The lighter the colour, the heavier the weight, and the thicker the scales of the beard, the better the quality usually proves, but analysis is the best guide. Caramanian valonia is very inferior.
The tannin contained in valonia is especially suitable in the manufacture of sole-leather. It deposits much bloom, and if used as a dusting material, has the characteristic of making the leather solid and compact, but leaves the grain somewhat rough and hard to work. In mixture with gambier and other materials, it is an excellent tannage for dressing leather, and with proper management deposits little or no bloom (cp. [p. 231]).
Q. infectoria ([Fig. 50]) is the source of the “Turkish” or Aleppo galls. Galls are caused by insects, principally of the genus Cynips, or gall-wasps, which lay their eggs in different parts of plants, and in some way cause an abnormal growth of the bud, leaf, or other part.
Fig. 50.—Gall Oak (Q. infectoria).
Aleppo galls are developed from the young shoot of the oak, are best before the insect has escaped, and contain in this stage up to 50 or 60 per cent. of gallotannic acid. When the insect has developed and escaped, the galls are of course perforated, much lighter, and more porous. These galls and those of Rhus semialata are the principal sources of the tannin of commerce.
The Q. infectoria also bears a large gall like an apple, “Apples of Sodom,” or “rove,” caused by a different insect. This, in a crushed condition, has been somewhat largely used as a tanning material, and contains 24-34 per cent. gallotannic acid.
English oaks have several species of galls and oak-apples, but they do not seem to be of much value for tanning.
Fig. 51.—Chestnut Oak (Q. prinus).