PIGMENTS 149
materials. Specific gravities of the more important pigments and inerts are listed
in the table of physical properties.
The oil absorption of a pigment is the amount of oil that is just required to wet
each of the pigment particles and to convert the mass into a mobile paste. Pig-
ments differ greatly in the amount of oil required for this purpose. It is often ex-
pressed as the number of grams of oil required to grind 100 grams of pigment into
a stiff, putty-like paste that does not' break ' or separate (see Gardner, pp. 539-
560). Oil absorption is no exact physical constant. It varies slightly from lot to
lot of pigment, with the kind and condition of the oil used, and with the degree
and duration of mixing and rubbing. Some pigments, like basic carbonate white
lead, are characterized by low oil absorption, which is generally as low as 9 to 12
per cent by weight of oil, to make it into a workable paste; raw sienna, on the other
hand, takes upwards of 50 per cent oil to grind. Pigments with low oil absorption
are favored, in general, because paints made from them have less tendency to
discolor as a consequence of the yellowing of the oil. Many of the pigments with
high specific gravity have low oil absorption. Gardner says (p. 544) that oil ab-
sorption is dependent essentially upon the total surface of the pigment, the inter-
facial tension relations between pigment and vehicle, particle shape, size, and
distribution, and the chemical nature of oil and pigment. All these are important
factors that have much influence on the plastic and flow properties of oil paints
(see also R. Houwink, Elasticity', Plasticity and Structure of Matter [[Cambridge:
University Press, 1937], pp. 311-327).
Pink (Dutch pink, Italian pink, brown pink), in addition to its meaning as a
tint of red, is also used for certain yellow lakes prepared from quercitron (see
Quercitron Lake), or from Persian berries (see Persian Berries Lake), or from
similar, natural, yellow coloring matters. (The Shorter Oxford English "Dictionary
says that the origin of the word in this connection is obscure.) Brown pink is a
deep variety of quercitron lake (Weber, p. 127).
Pipe Clay (see China Clay).
Plaster of Paris (see Gypsum).
Pompeiau Blue (see Egyptian Blue).
Pozzuoli Red is a red iron oxide of volcanic origin from Pozzuoli, near Naples.
Prussian Blue (Berlin blue, Paris blue, Antwerp blue, Chinese blue) is the
earliest of the modern synthetic colors. It is a complex chemical compound which,
technically, is ferric ferrocyanide, Fe4(Fe[CN]e)«, or a closely similar compound.
It is now commonly made by the action of an oxidizing agent, such as potassium
bichromate and sulphuric acid, upon a mixture of copperas (ferrous sulphate),
sodium ferrocyanide, and ammonium sulphate, giving a blue with the approxi-
mate formula, Fe(NH4)Fe(CN)6. The pigment which is precipitated from dilute
solutions of those salts is a deep blue, finely divided compound which, after it has
settled and after the mother liquor is drawn off, is washed, filtered, and dried.
(See Beam, pp. 85-92 for details.) The product is amorphous in colloidal ag-