Re: Demachy and red chalk
On Sun, 24 May 2009, Katharine Thayer wrote:
Very few if any paint manufacturers use natural iron oxide pigments mined from the earth any more; they're generally made synthetically now. PR 101, which is the pigment for all the venetian red, red oxide, red earth, red ochre, indian red, and such-named paints, is a manufactured rather than mined pigment. So Judy's caution that color can change from mine to mine is somewhat outdated, but the general caution still holds that PR 101 (red iron oxide) pigments can come in different shades of red and orange, and in varying degrees of transparency as well.In fact, my experience over the last 100 or so years (and even the last 2 or 3) is that a manufactured pigment can also change, not just from brand to brand, but from lot to lot. I probably mentioned my dismay when a black "gouache" I'd been using in gum changed for the worse when I got a new tube (same company), so much so that I abandoned that technique. There have been other variations as well, tho not so serious.
In any event, if you have a favorite/important paint or pigment, it might be a good idea to lay in a spare. Of course, unlike gum arabic by the gallon, paint may not have a lot number (I'm downstairs, the paints are upstairs... also, labelling practice varies widely from brand to brand, sometimes even year to year). Still, odds are good that same source same year would be same.