Recently, I have been experiencing trouble with my prints, ie the darkness is way too light. After hours of study from various books, websites and friends, and sleepless nights there was just one thing for it; do some tests. It would seem, that either the plate hasn't been in the rosin box* and/or the metal is milled densely, therefor the acid needs more time to bite into it. ugh.
We all stood round the plates, comparing a good plate to a weak one, our fingers stroking the rosined (aquatinted) areas. We looked like a Glen Baxter cartoon. After much chin stroking we deduced (like Sherlock) that a good strong aquatint tone feels like sandpaper on the plate, and so I had to bite the plate for much longer.
*big monster box with powdered pine sap whirling round and settling on plate like snow to make a resist against the acid, so making tiny dots of resist which helps the ink stick to and so making a lovely dark tone.
This one, 'Tabletop' (300x134mm), is from a drawing of the table in my flat, where breakfast, the computer, letters and a Xmas tree have to share the space. Not a lot of room. The first version of the print was very pale, the start of trouble with acid bite times. so, instead of re-applying the aquatint, I decided to do close hatching and redrawing with a compass point on soft ground, then put in the nitric acid for about 8mins. it worked and so everything is good again.