Leather consists of protein collagen fibres and the dyeing properties of leather are influenced by tanning agents such as tannin and chrome. Surface dyeing of chrome leather has traditionally been performed with acid dyes. Currently, however, penetration dyeing is employed prior to surface dyeing to improve colour fastness, thereby resulting in a complicated process. In this paper, we investigated the dyeability of chrome leather using a direct dye, curcumin S, on the basis of K/S value–λ curve and microscopic photographs of the dyed leathers. Consequently, we have found that curcumin S is an anionic surface-dyeing-type dye with a long molecular structure and that it forms coordinate bonds with chromium metal ions, which are present in large quantities on the leather surface. Therefore, curcumin S behaves like an acid dye during the initial and middle stages of dyeing and therefore exhibits excellent dyeing properties in a dyeing bath with a low pH. In addition, when the bonds of the pigment on the surface and chrome are saturated, ionic bonds between the sulfonic acid anion in the dye structure and the amino cation in leather collagen gradually develop over a long period of time.

In the practical dyeing of chrome leather in solutions with various curcumin S. concentrations, the surface colour of dyed chrome leather tends to become more reddish as the dye concentration is increased. However, the K/S value of the absorption peak wavelength (420 nm) changed only slightly. We also measured the colour fastness, of the dyed leather, to light, washing (dry and wet), perspiration and rubbing. The evaluations showed fairly good results, with ratings of 4–5, except in cases of exposure to alkaline sweat or rubbing, where the colour fastness decreased slightly.

Changes in the physical properties of underwear by repeated washing were examined using three types of knitted underwear fabrics from wool yarn, polyester-covered wool yarn and polyester yarn. Test results indicated the remarkable changes in the physical properties of wool underwear. Underwear from polyester-covered wool and polyester fiber yarn underwent little influence by washing. Moreover polyester-covered wool underwear was similar to wool underwear before washing. Wool yarn covered with polyester yarn resulted in similar physical property of the wool before washing. It was suggested that polyester-covered wool retains its good touch as wool underwear while suppressing the change of physical properties.