Stone lithography starts with a slab of limestone. Limestone is used because it has both water- and grease-loving properties, which properties are exploited to create the print. You need to grain or grind the stone down to a smooth, flat surface, using carborundum grit and a heavy steel disc with a handle called a levigator. Repeated passes of each grit with water eventually smooth out the stone surface.
Once the stone has been grained, the surface has been opened up and made very receptive to grease, so you have to be very careful not to touch it with your fingers. The image can be drawn or created using greasy materials such as lithographic crayons, liquid tusche, etc. Once the image has been completed, the stone has to be transformed with a chemical reaction which desensitizes the non-printing areas to ink (makes them more water loving) and enhances the image areas which are supposed to accept ink (makes them more oil loving). First, the image is protected with rosin, which helps keep the greasy image from breaking down during the etch. Next, talc is rubbed on to keep the printing area from "blooming" during printing (like what happens to newsprint wrapping your greasy fish & chips). Then an etch solution is mixed up with drops of nitric acid in gum arabic. The proportions are related to the type of stone, type of drawing, amount of etch required, etc. This etch solution is spread evenly over the stone over a three minute period, then the fine gum arabic film is "stretched" across the stone by buffing with tarlatan.
The image area is wiped out with paint thinner, then rubbed up with a very thin film of asphaltum and thinner. The gum film is washed out with water, and then the image is rolled up with a special leather roller & "short" (thick) roll-up ink. The image is proofed to determine whether the etch was successful, then a second etch is prepared, applied, and "stretched". This second etch is rubbed up with the colour of the ink to be printed; if printing with black, asphaltum can be used, and the gum film is washed out.
The stone has to be kept damp using a sponge with water during proofing and printing to keep the etched areas (non-greasy) from scumming up with ink. The print is proofed using newsprint, to get the ink layer up to the right level for printing on the editioning paper. The print is then editioned. Various registration methods can be used: we had a frame set up around the stone with two bars set at mid-point that bars and T-marks were placed. Bars were lightly penciled in at the back of the editioning paper at the mid-point; these were aligned with the registration marks on the frame, to (hopefully!) create a repeatable alignment of paper on the stone.
We first created a print with just brown ink, as that's what I'd had in my mind for the final print. Unfortunately, even though the roll up looked great, the roller used for editioning is a different surface, and apparently the roll up roller pushed the ink into the microscopic layers representing the mid-values. So the brown started to loose the mid-values really fast.
So we tried to do some just black prints using the roll up roller. It's too difficult to completely clean out the leather roller to use any other colour than black; it's scraped to remove the ink. The black prints resurrected the mid-values.
Next, Pat suggested that we do a "double hit"; print another colour ontop of the brown, directly. We chose black using the roll up roller. The colour result, while not obvious in the digital image, is much richer and denser. We ended up doing more brown prints then printed black on top to create the foundation for the edition.