The oxalic acid is produced by bacterial action, as has been shown by Zopf[31] and Banning[32]; but it is difficult to estimate it in the bate, and a reliable analytical method of doing so requires devising.
A further 30 per cent. to 40 per cent. of the lime in the skins is removed by the chemical action of the complex amines of the organic acids, and by the mechanical effect of the paddle or drum, so that the final result is more or less as follows:—
| Per cent. | |
| Lime neutralized by free acids | 9 |
| Lime dissolved by complex amines | 25 |
| Lime precipitated | 30 |
| Lime remaining in the skins | 36 |
| ── | |
| 100 |
The lime remaining in the skins is not in the caustic state, but principally in the form of neutral salts (see [footnote] to p. 24).
Fig. 7.
Some of these neutral salts appear to be absorbed by the skin during the course of the bating, for, on examining pieces of the same skin at frequent intervals during the operation, and determining the ash, it was found that a minimum point for ash content was reached in about ten minutes, after which the ash actually increased. The results are shown in the following table, and also graphically by the curves, Fig. 7—in which A is a “grain” previously washed as free from lime as possible; B, an unwashed “roan.” The effect of the absorption of inorganic matter is very noticeable in both cases. A part of the effect only is apparent since a certain amount of skin substance passed into solution.
Variation of Ash Percentage during Puering.