Knoppern are galls produced on the immature acorns of various species of oaks, principally Q. Cerris in Hungary, and were formerly largely used there for tanning, as they contain up to 35 per cent. gallotannic acid. They are now less abundant, and have been largely replaced by valonia, sometimes called orientalische Knoppern. Like all purely gallotannic materials, they naturally give a soft and porous tannage, ill-adapted for sole-leather purposes, which has led to the Austrian practice of drying, or rather stewing, the leather in very hot and damp stoves, which make it hard and brittle.
Chinese and Japanese galls are the product of the action of an aphis on a species of sumach, and will be mentioned again under [sumachs (Rhus)].
Djaft, dchift, jift, or jaft is a material apparently of Eastern origin, and said to be derived from an oak of Kurdistan. Dark red scales or fragments, origin uncertain, very astringent and darkish tannage, liquor when spilt dries whitish, apparently from crystallisation of some sort. It contains a large amount of tannin. It appears very irregularly in commerce and the writer would be glad to obtain further samples and details of origin. He once used 6 or 7 tons successfully in sole-leather tannage. It has also been attributed to a shrub allied to the Cæsalpinias ([p. 286]).
The most important American oaks are—Q. prinus (castanea, monticola), the Chestnut or Rock Oak ([Fig. 51]). About equal to our oak in strength, bark very thick, and infusion strongly fluorescent, especially in presence of ammonia. Source of chestnut-oak extract. The most important tanning oak-bark of the United States.
Q. alba, or “white oak,” is perhaps the most widely distributed and abundant of any of the American oaks, and very closely resembles the European Q. robur.
Q. tinctoria or nigra, Black or Quercitron Oak. Poor as a tanning material, but used for dyeing yellow, and for modifying the colour of hemlock tannages. The dyestuff, quercetin, is closely allied to that of fustic, and gives yellows with alum and tin mordants.
A good deal of information is given by Trimble[138] on American oaks and other tanning materials.
[138] ‘The Tannins,’ vol. ii., Lippincott, Philadelphia, 1894.
Important Indian oaks are Q. glauca, Q. lamellosa and Q. incana; bark of last said to yield 22 per cent. of tannin.