After using oil paint for well over a decade I am now interested and concerned about the differences in white oil paint and how the various kinds may react and age when used in plein air painting.
Last week I was looking at the surface of one of my plein air paintings done 3 years ago in Alaska. I discovered that small but visible cracks had appeared in the surface of paint in areas where I had applied a thick coat (impasto) of white paint. This discovery led me to research the possible causes of cracking. Several different possibilities exist:
1. Paint was too thick and on the dry side and I should have used a medium mixed with it before applying it impasto.
2. The paint could
have been from a bad batch and had insufficient oil binder mixed in with the
pigment. This can lead to a thin paint
film that is prone to cracking and flaking. Here is a photo of cracked white paint - look in the middle of the image and see the cracks in the brush strokes...
Here is a summary of the different types of White Oil Paint available and generally used by plein air painters and the paint characteristics:
Titanium White: Reflecting 97.5% of all available light, this most opaque white is the perfect choice for direct painting. Monet would have loved it because he wanted his paintings to look soft, like velvet. The covering power of Titanium White is useful for creating opaque layers, but Titanium-Zinc White is preferable for color-mixing.
Titanium Zinc White: Most useful all-purpose oil painting white. An excellent mixing white, T-Z White combines the soft texture and opacity of Titanium with the creamy transparency of Zinc for less "chalky" mixtures. Consider using T-Z White for color-mixing because it takes so much color to tint Titanium.
Zinc White: The most transparent white, Zinc is recommended for glazing, scumbling and alla prima painting. Compared with all other whites, Zinc White has less hiding power. Zinc White dries slowly, so painters who want to paint wet into wet over a long time will find it useful. Because it's brittle, painters should not consider it as a general painting white unless painting on panel.
I now plan to switch from using Titanium White to Titanium-Zinc White - which is a mix of Titanium and Zinc. Titanium has a great tinting strength and Zinc provides a more transparent quality to the white - it sounds like the best of all worlds when it comes to white oil paint. To read an in depth article about the different types of white paint I recommend "Getting the White Right" by Gamblin Color. The folks there really know their paint and are free with expert advice and opinion on any problems we may encounter with paint.