PIGMENTS 125
as a lake by precipitation on barytes or chalk. Lithol red is bluish red with a deep
blue-red undertone (see Beam, p. 144). It does not bleed in oil and has good
stability to light and heat. It has not been offered to the artists' trade under this
name, but no doubt is found in some cheap red paints as a substitute. It was first
made by Julius in 1899.
Lithopone is a co-precipitated pigment which is made by adding zinc sulphate
to barium sulphide in solution. The press cake, which is a mixture of zinc sul-
phide and barium sulphate (ZnS + BaS04), is dried, calcined at red heat, and
quenched, a process necessary to give it useful pigment properties (see Beam,
p. 53). The mixture of the two components, zinc sulphide and barium sulphate
(28 : 72), is so intimate that they can hardly be distinguished microscopically.
It is very finely divided, opaque, and without distinguishing optical character.
It has about the same whiteness but has greater hiding power than zinc white.
Lithopone is partially soluble in dilute mineral acids, with the release of hydrogen
sulphide from the zinc sulphide component; it is unaffected by alkalis and by
heat. This pigment, in the early days of its manufacture, had one serious defect,
a tendency to darken (gray or blacken) in strong light but to turn white again
in the dark (see Beam, p. 54). The trouble was traced to various causes, among
which were the presence of foreign metallic impurities, but, after years of re-
search, a lithopone is now produced which does not suffer change in light. The
so-called 'titanated lithopones,* which contain about 15 per cent titanium oxide
(see Gardner, pp. 1230-1232), have hiding power superior to that of straight
lithopone.
Lithopone was apparently first produced and patented by John Orr in Eng-
land about 1874. It is now industrially important and widely used in interior
paints, lacquers, and enamels, for it has a combination of exceptional whiteness,
brightness, and low cost. It has not been much used as an artist's pigment be-
cause, perhaps, of its unfortunate early history. It is used for poster colors and
for cheap water colors. One may expect to find it in the priming coats of modern,
prepared artists' canvas.
Litmus (archil) is a natural organic red coloring matter that is procured from
such lichens as Lecanora tartarea or Roccella tinctoria. It is extracted from the
dry plants by potassium carbonate solution in the presence of air. It is soluble
in water and in alcohol, giving a carmine red solution, and in alkalis, giving a
bluish violet color. Archil is quite similar to litmus, although obtained from other
species of Roccella and Lecanora (see Colour Index ^ p. 2,97).
Logwood is the name of a red dye that is extracted from the wood of Haema-
toxylon campechianum which is indigenous to Mexico, Central America, and the
West Indies. The interior of the live wood is yellow but changes rapidly to dark
brown on exposure to air. The leuco compound, haematoxylin (CieHuOe), changes
to red-brown haematein (CieH^Oe) on exposure to air. It is extracted by boiling
logwood chips in water over steam under pressure. The red-brown haematein
crystals are sparingly soluble in water. With sodium hydroxide, there is formed