I myself always thought "rosin" was what you soothed yr violin bow with, but here from the American Annual of Photography of 1925, article by Joseph Petrocelli, slightly abbreviated, is...........
Resinopigmentipia -- now called Resinotipia -- new photographic process by Prof Rodolfo Namias of Milan.
That bichromated gelatine paper exposed to light loses the property of swelling and adhesivity when immersed in hot water is basis of the process. So if we expose a piece of bichromated gelatin paper to light
under a POSITIVE, & then immerse in hot water, the gelatin in the highlights will be insoluble, in the half tones it will be less insoluble & in the shadows it will swell to the max.
If we spread colored pigments on the paper, they should in theory adhere 1st to the shadows & then proportionately to the half tones, but in practice the powders adhere almost all over & stain the paper -- which is why similar processes failed: they had a very short scale. Namias's experiments made his process workable.
In mixing colors with a greasy material, or RESIN (my emphasis) he got colored powders which don't stain & adhere only to the parts less acted on by light, inversely proportionate to exposure. So with these pigments & specially prepared gelatin paper, we get a long scale of tone, from deep black shadows to pure white highlights -- tho PURE whites only by manipulation. That there's always a veiled effect in the highlights is an advantage. ..because rapid, exceedingly simple manipulation is possible.
The greatest advantage however is that you print from a positive, so results are under control, especially from a positive on paper, because it can be modified by pencil, crayon, graphite, etc. It could even be a drawing, or whatever.
Pigments are spread with cheap, soft round brushes, allowing more control. Print takes a few minutes. Polychrome prints also possible. All the richness of a pigment image, but without the glaze or gloss some object to in multiple gum, oil or carbon. Rather it's a velvety, finely textured mat pigment of great depth, long scale, etc. etc. etc.
Paper is prepared by immersion in 5% solution ammonium bichromate or 4% potassium bichr. . (20 oz water w. 1 oz am bi will sensitize 6 sheets 11x14 inches.) Soak in solution 2-3 min, hang to dry in dark. Will keep in a dark envelope 20 days in winter, 10 days in summer. Print by contact from pos. on glass, film or paper, in diffused light or strong artificial light. For more detail, print film to film. Exposure ranges from 1 second to 1/2 hour, depending on density of positive & the light. Over-exposure destroys highlight detail, underexposure clogs shadows.
After exposure wash in running water til bichromate is gone, or a short wash followed by still soak for several hours.The paper can be dried promptly or swelled right away in hot water (ca 120 degrees). But the swelling has to be plainly visible. If not, probably over-exposed, or water not warm enough. Add a few drops of ammonia, but sparingly, otherwise gelatin will soften too much & image be lost.
If underexposed, use cooler water & no ammonia. Place the paper on plate glass, or other support, blot off surplus water with soft cloth -- then paper is ready for pigmenting. Dip a large round soft brush in the pigment & spread all over the paper with rotary motion, gently, but rubbing in. The image will readily come up. Add pigment for more intensity in areas, remove where not wanted. (Use fresh brush to remove..) Starting in highlights,.. Use plenty of pigment, excess can be re-used when dry & ground in a mortar. Can also retouch with an eraser or rubber when dry. Semi-dry print can be retouched with small wet brush. Completely dry prints can be immersed in cool water and retouched with small brush for pure whites. Fix the print while still damp over steaming water for 1-2 min. Can repeat these steps if result not satisfactory.
Does anyone have a clue what that "greasy material" or "resin" is that he mixes with the dry pigment?
PS. I tried to make a couple of points clearer -- this Petrocelli fellow is NOT your master explicator (but that "greasy material" could still be a stick of oleomargarine or pommade).