TITANIUM WHITE is an opaque non-yellowing white. Most brands contain some zinc white to aid in mixing (even if the label doesn't tell us that). Pure titanium white is chalky and hard to mix. Pure zinc white is a little too transparent for most uses.
I avoid using Lead White, Flake White or Cremnitz White, all of which contain toxic lead. But maybe now I'll have to rethink this.
The following material (in blue below) about Zinc White oil paint has been excerpted from an interview with Edward Hopper by John Morse, June 17, 1959:
"JOHN MORSE: Then back to your own techniques, you mentioned that only one painting that you know of of yours has had to have some attention, Nighthawks in Chicago. What was the occasion there?
Edward Hopper's "Nighthawks"
EDWARD HOPPER: Well, I think it was because in order to get a greater whiteness and brilliancy, I had used zinc white in a certain area of the picture. I think that had cracked or scaled, whereas the parts where I had used lead white did not. This is my remembrance of it.
JOHN MORSE: Was that due, do you think, to an inferior quality of the zinc white or to the nature of the materials?
EDWARD HOPPER: No, I don't think so. I think that zinc white has a property of scaling and cracking. I know in the painting of houses on the exterior, zinc white is apt to crack and scale, whereas lead white merely powders off.
JOHN MORSE: I see.
EDWARD HOPPER: So I think that the same would be true in pictures.
JOHN MORSE: And since that experience you have avoided zinc white?
EDWARD HOPPER: Yes, I use only lead white now."
Edward Hopper's "Pennsylvania Coal Town," 1947
Then I got an email from Hylla Evans (who really knows her paint chemistry) and have posted her remarks in green below:
"The problems with zinc white have been known for a long time. You know I like lead/flake white for oils and don't get it on my skin (or wash it quickly if I do).
The article casually mentions the interaction of the zinc oil paint with acrylic (presumably underpainted). This screams for the same attention to be published about reactivity of other oil paints with acrylic emulsions.
Here are my over simplified and obviously personal thoughts:
All oil paintings crack; it's just a matter of when - not if.
Painting on a rigid substrate will make for less humidity-related cracking than comes from expansion and contraction of canvas.
Stick to materials and practices that have been time tested. Meaning don't include acrylic in any aspect of your oil painting. It might be okay and it might not - but why have your art be the experiment?
Fat over lean; thick over thin, etc.
Linseed oil is good but will yellow over time with most whites (titanium especially). It's the oil that's yellowing. Put a yellowed painting in sunlight for a day or two and the yellow will disappear.
Except for the above, keep all oil paintings out of sunlight - direct or indirect - to extend the life of the work at its best.
That said, and other pigments not addressed, museums are best able to control environments for art.
True though that zinc is not a good paint ingredient and really never was.
Also, most drying agents that come in and out of fashion put paintings at high risk. As convenient as they are, they are a serious compromise compared to allowing drying time and natural resins that have been successfully used for hundreds of years.
I'm afraid we are all taking chances with alkyd and other modern resins."