Working with the antipathy between grease and water, the non-image areas of the surface of the stone have been chemically desensitized so that they will no longer be receptive to the greasy printing ink, whereas the image areas are still sensitive to the ink and will want to draw in the same amount of grease (ink) that will essentially replicate the tonalities of the original drawing when ink is applied, paper laid overtop, and the whole is run through the press. Essentially, the ink only sticks to the chemically desensitized/etched area.
