730

THEORY AND PRACTICE OF ENGINEERING.

Book II

tree with two oblique gutters to conduct the sap which flows from the wood into the boxor cavity below ; in about a fortnight the box becomes full, and a wooden shovel transportsit into a pail, and it is then put into a cask : the edges of the wound are chipped everyweek, and the boxes after the fust generally fill in about three weeks. The sap thusprocured is used as turpentine without any preparation, and is called pure dripping.

The Scrapings are the crusts of resin that are formed on the sides of the wounds;these are often mixed with the turpentine, which in this state is used in the manufactureof yellow soap, and is called Boston turpentine; in five or six years the tree is abandoned,and the bark never becomes sufficiently healed to allow of the same place being woundedtwice.

Spirits of Turpentine are principally made in North Carolina , and obtained by dis-tilling the turpentine in large copper retorts: six barrels of turpentine afford 122 quarts ofthe spirit; the residium after the distillation is resin, which is sold at ^ the price of theturpentine.

As soon as vegetation ceases in any part of a pine tree its consistence changes, the sapwood decays, and the heart becomes surcharged with resinous juice to such a degree as todouble its weight in one year; this accumulation increases.

Tar of the southern states of America is made from the dead wood of Pinus australis,obtained from trees prostrated by time, by fires annually kindled in the forests, or from thetops of those that are felled for timber, &c. : dead wood is productive of tar for severalyears after it has fallen from the tree. To procure the tar a kiln is formed in a part ofthe forest abounding in dead wood, which is collected, stripped of the sap wood, and cutinto billets of 2 or 3 feet long, and about 3 inches thick, a tedious and difficult task,rendered so by the numerous knots with which the wood abounds. The next step is toprepare a place for piling the billets, and for this purpose a circular mound is raised,slightly declining from the circumference in the centre, and surrounded by a shallow ditch ;the diameter of the pile is proportioned to the quantity of wood which it is to receive; tocontain 100 barrels of tar it should be 18 or 20 feet wide: in the middle is a hole with aconduit leading to the ditch, in which is formed a receptacle for the tar as it flows out.Upon the surface of the mound, after it has been beaten hard and coated with clay, thewood is laid round, in a circle like rays. The pile when finished may be compared to acone truncated at § of its height and reversed, being 20 feet in diameter below, 25 or 30feet above, and 10 or 12 feet high ; it is then strewed over with pine leaves covered withearth , and held together at the sides with a slight cincture of wood ; this covering isnecessary in order that the fire kindled at the top may penetrate downwards towards thebottom with a slo\v and gradual combustion, for if the whole mass were rapidly inflamedthe operation would fail, and the tar would be consumed instead of distilled ; in fine, thesame process is observed as in Europe for making charcoal; a kiln which is to afford 100or 130 barrels of tar is eight or nine days in burning.

Strasburgh Turpentine , to be good, ought to be clear, free from impurities, transparent,and of the consistence of syrup, with a strong resinous smell, and rather a bitter taste: itis the only turpentine produced by any kind of pine or fir tree, which is used in thepreparation of clear varnishes, and its oil sells at a higher price than any other. The pro-portions for making the oil are 5 lbs. of liquid resinous juice to 4 pints of water distilledin a copper alembic; this is the Essential oil of turpentine; and if 1 pint of it be redistilledwith 4 pints of water it is called rectified or a;thereal oil of turpentine.

Lamp-black -The apparatus employed for this purpose consists of a furnace, a chimney,

and a small chamber or box for collecting the soot: the furnace is about 2 feet 6 incheswide, C or 4 feet long, and 2 feet 6 inches high, and it is usually set in brick : on each of thelong sides this furnace has an opening near the bottom, which can be shut up at pleasure,by means of a little door attached to it. The furnace has a brick chimney made almosthorizontal, to conduct the smoke into the chamber or box: the chimney is from 14 to16 inches long, and 12 inches broad and high ; at the place where the pipe of the chimneyterminates is constructed a chamber or box, into which the pipe should enter some inches,so as to carry the smoke into its centre. This chamber is generally about 12 feet square,and 9 feet high in the roof; there is a door on one side, and in the upper part or ceilingan opening 5 or 6 feet square. The walls of the chamber are lined with thin planks ofwood or plastered very smooth, and the door is fitted closely into a groove: over theopening in the roof is placed a flannel bag, supported by rods of wood in the form of apyramid, composed of four pieces of coarse flannel sewed together.

When the lamp-black is to be made, a little of the straw through which the resin and tarhave been strained, and some of the other refuse, are put into the furnace and lighted, freshstraw, impregnated with tar, being strewed over the fire, as fast as the other is consumed.

The smoke passes into the chamber, and deposits its soot on the walls, and on the flannelbag, from both of which it is detached, after the whole of the straw and refuse have beenburned, by striking the outsides smartly with a stick. The flannel pyramid acts as a filterto the lighter part of the smoke, retaining the soot, and permitting the heated air to escape