The shoes on your feet are made of leather. The hemlock trees that grow on the hills were stripped of their bark by peelers in early spring. Black oak and chestnut oak are also stripped in our woods. Carloads of bark are shipped to the tanneries to be used in the tanning of skins which changes them into leather.
That beautiful book upon the table is bound in Russia leather. The acorn cups of a European oak were used to tan the skins that made this leather so much more beautiful than that of your shoes. Your gloves are made of kid skins tanned in Europe. For this particular work the nut-like galls that grow on certain oak trees are gathered in the woods.
Tannin is the substance in oak bark which makes it valuable in tanning leather. A high percentage of tannin is found in oak galls. For this reason they are gathered in many countries, and are among the most valuable and high-priced supplies for the establishments that tan skins for gloves. The most expensive inks and dyes, those that do not fade, but are practically permanent, are made from selected oak galls.
Oak apples are a strange fruit, found in more or less abundance on the leaves of our own oak trees in autumn. You have seen them in summer time, plump, green balls, sometimes as large as a hen’s egg, but globular, sitting upon a leaf. In autumn the balls take on the colour of the dying leaves.
The same tree may have hard little marble-like balls growing on its twigs. These are of different sizes, and it is not unusual to find a hole in the side of each.
All such outgrowths on the leaves and twigs of oaks are called galls, and they are chiefly caused by winged insects called gall-gnats. An egg is laid in early spring, in a slit pierced in the twig or leaf. As this egg hatches, the tissue about it is disturbed in its growth by the presence of this feeding grub. The soft leaf pulp, or the tender tissues of the twig that surround it, are exactly what the grub likes to eat. Food and drink are all about it, and as it feeds, it grows. The leaf swells, and so surrounds the grub with an abnormal growth. The grub still feeds, and the swelling becomes larger, until finally, when the insect ceases to eat, it is housed in the peculiar ball which we know as an oak gall. Each species of gall-maker is known by its house.
The oak apples are of several kinds. Some are empty except for a little shell in the centre, in which the fat grub sleeps. Sometimes the substance within the “apple” is corky, sometimes spongy. Bullet galls, which form on twigs like little marbles, are usually solid to the centre, where the grub lies until the time comes for it to bore its way out to the surface, and fly away, to lay eggs which will produce other galls. Usually oak galls, found in winter, contain the sleeping grub, whose transformation into a winged insect waits until the coming of spring.
The cork in your ink bottle is the bark of an oak tree. Go to Portugal or to Northern Africa, and you may see the cork harvest in progress in July or August. There is no place to go for genuine cork except to a small evergreen oak that rarely reaches a height over thirty feet. When these trees are twenty-five years old, a hard, thin layer of bark is stripped off. This is a valuable tan bark, but it is not in the least corky. The tree now produces a spongy bark entirely different from the first. It is not disturbed for eight or ten years. This is stripped off. It is the poor quality of bark which fishermen use to float their nets with.
Ten years later the bark is stripped again. It is better in quality than the first. Each ten years brings the bark stripper again to the tree. In the fiftieth year, the bark is of the finest quality, and for fifty years that follow there are five strippings of bark of the highest grade. Then the quality becomes poorer. The trees are cut down, the bark is sold to the tanners, and the wood is used for charcoal or for fuel.
It is a very particular job to get the cork off and leave the under layer uninjured. The trunk is stripped from the ground to the point where it branches, and the inner “mother bark” must not be bruised, for no more cork will grow on any bruised spot. Two circular cuts are made, one at the top, one at the bottom of the columnar trunk, then two opposite slits are made dividing the bark of the trunk into two halves. These curved plates are worked off by inserting a wedged-shaped tool between the bark and the trunk, and gradually working it further in until the whole curved plate of cork comes off. These two big sheets are steamed and flattened, then bound in bundles, and shipped to wholesale dealers in cork.