Fig. 54.—American Sumach (Rhus glabra).
R. glabra, Southern States, U.S.A. ([Fig. 54]). Very largely used in the States to take the place of Sicilian sumach. A sample collected by the late Professor Trimble, and analysed in the Leather Industries Laboratory, contained 25 per cent. of tannin and produced a leather of very much darker colour than Sicilian.
R. typhina, “staghorn” or Virginian sumach, contains 10-18 per cent. of tannin. A sample from same source as above contained 13 per cent.
R. cotonoides, U.S.A. The analysis of a sample of this material gave 21 per cent. of tanning matter, and leather tanned with it was almost equal in colour to that from R. glabra.
Other sorts found in States: R. semialata (5 per cent. tannin); R. aromatica (13 per cent. tannin); R. metopium (8 per cent.); R. copallina, R. pumila, R. canadensis; R. toxicodendron is the well-known “poison ivy,” a climbing plant which causes a severe and irritating eruption if touched.
R. glabra and R. copallina are chiefly recommended for extended cultivation in the United States.
In Virginia, the leaves are collected and cured by the country people, and sold and delivered to owners of mills for grinding. Their particular object being to secure the largest possible quantity of product at the lowest cost, little attention is given to the quality obtained, or the manner of collecting. The most intelligent dealers in the raw material urge upon collectors to observe the following particulars:—To ensure a maximum value for tanning purposes, the leaf should be taken when full of sap, before it has turned red, has begun to wither, or has been affected by frost. Either the leaf-bearing stems may be stripped off, or the entire stalk may be cut away, and the leaves upon it allowed to wither before being carried to the drying shed; but care must be observed that they are neither scorched nor bleached by the sun. When wilted, they are carried to a covered place, and spread upon open shelving or racks to dry, avoiding the deposit in any one place of a quantity so great as to endanger the quality of the product by overheating and fermentation. Sumach should be allowed to remain in the drying-house for at least one month before sending to the market; in case of bad weather, a longer period may be required. When ready for packing for shipment, it should be perfectly dry and very brittle, otherwise it is likely to suffer injury in warehouses from heating and fermentation.
Buyers of sumach leaves for grinding depend largely upon colour for the determination of the value; the leaves should, therefore, when ready for market, present a bright-green colour, which is evidence that they have suffered neither from rain after being gathered, nor from heating during the process of drying. Leaves having a mouldy odour or appearance are rejected. The Virginian crop reaches 7000-8000 tons, and is collected at any time between July 1st and the appearance of frost.
There is an important difference in the value of the European and American products. The proportion of tannic acid in the latter is generally lower than that found in the former, which is much preferred by tanners and dyers. By using Sicilian sumach it is possible to make the finest white leathers, while by the employment of the American product, the leather has a disagreeable yellow or dark colour, apparently due to a colouring matter, which exists in larger quantity in the American variety than in the Sicilian.
Experiments upon the presence of colouring matters made by treating an infusion of sumach with a solution of gelatine, gave the following results:—