428 ETCHING. 



already described must be put into lukewarm 

 water to soften it, and render it easily worked by 

 the hand. When sufficiently pliable, it must be 

 drawn out into long rolls, and put round the edges 

 of the plate, pressing it down firm, and forming it 

 with the fingers into a neat wall or margin. A spout 

 must be formed in one corner, to pour off the aqua 

 fortis by afterwards. 



The nitrous acid (spirits of nitre) is now to be 

 diluted with four or five times as much water, or 

 more (according as you wish the plate to be bit 

 quick or slow,) and poured upon the plate. In a 

 few minutes you will see minute bubbles of air 

 filling all the lines that have been drawn on the 

 copper, which are to be removed by a feather ; and 

 the plate must be now and then swept, as it is 

 called, or kept free from air bubbles. By the more 

 or less rapid production of these bubbles, you 

 judge of the rapidity with which the acid acts upon 

 the copper. The biting-in of the plate is the most 

 uncertain part of the process, and nothing but very 

 great experience can enable any one to tell w^hen 

 the plate is bit enough, as you cannot easily see 

 the thickness and depth of the line till the ground 

 is taken off. 



When you judge, from the time the acid has 

 been on, and the rapidity of the biting, that those 

 lines which you wish to be the faintest are as deep 

 as you wish, you pour off the aqua fortis by the 

 spout, wash the plate with water, and dry it, by 

 blowing with bellows, or by the iire, taking care 

 not to melt the ground. 



Those lines that are not intended to be bit any 

 deeper, must now be stopped up with turpentine- 

 varnish mixed with a little lamp-black, and laid on 

 with a camel's-hair pencil ; and when this is 



