The longer the etch (time in the bath) the deeper the etch, the more ink it will hold and so it will be a darker line in your print.
Here are examples of my first line etching. The image is based on an etching from 1516. When I make these prints I felt in direct contact with the people who worked in the 16th century. I like that feeling.
sepia ink on stonehenge paper
Today we started aquatints. Using black spray paint, we can add tones to our plate. I made a new line etching, and then covered the areas I wanted to remain the brightest/whitest with soft ground. Then I sprayed the plate really lightly with black spray paint. It was very grainy and you could see the plate below it as well as my etched line drawing. After this, I dropped the plate into the acid bath and etched it for 1 minute. I then covered the areas I wanted to remain light grey with ground and etched it again for another minute. Once more I covered the next darkest tonal areas with ground and brought it into the acid bath for one last etching of 3 minutes to create the darkest darks. In this way it's just like the reduction woodblock prints. You are always taking away from your image until you are done. I haven't printed my plate yet but here's a look at it. It has a lot of gradations of tone. I can't wait to print it.
Bad Dreams No.2
aquatint etching on zinc plate