The process of chromolithography is chemical, because an image is applied to a stone or zinc plate with a grease-based crayon. (Limestone
and zinc are two commonly-used materials in the production of
chromolithographs.) After the image is drawn onto stone, the stone is
gummed with gum arabic solution and weak nitric acid, and then inked with oil based paints
and passed through a printing press along with a sheet of paper to
transfer the image to the paper. Colors may be added to the print by
drawing the area to receive the color on a different stone, and printing
the new color onto the paper. Each color in the image must be
separately drawn onto a new stone or plate and applied to the paper one
at a time. It was not unusual for twenty to twenty-five stones to be
used on a single image.
Each sheet of paper will therefore pass through the printing press as
many times as there are colors in the final print. In order that each
color is placed in the right position in each print, each stone or plate
must be precisely ‘registered,’ or lined up, on the paper using a
system of register marks." (1)It is easy to determine a stone or plate chromolithograph from new or reproduction lithography (or photo copies) using a 10x jewelers loop. Looking through a 10x loop at any paper sign utilizing stone lithography you will see crayon type strokes or inconsistent size dots often overlapping or on top of a different color to produce different colors or color variations. Newer photolithograpy will show up as just uniform dots or a hexagon with a dot in the center, the tighter the pattern, the newer the lithography. The best way to become familiar with progressive printing processes is to visit an antique mall that has a vendor that specializes in old post cards and view them with the 10x loop starting with those dated from the 1800's to c.1970 or later.
To determine laser scanned copies look for very fine horizontal lines (requires very close examination and sometimes a 20x loop when a high quality scanned laser prints).
Another way to determine if the paper is old is by utilizing a good black light. Paper produced prior to World War I used pure pulp. Paper produced after 1914 used recycled bleached paper due to a paper shortage and bleached paper fluoresces under a black light. (Note: This process usually is effective in determining if the is or is not old, however, pre-1914 unused paper stock has been found in old warehouses and used to reproduce some of the rarer signs so always check the lithography with a 10x loop as well as with the black light).(1)