of tannin can be detected; and the yield (of leather, based on the pelt
employed), which, from a practical standpoint, is so important, is so
very low that it is hardly possible to speak of it as a tannin in the
ordinary sense of the word. Formaldehyde must, therefore, be termed a
pseudo-tannin.

The tanning effect of formaldehyde is, according to Thuau, [Footnote:
_Collegium_, 1909, 363, 211.] increased by those salts which bring
about colloidal polymerisation of the formaldehyde, the resultant
compounds being absorbed by the hide fibre. Fahrion considers this to be
a true tannage, and is supported by Nierenstein [Footnote: _Ibid._,
1905, 157, 159.]:--

R.NH_2 R.NH-|
+O.C.H. = CH_2 + H_2O
R.NH_2 | R.NH-|
(Hide.) H (Leather.)

A peculiar combination between true tannage and pickling is to be found
in the tawing process (tannage with potash, alum, and salt), whereby,
firstly, the salt and the acid character of the alum produce a pickling
effect, and secondly, the alum at the same time is hydrolysed, and its
dissociation components partly adsorbed by the hide, thereby effecting
true tannage. This double effect is still more pronounced in the
synthetic tannins which contain colloidal bodies of pronounced tanning
intensity on the one hand, inorganic and organic salts on the other,
which then act as described above. Their real mode of action can only
be explained with the aid of experimental data. The following chapters
will deal with the different behaviour of the various groups of
synthetic tannins.