There are always some classic pieces that we find in our life that make us smile every time we think of them. Like that super comfy white leather boots in your shoe cabinet. However, it frets you sometimes to remember that over time, your favorite boots will no longer be as white and shiny, and would gradually become old and yellowish. Now let’s find out what’s behind of the yellowing of white leather—— In 1911 AD Dr. Stiasny has developed a novel synthetic tannin that could replace vegetable tannin. In comparison with vegetable tannin, synthetic tannin is easy to produce, has great tanning property, light color and good penetrability. Thus it has come to occupy an important position in the tanning industry over the hundred years of development. In the modern tanning technology, this type of synthetic tannin is used in almost all the articles. Due to its different structure and application, they are often called synthetic tannin, phenolic tannin, sulfonic tannin, disperse tannin, etc. The commonality of these tannins is that their monomer is usually of phenolic chemical structure.