Sometimes valonia is attacked by a sort of honeydew, probably caused by an aphis, which renders it very sticky, and perhaps more liable to heat, but does not in itself damage its tanning properties. 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. [Illustration: 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. [Illustration: FIG. 51.--Chestnut Oak (_Q. prinus_).] _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_).