Leather 101
leather from earliest times. It has been suggested that these were first used to
impart colour to oil-tanned or pseudo-leathers. Indeed, until the development
of what were known as “chemical tannages” in the nineteenth century, the verb
“to tan” was often reserved for the process of vegetable-tanning. It is now known
that these aqueous extracts contain many different organic compounds, and that
those that react with the collagen are polyphenolic in nature. These are termed
tannins and have molecular weights between 500 and 3000. Polyphenols with
smaller molecular weights lack tanning power and those with larger are unable
to penetrate into the skin’s fibre structure.
The vegetable tannins can be divided into two main classes, i.e.hydrolysable
and condensed. The hydrolysable tannins are obtained commercially from, for
example, sumac leaves, tara pods, myrabolam fruits, Turkish or Chinese galls or
oak bark. Examples of condensedtannins are those from quebracho wood,
mimosa bark and gambier leaves and twigs.
Hydrolysable tannins are more or less complex gallic acid esters of glucose
(Figure 3) or compounds related to these based on ellagic acid (Figure 4) or
chebulic acid (Figure 5). Their name derives from the fact that in solution they
are readily hydrolysed by heat, acids or microbiological action to precipitate out
the esterifying acids.
O
GOGGO
GO
GGOGOGHOHOHOCOG =HOHOCOO O
COH
OHG G=HOFigure 3The major component of Chinese gall extract (tannic acid)