Here's some more from my old writings:
Prussian blue is made with a non-drying petroleum based vehicle. If kept in a closed container and away from oxygen, it may keep its consistency for generations. I have remnants of tubes from my apprentice days in the early '60's that still spreads acceptably. Some are Dykem some are Permatex. I prefer Dykem. The Canode stuff works well but when it comes to the realy touchy finishing operations I still fall back on Prussian blue and red lead.
Let's define a few things. Prussian blue is usually used as a transfer medium, that is, it's applied in a thin uniform film to the reference which is then rubbed on the work. Prussian blue is thereby transferred to the work's high spots. The scraper hand works these down and repeats until the entire surface of the work is freckled.
Sooner or later the scraper hand has to reduce the film thickness to achieve greater sensitivity. The smaller indications may disappear against the glare and flash of the scraped surface and to remedy that we use a contrast medium. The chromatic opposite of blue is orange. For many generations the complement of Prussian blue has been red lead oxide. Two of the virtues of Prussian blue and red lead oxide is their low abrasion characteristics and the density of very thin films that appear dull black against the work. Red lead oxide is entirely too toxic to use in present day industry although I've used it for over thirty years and I'm alright (twitch!) all right (twitch!) all right (twitch!) SLAP!! I'm fine.
For those who may be unaware of the term "contrast medium," it's a color in oil applied to the work as a means of making the print indications transferred from the scraping reference more visible. I prefer orange (red lead oxide is perfect except for heavy metal toxicity) or chrome yellow when contrasted with prussian blue. The chrome yellow can be applied as thinly as red lead and the indications come out with a greenish cast that makes them more readily distinguishable from false indications like fuzz and debris. To answer the question, you can't buy chrome yellow contrast medium. You have to make it from the dry artist's pigment mixed with short fiber bearing grease to the consistency of spreadable butter or tooth paste.
The film thickness of the contrast media can be adjusted with the edge of the hand. There's nothing like clean dry skin for this. I prefer a film thickness that can be described as a faint haze barely tinged with color.
In some spasm of generosity an old time paint store owner gave the opera company a supply of dry paint pigments. I experimented with them and with one exception they made poor transfer or contrast media for scraping. Chrome yellow makes an excellent contrast medium. Its abrasion is low meaning your scrapers will maintain their edge and it's bright yellow color makes for olive green indications at least as well as the dark grey ones from red lead.
It's interesting if time consuming to experiment with things like the perfect scraping pigment media. Lamp black works well but it's very difficult to acheive a smooth media with it.
There are many choises for vehicle. Some use a little oil others a volatile solvent. For what it's worth I use a short fiber soft ball bearing grease, a putty knife and a glazed floor tile as a mixing pallet. Add pigment to the grease folding it in and smearing to ensure distribution and smoothness. Fold in as much pigment as the grease will take without the stuff getting cakey or hard to spread. If your color isn't smooth and uniform and your wrist doesn't get sore you're not working the process hard enough. Mix, fold, scrape the stuff to the center and start over. Sooner or later you'll get a smooth densly colored paste that suits scraping needs.
