to cool upon the table. But under this treatment many joints made of soda gla.s.s which are not quite uniform in substance, but otherwise serviceable, will break down. In gla.s.s-works the annealing is done in ovens so arranged that the gla.s.s enters at the hottest end of the oven where it is uniformly heated to a temperature not much below that at which it becomes viscous, and slowly pa.s.sed through the cooler parts of the chamber so that it emerges cold at the other end. This method of annealing is not practicable in a small laboratory. But fortunately very good results can be obtained by the following simple device, viz.:--

By wrapping the hot apparatus that is to be annealed closely in cotton wool, and leaving it there till quite cold. The gla.s.s should be wrapped up immediately after it is blown into its final shape, as soon as it is no longer soft enough to give way under slight pressure. And it should be heated as uniformly as possible, not only at the joint, but also about the parts adjacent to the joint, at the moment of surrounding it with the cotton. Lead gla.s.s appears to cool more regularly than soda gla.s.s, and these precautions may be more safely neglected with apparatus made of lead gla.s.s; but not always. At the date of writing I have had several well-blown joints of thick-walled capillary tube to No. 16 (see diagram, p. 82), break during cooling, in consequence of circ.u.mstances making it dangerous to heat the neighbourhood of the joint so much as was necessary.

The black carbonaceous coat formed on hot gla.s.s when it is placed in cotton wool may be removed by wiping with methylated spirit, or, if it be very closely adherent, by gently rubbing with fine emery, moistened with the spirit.

Cotton wool is rather dangerously inflammable; it should therefore be kept out of reach of the blow-pipe flame, and care should be taken that the gla.s.s is not placed in contact with it at a sufficiently high temperature to cause its ignition.

Another method of annealing is to cover the hot gla.s.s with hot sand, and allow it to cool therein.

As in the case of lead gla.s.s, so with soda gla.s.s. A thorough acquaintance with the effect of the various parts of the flame upon it should be gained before further work is entered upon, for which purpose an hour or more spent in observing its behaviour in the flame will be fully repaid by increased success subsequently.

=The Use of Combustion Tube.=--It is often necessary to construct apparatus of what is known as hard gla.s.s or combustion tube. It is almost as easy to work combustion tube as to deal with lead and soda gla.s.s if the oxy-hydrogen flame be employed.

It is not necessary to set up a special apparatus for this purpose; many of the ordinary blow-pipes can be used with oxygen instead of with air.

It is only necessary to connect the air-tube of the blow-pipe with a bottle of compressed oxygen instead of with the bellows. The connecting tube should not be too wide nor too long, in order to avoid the acc.u.mulation in it, by accident, of large quant.i.ties of explosive mixtures.

Two precautions are necessary in manipulating hard gla.s.s in the oxy-hydrogen flame. The gla.s.s must _not_ be overheated. At first one is very apt to go wrong in this direction. The supply of oxygen must _not_ be too great; a small hissing flame is not what is wanted. If either of these precautions are neglected most gla.s.s will devitrify badly. With a little care and experience, devitrification can be absolutely avoided.

Ordinary combustion tube can be used, but I find that the gla.s.s tube (Verbrennungsrohr) made by Schott & Co. of Jena, which can be obtained through any firm of dealers in apparatus, is far better than the ordinary tube.

By following these instructions, any one who has learned how to work with lead or soda gla.s.s will find it easy to manipulate hard gla.s.s.

CHAPTER III.

_CUTTING AND BENDING GLa.s.s--FORMING GLa.s.s APPARATUS BEFORE THE BLOW-PIPE--MAKING AND GRINDING STOPPERS TO APPARATUS, ETC._

In the later pages of this Chapter it will be a.s.sumed that the operations first described have been mastered. The beginner should therefore practise each operation until he finds himself able to perform it with some degree of certainty. Generally speaking, however, after the failure of two or three attempts to perform any operation, it is best to give up for a few hours, and proceed to the work next described, returning to that upon which you have failed subsequently. If, unfortunately, it should happen that the work next in order involves the performance of the operation in which the failure has occurred, it is best to pa.s.s on to some later work which does not demand this particular accomplishment, or to rest a while, and re-attack the difficulty when refreshed.

=Cutting Gla.s.s Tubes.=--The simplest method of cutting a gla.s.s tube is to make a sharp scratch with a file across the gla.s.s at the point where it is desired to cut it, and on pulling apart the two ends, it will break clean off. It is important that the file be sharp. In pulling apart the ends the scratch should be held upwards, and the pull should have a downward direction, which will tend to open out the scratch. In the case of a large tube, a scratch will not ensure its breaking clean across. The tube must be filed to some depth, half-way, or even all round it. A good way of breaking a tube is to place the file in the table after scratching the gla.s.s, to hold the gla.s.s tube above its edge with one hand on each side of the scratch, and to strike the under side of the tube a sharp blow upon the edge of the file, directly beneath the scratch. In this way very even fractures of large and moderately thin tubes may be made. It answers particularly well for removing short ends of tube, not long enough to hold; the tube is held firmly upon the file, and a sharp blow given to the short end with a piece of large tube or a key.

A file whose faces have been ground till they are nearly smooth, so as to leave very finely-serrated edges, will be found useful for cutting gla.s.s tubes. Such a file should be used almost as a knife is used for cutting a pencil in halves.

The simple methods just described are too violent to be applied to delicate apparatus, too tedious when employed upon the largest tubes, and very difficult to apply when the tube to be cut is very thin, or too short to permit the operator to get a good grip of it on either side of the file mark. In such cases, one or other of the following methods will be useful:--

1. Make a scratch with a file, and touch it with the end of a _very small_ piece of gla.s.s drawn out and heated at the tip to its melting point. It is important that the heated point of gla.s.s be very small, or the fracture is likely to be uneven, or to spread in several directions. Also, it is best to use hot soda gla.s.s for starting cracks in tubes of soda gla.s.s, and lead gla.s.s for doing so in lead gla.s.s tubes. If the crack does not pa.s.s quite round the tube, you may pull it asunder, as previously described, or you may bring the heated piece of gla.s.s with which the crack was started to one end of the crack, and slowly move it (nearly touching the gla.s.s) in the required direction; the crack will extend, following the movements of the hot gla.s.s.

Instead of hot gla.s.s, pastils of charcoal are sometimes employed for this purpose. They continue to burn when once lighted, and there is no need to re-heat them from time to time. They should be brought as close to the gla.s.s as is possible without touching it, and, when no longer needed, should be extinguished by placing the lighted end under sand, or some other incombustible powder, for they must not be wetted.

2. A method much practised by the makers of sheet gla.s.s, and suitable for large objects, is to wrap a thread of hot gla.s.s round the tube, at once removing it, and touching any point of the gla.s.s which the thread covered with water or a cold iron, when a crack will be started and will pa.s.s round the gla.s.s where it was heated by the thread.

3. Tubes which are large and slightly conical may have a ring of red-hot iron pa.s.sed over them till it comes into contact with the gla.s.s, then, the iron being removed, and a point on the heated gla.s.s being at once touched with cold iron as before, it will break as desired. Or a string, moistened with turpentine, may be loosely twisted round the tube, and the turpentine ignited, afterwards the application of sudden cold to any point on the zone of hot gla.s.s will usually start a crack, which, if necessary, may be continued in the usual manner. The last three methods are chiefly useful in dealing with the largest and thickest tubes, and with bottles.

A fairly stout copper wire, bent into the form of a bow so that it can be applied when hot to a considerable surface of a gla.s.s tube, will be found superior to the point of hot gla.s.s or metal usually employed, for leading cracks in gla.s.s tubes. With such a wire a tube can be cut so that the cross section of the end is at any desired angle to the axis of the tube, with considerable precision. I am indebted for this suggestion to Mr. Vernon Boys and Dr. Ebert.

=Bending Gla.s.s Tubes.=--The blow-pipe flame is not a suitable source of heat for bending tubes, except in certain cases which will be mentioned in a subsequent paragraph. For small tubes, and those of moderate size, a fish-tail burner, such as is used for purposes of illumination, will answer best. Use a flame from one to two inches in breadth--from _A_ to _A_ (Fig. 6), according to the size of the tube which is to be bent. If the length of tube that is heated be less than this, the bend will probably buckle on its concave side.

[Ill.u.s.tration: FIG. 6.]

The tube to be heated should be held in the position shown in Fig. 6, supported by the hands on each side. It should be constantly rotated in the flame, that it may be equally heated on all sides. In the figure the hands are represented above the tube, with their backs upwards. A tube can be held equally well from below, the backs of the hands being then directed downwards, and this, I think, is the more frequent habit. It is difficult to say which position of the hands is to be preferred. I lately observed how a tube was held by three skilful amateurs and by a professional gla.s.s-blower. All the former held the tube with the hands below it. The latter, however, held it from above, as in Fig. 6. He, however, was working with a rather heavy piece of tube, and I am inclined myself to recommend that position in such cases. During a long spell of work, the wrist may be rested from time to time by changing the position of the hands.

When the tube has softened, remove it from the flame, and gently bend it to the desired angle. The side of the tube last exposed to the flame will be slightly hotter, and therefore softer, than that which is opposite to it. This hotter side should form the concave side of the bent tube.

[Ill.u.s.tration: FIG. 7.]

The exact condition in which the gla.s.s is most suitable for bending can only be learned by making a few trials. If it is too soft in consequence of being overheated, the sides will collapse. If, in the endeavour to heat the side _A_ of Fig. 7 a little more than _B_, _B_ is insufficiently heated, the tube will be likely to break on the convex side _B_. If the bent tube be likely to become flattened, and this cannot always be prevented in bending very thin tubes, the fault may be avoided by blowing gently into one end of the tube whilst bending it, for which purpose the other end should be closed beforehand. A tube already flattened may, to some extent, be blown into shape after closing one end and re-heating the bent portion, but it is not easy to give it a really good shape.

When making a bend like that in Fig. 7, to secure that the arms of the tube _C_ and _D_, and the curve at _B_, shall be in one plane, the tube should be held in a position perpendicular to the body, and brought into the position shown in the figure during bending, by which means it will be found easy to secure a good result. Lead gla.s.s tubes must be removed from the flame before they become hot enough to undergo reduction. If they should become blackened, however, the stain may be removed by re-heating in the oxidising flame (see p. 18).

When a very sharp bend is to be made, it is sometimes best to heat a narrow zone of the gla.s.s rather highly in the blow-pipe flame, and to blow the bend into shape at the moment of bending it, as previously described, one end having been closed for that purpose beforehand. Lead gla.s.s should be heated for this purpose in the oxidising flame (pp. 17 to 22).

The processes of bending large tubes, making U-tubes and spiral tubes, are more difficult operations, and will be explained in Chap. IV.

=Rounding and Bordering the Ends of Tubes.=--After cutting a piece of gla.s.s tube in two pieces, the sharp edges left at its ends should be rounded by holding them in a flame for a few moments till the gla.s.s begins to melt. The oxidising point of a pointed flame may be used for both kinds of gla.s.s. The flame will be coloured yellow by soda gla.s.s at the moment of melting. This indication of the condition of soda gla.s.s should be noted, for it serves as a criterion of the condition of the gla.s.s. The ends of soda gla.s.s tubes may also be rounded in the flame of a common Bunsen's burner.

When the end of a tube is to be closed with a cork or stopper, its mouth should be expanded a little, or =bordered=. To do this, heat the end of the tube by rotating it in the flame till it softens, then remove it from the flame, at once introduce the charcoal cone (Fig. 5, p. 11), and rotate it with gentle pressure against the softened gla.s.s till the desired effect is produced. In doing this it is very important that the end of the tube shall be uniformly heated, in order that the enlargement shall be of regular form. If the tube cannot be sufficiently expanded at one operation, it should be re-heated and the process repeated.

Borders, such as are seen on test-tubes, are made by pressing the softened edge of the tube against a small iron rod. The end of the rod should project over the softened edge of the tube at a slight angle, and be pressed against it, pa.s.sing the rod round the tube, or rotating the tube under the rod.

=Sealing=, that is closing the ends of tubes, or other openings, in gla.s.s apparatus.

In performing this and all the other operations of gla.s.s blowing, the following points must be constantly kept in mind:--

(_a._) That it is rarely safe to blow gla.s.s whilst it is still in the flame, except in certain special cases that will be mentioned subsequently. Therefore always remove apparatus from the flame before blowing.

(_b._) That when heating gla.s.s tubes, unless it is specially desired to heat one portion only, the tube must be constantly rotated in the flame to ensure that it shall be uniformly heated, and to prevent the tube or ma.s.s of gla.s.s from a.s.suming an irregular form.

(_c._) Always blow gently at first, and slowly increase the force applied till you feel or see the gla.s.s giving way. It is a good plan to force the air forward in successive short blasts rather than in one continued stream.

(_d._) When it is necessary to force air into tubes of fine bore, such as thermometer tubes, the mouth must not be used, for moisture is thereby introduced into the tube, which it is very difficult to remove again in many cases. All tubes of very small bore should be blown with the aid of an india-rubber blowing-bottle, such as are used for spray-producers, Galton's whistles, etc. The tube to be blown must be securely fixed to the neck of the bottle, which is then held in one hand, and air is forced from it into the tube as it is required. These bottles are frequently of service to the gla.s.s-blower--_e.g._, when tubes of very fine bore have to be united, it is necessary to maintain an internal pressure slightly exceeding that of the air throughout the operation, in order to prevent the viscous gla.s.s from running together and closing the tube. An india-rubber blowing-ball is very convenient for this purpose.

To seal the end of a gla.s.s tube (Fig. 8), adjust the flame so that it will heat a zone of gla.s.s about as broad as the diameter of the tube to be sealed (see _A_, Fig. 8). Hold the tube on each side of the point where it is to be sealed in the manner described in the description of bending gla.s.s tubes (p. 28). Bring the tube gradually into the flame, and heat it with constant rotation, till the gla.s.s softens (for lead gla.s.s the oxidising flame must be used, as has been already explained).[6] When the gla.s.s begins to thicken, gently pull asunder the two ends, taking care not to pull out the softened gla.s.s too much, but to allow the sides to fall together, as shown at _A_. When this has occurred, heat the gla.s.s at the narrow part till it melts, and pull asunder the two ends. The closed end should present the appearance shown at _D_. If the gla.s.s be drawn out too quickly its thickness will be unduly reduced, and it will present the appearance shown at _B_. In that case apply a pointed flame at _b_, and repeat the previous operation so as to contract the tube as at _c_, taking care not to allow the gla.s.s to become much increased nor decreased in thickness.

[6] Remember that when the lead gla.s.s is heated to the proper temperature it will present an appearance which may be described as a greenish phosph.o.r.escence. At higher temperatures it a.s.sumes an orange-red appearance. If it loses its transparency and a.s.sumes a dull appearance, it must be moved further into the oxidising parts of the flame.

If a considerable ma.s.s of gla.s.s be left at _d_, it may be removed by heating it to redness, touching it with the pointed end of a cold gla.s.s tube, to which it will adhere, and by which it may be pulled away.