Chris Hogg wrote:

Traditionally the main white pigment in paint was white lead

See http://tinyurl.com/opgrn92

"White lead mixes well with linseed oil - with which it has what is
sometimes described as a natural affinity-other oils, and turpentine.
It is used both as a white paint and as a base for mixed paints, for
interior or exterior application. For certain purposes, such as, for
example, on out*side woodwork, it has few equals and no superior."

I dragged an old (20 year?) tin of white undercoat out of the shed a
couple of days ago, there was a deep layer of what smelt like linseed
oil on top, and the solids were beige and lumpy like curdled milk, it
mixed back together pretty well and became white, no problems at all
using it ...