英文科學讀本 第五冊·Lesson 47 Tar and Pitch

Lesson 47 Tar and Pitch

Our lessons on coal introduced us to tar as one of the off-products formed from that mineral in the manufacture of coal-gas. There is another kind of tar—not black like coal-tar—but of a dark brown color. This is made from wood.

If we examine it we shall find it to be a dense, heavy, sluggish liquid, having something of the consistency of molasses. It has a powerful smell, and a bitter, unpleasant taste. It is highly inflammable, and burns with a bright flame, giving off volumes of dense smoke. If we let fall a small quantity in a vessel of water, it sinks to the bottom, because it is heavier than the water. It will not mix with the water, and it is insoluble in water; but it is soluble in spirits of turpentine, and in all fats and oils.

We always employ either oil of turpentine or grease of some sort to remove tar stains from the hands. Water will not do it.

In its properties, therefore, this wood-tar resembles the coal-tar of our early lessons.

It is the property of not mixing with water that has caused tar to be so extensively used in preserving wood-work and other materials.

We tar wooden sheds, fences, and the outside of ships. We steep the ship's cordage in tar; we tar sheets of canvas to make water-proof tarpaulings, and we steep in tar great beams of timber, which are required to resist the action of water. When a wooden post is to be fixed in the earth, the lower end of it is first steeped in tar. The reason is simple. The moisture in the earth cannot enter the wood to rot its substance, because it cannot mix with, or pass through the outside coating of tar. A well-tarred shed will last for many years in spite of rain, because the rain simply runs off the tarred surface, and cannot penetrate.

Pitch is a black, solid substance. It is very brittle, and when broken has a bright shining surface. It becomes liquid and boils with a slight heat. Like tar, it is impervious to water. Tar and pitch, like resin and turpentine, come from the same source. Indeed, tar and pitch are the products of some of the same family of the cone-bearing trees from which we get resin and turpentine.

The particular kind of pine which yields tar and pitch is known as the Scotch fir, and is grown chiefly in Norway, Sweden, and Russia. In our country the swamp pine of North and South Carolina, Georgia, and Alabama yields tar. Tar, although a secretion, does not flow naturally from trees like turpentine. It is stored up in the roots, and the supply can only be obtained by the destruction of the tree. When the tree is fit for the purpose it is felled, and the roots are cut up into small logs. A circular pit, tapering towards the bottom, is next dug on a sloping bank or hillside, the sides of the pit being beaten hard and smooth. The bottom of the pit is made to communicate, by means of a pipe, with a tank placed below.

The little logs of pine roots are then carefully packed in the pit, and when it is full a fire is lighted on the top. As soon as the whole mass is fairly lighted, the mouth of the pit is covered with earth and tufts of grass. In this way the burning goes on in a slow, smouldering sort of way, because it is partially smothered and confined.

The result is that the wood becomes charred and converted into charcoal, and a dense, molasses-like liquid runs downwards through the mass, and passes into the tank below. This is the tar of commerce.

The tar, when heated in a retort, distils over a volatile spirit—oil of tar—and leaves the black, solid substance—pitch.