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CHAPTER II.

VARIETIES OF GLASS AND THEIR MANAGEMENT.

All the varieties of glass that are ordinarily met with contain silica (SiO₂) 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]—

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]

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