In a painting, cracking can easily occur when the deep layers and middle layers painted first dry more slowly than the superficial layers painted last. The paints containing a high amount of oil dry more slowly than paints that contain less oil. While painting the final layers, continuing to adhere to the “fat over lean” principle, the painter can help prevent cracking paint later.
Fat over Lean, What Does it Mean?
Whether the artist is scumbling, adding highlights or glazing these most superficial (or top) layers, the fattest paint is recommended: that is the paint with the most oil content (compared to the previously painted, deeper underlying layers). Fat or oil content is relative to the layers in that particular painting. Pure White Paint in Final Layers
Zinc white dries more slowly than titanium white which dries more slowly than lead white. Zinc white, as the slowest drying white paint, is theoretically best suited for final layers. Although I have not tested this myself, zinc is thought to be unstable as a stand alone paint.* If pure white is needed, I add a bit of another white, such as titanium white to stabilize the zinc. Titanium does make the zinc less transparent and more opaque. If I am glazing and want to maintain or stretch zinc’s transparency, I mix a glaze medium containing turpentine, stand oil, and damar varnish. I have chosen this glaze because it dries much more slowly than a glaze medium such as Windsor Newton Original Liquin. And I want this last layer to dry the most slowly of all.
Zinc White with Other Pigments
In my opinion, when zinc white is mixed with other paint colors it can be used freely in the last layers of an oil painting. I believe it becomes more stable as the mixture incorporates the properties of the added paint. Another note on using zinc white as opposed to titanium or lead: Zinc affects the chroma of the paint it is mixed with more subtly compared to lead or titanium. This is illustrated in the photo above in which three different whites are mixed in equal proportions with Gamblin Prussian Blue, a semi-transparent paint. The zinc mixture on the far right has knocked the chroma down to a lesser extent than the titanium in the middle and the lead or flake white on the far left. I have also found that the use of zinc, a semi-transparent paint, in a mixture on final layers allows a glow that I have more difficulty achieving with titanium. Titanium, an opaque paint, can leave a flat chalky quality in some mixtures. That being said, titanium's covering capacity is superior.
No white paint is the perfect choice in all situations. By understanding a pigment's properties the painter can elicit desired effects more readily. For more about the properties of white paint, click here: Lead, Titanium, Zinc Properties. A white paint can be used in its pure form or can be combined with another white in an infinite number of ratios. The artist has quite a varied toolbox of whites that can be used in multiple situations depending on the artist's needs.
*http://www.dickblick.com/items/01571-1033/#colorpigments, 9/21/10
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