1. How Tanning Keeps Cow Hide from Rotting and Makes Brown or Blue-Green Leather
If left alone, cow hide rots and becomes hard like a board when it dries. To prevent this, chemicals are soaked into the hide to make it soft and resistant to decay. This process is called tanning [1]. According to Tokyo Metropolitan Government materials, animal brains, plant barks, roots, and fruit extracts were used in the past. Today, salts of a metal called chrome are the most commonly used [1]. The tannin component found in plant extracts creates brown-toned leather when used for tanning, while chrome produces blue-green leather [1].
Both methods work by binding to the protein fibers (collagen) in the hide, which helps make it resistant to decay. However, the way they bind differs by type. Wikipedia notes that there are differing explanations for the exact mechanism of how chrome binds [3]. Looking at just the color might make it seem like a matter of personal preference. But when leather is placed in hot water, the difference between the two types becomes clear.
2. Comparing Vegetable and Chrome Leather by Shrinkage Temperature in Hot Water
One way to measure the quality of tanning is by checking the shrinkage temperature. Small pieces of tanned leather are placed in water, and the temperature is raised slowly. At a certain point, the leather suddenly begins to shrink. This is the shrinkage temperature. It is not the melting point, but the temperature at which shrinking begins [1]. The Tokyo Metropolitan Government table lists chrome-tanned leather as having a shrinkage temperature of 90°C (about 194°F) or higher, while vegetable-tanned leather is around 80°C (about 176°F) [1]. These are rough guides; the actual numbers change depending on the raw material, tanning conditions, and measurement method.
Leather that resists heat better is thought to hold its shape well when exposed to steam or an iron. If this were the only factor, chrome tanning would seem like the winner. Yet, vegetable tanning is still produced today. Why?
3. Why Slow Vegetable Tanning Is Still Used Today
Vegetable tanning takes time. Traditionally, hides stretched on wooden frames are soaked for weeks in tanks where the concentration of tannins is raised gradually [3]. A newspaper article about a new brand in Himeji explains that using traditional methods to tan "nume-gawa" (natural vegetable-tanned leather) takes nearly one month, resulting in leather that holds its shape well and feels sturdy [4]. This is an example from one company; not all vegetable tanning takes exactly one month.
In contrast, Tokyo Metropolitan Government materials summarize chrome tanning as having a short, easy-to-manage process, being cheaper, and offering high heat resistance and strength [1]. Its practical use advanced in the late 19th century. A key moment was in 1893, when US-based Denis developed a "one-bath" method that was easier to use on-site [2]. As a result, chrome tanning came to dominate much of the leather industry. The proportion varies by source, but it is often stated that about 80–90% of the world’s leather is chrome-tanned [3].
The difference between days and weeks can affect price and production volume. Vegetable tanning survives because the leather made over time has a unique character that appears as it ages.
[SAFETY NOTE: The older two-bath method involved risks, but the one-bath method made it easier to handle [2].]
4. How Vegetable-Tanned Leather (Nume-gawa) Changes as You Use It
Natural vegetable-tanned cow hide is called "nume-gawa." Its feature is a natural texture without dye or paint, and it turns an amber color as it is used [5]. It is strong but is said to be weak against water [5]. This color change is called "aging" or "patina." The Himeji newspaper article also lists enjoying this aging process as a charm, alongside its shape retention [4].
On the other hand, chrome-tanned leather is used in many products because it is heat-resistant and quick to produce [1]. Some chrome-tanned leathers retain a natural texture, while others are dyed. The choice depends on how the product will be used.
One can say that nume-gawa is "leather whose expression changes with time," while chrome-tanned leather is "leather that resists heat and is quick to make." Neither is better; they simply have different characteristics. Can these characteristics be combined in one piece of leather?
5. Composite Tanning, Which Combines Tanning Agents, and Efforts Regarding Chrome
There is also a method called composite tanning, which combines two or more tanning agents. This is likely done to incorporate the heat resistance of chrome and the character of vegetable tannins as needed.
There is also a history of efforts regarding chrome. The trivalent chromium used in tanning has much lower toxicity than hexavalent chromium, which is highly toxic. However, Tokyo Metropolitan Government materials state that hexavalent chromium can be produced if sludge containing chromium is burned in an oxygen-rich environment [1]. These materials also explain that interest in "low-chrome" and "chrome-free" tanning has grown because regulations now target wastewater containing trivalent chromium, leading to efforts to reduce usage amounts [1].
6. How to Tell Vegetable-Tanned and Chrome-Tanned Leather Apart at Home
You can compare items at home. Ask family members to show you leather goods like shoes, bags, or wallets. Try sorting them by texture and color: those with uniform, even colors versus those where wood grain or patterns are visible. Keep in mind that visual appearance alone does not always reveal the tanning method. If you find a product labeled "nume-gawa," ask the owner what color it was when new compared to now.
You can also look in stores. Check tags for labels like "nume-gawa" or "chrome-tanned." Some products have no label, and labels alone do not always reveal the tanning method. Asking a store clerk is a good way to learn. Please do not pour water on items or scratch them with fingernails.
If you want to read more, the public relations magazine *Kawa to Kimono* (Leather and Clothing) from the Tokyo Metropolitan Leather Technology Center summarizes leather types and tanning agent differences in Japanese [1].