A similar effect, though of opposite nature from a tanning standpoint, is exhibited by sulphonates on certain colloidal dark coloured substances. A phenolsulphonic acid, which had been overheated during sulphonation and subsequently condensed (crude Neradol), imparted a conspicuous greyish-brown colour to the leather; samples of this crude product were then partly neutralised with varying amounts of alkali, and these samples (containing increasing quantities of salts) tested for tannin and colour effects. It was found that the more highly neutralised samples imparted a darker colour to the solutions, but these dark products did not deposit the dark impurities on the pelt. One may therefore assume that tannoid substances are colloidally suspended, and when converted into true solutions are incapable of being fixed in insoluble form by the pelt.
Just as, by adding Neradol D to a tanning extract, the phlobaphenes are solubilised and a dark coloured extract results, it is also possible to remove the mechanically deposited phlobaphenes and oxidised tannins from the finished leather, and, as a consequence, lighten the colour of the leather. For practical purposes, bleaching with Neradol D is carried out by brushing over the darkly coloured leather with a 2°-3° Bé. solution of Neradol D, and then rinsing well with water, in order to remove the solubilised tannin. A lighter colour may also be obtained by immersing the leather in a liquor of the strength mentioned above for several hours, and then rinsing with water, but by this procedure not only the surface tannin is removed, but also tannin from the leather substance itself; this method is therefore not suitable for heavy leathers which are sold by weight.
The advantage of employing Neradol D as a bleach in this way is to be found in the fact that, on the one hand, the bleaching sulphonic acid attacks the leather to a much slighter extent than is the case with inorganic acids usually employed for this purpose; on the other hand, the method of brushing the sulphonic acid on the leather only introduces small amounts of sulphonic acid in the leather, thus lessening the harmful effects of acids upon leather. Furthermore, the common methods of using alkalies as tannin-solubilising agents with the consequent running off and waste of alkaline tan liquors are here substituted by a method leaving liquors rich in tannin and Neradol, and which may be used in the ordinary procedure of tannage.
Since Neradol D contains neutral sodium sulphate (about 3 per cent.), and the latter, by precipitating colouring matters present in tan liquors, may slightly bleach these, it was of interest to determine whether the sodium sulphate plays any part in the bleaching effected by Neradol. Mixtures of chestnut and quebracho extracts were prepared, to which were added:—
(1) 5 per cent. Neradol D. (2) 5 per cent. Neradol D. free from Na_2SO_4. (3) ° 15 Per cent. sodium sulphate (corresponding to above Neradol D).
These mixtures were allowed to act upon pelt alongside of comparison tests using quebracho and chestnut extracts only, the strength of the liquors in all cases being 1.5° Bé; the pelt was left in the solution till tanned through. The following results were obtained:—
(1) Quebracho tanned leather was darker; no difference in colour by chestnut extract. (2) Similar to (I). (3) Same colour as given by the original extracts.
This experiment demonstrates that absence of sodium sulphate in the mixture is without influence on the colour of the resulting leather, and that an addition of sodium sulphate to natural extracts does not affect the colour imparted by them to pelt
5. Effect of Neradol D on Pelt
Being a sulphonic acid derivative, the chemical constitution of Neradol is obviously considerably different from that of the natural tannins, and the question has been asked: Will Neradol D, in its concentrated form, attack the hide substance?[Footnote 1: Collegium, 1913, 521, 487.] Bearing in mind that concentrated extracts of vegetable tannins in some circumstances effect a "dead" tannage (cf. case-hardening) and hence reduce their practical value, and that for this reason it is impossible to allow either concentrated extracts or concentrated Neradol D to act upon pelt, the author still decided to carry out some experiments in this direction. Concentrated Neradol D (33° Bé.) and strong aqueous solutions of this material in strengths of—