PAINTING MATERIALS
gregates and so finely divided that it has almost the characteristics of a dye. By
control of conditions of precipitation and oxidation, variations in shades and
physical characteristics of the blue may be had. (Antwerp blue is a light shade of
Prussian blue, made by precipitating zinc along with the iron, or it may contain
inerts like gypsum, barytes, etc.) The dry powder or lump form is dark blue, al-
most blue-black, but some varieties, especially those prepared with bleaching
powder as the oxidizing agent, have a reddish, coppery lustre. It is a transparent
color but has very high tinting strength. One part of Prussian blue will render 640
parts of white lead perceptibly blue (see Colour Index, p. 309). The color by trans-
mitted light is green-blue which is also its color in tint. In an oil film, discrete
particles can not be seen, even at high magnification. De Wild has compared its
appearance in oil with that of indigo (p. 32). It seems to form thin smears on the
surfaces of other pigment grains (see Chrome Green).
Prussian blue is fairly permanent to light and air. Laurie mentions (The
Painter's Methods and Materials^ p. 94) a ceiling painted with it in the first part
of the XIX century in which the color is still good. It sometimes acquires a
peculiar, metallic bronze cast when subject to out-of-door weathering, and some-
times paint films that contain it turn green because of yellowing of the oil. The
blue is unaffected by dilute mineral acids, but it is very sensitive to alkalis which
cause it to turn brown; for this reason, it can not be used in true fresco. It is
soluble in 10 per cent oxalic acid (Beam, p. 89). It decomposes rapidly on ignition
and leaves a residue of ferric oxide.
Huge quantities of Prussian blue are now used in the paint and printing ink
trades; the most important commercial green pigment, chrome green (see Chrome
Green), is made by adding it to chrome yellow.
Prussian blue has a conspicuous place in the history of painting materials
because it is the first of the artificial pigments with a known history and an
established date of first preparation. Moreover, it is a material so complex in
composition and method of manufacture that there is practically no possibility that
it was invented independently in other times or places. It was first made in Berlin
on or about the year 1704 by Diesbach, a dyer or color maker. Kopp (IV, 369) says
that it was first mentioned in an anonymous communication entitled c notitia
coeruki Berolinensis nuper inventi, in a report of the Miscellanea Berolinensis,
1710. In that notice the beauty of the color was praised and it was claimed that it
was useful as a painting pigment. It was said then to be for sale by the book dealers
of the Berlin Academy. In that account there was nothing about the discoverer
or the method of preparation. Stahl (Georg Ernst, 1660-1734) gave a more ac-
curate and detailed report of the discovery in his Experiments*> Observationibusy
animadversionibus CCC, etc. (1731). According to him, it came from an accident
which resulted when Diesbach wished to prepare Florentine lake by the precipita-
tion of an extract of cochineal with alum and iron vitriol and a fixed alkali. He
asked the well known alchemist, Johann Konrad Dippel (1673-1734), to let him