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BOOK ILLUSTRATIONS 273
cooling, the copper first assumes a pale yellow colour and finally becomes almost chocolate coloured.
There is often a slight scum of glue which it is necessary to get rid of at the end of this " burning in " process. This is done by subjecting the plate to a cleansing solution composed of one dram of chromic acid, one dram of sulphuric acid, and a pint of water. The plate is then read}7 to be etched. Should it, however, be desired to make any portion a solid black, then the corresponding part of the plate must be suitably painted before the etching is commenced.
This, as before stated, is done by subjecting the plate to the action of dilute nitric acid. Formerly this process was performed by placing the plate in a trough containing the acid and rocking to and fro for a few minutes, the operation being repeated until a satisfactory result was obtained.
At the present day, however, when so much depends upon saving time, it is not surprising that machine ry should have been called in to help in this direction. One of the most recent and likewise most satisfactory of these machines is that shown in fig. 111. This " Holt Etching Machine " occupies the extremely small floor space of about 16 square feet, and requires only J horse-power wThen in motion. The part at the top of the table, with raised lids, contains the etching solution, and this solution is kept in constant motion by means of paddles attached to a motor-driven spindle. These paddles are so arranged that the acid is kept in constant motion, but the plates which are placed in the bath remain in precisely the same position the whole time. The " floating board " upon which the plates are placed measures 26 inches by 20 inches. This board rises automatically to the surface when the machine is opened, and by closing the lids the board with the plates is forced beneath the etching fluid. The machine is simultaneously set in motion causing the disc s