The above solutions were made up and allowed to act upon pelt pieces weighing 15 gm.; whereas Solution I. remained clear throughout the experiment, Solution II. became somewhat clouded, and Solution III. assumed a milky appearance. The pelts were tanned through in seven days and yielded leathers which, after drying and finishing, possessed yellow colour, long fibre, and good tensile strength, but a rather empty feel.
To prevent separation of insoluble matter during tannage, another experiment was carried out, in which the pelts were first submitted to the action of formaldehyde (10, 20, and 40 gm. in 500 c.c. water) for three days, being subsequently removed to fresh solutions of partly neutralised phenolsulphonic acid (cf. above). Similar results were obtained, but the leather felt even more empty than those obtained by the former experiment.
Attempts at converting pelt into leather by first immersing the pelt in a partly neutralised solution of phenolsulphonic acid, and subsequently transferring it to fresh solutions of formaldehyde, gave merely negative results; the phenolsulphonic acid effected pickling action upon the pelt, but was subsequently quickly replaced by the formaldehyde, before the latter had penetrated the pelt in sufficient quantity to induce condensation, thereby exerting tanning action.
To explain the tanning effects of these mixtures, the author analysed the leathers resulting from the effects of the latter, and was able to show, that in these cases also, condensation of phenolsulphonic acid and formaldehyde takes place inside the pelt, since on the one hand the analyses left no doubt but that true tannage had been effected, and on the other hand an ammoniacal extract of the leathers gave the typical reaction for condensation products of phenolsulphonic acid, with aniline hydrochloride. [Footnote: Collegium 1913, 516, 142.]
The leather analyses gave the following figures:—
Moisture - - - 18.30 per cent.
Fats - - - - 0.47 "
_ Ash - - - - 0.98 "
Leather { Tannin - - - 26.37 "
substance { Hide substance - - 53.88 "
A characteristic feature is the low value of tannin, which is considerably higher [Footnote: Ibid., 1913, 521, 478.] where condensation products of phenolsulphonic acids are used as tanning agents; the action effected by the separate constituents, therefore, is more that of pickling.
2. Mixture of Phenolsulphonic Acid and Natural Tannins
A piece of pelt was immersed in a half-neutralised solution, measuring 6° Bé., of phenolsulphonic acid, and left sixteen hours in the solution, which completely penetrated the pelt during this time; it was then transferred to a 12° Bé. solution of a mixture of quebracho and chestnut, which in two days converted the pelt into a light coloured leather possessing good tensile strength.
By using a bath composed of half-neutralised phenolsulphonic acid and quebracho extract in 7° Bé. solution, another piece of pelt was completely tanned in two days. The same result was obtained by first half neutralising the phenolsulphonic acid and then adding sulphited quebracho extract till a 5° Bé. solution was obtained.