Aquatint: Step 8, the last tone can be very dark, almost black however be careful, a zinc plate with the acid will eventually (in 5-10 minutes depending upon rosin particle size, acids age, etc) totally burn away the islands of rosin protected metal, leaving areas of white plate when etched. The only way to determine how much more time the plate can withstand is to use a microscope or a jewellers loupe to check the state of the plate.