DECOLORIZERS
Decolorizers are the agents employed by the glass maker to neutralise or
subdue the objectionable tints given by the colouring action of small
traces of iron oxide, which exists as an impurity present in the
materials used or otherwise become accidentally admixed during the
process of the manufacture of glassware.
The small additions of manganese dioxide, arsenic, nitre, nickel oxide,
selenium, antimony, oxide, etc., to glass batches may be considered as
decolorizers. The most commonly used of these materials is manganese
dioxide, so the action of this material will be explained. Every glass
maker finds that one or other of the raw materials he uses may contain
impurities. It is seldom that glass makers’ sand can be obtained that
does not contain traces of iron oxide present as an impurity. Again, the
cullet collected from the glass house often contains iron scale or rust
from the blowing-irons, which firmly adheres to the glass and gets
admixed with the batch for re-melting. The presence of even very small
traces of iron in glass becomes evident as a pale sea-green tint when
viewed through any thickness of metal. The chemical action of the glass
upon the walls of the pot is continually dissolving a minute quantity of
iron from the and diffusing it throughout the metal, giving it a
tendency to the pale-green tint.
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.
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