In an earlier post you can find information about the appropriate white paint to use on the initial layers: Leanest White Paints. The next layers (after the initial layers) in the painting ought to contain a less lean paint or a fatter paint with more oil. This technique helps prevent cracking over time.
Middle Layer White Oil Paint
Painting over the foundation or underpainting requires pigments that are slower drying than these early layers. Slower drying is fatter or contains more oil.
~Titanium white is slower drying than lead white and suitable to use here. If lead white was used in the early layers, lead plus titanium or just titanium white can be used. If titanium white is to be used in the early layers, it is advisable to ensure that the first layers are dry before applying the subsequent layers of titanium white or mix a bit of zinc white in to slow down drying of the middle layers. Also note that titanium white is neither cool nor warm and is generally the brightest white.
Wet Into Wet
~Titanium white can also be used to paint wet into wet. If one would like to keep the painting open longer, zinc white, which is the slowest drying white can be mixed into the titanium. Note, however that the zinc is semi transparent and will lessen the covering capacity of the titanium proportionally to the amount of zinc used. Since titanium white can have a chalky quality, mixing it with another white can decrease this (lead white for early layers and zinc white for later layers).
~Zinc white can also be used successfully to paint wet into wet. Notice, however, that compared to titanium white zinc's tinting strength is much weaker. This is an advantage if one wants to use zinc mix subtle hues without knocking down the brightness too far.
Glazing with Oil Paints
When applying transparent or semi-transparent layer(s) the white to use should have the property of semi-transparence.
Example: a palette of transparent pigments such as Prussian Blue, Madder Red, and Hansa Yellow will behave better for glazing when the white used in the glazing palette is semi-transparent.
~ Zinc white is the only semi-transparent white (sometimes listed as semi-opaque) available.
Also, glazing over an area to lighten and lesson color can be done with zinc white or zinc white mixed with a medium. Liquin as the medium will lessen zinc’s slow drying time. Dick Blick* states that unmixed zinc white may be prone to cracking over time. Also remember that zinc can lean to the cool side of the palette with a bluish tint.
* http://juliesusanne.blogspot.com/2010/04/foundation-white-ground-underpainting.html July, 6, 2010
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