Whereas the colour of leathers tanned with Neradol D only is nearly a pure white, those tanned with mixtures of Neradol D and vegetable tanning materials are more or less light coloured according to the quantity of Neradol D present, as has been explained when discussing the phlobaphene-solubilising action of Neradol D. In any case, all leathers tanned with Neradol D possess fibre of remarkable length, which explains their increased tensile strength and elasticity. The tensile strength of a leather tanned with a mixture of Neradol D and vegetable tannins was 3.7 per cent, as compared to 3 per cent when no Neradol was used; the extension was 56 per cent, when tanning with Neradol D as against 36 per cent, without the latter.
The sensitiveness to light of leathers tanned with Neradol D may be mentioned. Exposed to direct sunlight, the surface of the leather assumes a yellowish colour after two days' exposure, and assumes a pure yellow colour after a further three days. A further fifteen days' exposure only darkens the leather slightly, the final colour being very little different from the one obtaining after five days' exposure.
In passing, it may be remarked that this yellow colour is observed on the surface only, the grain otherwise possessing that pure white colour characteristic of Neradol D tanned leather. Further, it may be noted that leathers tanned—with Neradol D fix basic coal-tar dyes excellently, whereas acid and substantive dyestuffs are fixed with other than their natural shades.
The author has analysed a leather exclusively tanned with Neradol D, and has obtained the following results:—[Footnote: Collegium, 1913, 521, 478.]
Moisture - - - - - 15.53 per cent.
Ash - - - - - - 0.93 per cent.
Fats- - - - - - 1.26 per cent.
Extraneous matters - - - 0.00 per cent.
Leather Substance |Tanning matters- 36.92 per cent.
Leather Substance |Hide substance - 45.36 per cent.
———————-
100.00 per cent.
[Footnote: Sp. gr., 0.642.]
From these figures, those of "degree of tannage" and "yield" (pelt—>leather) are calculated as 81.4 and 220 respectively.
These figures correspond closely to those obtained by the analysis of leathers tanned with vegetable tanning materials, and this proves the similarity between the Neradol D tannage and a vegetable tannage in their chemical aspects.
11. Neradol D Free From Sulphuric Acid
In order to prepare phenol and cresulphonic acids, such quantities of technical sulphuric acid are used as do not allow of the assumption of complete utilisation of the sulphuric acid; hence it was of theoretical interest to remove eventual traces of free sulphuric acid from the product. For this purpose, the author diluted crude Neradol to 20° Bé. and gradually added small quantities of milk of lime; the precipitates were freed from the liquid by suction and washing, and a Neradol free from sulphuric acid resulted, which was then brought to the acidity of Neradol D with the calculated amount of alkali. From the calcium sulphate precipitate, the amount of sulphuric acid originally present was calculated, and was found to be only 4 per cent.
The acid-free sample of Neradol was tested with regard to its suitability as a tanning agent; leather tanned with this sample differed from one tanned with an untreated sample (Neradol D) by being harder and possessing a pronouncedly greyish colour. This difference, however, may not be due to the absence of sulphuric acid but to the presence of the slightly soluble calcium sulphate in the sample treated with milk of lime. To prove this point, another way of preparing Neradol D free from sulphuric acid was looked out for. Sodium acetate was added to a solution of crude Neradol until the latter was no longer acid to congo-red; at this point no free sulphuric acid can be present in the solution. The product, partly neutralised till the acidity of Neradol D was reached (part of the acidity then being due to liberated acetic acid), yielded a leather which neither in colour nor in feel differed from the usual Neradol D tanned leather. This proves that the grey colour and the hardness of the leather described in the former experiment is due to the presence of calcium sulphate.