Some planters do not use acetic acid, but allow the latex to coagulate of itself. The objection is that it takes a much longer time to set, and the latex is liable to suffer from the decomposition of the proteids. Rubber is also sometimes smoked over a wood fire. This accelerates the drying but darkens it and sometimes causes a small reduction in value.
Sheet rubber is made in the same way but in long, flat trays. Crepe is made in a machine in vented in the Malay states, the rollers of which are grooved and tear up and press the soft rubber together again, making it of a lace-like appearance.
There is a slight preference at present for bis cuit and sheet rubber over crepe, but the latter has the advantage of drying more rapidly. Block rubber has recently come into prominence.
Scrap is the waste bits of rubber derived from the cuts when reopening, and any other bits which Cannot be made into biscuits. The washings of the cups and splashes of milk, and in fact every drop of latex, collected into a wooden tub and coagu lated with acid, go into the scrap.
Returns and profits.—Every well-grown tree of six years (in the Straits Settlements) should give three-fourths to one pound of dry rubber per year, and increase as the tree grows. The price of plan tation rubber has been extraordinarily high of late, reaching as much as seven shillings a pound. Although it is difficult to forecast even an average price of the product, at a reduction of one-half of the present value the planter would still gain a large profit. It is estimated in the East that the cost of making the rubber and putting it on the market is five to ten cents Mexican, or one to two pence per pound ; in Trinidad it is eight pence to one shilling. The scrap, if tolerably clean, is worth one or two shillings less per pound, but usually brings a higher price than the best African rubber.
Central American or Panama rubber (Castilloa elastica). Fig. 794 ; Fig. 120, Vol. I.
The Central American rubber, a tree allied to the bread-fruit, is a native of the northern parts of South America and Central America, and is more suited for cultivation in latitudes ten degrees north of the equator. It does not seem ever to have been grown successfully along the equator. The
area of its successful cultivation lies north of the region for the latter plant. Cultivated trees reach a height of sixty feet, with a diameter of eighteen inches, in twenty years.
The plant is raised from seed in nursery-beds, and when about a foot tall is removed to the plan tation. It thrives best when planted not too thickly with other trees. The tree can be tapped in the same way as Para rubber at about eight years of age. A spiral form of cut is often used (see Vol. I, page 108), but is not recommended. With this tree, as with all others, it is best not to tap too early, as such treatment is likely to affect later production. A better quality of rubber is pro duced as age advances. The latex is coagulated by adding boiling water to it, and, after straining, by adding eight ounces of formaldehyde to a barrel ful. Then the creamy mass is washed again and rolled out, or it may be mixed with water in a bar rel with a tap at the bottom. This water is drawn off in about twelve hour s, and the opera tion repeated two or three times, when the cream is allowed to coagulate and is then rolled out.
Separation can also be ef fected by cen trifugal action, but the frequent stoppages neces sary are an ex pensive waste of time. By this process the rub b e r is rapidly brought to the surface of the vessels used, and requires only to be dried. A con venient method of coagulating and drying is by means of the "sand filter ," which can be used in connection with either the creaming or the centrifugal process. A centripetal method is now under trial, which is inexpensive and is expected to work with great economy. If the latex is left in the original fluids after strain ing through a fine mesh, it quickly ferments and becomes putrid ; the rubber will then coagulate and float on the surface, and there is but trifling loss. The rubber thus produced is dark in color, but of good quality, free from resinous matter and keeps well. The method, however, is tedious, repul sive, and takes considerable time. (Hart.) The amount of rubber from a tree is variously stated. An eight-year-old tree probably gives about six pounds (or less) a year on an average. The rubber brings a lower price than Para.