An unusual white that is not often found as a single-pigment paint but may be lurking in other paints as an undisclosed ingredient. Described as "An insoluble mixture of barium sulfate and zinc sulfide" -Artiscreation. This has zinc in the name but it's not the same zinc white that causes cracking, which is called zinc oxide.
Lithopone is a slow drying color. It’s a gentler white than titanium and is sometimes mentioned in the more obscure technical discussions on paints. In a world where PW4 is on everyone's radar, we've been looking for white pigments that will yield that extra bit of cool blueness in cyan tints. Lithopone had been suggested as a potential route, as it's not as opaque as titanium and is somewhat similar to zinc white, however we're awaiting more information about lithoopne’s lightfastness over time.
A group of pigments fared a bit worse in terms of lightfastness in a lithopone-safflower blend (see Golden's lightfastness testing). The binding oil does matter, so bear in mind this was in safflower oil. Their findings stated, "...In fact, there were a few instances when mixes with [lithopone] Porcelain White were less lightfast than corresponding mixes with other safflower whites. The existence of this group—composed of Permanent Green Light (PG7/PY3), Ultramarine Blue (PB29), Permanent Yellow Light (PY3), Egyptian Violet (PV23), and Fanchon Red (PR112)—suggests that lithopone (in safflower oil) may reduce the lightfastness of certain pigments. It’s also notable that Green Gold (PY129) faded about twice as much with Porcelain White as with Titanium White Safflower, though both tints exhibited relatively low lightfastness."
Interestingly, The Artist's Handbook mentions a certain yellow-greenness in oils, which is something we had not heard mentioned anywhere else but makes some sense based on experience. It also mentions that this pigment is sometimes used to "bulk out" other white paints as it is not very expensive. That could be interesting or important when it comes to the lightfastness of a given mixing white, especially if the lithopone is not stated on the label.
While pure versions are not soluble some grades may contain toxic impurities. This is mentioned in Monona Rossol's recent work, which can be found through her site. Additional information on zinc and barium pigments can be found in her book, The Artist's Guide to Health and Safety.
