A siccative is a solution of metal
compounds that is used to speed up the chemical drying process of
oil paints. The drying of oil paints can occur on the surface and
deep in the paint itself. Depending on the metal compounds that
make up a siccative, either the depth drying or the surface drying
will be stimulated.
The light siccative, Siccative Courtrai, increases the drying in
depth in particular, whereas the darker Siccative Harlem increases
the surface drying. Siccative Courtrai consists exclusively of a
solution of metal compounds, whilst Siccative Harlem also includes
a certain percentage of oil. Siccative Harlem is therefore fatter
and can be regarded as a quick-drying medium.
The choice of siccative depends on the composition of a painting.
As layered painting always requires the "fat-over-lean" technique
to be followed in connection with a good adherence between the
paint layers, it is not advisable to use the fatter Siccative
Harlem in the lower layers. Optimal adherence of the next layer
would moreover be less likely due to the speeded up drying of the
surface of the layer. The incorrect use of siccatives can lead to
problems. Apart from the drying process, the addition of a
siccative also speeds up a painting's ageing process. To avoid
problems, the paints must never have more than 2% Siccative
Courtrai added to it and never more than 10% of Siccative Harlem.
Generally speaking the use of siccatives must be kept to a