the proportions somewhat, either to suit their furnaces or through
preference for some particular set of proportions, but the formula
given is an example of one yielding a good glass, soft enough for
flame-working while possessing good durability.
Glass of this type was made in the early part of the war, and would have been continued, as one meeting many scientific and industrial requirements, had it not been for the necessity of conserving supplies of potassium compounds, of which the amounts that could be ap- portioned for use in glass manufacture were sufficient only for the production of certain optical glasses. Without potassium compounds these optical glasses could not be made having the constants required by the optical industry. Manufacturers of other scientific glass- ware had, therefore, to search for methods of producing soft work- able glasses without employing quantitites of manufactured po- tassium compounds. To some extent nitre was available and was used. Potash felspar, which for long had been an ingredient in certain glasses, was a convenient form of aluminium compound for introducing alumina. The amounts of this material employed varied between wide limits, and glasses of good working qualities were ob- tained. Good examples of potash felspar contained about 10% of KO. If in the above formula all the alumina be introduced in the form of such felspar, about 2 % of K 2 O is also introduced into the glass. Glasses having many good qualities were made with enough felspar to yield from 3 to 4 % of K 2 O in the resulting glass, but the amount of alumina introduced rendered the glass too stiff, and liable also to give a roughened surface if long worked in the blowpipe flame. Such roughening could be removed by heating to a higher temperature, but its occurrence was a decided objection, and, more- over, flame-workers were placed at some disadvantage in respect of the time occupied in the production of blown vessels and ap- paratus. To remedy these defects varying proportions of borax were employed, and in this way sodium-potassium-calcium-alumin- ium-bpro-silicate glasses of good working qualities and of marked durability were produced, which met many of the requirements of laboratory workers. Some investigators and manufacturers of scientific glassware, however, looked upon these glasses as temporary expedients, and only awaited supplies of potassium compounds to return to the earlier type.
It is not found convenient by glass-makers to have to produce a very great variety of glasses. Unless a glass is generally suitable for the needs of laboratories and of industries where ready and kindly working in a flame, along with good durability, is required of it, the glass fails to fulfil the requirements it may reasonably be expected to meet. For example, the boro-silicate glass referred to possessed many desirable properties, and articles made from it in the flame, and also by blowing into moulds, left little to be desired when the glass was well made and the necessary technical skill had been acquired. It failed, however, when used for X-ray tubes. Good and workable bulbs and tubing could be made from it, but experience showed that X-ray tubes of this glass took longer to exhaust, and that there was a lack of stability in the vacua obtained. Investiga- tion left little doubt that the glass parted with water vapour under electrical bombardment, and the results of numerous experiments proved that borax was an undesirable ingredient in glass intended for X-ray work. A glass of the general type indicated in the formula above is quite suitable for such work, and hence X-ray bulbs and tubing can be made from it in the course of working a pot for a variety of other articles. It may be mentioned here that, unless manganese in small quantity be present in the glass, an X-ray tube in use does not exhibit the green phosphorescence with which workers with X-rays appear to have become accustomed. As manganese dioxide is generally added as a so-called decolorizer, only one type of glass need be made for practically all the scientific pur- poses and many of the industrial purposes to which a comparatively soft glass is put. Experience so far appears to show that the best type is on the lines of the formula given, and that the presence of notable proportions of the oxides of aluminium and potassium are essential. It is unnecessary to go into details about the form in which each ingredient of the glass is introduced in the batch mixture. Potash felspar has been mentioned as a convenient source of alumina, and part of the alkalies may be usefully added as nitrates. In general, all the materials of the batch mixture should be as pure as can be obtained commercially, so that the composition of the glass may depart as little as possible from that which it is intended to have, and which has been proved to give satisfactory results.
Before proceeding to other types of scientific glassware which were called for during the war, one or two remarks which are relevant for almost all glasses may be made here.
As far as it is possible to obtain and to store them, all the sub- stances for a batch mixture should be free from water. In several instances it has been shown that a glass made from anhydrous ma- terials differs from one calculated to give the same composition finally, but produced from a wet batch, or from one containing an ingredient having a notable proportion of combined water. In addi- tion to some lack of general stability, the glass from a wet batch may show, and in many instances has shown, a greater tendency to devitrification when heated in a flame or by radiation. The amount of water left in a glass may be very small, but it has been shown to
be sufficient to affect the behaviour of the glass. The only reserva- tion to the statement that to produce the best glasses the materials should be dry is that the action of water to effect change in glasses either during their production or on subsequently heating them is, if not imperative, at least an advantage in respect of the production of certain coloured glasses and apparently of some opal glasses.
The other remark is about homogeneity. Apart from optical glasses, which must have the same composition throughout, all glasses for laboratory use should be made in such a way as to secure the greatest possible homogeneity. It is a matter of experience that glass which has been kept heated for some time, even after it is ap- parently " fined " and ready for working, is more resistant to heat changes and is also more generally stable than the same glass less well founded. Attempts to secure the thorough incorporation of all the ingredients by making the glass at a very high temperature were not altogether satisfactory, since there was greater attack of the pot, and, in many cases, too much loss of some of the more volatile con- stituents. Some glasses require very high temperatures, and prob- lems connected with them led to investigations on materials for pots and furnaces to improve their refractory nature and so to make the production of such glasses possible. The remark about long heating applies to these glasses as well, but the attempt to substitute heating through a relatively short period of time at a very high tem- perature, for long-founding of glasses which only needed a mod- erately high temperature, led to uncertainty of composition and failed to secure the homogeneity aimed at. It is perhaps unwise to dogmatize on this matter, having in mind certain exceptions, but as a general rule it may be said that in the present state of our knowl- edge the long-founding so much insisted upon by many experienced glass manufacturers cannot be dispensed with if the nicest possible refinements of a good glass are to be realized.
Stirring to secure homogeneity is a necessary operation in making optical glass. It is not customary to stir glass for laboratory use, but this is not to say that such glass would not be improved by being stirred if it were economically possible to do so. Although it is out- side the range of scientific glasses, the opportunity may be taken here of drawing attention to an instance in which perfect homo- geneity in glass does not appear to all eyes as an advantage. The instance is that of coloured glasses used for decorative purposes, such as windows. Some of the charm of old glass seems to be asso- ciated with a marked lack of identity of composition, and, therefore, of regularity of optical properties throughout the glass. From a glass-maker's point of view it was an imperfect manufacture, but those who find depth and life in the less perfect production may ask, " Would it be imperfect manufacture to take advantage of the pos- sibilities in a glass-melting to secure a more perfect fitness and suitability for the purposes for which such a glass is designed?" Certainly the control which modern manufacturers have over glass, and the knowledge and experience which they possess, would make it possible to secure a great variety of pleasing results.
The subject of annealing has, in recent years, been given much attention, and several investigations have been carried out. Results of much interest and importance have been obtained, dealing with the conditions for removing strain in glass and with the problem of annealing, both from the theoretical and a practical point of view. Consideration of these results serve to emphasize the importance of thoroughly annealing any glass articles which are required to with- stand marked changes of temperature, and of arranging that any vessels, etc., which in the course of production are re-heated locally, shall be re-annealed. Tubing is not customarily annealed as part of the process of manufacture, but for certain purposes, notably with tubes which are to be ground, it is an advantage to anneal them.
" Resistance " Glass. Laboratory glassware, to deserve this description, must possess great stability, and must part with only minute traces of any of its ingredients when it is exposed to the action of the majority of solutions and liquids used in a chemical laboratory. In the early days of the war the production of such glassware was undertaken by British manufacturers. The chief varieties made can be included in two types: one containing compounds of zinc and the other free from this metal. In neither type is the inclusion of arsenic or antimony considered to be permissible.
The following formulae, illustrative of these two types, give ap- proximate proportions for batch mixtures expressed in percentages of the oxides contained in the various ingredients of a batch:
(A) SiO 2 66 (B) SiO 2 66
B 2 O 3 8 B 2 O 3 9
A1 2 O 3 9 A1 2 O 3 2-5
CaO 5 ZnO 8
MgO i MgO 5
Na 2 O 8 Na 2 O . . 9^5
K 2 3
It is to be understood that adjustments of the proportions given can be made to suit different furnaces and also to fit in with the amount of broken-up glass from previous meltings (cullet), which is incorporated in the batch. The addition of cullet is customary on
