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I made a white test of zinc white (Beckers "A", here). In one patch I added linseed oil. It has experienced clearly visible yellowing after half a year. Another patch was mixed with Liquin (alkyd medium). It also became more yellow, but decidedly less so. The third patch consisted of zinc white directly out of the tube. It has experienced even less yellowing. The binder is modified soya bean oil. So it seems to be a good oil. I don't know why it isn't used more.

Anyway, this proves that Liquin actually yellows with time, but not as much as linseed oil. Zinc oxide has the advantage that it is entirely photo resistant, which means that the pigment, in itself, always remains entirely white. Titanium pigment, however, undergoes yellowing, regardless of binder. This is why it's nearly always mixed with zinc oxide to lessen yellowing. Interestingly, I also left such patches out in the sunshine. Neither of them has undergone much yellowing, because sunlight bleaches oil. Still, the patch directly out of the tube is whitest.

The conclusion is that one should never mix white with linseed oil if one paints a white area. Use alkyd medium instead, or modified soya bean oil.

Mats Winther
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