pigments	121
and was used in Italian painting certainly as early as the XV century and prob-
ably even much earlier.
A coloring material much like indigo has been observed in a blue layer be-
neath an azurite film in a Sienese painting. (See R. J. Gettens, 'Microscopic
Examination of Specimens from an Italian Painting/ Technical Studies, III [1935],
pp. 165-173.) De Wild lists (p. 31) four paintings, three by Frans Hals and all
of the XVII century, in which he found indigo. Perkin and Everest say (p. 475):
Its employment in Europe was very limited until in 1516 when it began to be imported from India
by way of the Cape of Good Hope, but its introduction in large quantity did not occur until about
1602. Owing chiefly to the opposition of the growers of woad, its European rival as a dyeware, it
met with much opposition, and various laws were enacted both on the Continent and in England
prohibiting its use. It was called a 'devilish drug' and was said to be injurious to fabrics. In 1737
its employment was legally permitted in France, and from this period its valuable properties appear
to have become gradually recognized throughout Europe.
In the Far East, indigo was used as a pigment as well as for dyeing cloth;
It was identified in a blue layer beneath azurite on a Chinese painted clay statue
of the Tang Dynasty from Tun Huang in Central Asia, and also on a wall
painting of later date from Kara Khoto in Central Asia.
Inert is the name given to any inactive white pigment which has little or no
hiding power or tinting strength when it is used in a paint vehicle. Examples are
gypsum, barium sulphate, chalk, etc. These generally have a refractive index
below 1.70. They may have a considerable whiteness when used with tempera
medium, but in oil they are nearly transparent and give only dull yellow films.
Inerts are employed for ground and priming materials (see Gesso). They may
be used as extenders (see Extender) for pigments with high tinctorial power, and
they may also be used for the bases, carriers, or substrates of lake pigments
(see Lake).
Infusorial Earth (see Diatomaceous Earth).
Ink is a liquid or viscous material used for writing, printing, lithographing,
stamping, and staining. Inks are made from dyes and from pigment suspensions
like carbon black. Those used for printing and lithographing are made by grind-
ing pigments in oils and varnishes. Ordinary writing inks are iron gall inks in
which the color and stain are formed by the combination of gallotannic acid from
oak galls and green vitriol (ferrous sulphate) in the presence of air (see Iron Gall
Ink and Chinese Ink). Both vegetable and aniline dyes are used for special,
colored writing inks (see Mitchell).
Inorganic Pigments are those natural pigments prepared from minerals and
ores (see Earth. Pigments) or those synthetically made which are chemically pre-
pared from the metals. The most stable and inert pigments are in this class.
Iris Green is an organic dyestuffof natural origin from the juice of iris flowers.
Thompson says (The Materials of Medieval Painting, p. 171) that it was used
extensively in the XIV and XV centuries, particularly in manuscript painting.
The beautiful green color was best developed by mixing with alum.