The whole art of drawing, painting, limning, and etching collected out of the choicest Italian and German authors : to which is added exact rules of proportion for drawing the heads of men, women and children , of what bigness soever / originally invented and written by the famous Italian painter Odoardo Fialetti, painter of Boloign ; published for the benefit of all ingenuous gentlemen and artists by Alexander Brown ...

Observations by which you may know when it is deep enough.

When the Aqua fortis hath layn upon the Plate a little more then a quarter of an hour, or half an hour; there being no certainty in time, because sometimes the Aqua fortis will work stronger then at other; therefore when you think it is deep enough, pour off the Aqua fortis from the Plate into a Glass, then wash the Plate with a little fair water, then take a Knife and scrape off a little piece of the ground where it is hatcht, and may be least prejudicial to the Plate, and if you perceive it to be deep enough; warm the Plate and rub the ground off as before-written; but if you finde it not deep enough, take a little Candle-tallow and melt it in a Spoon, and while it is warm take a Pensil and cover the place with it where you scraped the ground off, then pour the Aqua fortis upon it again, and let it lie till you guesse it to be deep enough; then pour the Aqua fortis from the Plate as afore∣said, and at any time when you perceive that the Aqua fortis doth not work strong enough, you pour off half the old, and refresh it with some new, for when the Aqua fortis hath been upon the Plate a∣bout half an hour, it will be much the weaker, because the strength of it doth evaporate away, and by a little practice you will come to the certain knowledge when the Aqua fortis hath eaten deep enough.