The Stiasny number reaction was used to determine the reactivity
of tannins towards formaldehyde. According to Yazaki and
Collins (1994) about 0.2 g (oven-dry mass) of tannins sample,
5ml of 37% aqueous formaldehyde and 5ml of 10M hydrochloric
acid solution (HCl) were mixed together and heated under
reflux for 30 min. At the end of this reaction, the mixture was filtered
through a sintered glass filter (filter no. 3) while it was still
hot. The precipitate was dried to a constant weight in an oven at
105 ◦C. The Stiasny number was determined as a percentage of
the weight of the precipitate to the weight of the starting sample.