CHAPTER II.

_VARIETIES OF GLASS AND THEIR MANAGEMENT._

All the varieties of glass that are ordinarily met with contain silica (SiO_{2}) associated with metallic oxides. In a true glass there are at least two metallic oxides. The unmixed silicates are not suitable for the purposes of glass. They are not so capable of developing the viscous condition when heated as mixtures--some of them are easily attacked by water, and many of those which are insoluble are comparatively infusible. There is generally excess of silica in glass, that is, more than is necessary to form normal silicates of the metals present. The best proportions of the various constituents have been ascertained by glass-makers, after long experience; but the relation of these proportions to each other, from a chemical point of view, is not easy to make out.

The varieties of glass from which tubes for chemical glass-blowing are made may be placed under three heads, and are known as[2]--

Soft soda glass. Also known as French glass. Lead glass. Also known as English glass. Hard glass.

[2] For details of the composition of the various glasses, some work on glass-making may be consulted.

In purchasing glass tubes, it is well to lay in a considerable stock of tubes made of each of the two first varieties, and, if possible, to obtain them from the manufacturer, for it frequently happens that pieces of glass from the same batch may be much more readily welded together than pieces of slightly different composition. Yet it is not well to lay in too large a stock, as sometimes it is found that glass deteriorates by prolonged keeping.

As it is frequently necessary to make additions, alterations, or repairs to purchased apparatus, it is best to provide supplies both of soft soda glass and lead glass, for though purchased glass apparatus is frequently made of lead glass, yet sometimes it is formed from the soda glass, and as it is a matter of some difficulty to effect a permanent union between soda glass and lead glass, it is desirable to be provided with tubes of both kinds.

Many amateurs find that soda glass is in some respects easier to work with than lead glass. But, on the other hand, it is somewhat more apt to crack during cooling, which causes much loss of time and disappointment. Also, perhaps in consequence of its lower conductivity for heat, it very often breaks under sudden changes of temperature during work. If, however, a supply of good soda glass is obtained, and the directions given in this book in regard to annealing it are thoroughly carried out, these objections to the use of soda glass will, to a great extent, be removed. I find, however, that when every precaution has been taken, apparatus made of soda glass will bear variations of temperature less well than that made of lead glass. Therefore, although the comparatively inexpensive soda glass may be employed for most purposes without distrust, yet I should advise those who propose to confine themselves to one kind of glass, to take the small extra trouble required in learning to work lead glass.

In order to secure glass of good quality, a few pieces should be obtained as a sample, and examined by the directions given below. When the larger supply arrives, a number of pieces, taken at random, should be examined before the blow-pipe, to compare their behaviour with that of the sample pieces, and each piece should be separately examined in all other respects as described subsequently.

Hard glass is used for apparatus that is required to withstand great heat. It is difficult to soften, especially in large pieces. It should only be employed, therefore, when the low melting points of soda or lead glass would render them unsuitable for the purpose to which the finished apparatus is to be put. What is sold as Jena combustion tube should be preferred when this is the case.

=Characters of good Glass.=--Glass tubes for glass-blowing should be as free as possible from knots, air-bubbles, and stripes. They should be in straight pieces of uniform thickness, and cylindrical bore. It is not possible to obtain glass tubes of absolutely the same diameter from one end to the other in large quantities, but the variations should not be considerable.

When a sharp transverse scratch is made with a good file on a piece of tube, and the scratch is touched with a rather fine point of red-hot glass (this should be lead glass for a lead glass tube, and soda glass for a tube of soda glass), the crack which is started should pass round the glass, so that it may be broken into two pieces with regular ends. If the crack proceeds very irregularly, and especially if it tends to extend along the tube, the glass has been badly annealed, and should not be employed for glass-blowing purposes. It is important that the point of hot glass used shall be very small, however. Even good glass will frequently give an irregular fracture if touched with a large mass of molten glass.

Finally, glass tube which is thin and of small diameter should not crack when suddenly brought into a flame. But larger and thicker tubes will not often withstand this treatment. They should not crack, however, when they are brought into a flame gradually, after having been held in the warm air in front of it for a minute or so.

Good glass does not readily devitrify when held in the blow-pipe flame. As devitrified glass very often may be restored to its vitreous condition by fusion, devitrification most frequently shows itself round the edges of the heated parts, and may be recognised by the production of a certain degree of roughness there. It is believed to be due to the separation of certain silicates in the crystallised form. Hard glass, which contains much calcium, is more apt to devitrify than the more fusible varieties.[3]

[3] The presence of silicates of calcium and aluminum are considered to promote a tendency to devitrification in glass; and glasses of complex composition are more apt to devitrify than the simpler varieties. See _Glass-making_, by Powell, Chance, and Harris, Chap. IV.

Glass tubes are made of various sizes. When purchasing a supply, it is necessary to be somewhat precise in indicating to the vendor the sizes required. I have therefore placed at the end of the book, in an appendix, a table of numbered diagrams. In ordering tubes it will usually only be necessary to give the numbers of the sizes wished for, and to specify the quantity of each size required. In ordering glass tubes by weight, it must be remembered that a great many lengths of the smaller sizes, but very few lengths of the larger sizes, go to the pound. Larger-sized tubes than those on the diagram are also made. In ordering them the external diameter and thickness of glass preferred should be stated.

=Cleaning and Preparing a Tube.=--It is frequently much easier to clean the tube from which a piece of apparatus is to be made than to clean the finished apparatus. A simple method of cleaning a tube is to draw a piece of wet rag which has been tied to a string through the tube once or twice, or, with small tubes, to push a bit of wet paper or cotton wool through them. If the dirt cannot be removed in this way, the interior of the tube should be moistened with a little sulphuric acid in which some bichromate of potassium has been dissolved. In any case, it must finally be repeatedly rinsed with distilled water, and dried by cautiously warming it, and sucking or blowing air through it. In order to avoid heating delicate apparatus which has become damp and needs drying, the water may be washed out with a few drops of spirit, which is readily removed at a low temperature.

Before using a glass tube for an operation in which it will be necessary to blow into it, one end of it must be contracted, unless it is already of such a size that it can be held between the lips with perfect ease; in any case, its edges must be rounded. For descriptions of these operations, see page 35. The other end must be closed. This may be done by means of a cork.

=Presenting Glass to the Flame.=--Glass tubes must never be brought suddenly into the flame in which they are to be heated. All glass is very likely to crack if so treated. It should in all cases be held for a little while in front of the flame, rotated constantly in the hot air and moved about, in order that it may be warmed over a considerable area. When it has become pretty hot by this treatment, it may be gradually brought nearer to the flame, and, finally, into contact with it, still with constant rotation and movement, so as to warm a considerable part of the tube. When the glass has been brought fairly into contact with the flame, it will be safe to apply the heat at the required part only. Care must be taken in these preliminary operations to avoid heating the more fusible glasses sufficiently to soften them.

=Methods of working with Lead and soft Soda Glass respectively.=--When lead glass is heated in the brush flame of the ordinary Herapath blow-pipe, or within the point of the pointed flame, it becomes blackened on its surface, in consequence of a portion of the lead becoming reduced to the metallic state by the reducing gases in the flame. The same thing will happen in bending a lead glass tube if it is made too hot in a luminous flame. A practical acquaintance with this phenomenon may be acquired by the following experiment:--

Take a piece of lead glass tube, bring it gradually from the point of a pointed flame to a position well within the flame, and observe what happens. When the glass reaches the point _A_ (Fig. 3), or thereabouts, a dark red spot will develop on the glass, the area of the spot will increase as the glass is brought further in the direction _A_ to _B_. If the glass be then removed from the flame and examined, it will be found that a dark metallic stain covers the area of the dark red spot previously observed. Repeat the experiment, but at the first appearance of the dark spot slowly move the glass in the direction _A_ to _C_. The spot will disappear, and, if the operation be properly performed, in its place there will be a characteristically greenish-yellow luminous spot of highly heated glass. In this proceeding the reduced lead of the dark spot has been re-oxidised on passing into the hot gases, rich in oxygen, which abound at the point of the flame. If one end of the tube has been previously closed by a piece of cork, and if air be forced into the tube with the mouth from the open end before the luminous spot has become cool, the glass will expand. If the experiment be repeated several times, with pointed flames of various sizes, the operator will quickly learn how to apply the pointed flame to lead glass so that it may be heated without becoming stained with reduced lead.

If the spot of reduced metal produced in the first experiment be next brought into the oxidising flame, it also may gradually be removed. On occasion, therefore, apparatus which has become stained with lead during its production, may be rendered presentable by suitable treatment in the oxidising flame. The process of re-oxidising a considerable surface in this way after it has cooled down is apt to be very tedious, however, and, especially in the case of thin tubes or bulbs, often is not practicable. In working with lead glass, therefore, any reduction that occurs should be removed by transferring the glass to the oxidising flame at once.

Small tubes, and small areas on larger tubes of English glass, may be softened without reduction by means of the pointed oxidising flame; but it is not easy to heat any considerable area of glass sufficiently with a pointed flame. And though it is possible, with care, to employ the hot space immediately in front of the visible end of an ordinary brush flame, which is rich in air, yet, in practice, it will not be found convenient to heat large masses of lead glass nor tubes of large size, to a sufficiently high temperature to get the glass into good condition for blowing, by presenting them to the common brush flame.

It may seem that as glass which has become stained with reduced lead can be subsequently re-oxidised by heating it with the tip of the pointed flame, the difficulty might be overcome by heating it for working in the brush flame, and subsequently oxidising the reduced lead. It is, however, difficult, as previously stated, to re-oxidise a large surface of glass which has been seriously reduced by the action of the reducing gases of the flame, after it has cooled. Moreover, there is this very serious objection, that if, as may be necessary, the action of the reducing flame be prolonged, the extensive reduction that takes place diminishes the tendency of the glass to acquire the proper degree of viscosity for working it, the glass becomes difficult to expand by blowing, seriously roughened on its surface, and often assumes a very brittle or rotten condition.

When it is only required to bend or draw out tubes of lead glass, they may be softened sufficiently by a smoky flame, which, probably owing to its having a comparatively low temperature, does not so readily reduce the lead as flames of higher temperature. But for making joints, collecting masses of glass for making bulbs, and in all cases where it is required that the glass shall be thoroughly softened, the smoky flame does not give good results.

In the glass-works, where large quantities of ornamental and other glass goods are made of lead or flint glass, the pots in which the glass is melted are so constructed that the gases of the furnace do not come into contact with the glass;[4] and as the intensely-heated sides of the melting-pot maintain a very high temperature within it by radiation, the workman has a very convenient source of heat to his hand,--he has, in fact, only to introduce the object, or that part of it which is to be softened, into the mouth of the melting-pot, and it is quickly heated sufficiently for his purpose, not only without contact of reducing gases, but in air. He can therefore easily work upon very large masses of glass. In a special case, such a source of heat might be devised by the amateur. Usually, however, the difficulty may be overcome without special apparatus. It is, in fact, only necessary to carry out the instructions given below to obtain a considerable brush flame rich in air, in which the lead glass can be worked, not only without discoloration, but with the greatest facility.

[4] See _Principles of Glass-making_, p. 31.

_To Produce an Oxidising Brush Flame._--The blower used must be powerful, the air-tube of the blow-pipe must be about half as great in diameter as the outer tube which supplies the gas. The operator must work his bellows so as to supply a strong and _steady_ blast of air, and the supply of gas must be regulated so that the brush flame produced is free from every sign of incomplete combustion,[5] which may be known by its outer zone being only faintly visible in daylight, and quite free from luminous streaks (see Fig. 4, p. 9). When a suitable flame has been produced, try it by rotating a piece of lead glass at or near the end of the inner blue part of the flame (_A_ Fig. 4); the appearance of the glass will quickly indicate reduction. When this occurs move the glass forward to the end of the outer zone _B_, but keep it sufficiently within the flame to maintain it at a high temperature. If all is right the metallic reduction will quickly disappear, the glass will become perfectly transparent once more, and will present the appearance previously observed in the experiments with the pointed flame, or, if very hot, assume a brownish-red appearance. If this does not occur, the supply of air must be increased or the supply of gas diminished until the proper effects are secured.

[5] Nevertheless the supply of air must not be so excessive as to reduce the temperature of the flame sufficiently to prevent the thorough softening of the glass, which will occur if the bellows is worked with too much zeal.

In working upon lead glass with the highly oxidising brush flame, it is a good plan to heat it in the reducing part of the flame _A_ for thoroughly softening the glass, and to remove it to the oxidising flame _B_ to burn away the reduced metal. In prolonged operations, in order that reduction may never go too far, hold the glass alternately in the hot reducing flame and in the oxidising flame. The inferiority of the outer oxidising flame to those portions nearer the inner blue zone for softening the glass, may perhaps be accounted for by the presence of a larger proportion of unconsumed air in the former, which being heated at the expense of the hot gases produced by combustion, thereby lowers the temperature of the flame. At or near _A_ (Fig. 4) where the combustion is nearly complete, but no excess of air exists, the temperature will naturally be highest.

If a very large tube be rotated in the oxidising flame at _B_ (Fig. 4) it may happen that the flame is not large enough to surround the tube, and that as it is rotated those parts of it which are most remote from the flame will cool down too considerably to allow all parts of the tube to be simultaneously brought into the desired condition. This difficulty may be overcome by placing two blow-pipes exactly opposite to each other, at such a distance that there is an interval of about an inch between the extremities of their flames, and rotating the tube between the two flames. It may be necessary to provide two blowers for the blow-pipes if they are large.

Again, if a very narrow zone of a tube of moderate size is to be heated, two pointed flames may be similarly arranged with advantage. Occasionally more than two flames are made to converge upon one tube in this manner.

Another method of preventing one side of a tube from cooling down whilst the other is presented to the flame, is to place a brick at a short distance from the extremity of the flame. The brick checks the loss of heat considerably. A block of beech wood may be used for the same purpose, the wood ignites and thereby itself becomes a source of heat, and is even more effective than a brick.

Fuller details of the management of lead glass under various circumstances will be found in the subsequent descriptions of operations before the blow-pipe.

Before proceeding to work with soda glass, the student should not only verify by experiments what has been already said, but he should familiarise himself with the action of the blow-pipe flame on lead glass by trying the glass in every part of the flame, varying the proportions of gas and air in every way, repeating, and repeating, his experiments until he can obtain any desired effect with certainty and promptitude. He should practice some of the simpler operations given in Chapter III. in order to impress what he has learned well on his mind.

=Management of Soda Glass.=--In working with soda glass the following points must be constantly kept in mind. That as it is much more apt than lead glass to crack when suddenly heated, great caution must be exercised in bringing it into the flame; and that in making large joints or in making two joints near each other, all parts of the tube adjacent to that which, for the moment, is being heated, must be kept hot, as it is very apt to crack when adjacent parts are unequally heated. This may be effected by stopping work at short intervals and warming the cooler parts of the tube, or by the use of the brick or block of wood to check radiation, or even by placing a supplementary blow-pipe or Bunsen burner in such a position that its flame plays upon the more distant parts of the work, not coming sufficiently into contact to soften the glass, however, but near enough to keep it well heated. Lastly, to prevent the finished work from falling to pieces after or during cooling, the directions given under the head of annealing must be carefully carried out.

In very much of his work the glass-blower is guided more by the _feel_ of the glass than by what he sees. The power of feeling glass 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 glass to a higher temperature than is necessary, and that they employ larger flames than are wanted. If glass 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 glass, attempt to blow the glass 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 glass for blowing it, become almost automatic.

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

=Annealing.=--If apparatus, the glass 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 glass 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 massive; 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 glass shall cool as regularly as possible. And this is particularly important in the case of articles made of soda glass. Some glass-blowers content themselves with permitting the glass 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 glass which are not quite uniform in substance, but otherwise serviceable, will break down. In glass-works the annealing is done in ovens so arranged that the glass 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 passed 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 glass 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 glass appears to cool more regularly than soda glass, and these precautions may be more safely neglected with apparatus made of lead glass; 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 circumstances making it dangerous to heat the neighbourhood of the joint so much as was necessary.

The black carbonaceous coat formed on hot glass 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 glass 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 glass with hot sand, and allow it to cool therein.

As in the case of lead glass, so with soda glass. 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 glass or combustion tube. It is almost as easy to work combustion tube as to deal with lead and soda glass 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 accumulation in it, by accident, of large quantities of explosive mixtures.

Two precautions are necessary in manipulating hard glass in the oxy-hydrogen flame. The glass 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 glass 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 glass tube (Verbrennungsroehr) 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 glass will find it easy to manipulate hard glass.