| Virginia, | mixed, | collected in | June, | gave | A nearly white precipitate. |
| „ | „ | „ | July, | „ | A decidedly yellowish-white precipitate. |
| „ | R. copallina | „ | August, | „ | A dirty-yellow precipitate. |
| „ | R. glabra | „ | „ | „ | A very dirty-white precipitate. |
| Fredericksburg, | mixed | „ | „ | „ | A dirty-yellow precipitate. |
| Sicilian | „ | „ | „ | A slightly yellowish-white precipitate. | |
For the purpose of tanning white and delicately coloured leathers, therefore, the collection should be made in June; while for tanning dark-coloured leathers, and for dyeing and calico-printing in dark colours, where the slightly yellow shade will have no injurious effect, the collection may be made in July. It appears that for all purposes, the sumach collected after the 1st of August is inferior in quality.
Experimental results as regards percentage of tannin obtained by collecting sumach at different seasons showed:—
| Per Cent. of Tannic Acid. | |||||
|---|---|---|---|---|---|
| Virginia, | mixed, | collected in | June, | gave | 22·75 |
| „ | „ | „ | July, | „ | 27·38 |
| „ | R. glabra | „ | August, | „ | 23·56 |
| „ | R. copallina | „ | „ | „ | 16·99 |
| Sicilian, | R. coriaria | „ | „ | „ | 24·27 |
It is evident, therefore, that in order to secure the maximum amount of tannic acid, the sumach should be collected in July, but the colouring matter of the leaves has an important influence upon the value of the product. The leaves of the upper extremities of the stalks are always richer in tannic acid than those of the base; and the increase of age of the plant is accompanied by a general diminution of this acid.
The mill used for grinding sumach leaves consists of a heavy, solid, circular, wooden bed, 15 feet diameter, with a depression around the edge a few inches deep and 1 foot wide, for the reception of the ground sumach from the bed, and two edge-rollers, weighing about 2500 lb. each, 5-6 feet diameter, and provided with numerous teeth of iron or wood, thickly inserted. In Europe and in some parts of the Southern States, sumach is still ground by stones revolving on a stone bed, and the sifting is often done by hand.
Fig. 55.—Venetian Sumach (Rhus cotinus).
R. cotinus, Venetian sumach. Fr. Arbre à perruques; Ger. Perrukenstrauch ([Fig. 55]). More important as a dyeing than as a tanning material, its twigs and wood, “young fustic,” containing a large proportion of a colouring matter (fisetin), which with tin and alumina mordants dyes bright yellows; and much resembles, but is not identical with the myricetin present in R. coriaria.[144] Its leaves, known as Turkish or Venetian sumach, contain about 17 per cent. of tannin, and are used for tanning.
[144] Perkin and Allen, Trans. Chem. Soc. 1896, 1299.