1. How Does the "Suntan Pattern" Leather Accessory Get Its Sun-Tanned Look?
nendo’s leather accessory, "Suntan Pattern" is made by applying sun-tanning oil to the entire piece of leather. To create the pattern, they use SPF 50 sunscreen and leather-colored powder. As a result, patterns that were initially invisible emerge after the item is used and exposed to sunlight. Over time, the color fades and the pattern disappears [1]. However, this is not a natural change in the leather; it is a design feature. It is a design that takes advantage of the way leather changes color in sunlight. So, what happens with ordinary leather that has no such design tricks?
2. What Are Tannins, the Plant Substances That Turn Skins into Leather and Give It Color?
Tannins are plant components that turn raw animal skins into leather and make them difficult for microorganisms to break down. They are a group of substances found in plants that bind strongly with proteins in the skin, especially collagen [2]. Tannins also leave color on the leather. Technical documents from the Tokyo Metropolitan Leather Technology Center state that vegetable tanning is a typical non-chrome tanning method, and that tannins leave their color on the leather [3]. In other words, the tanning agent protects the leather and also gives it color. Vegetable tannins are broadly divided into two groups: "hydrolyzable" and "condensed." Condensed tannins overlap to form a brown substance called phlobaphene, which is difficult to dissolve in water [2]. Because the properties differ by tannin type, the way the color changes is also thought to differ. The next section looks at experiments that investigated this.
3. How Does Color Change in Sunlight Differ Between Six Types of Vegetable Tannins?
A research group from Ege University in Turkey and others conducted an experiment in 2008. They tanned cowhide with six types of vegetable tannins and measured color changes by exposing them to artificial xenon lamp light. This discussion is limited to the scope of this experiment. The condensed types were quebracho, mimosa, and gambier. The hydrolyzable types were tara, chestnut, and valonea [4]. The results showed that color changes in hydrolyzable types were significantly smaller than in condensed types [4]. Condensed types gambier and quebracho changed suddenly in the direction of red and yellow. Mimosa changed slowly, and hydrolyzable tara changed even more slowly. In terms of measurements, condensed types showed a tendency for both redness (a) and yellowness (b) to increase. Hydrolyzable types showed a tendency for yellowness (b) to increase [4]. Japanese documents agree with this direction. Technical documents from the Tokyo Metropolitan Leather Technology Center state that mimosa and quebracho are prone to sun tanning, while tara tannin has better light resistance than these [5]. This matches the direction of the experiment.
4. Does Leather's Color Keep Changing the Longer It Is Exposed to Light?
In this experiment, color did not move in only one direction. Color continued to change for up to 24 hours of light exposure, and the magnitude of the change peaked around 24 hours. With longer exposure, the change decreased. The way darkness changed varied slightly by tannin type [4]. The 24 hours mentioned here refers to the time of the xenon lamp test, which is different from 24 hours of daily life. It cannot be said from this experiment alone whether used leather changes in the same order. So, in leather actually used, when and how much is the color change noticeable?
5. Does Leather Change Color Most in the First 2 Years? When Is Leather’s Color Change Most Noticeable?
The Italian Association of Vegetable-Tanned Leather (Consorzio Vera Pelle Italiana Conciata al Vegetale) states that temperature, heat, and light affect color change. Frequency of use, surrounding light, humidity, and care products like wax are also involved [6]. In the same survey, it is stated that color change decreases over time and is mainly noticeable in the first two years of use. It also says that the strong properties of tannin extracts appear over time, shifting the color toward brown tones [7]. Leather dyed brown showed the least change, while undyed natural color leather showed the greatest change [6]. When reading this, note that this association promotes vegetable-tanned leather. The "2 years" is an observation from the survey and does not apply to all leather. However, when viewed alongside differences by tannin type from the 2008 experiment, it is thought that tannin type and usage conditions are involved in how color changes.
6. How Can You Investigate Leather Color Change at Home?
What you can do today: Ask an adult for permission to look at leather products in your home, such as wallets, belts, or shoes. Compare the sides that usually get sunlight with the inside of folds or parts that are usually hidden. Write down the differences in color in words. You cannot tell the type of tannin or whether it is dyed just by looking. It is enough to observe where and how color differences appear. Avoid placing items in strong sunlight to force a test. If you want to know more: If a leather product is labeled "vegetable tanned," you can look up what kind of color change might occur using information from shop staff or official leather technology center documents.