To subdue or neutralise this objectionable tint in the glass, the glass maker uses certain metallic oxides which give delicate counter-tints. Only those glasses which are made from the purest materials can be decolorized to become sufficiently clear to use in making the best table glassware. In optical glassware, where the use of manganese is not permissible, the greatest care has to be taken in the selection and testing of the materials to be used. If manganese oxide be used in making optical glass, although the eye may not be sensitive enough to observe the actual color absorption, glass is produced in which the solar rays are obstructed, and much less light is transmitted by the glass when used as an optical lens or prism. Therefore the optician avails himself of those glasses which have not been decolorized as being more satisfactory for his purpose, as more light is transmitted by such glasses.
Apart from the pale sea-green tint given to glass by the presence of small traces of iron, certain of the silicates themselves produce natural colors. The soda silicate present in soda-lime metal tends to give a pale bluish-green tint when viewed through any thickness of glass. The lead silicate has a yellowish hue. Each of these influences has to be counteracted if clear crystal glass is desired. The decolorization of glass by manganese dioxide depends upon the purple tint it gives to glass. This purple color, being complementary to the pale green color given by the presence of iron, serves and acts as a counter-tint, and by the absorption of the green light, a less perceptible coloring is produced. In the case of the decolorization of glass, we get the red and blue of the purple subduing the blue and yellow or green tint given by the iron. But certain other factors are necessary. The purple color from manganese oxide is given only to glass in the presence of oxidizing agents; and the absence of sufficient oxidising agents in the glass batch, the purple manganese colour is unstable and its action as a counter-tint is lost. Therefore, the glass maker uses strong oxidising agents in his glass mixtures for crystal effects, usually in the form of potassium nitrate and red lead, which liberate oxygen. Whilst undergoing decomposition in the glass melt, the presence of this free oxygen keeps the manganese used in a higher state of oxidation, and gives the necessary purple coloration. It is also evident that, if the glass melting in the pot is kept at a high temperature for any considerable length of time, this period of oxidation cannot last, and, after all the free oxygen gas has been evolved, any further heating tends to turn the glass greenish again or of poor colour, by the conversion of the manganese into the lower state of oxidation in which the purple colour is not evident. If by chance the glass maker has added too much manganese to the glass, and the purple colour becomes too evident, he resorts to the use of a small amount of carbonaceous reducing agent, such as a piece of charred wood or potato, which he plunges or pushes to the bottom of the pot by means of a forked iron rod or pole, where it vaporises, giving off moisture and carbonaceous gases which reduce the manganese purple colour to a lower oxidised colourless state, and in a very short time the excess of purple colour has disappeared and the glass appears colourless.
Much of the success of crystal glassmaking depends upon the proper adjustment of the decolorizers used and obtaining the best colourless effect. The quality of the manganese is important; only pure manganese dioxide should be used. In many cases the mineral ore, pyrolusite, is used on account of its cheapness. This is objectionable, as much iron may be present in the ore, when its use as a remedy is worse than the defect. The necessity of taking advantage of the services of a consultant chemist here becomes apparent, for, if glass manufacturers would only have their different consignments of materials examined and tested from time to time, many of the disappointments and difficulties experienced by them at present would be obviated. A considerable saving in the cost of batch materials can be made by the judicious selection of more suitable qualities in preference to inferior or adulterated varieties. In many cases, a chemist can substitute for certain of the expensive batch materials other cheaper materials introducing the same elements at less expense, and still retain the same quality in the glass produced.
CHAPTER VII
THE REFRACTORY MATERIALS USED
Of the greatest importance to the glass manufacturer are the refractory materials upon which the life of his furnace and pots depends. A few notes giving a description of them and dealing with the manufacture of the fire-resisting blocks used in building the furnaces will be of interest.
The chief and most generally used of such materials are the goods. The best known deposits of fire-clays in this country are those in the Midlands, Stourbridge, Leeds, and Glasgow districts. In each of these districts the mining of fire-clays and the manufacture of fire-resisting goods for furnace work forms an important industry. The theoretical composition of a true would be a double silicate of alumina, and in this pure state it would be of a very refractory nature. But, naturally, fire-clays show the presence of other bases, such as iron, lime, magnesia, titanium, and alkalies, which, if present to any appreciable extent, lower the degree of resistance to heat or refractoriness of the clay. These other bases may be considered as impurities or natural fluxing agents. The characteristics of a highly refractory clay suitable for glass manufacturers’ requirements would be: (a) that such a clay should show no signs of softening at the highest heat of the furnace; (b) a squatting point not below Cone 31 or 1690° Centigrade; (c) a high alumina content not below 30 per cent.; (d) the greatest freedom from impurities; (e) a fine-grained texture; and (f) a high degree of plasticity. These are the qualities most essential for glass house work. The figures given by the chemical analyses of good fire-clays would probably fall within the following limits—
| Silica | 49% to 65% |
| Alumina | 48% to 31% |
| Ferric Oxide | 0·5% to 1·5% |
| Titanium Oxide | nil to 1·5% |
| Lime | nil to 0·5% |
| Magnesia | nil to ·2% |
| Total Potash and Soda | 0·5% to 1·8% |
Clays of higher silica content than 70 per cent. would not be considered suitable as pot-clays owing to the case in which glass attacks silicious clays. It is important that chemical analyses of fire-clays should be compared with results obtained from the analysis of fired or burnt samples, or they should be recalculated to allow of such comparison, so as to exclude the figures for the hygroscopic and chemically combined water of the clays.
The writer gives the following particulars of a very suitable for glass house pot-making. It is plastic and highly refractory, and is now being considerably used by the trade. The clay is supplied by Mansfield Bros., Church Gresley. The figures are from a report made by Mr. J. W. Mellor, D.Sc., of the County Laboratory, Stoke-on-Trent, and are as follows—