Description of sorts of lithographic stones. (source: Tamarind) (Inglès)
Lithography stones have a close and compact, yet porous structure. They should be handled with care, because they are very hard and brittle; when they break, they part cleanly with a conchoidal fracture. Stones must have a certain thickness , depending on over-all size, to withstand the pressures of the printing press without breaking.
The color of a stone indicates its hardness and quality. The following seven grades of color are recognized for lithographic purposes:
- Blue stones. These are the most ancient, most compact, and darkest of all the lithographic stones. They were subjected to the greatest pressures and heaviest infusions of mineral pigmentation during their formation. Being denser, purer, and rarer than other grades, these stones were in the past highly prized for engraving purposes. They are generally too dark for artists' drawings and are too heavy for general lithographic use today.
- Dark gray stones.These stones also are rare; they are somewhat lighter in color than blue stones, but usually too dark for the artist to develop tonal gradiations with ease. They are excellent stones for engraving, for solids, and for linear work.
- Medium gray stones.These are the most desirable stones for use in lithography. The tonal range of a drawing can be clearly seen. Though less hard than the previous categories, these stones are of ample density to withstand strong lithographic etches enely and without coarsening. Today, stones of this quality are rare (particularly in large sizes) and are highly prized. They make possible the printing of very large editions with remarkable stability.
- Light grey stones.These stones are a light, cool grey. They are the youngest and softest of the limestones in the gray category. These are found in greater numbers than the medium gray stones and can be used with excellent results for all varieties of work. Occasionally, marine fossils are found imbedded in stones of this color.
- Hard yellow stonesThese stones have a warm tan color. They are less dense than the grey varieties, although they are still capable of producing a satisfactory range of work. Most of the large stones brought to the USA were of this quality. Being relatively soft, stones of this quality cannot withstand strong etches without some coarsening. In addition, the sharp granularity that is characteristic of grey stones cannot be produced on hard yellow stones; crayon drawings with fine tones and delicate tusche washes executed on such stones therefore appear somewhat less crisp.
- Soft Yellow stones.These stones are lighter in color and even softer than the previous category. They usually have an uneven density and are composed of greater percentages of silica, ferrous oxide, and other impurities. Some of these impurities will resist the action of the etch more than others, thus roughening the surface of the stone and undermining parts of the image. Soft yellow stones are suitable only for the coarsest type of drawings and are not recommended for processes that require extensive corrective procedures. Linear work and autographic tranfers print fairly well from soft yellow stones.
- White stones.These are the most recently formed, the softest, and the least desirable of all the grades of lithographic stone. They are chalky white and invariably contain large percentages of impurities. White stones are generally incapable of producing dependable results for any type of lithographic use. They are used only because of the general shortage of all grades of stone, and only for the simplest types of printing.
In addition to color, lithography stones display other physical characteristics that govern their quality and performance:
Iron marks These may appear on the surface of the stone as bands, ribbons, or streaks of dark grey, bluish grey, or pale reddish brown. They result from iron oxides in differing states of oxidation reduction which were present during the formation of the stone. Though sometimes visually distracting, they seldom interfere with the lithographic properties of the stone.