Tannin classification:
According to the properties-
1. Cate-chol:
Extracted from bark, woods.
Example - Di-hydroxy-benzene.
2. Pyrogallol:
Extracted from leaves, fruits, pods.
Example - Tri-hydroxy-benzene.
According to the modern classification-
1. Hydrolysable tannin
2. Condense tannin.
1. Hydrolysable tannin:
Undergo hydrolysis at aqueous solution at elevated temperature (60°c) & splitted up into their different constituent.
Pyrogallol - Ex: Valonia, Chestnut, Galls, Myrobalan etc. (Short key - "VC" & "GM")
Properties:
- Gives yellow to tan-brown color.
- Contain high acidity.
- Contain high sugary matters.
Penetration of tannin is low.
- Tent to hydrolysis & deposit insoluble matter usually Chebulic acid & Chebulinic acid as yellow white patches known as bloom.
2. Condense tannin:
Properties:
- Gives reddish-brown color.
- Contain low acidity.
- Contain sugary matter.
- High penetration.
- Higher weight yield leather.
- Relatively stable on hydrolysis but prone to oxidation when exposed to light & heat.
- On exposure produce dark color leather (shrinkage temperature 85°c).