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Chestnut tannin classification
Astringent and sweetened extracts
Astringency is the main characteristic which determines the tanning properties of different types of extracts. The astringency depends upon a number of factors which can be varied. It is evident how it is possible to obtain different chestnut extracts with a lower degree of astringency and, consequently, with a different tanning behaviour.
Chestnut extracts can be classified in two groups:
Astringent chestnut extracts (Chestnut N - Chestnut N2) present a marked astringency, a balanced tannin non-tannin ratio, a remarkable capacity to combine with hide fibres ensuring a particularly tight and elastic grain. They also have excellent light fastness because hydrolysable tannins are not subject to oxidation.
Sweetened chestnut extracts (Dulcotan RN - Dulcotan Special), with a higher pH compared to the other ones, are less astringent and therefore penetrate more easily and rapidly. Although the sweetened extracts for some characteristics are completely different from the astringent types, they have the same light fastness and capacity to become fixed to the hide, while producing a more mildly tanned leather.
Astringent and sweetened chestnut extracts can be blended either by themselves or with cathecol type extracts, phenolic syntans, naphthalene and phenol-naphthalene syntans.
The composition of these blends must be made not only in relation to the characteristics required in the finished leather, but also to the tanning system used. This is especially important for tanning sole leathers.