In this study, we have investigated the effect of lake formation on the light stability of lake pigment, comparing structural and spectroscopic properties of eosin dye and its aluminum lake, i.e., geranium lake pigment. XRD and SEM observation confirmed that they both have crystalline structure, but their particle size is smaller in geranium lake pigment than in eosin. Their UV-VIS reflectance spectra showed slight differences, such as a slight shift of absorption edge wavelength, while no significant difference was observed by FT-IR and Raman spectra. In the light stability test of their oil paint, the color change of geranium lake paint was about four times less than eosin paint. These results imply that, in geranium lake pigment, the bond formation of eosin with aluminum ion causes the modification of its electronic state, which contributes to the improvement of light stability. We have also investigated their discoloration mechanism. FT-IR analysis showed the dissociation of C-Br bonds and the structural modification of xanthene ring in both samples. Their cross-section and micro-FT-IR analyses revealed that a thin layer was formed at the paint surface, and the photodegradation occurred only at this thin top layer.
The main component of oriental lacquer is urushiol and is a kind of catechol. The oriental lacquer is usually cured by using an enzyme called laccase. Thus, it takes a long time for preparing lacquer wares. Rapid curing by UV irradiation was tried for the main component of oriental lacquer, i.e, urushiol in this study. However, the surface gloss of the film decreased. The bromination of urushiol within ten seconds before UV irradiation could prevent the decrease of gloss. Further, the film obtained by this method was never worse than that by the conventional method on the factors of color such as lightness and chroma. This fact suggests the high-speed production of lacquers wares by bromination and UV irradiation.