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Many types of limestone have been used in lithography, but by far the best is the stone that comes from Solnhofen in Bavaria, sometimes called Kellheim stone. The superiority of this stone is due to its fine and uniform molecular structure, which leads to a stable and consistent reaction to the processes of drawing, etching, and printing. The smallest stones were originally used for vignette drawings and illustrations, the largest for posters and billboards. In limited quantities, some stones are still being quarried today. Through the years, stones of the larger sizes have become increasingly difficult to find; and the shortage may be expected to become still more critical in the future.

Solnhofen stones are harder and more brittle than other limestone. Because of this, they are easily chipped and broken, and should be handled with care. Their texture is compact yet porous and they will retain moisture on their surface for considerable periods of time.

Stones vary in color from yellow or buff to a blackish gray. Their color is an index to their hardness: the darker the color, the harder the stone. In general, the harder stones are best suited to lithographic printing. Because of the difhculties encountered in working on dark gray stones, particularly the distortion of value relations, light gray stones should be used whenever possible for delicate or complex work. The softer buff and yellow stones should be reserved for drawings having relatively simple tonal values. Few stones are perfect. Many stones, however, have only minor defects to which no special attention need be paid. Many light-colored stones have red iron marks or stains. These have no effrect on drawing or printing, and such stones are completely usable. "Chalk marks," or a gray-wbite mottling on the surface, present a more difficult problem. The whitish areas are frequently soft and tend to grind and etch unevenly. Stones so marked should be discarded or used only for simple work. Feldspar crystals often appear in the surface of stones. These crystals, since they have no affinity for grease, will print as white spots. Usually they appear in small groups or clusters, making it possible to work around them. When this cannot be done, another stone should be chosen.

The most common defects found in stones are tiny veins, which appear as hairline cracks differing from the surrounding stone in color and hardness. These may be very troublesome. At times they will print as faint white lines; at times they will take ink and print as black lines; at other times they will not affect the work. They are to be tolerated unless their effect is disruptive to the image.

Most limestone was formed during the Jurassic middle period of the Mesozoic era, between 136 million and 190 million years ago. Limestones used for lithography are described as being nonclastic, i.e., not composed of fragments of existing stone.
Instead, they contain microfragmentations of various forms of marine life (minute crustacea, protozoa with calcite shells, and certain coral-forming organisms) deposited in calcareous mud (micrite) on the sea floor. Pressure, heat, and chemical reaction compacted the calcareous deposits. In the formative cycle, one stratum of sediment was deposited over another, further compacting the earlier layers. During diastrophic periods, the earth's crust was deformed, and the sediments of the sea floor were elevated and subjected to subatrial erosion. During the periods of elevation, rivers and streams intermixed other chemical components with these deposits. After formation, the limestone strata in most other parts of the world were subjected to additional cyclic subsidence and uplift. Each such disturbance produced joints and faults in the normally horizontal stratum, permitting the formation of other minerals by hydrothermal alteration and weathering.
Two major geologic factors seem to be responsible for the unique properties of the Solnhofen deposits, First, owing to unusual regional circumstances, only a very small percentage of mineral impurities combined with the calcareous deposits during their formation. Second, the horizontal strata of the deposits of limestone remained relatively stable during the various epochs of cyclic upheaval.
Massifs of coral limestones correspond to the position of ancient reefs and are in juxtaposition with well stratified, fine grained limestones, exploited as lithographic stone. The famous quarries of Solnhofen have provided rare but precious impressions of swimming and aying animals (fish, crustaceans, flying reptiles), fallen on the muddy bottom and fossilized with an extraordinary delicacy of detail. There existed at that point, no doubt, tranquil lagoons in the middle of atolls or protected by reef barriers.

Lithographic stones have a close and compact, yet porous texture. They should be handled with care, because they are very hard and brittle; when they break, they part cleanly with a conchoidal fraeture.

Stones must have a certain thickness, depending on over-all size, to withstand the pressures of the printing press without breaking. Small stones up to 22 x 35cm should have a minimum thickness of 5 cm. Larger stones require 7 to 10 cm of thickness for safe printing. Thinner stones are often backed with slate to prvide sufficient bulk to withstand printing and to prolong their usefulness.
The color of a stone indicates its hardness and quality.

Chemically, lithographic stones of the finest quality contain approximately 94 to 98 % calcium carbonate and carbon dioxide. The remain ing 2 to 6 per cent of foreign matter is mostly composed of silica, iron, manganese, and aluminum oxide. Because of its extremely small percentage of chemical impurities, the stone can absorb grease or water with equal affinity; meanwhile it responds to the chemicals used in lithography with maximum sensitivity.

When the stone is etched with solutions of gum arabic and nitric acid, its chemical compsiticon enables the printing image to be established in the following two ways:

  1. The fatty bodies of the drawing materials are con-verted by chemical action into fatty acids; these combine with the calcium of the stone to form insoluble lime soaps that are highly receptive to greasy printing ink.

  2. The surfaces without grease drawing are changed from calcium carbonate to calcium arabinate by virtue of the adherent gum arabic film produced by etching and drying the stone. The adsorbent gum has the property of keeping the stone surface moderately damp when mois- tened with water. In this way the stone's natural affinity for water retention is increased considerably by the etch-ing process. When the residue of the drawing components is washed away with water and turpentine, the latent lithographic image can be faintly seen like a photographic negative. The difference in color between the image and nonimage areas is the result of the calcium oleate image formation and the calcium arabinate nonimage formation.

The reliability of these lithographic functions is related directly to the chemical composition of the stone itself.

Both image and nonimage formations are impaired if high percentages of foreign impurities are present. For example, concentrations of silica, alumina, magnesia, or quartz, being less porous than calcium carbonate, will resist somewhat the absorption of fatty image particles as well as the chemical reaction of desensitizing etches. Thus, image formation in areas containing such impurities will be weak or nonexistent and the tonal gradations of the drawing will be coarsened or disrupted. These conditions are particularly evident when working with soft yellow and white stones, which are high in impurities. It should now be evident why other types of limestone, which have even greater percentages of impurities, are unsatisfactory for lithography.