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.