Whether in the manufacture of extracts, or for direct use in the tannery, the temperature at which tanning materials are extracted is of prime importance. It is a common mistake to assume that the largest amount of tannin is extracted by boiling. Mr. A. N. Palmer has pointed out that this is by no means the case, but that each material has an _optimum_ temperature of extraction, at which more tannin is extracted than at any other; and the question has been carefully investigated by J. G. Parker and the author,[159] with results which are given in the following tables. For many purposes the colouring matter which accompanies the tannin is a serious disadvantage, and it is usually most extracted at the higher temperatures; and on this account it is necessary for the tanner who will work his leaches economically to ascertain at what temperature he can extract the largest amount of tannin combined with no more colouring matter than he can permit to enter his leather. Most materials are satisfactorily extracted at 50°-60° C., but as a general rule it is best to begin cold or nearly so, and only raise the temperature as the extraction proceeds. The tables show the percentages of tanning matter, and the amount of colour (as measured by Lovibond’s tintometer), obtained by extracting materials in a Procter’s extractor (p. 306 and L.I.L.B., p. 102) so long as any colour or tannin could be obtained. [159] Journ. Soc. Ch. Ind., 1895, 635. BELGIAN OAK BARK. -----------+---------+-----------+---------+---------------+--------- Temperature| Tanning | Soluble |Per cent.| Colour of ¹⁄₂ |Per cent. of | Matters |Non-tanning|of Tannin| per cent. |of Colour Extraction.|absorbed | Matters. | on | Solution in | on | by Hide.| | Maximum |¹⁄₂ inch Cell. | Maximum | | | Yield. +-------+-------+ Yield. | | | | Red. |Yellow.| -----------+---------+-----------+---------+-------+-------+--------- °C. |per cent.| per cent. | | deg. | deg. | 15 | 5·9 | 5·1 | 61·9 | 8·6 | 23·1 | 57·4 15-30 | 6·8 | 5·5 | 70·7 | 9·2 | 26·4 | 64·5 30-40 | 8·0 | 5·5 | 83·5 | 11·6 | 30·4 | 76·1 40-50 | 8·2 | 5·7 | 84·2 | 12·0 | 32·1 | 80·0 50-60 | 8·5 | 5·8 | 87·6 | 12·5 | 36·0 | 84·0 60-70 | 9·1 | 5·9 | 95·5 | 13·1 | 38·1 | 92·7 70-80 | 9·2 | 6·0 | 95·7 | 14·7 | 38·9 | 98·7 80-90 | 9·6 | 6·0 | 100·0 | 14·0 | 36·9 | 93·2 90-100 | 9·6 | 6·1 | 100·0 | 14·0 | 41·2 | 94·6 Boiled | | | | | | ¹⁄₂ hour | 9·1 | 6·6 | 93·7 | 15·0 | 42·6 | 100·0 -----------+---------+-----------+---------+-------+-------+--------- MYROBALANS.