Summary: Reversible darkening in daylight has been detected in three types of artificial material containing rutile: Anatase partly recrystallized by heat-treatment, under ‘dry’ conditions in the presence of iron or manganese, with the production of rutile. Certain iron-containing samples show a strikingly rapid darkening in daylight. This darkening is more pronounced in violet than in red light and is reversed on heating to below 100° C. Anatase completely recrystallized in the presence of molten sodium chloride. The resulting product contains futile and a sodium titanate in varying proportions. Anatase converted into rutile by heating, in the presence of iron, in an atmosphere containing hydrogen chloride.

The photosensitivity is held to depend on the entry of impurities into the rutile crystals. Tentative explanations of it are given and the possibility of a connexion with the known photosensitizing activity of titanium dioxide is stressed.