PURPOSETraditional intaglio asphaltum and shellac stopouts stop out completely, regardless of the thickness of the coating, offering a high degree of control but only flat tones; an "all or nothing" situation. Permeable grounds offer a field of activity between "all" and "nothing". They are applied to the plate in a film, or "layer" (as in a brush stroke) that has varying thicknesses. Where the film of ground is thin the etchant breaks through quickly and etches a tone and where it is thicker the etchant breaks through more slowly and creates less tone or no tone at all. This opens up a world of looser, more spontaneous and painterly effects to intaglio etchers who otherwise are more or less confined to techniques that tend to be mechanically rigid and labor-intensive.
Permeable grounds are non toxic.
Application of White Ground is discussed in detail in Frank Cassara's article in "Artist's Proof" where it was originally published in 1963, and in Ruth Leaf's book, "Intaglio Printmaking Techniques", 1976. Basically, it is thinned with water to whatever consistency is desired and applied with a brush, sponge, rag, etc.
I usually use them on top of an aquatint, though they also are commonly applied directly to the bare metal plate or have aquatint applied on top of them. If you first apply an aquatint and then apply the white ground on top of it, the aquatint creates a tone and the white ground creates lights and darks in that tone; as any stopping out of an aquatint will do. If you apply it directly to the metal with no aquatint, you get more pitting, flat biting, and textural effects as opposed to the tonal effects you get if you use it on top of an aquatint. If the aquatint is applied on top of the permeable ground, you get tones and/or textures with heightened contrast.
There is a learning curve for using permeable grounds, and an etcher should be familiar with the way they behave (i.e. do some practice plates and develop a feel for how thickly or thinly to apply them) before using them to create imagery on a plate. I have used them for years on zinc etched with nitric acid, copper etched with ferric chloride, and copper etched with Edinburgh Etch. They tend to be more permeable on zinc/nitric than on copper/ferric chloride or Edinburgh Etch, but otherwise behave the same.
They are water soluble until they have been immersed in acid, ferric chloride or vinegar for at least 10 or 20 seconds or more, after which they are waterproof, which makes it possible to rinse off plates in water between step bites. They dry to a soft greasy coating, which can be further manipulated by scratching or scraping with a wooden point or the corner of a piece of cardboard, blended with cotton swabs or stumps and can be removed with mineral spirits or detergents after etching is complete.
Permeable grounds can be used by themselves in thick or watery consistencies to create imagery, or can be combined with other resists such as airbrushed asphaltum or spray enamel to soften, intensify, or otherwise modify their effects. They can be used for semi-controlled textural effects by laying down a field of thin watery permeable ground and spraying or spattering wet-in-wet incompatible liquids like watery asphaltum dissolved in turpentine or mineral spirits into them, or spritzing them with kerosene from a spray bottle. They can be used as a base for sticking other materials to the plate for use as a resist, for instance you can get a good rock texture by laying down a very thin (watery) field of wet permeable ground and sprinkling particles of crunched up lump rosin into it and letting it dry, then etching. I usually do this on top of a rosin/box aquatint which gives a tonal effect, but a more textural effect could be achieved by doing it without one or by putting the aquatint on top of the ground.
The actual etching part of the process, as is always the case with etchings, is a highly critical guessing game. The question is, how dark do you want it to be and how long do you leave it in the etching solution to have it be that dark. This guess (timing) is based on three primary factors: a. the strength of the etchant, b. the character (fineness or coarseness) of the aquatint that's on the plate (a fine aquatint will max out to black in the acid and start to break down much sooner than a coarser one will, and an aquatint whose particles are more sparse and segregated on the plate will max out sooner than one whose particles are so dense that they're all starting to run together), and c. the amount (thickness) of permeable ground on the plate. The etcher has to decide what amount of time in the acid ("maximum etch time") is going to give a 100% black.
This "maximum etch time" is the basis for timing charts which I devised to break down the total etch time into 10% increments of grayscale. It takes much less time in the same tray of acid to get an aquatint to darken from 10 to 20% than it does to get it to darken from say 80 to 90%. These timing charts are a way to subdivide a given total amount of clock time (the "maximum etch time", or black) into individual etch (step bite) times that will create an even progression of values from white to black.