INDANTHRENE BRILLIANT PINK

Of the German "universal colors", is a color expression between vermilion and light madder lake. The latter has an admixture of ultramarine blue to give it a cooler quality. the colors are useful in all techniques including indoor fresco. When heated, all coal-tar pigments, because they consist of organic bodies, turn black. The base and filler material remain white.

INDIAN YELLOW

Or, magnesium euxanthate, a natural organic lake, is said to have been made from the urine of cows which have been fed on mango leaves. The raw product, Monghyr purree (after a city in Bengal), in the form of yellowish brown lumps, betrays its origin by its odor. When cleaned and powdered, it is a beautiful golden yellow glazing color, which is quite permanent and can be used in all techniques but fresco. Indian yellow is slightly soluble in water, but when boiled in water with added hydrochloric acid, the yellow if genuine Indian yellow, disappears. Substitutes remain in the water. Like many organic substances, it leaves ash when burned, but a white ash which is soluble in hydrochloric acid; otherwise the product has been adulterated. The cleaner and more golden yellow, the more valuable, brownish varieties being less valuable. It requires 100% oil, and needs an addition of varnish as it is a poor dryer. There also is no substitute for this peculiar color. It is very expensive, and the expected adulterations because of that had a bad effect on its reputation. Today coal-tar colors such as naphthol yellow are called Indian yellow by manufacturers.

INDIGO

The two great colors of medieval painting were azurite and indigo, both of which lost their importance with the inception of oil paint as neither hold up in an oil base. As a dyeing medium, indigo was imported by the ancient Egyptians (and later by the Greeks and Romans) from India, hence the name. The importation of indigo from the orient continued through the Middle Ages, and the best qualities in the European market were known as Baghdad indigo or Gulf indigo. A by-product of this natural plant dye formed a pigment which is heavy and impermanent, therefore cumbersome to use, along with Thioindigo, a red-violet coat tar pigment which is permanent, though only in watercolor. Following is an interesting passage concerning indigo: Unknown, Cambridge, twelfth century: "Take white marble and put it into hot dung for a day and a night, and then take it out and find it on another marble strongly, and make a very fine powder. And then take the foam which is found in the cauldron in which clothes are dyed the color of indigo, and put it on this powder, and work it up for a long time. And when it is dry, add more of the foam, until it acquires a good azure color. Then after it is thoroughly dry, powder it finely and wash it with water in a basin; and then let it settle and pour off the water, and after pouring off the water let it dry. When it is dry, powder it very finely on the marble, and put it away in a little bag. And know that the foam is better before the cloths are dyed. Know too, that you can do all this with white lead as well as with the powder of marble." (We are not sure that the source of the color in the dye van was woad and not oriental indigo.)

INFUSORIAL EARTH

Fuller's earth.

IRGANZINE YELLOW

No information available on history or chemical make-up; seems to be a recently made color. Very acidic dark green-yellow. When mixed with linseed it becomes a very dark green with a highly acid yellow staining power. Works best as a yellow stain in zinc or titanium white. Also uses about 60% oil from my experience.

IRIS GREEN

This was the chief rival of sap green in late medieval manuscript painting, made from the juice of iris flowers, and mixed with alum and thickened like sap green, but more often prepared as a clothlet. (It is the alum that causes the color to turn green.) Bits of cloth were dipped in alum solution and dried, and then dipped into the juice of the flowers and dried, over and over until they contained a sufficient quantity of color. The dark blue flowers do not appear to be a likely source of green; the purplish color that is first squeezed out is not promising; but as soon as it combines with alum, a clear and beautiful green appears. It was greatly used in the fourteenth and fifteenth centuries as far as we can tell as there is no method worked out for distinguishing it from sap green in manuscript paintings. We cannot estimate their relative importance in the works of different schools, but it is possible that iris green precedes the use of sap green and may have been more generally employed. In addition to the recipes for making greens, there are fourteenth and fifteenth century rules for making iris blues. The secret, according to a manuscript in Florence, is to take out "the yellow things in the flowers". (pollen).

IRON GALL INK

Made from tannin or gallotannic acid which is derived from oak galls. When this is combined with ferrous sulphate, ferrous gallotannate is formed (a colorless compound) which develops a black color on exposure to air because of oxidation to ferric gallotannate. 7 to 10 days are required for complete oxidation, therefore a dye or other provisional coloring matter is added to the ink to give it an immediate color. The ink ingredients are suspended in a solution of gum and water. We do not know when iron gall inks came into use, except that it was at some point in the Dark Ages or early Medieval times.

IRON OXIDE BLACK

A dependable black in fresco. When strongly heated, it turns a very dark red-brown.

IVORY BLACK

Prepared by charring bones, horns etc., in the absence of air. It is the purest and deepest black and is the best dryer. It is partially soluble in acids and can be used in all techniques. When used by itself over a smooth white ground of for example, lead or cremnitz white, it cracks, but not when slightly mixed with other colors. Ivory black was established in antiquity by the example of Apelles; but there is no evidence that it was continued in the Middle Ages. Where it picked up popularity again I have not fully established yet. Eugene Fromentin 1867: (In discussing the pigments of Rubens): "If you examine his blacks they are made of ivory-black, and serve with white to make all the imaginable combinations of heavy and light gray."