468 JOURNAL OF COSMETIC SCIENCE potassium) to make them more soluble in water. However, the insoluble pigments ren- dered after the addition of lead oxide or aluminum oxide to xanthene dye are not used as cosmetic pigments in the United States and Korea. Although salt forms of fluoresceins exist mainly in quinoid form, lactoid structures are still reported in the literature. How- ever, fluoresceins (e.g., fluorescein, tetrabromofluorescein, and tetrachlorotetabromofluo- rescein) may exist in three tautomeric forms: lactoid, quinoid, and zwitterion (Figure 21). LAKE PIGMENTS There is no current agreement regarding a universal definition of lake pigments. It is liable to cause confusion with other pigments toners and true pigments. In the organic pigments, there are three types of pigments: lake, toner, and true pigment (106). How- ever, as these pigments are mostly used with a diluent, the term “lake” is often used in some countries or industries for all three pigments (106,107). Historically, although the term “lake” refers to dyes adsorbed on the substratum, the current usage of this term may refer to the insoluble metal salt pigments (34). Numerous definitions exist for lake pigments depending on their industrial use. The definition of a lake pigment in the cosmetic industry is a water-insoluble pigment com- posed of a water-soluble straight color strongly adsorbed onto an insoluble substratum by a precipitant (108). The straight color is adsorbed onto the substratum by means of various interactions including ionic bonds, hydrogen bonds, and van der Waals forces (108). The overall process of lake pigment formation consists of three steps: substratum Figure 21. Salt forms of xanthene dyes.

469 COSMETIC COLORATION: A REVIEW formation, dye adsorption, and dye precipitation. In more detail, a lake pigment is pre- pared by adding water-soluble dye into an aqueous slurry of inorganic substratum then, it is precipitated with a metallic salt to make a lake pigment (Figure 22). In the United States, lake pigments are named using a combination of the name of a dye and the cation of the precipitant. For instance, FD&C Yellow No. 5, which is pre- cipitated with aluminum chloride, can be identified as FD&C Yellow No. 5 Aluminum Lake without the knowledge of what kind of substratum was used. From the name of the lake pigments, it is difficult to recognize what substratum was used. Previously, an alumina substratum was the only substratum used, so there was no need to put it into the nomenclature. Nowadays, substrata other than alumina–barium sulfate, and calcium carbonate–are used widely. Even though the substratum comprises of 56–89% of the lake pigment, it is difficult to notice which substratum was used with the current nomencla- ture (108). Although United States and China list lake pigments on product labels, the EU and Korea do not distinguish lake pigments from pure dyes on labels. Toners are pigments made by replacing of sodium metals with metals of higher valence, such as barium and calcium, without using a substratum to achieve the desired high insolubility (104). A toner is distinguished from a lake pigment by the absence of a sub- stratum. When a dye combines with higher valence metals to become a toner, it renders increased lightfastness, higher color strength, and modified shade (109). For example, a calcium cation substituted for the sodium ion in the pigments Deep Maroon (CI 15880, D&C Red No. 34) and Lithol Rubine BCA (CI 15850, D&C Red No. 7) are the most representative toner pigments. True pigments are insoluble (water, oils, and other solvents) compounds that contain no salt-forming group (e.g., salt, acid, or lactone group) to promote their solubility (106). They have no metal ions or substrata. These pigments are known as the most stable organic pigments, although their existence is relatively rare (19,106). Helidone Pink CN (CI 73360, D&C Red No. 30) and Permaton Red (CI 12085, D&C Red No. 36) are true pigments. Any certified colorants (toners or true pigments) mixed with a diluent are considered lake pigments in the United States (10,106,107). For instance, Japan does not permit lake pigment D&C Red No. 30 (CI 73360) to be used in cosmetics, but in the case of D&C Red No. 30 (CI 73360) being mixed with a diluent, it is regarded as a lake pigment in the United States, but a mixture produced by combining a true pigment and a diluent in Japan. Certain pigments like toners and true pigments can have different labels compared to those of the United States, based on its own regulations. In summary, the use of a different type of metal salt results in different shades and solu- bility from pure dye colorants. It is essential to have a distinct nomenclature for colorants in which different cations are combined with pure dye. Moreover, a universal definition Figure 22. Preparation of lake pigments.