NATURAL PHILOSOPHY. 195 



Last operation of Ming in. This operation requires some of the 

 re-agents, before named, and also, 



1. A siccative ink, made of linseed oil, rendered very siccative by 

 boiling it sufficiently with litharge ; it may be thickened with calcined 

 lamp-black. 



2. An electrotype apparatus, and some solution fit to gild and cop- 

 per the plate. 



Means of operation. The plate must be inked as copper-plate 

 printers do, taking care to clean off the white parts more perfectly 

 than usual ; the plate is then to be placed in a room sufficiently warm, 

 until the ink is well dried, which requires more or less time, according 

 to the nature of the oil employed. The drying of the oil may be hast- 

 ened by heating the plate upon the stand with the lamp, but the slow 

 process is more perfect and certain. 



When the ink is well dried, the white parts are cleaned again by 

 polishing the plate with cotton and pounce, or any other polishing pow- 

 der ; a ball of cotton, or any other matter, covered with a thin piece 

 of caoutchouc or skin, can be used for this purpose. When polished, 

 the plate is ready to receive the electro-chemical coating of gold, which 

 will protect the white parts. 



Gilding. The gilding is obtained by any of the various processes 

 of electrotyping which are known. The only indispensable condition 

 is, that the surface obtained by the precipitation must not be liable to 

 be attacked by any weak acid ; a solution answering this purpose is 

 made of ten parts (by weight) of ferrocyanide of potassium, 1 part of 

 chloride of gold, and 1000 parts of water, used with a galvanic battery. 

 During the gilding, the plate must be turned in several positions, in 

 order to regulate the metallic deposit. In some cases the gilding may 

 be made more perfect, if the plate is covered with a thin coating of 

 mercury before being put in the gilding solution. 



When the plate is gilded, it must be treated with the boiling caustic 

 potash, by the process already indicated for the preparatory engrav- 

 ing, in order to cleanse it from all the dried oil or ink which fills the 

 hollows. The plate is then washed and dried, and when the oil em- 

 ployed has been thickened with the lamp-black, the surface of the plate 

 is rubbed with crumbs of bread, in order to cleanse and take off the 

 black remaining ; then, the white parts being covered and protected 

 by a varnish not apt to be cracked, and the black parts uncovered and 

 clean, the plate can be bitten in by aquafortis, according to the ordi- 

 nary process used by engravers. 



This operation must be done upon the stand, and not by immersing 

 the plate in the solution. 



Before this last biting-in, if the preparatory engraving has not suc- 

 ceeded well, and the plate still wants a sufficient grain, it can be given 

 by the various processes of aquatint engraving. 



Before submitting the plate to the operation of printing, in order 

 to secure an unlimited number of copies, it is necessary, as before 

 stated, to protect it by a slight coating of copper, which is obtained by 

 the electrotype process ; otherwise the printing will soon wear the 



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