Paints
have been made for centuries by mixing pigments such as
red lead, white lead, and umber with drying oils and it
became obvious that these paints dried faster than raw oils.
Eventually it was discovered that oils stored in presence
of lead or manganese compounds (e.g. red lead or manganese
oxide) or better still if heated in presence of these compounds
developed improved drying properties. This formed the basis
of the production of boiled linseed oil, one of the foundations
of paint formulation. Boiled oil, varnishes, lead and manganese
compounds sufficed as driers, until the turn of the century
when the introduction of Zinc Oxide as a pigment gave only
a very slow drying paint.
A
search for improved driers for Zinc Oxide paints resulted
in the discovery of Cobalt as the most active metal.
Until
such time driers were incorporated into oil or varnish either
by cooking directly into the medium or alternatively by
cooking metal oxides into a smaller proportion of oil or
into rosin, to give a concentrated linoleate or rosinate
drier. Linoleate and rosinate driers deteriorate during
storage because oxidation of the unsaturated acid content
causes loss of solubility in solvents and oils.
T
he major advance in drier technology occurred in the 1920s
with the preparation of the metal naphthenates. The naphthenic
acid is a mixture of acids of the same general structure.
As the mixture would vary, so would the acid number, so
that the metal content of the drier was not always the same
from batch to batch. The next major advance in the technology
was the preparation of solvent solutions of these metal
naphthenates which allowed the metal content to be standardized.