in a nut shell (Using Photoshop),
1 Make a selection (mask) and invert that. This is the white. Adjust the curved to fill in a little in the middletone to highlight range.
2 Now taking the original art, double click on the channel and assign it a middletone gray color (but do not change the image.
3 copy and past into a new channel. Double click to change/assign the color, (This will be your black). Now, on this one, adjust the curves to burn out most of the highlights and into the mid tones. Leaving some good shadow tones.
4, Copy and past the black channel and adjust the curved to burn out more of the "halftones" so that you are leaving mostly solid black. Now control click on that channel to make a selection of all pixels. with it still selected, move to the mid tone gray channel and delete that selection of the black...out from the middletown gray so you do not have too much ink build up.
Done.
I would make sure to do some other adjustments but those are finer tuning the seps and are on a case by case basis.
