Description—Olibanum as found in commerce varies rather considerably in quality and appearance. It may in general terms be described as a dry gum-resin, consisting of detached tears up to an inch in length, of globular, pear-shaped, clavate, or stalactitic form, mixed with more or less irregular lumps of the same size. Some of the longer tears are slightly agglutinated, but most are distinct. The predominant forms are rounded,—angular fragments being less frequent, though the tears are not seldom fissured. Small pieces of the translucent brown papery bark are often found adhering to the flat pieces. The “Lubân Fasous Bedow” as exported from the Mijjertheyn district, in the eastern part of the Somali Country, is in very fine large tears.
The colour of the drug is pale yellowish or brownish, but the finer qualities consist of tears which are nearly colourless or have a greenish hue. The smallest grains only are transparent, the rest are translucent and somewhat milky, and not transparent even after the removal of the white dust with which they are always covered. But if heated to about 94° C., they become almost transparent. When broken they exhibit a rather dull and waxy surface. Examined under the polarizing microscope no trace of crystallization is observable.
Olibanum softens in the mouth; its taste is terebinthinous and slightly bitter, but by no means disagreeable. Its odour is pleasantly aromatic, but is only fully developed when the gum-resin is exposed to an elevated temperature. At 100° C. the latter softens without actually fusing, and if the heat be further raised decomposition begins.
Chemical Composition—Cold water quickly changes olibanum into a soft whitish pulp, which when rubbed down in a mortar forms an emulsion. Immersed in spirit of wine, a tear of olibanum is not altered much in form, but it becomes of an almost pure opaque white. In the first case the water dissolves the gum, while in the second the alcohol removes the resin. We find that pure olibanum treated with spirit of wine leaves 27 to 35 of gum,[545] which forms a thick mucilage with three parts of water. Dissolved in 5 parts of water it yields a neutral solution, which is precipitated by perchloride of iron as well as by silicate of sodium, but not by neutral acetate of lead. It is consequently a gum of the same class as gum arabic, if not identical with it. Its solution contains the same amount of lime as gum arabic affords.
The resin of olibanum has been examined by Hlasiwetz (1867), according to whom it is a uniform substance having the composition C₂₀H₃₀O₃. We find that it is not soluble in alkalis, nor have we succeeded in converting it into a crystalline body by the action of dilute alcohol. It is not uniformly distributed throughout the tears; if they are broken after having been acted upon by dilute alcohol, it now and then happens that a clear stratification is perceptible, showing a concentric arrangement.
Olibanum contains an essential oil, of which Braconnot (1808) obtained 5 per cent., Stenhouse (1840) 4 per cent., and Kurbatow (1871-1874) 7 per cent. According to Stenhouse it has a sp. gr. of 0·866, a boiling point of 179·4° C., and an odour resembling that of turpentine but more agreeable. Kurbatow separated this oil into two portions, the one of which has the formula C₁₀H₁₆, boils at 158° C., and combines with HCl to form crystals; the other contains oxygen. The bitter principle of olibanum forms an amorphous brown mass.
The resin of olibanum submitted to destructive distillation affords no umbelliferone. Heated with strong nitric acid it develops no peculiar colour, but at length camphretic acid (see Camphor) is formed, which may be also obtained from many resins and essential oils if submitted to the same oxidizing agent.
Commerce—The olibanum of Arabia is shipped from several small places along the coast between Damkote and Al Kammar, but the quantity produced in this district is much below that furnished by the Somali Country in Eastern Africa. The latter is brought to Zeyla, Berbera, Bunder Murayah, and many smaller ports, whence it is shipped to Aden or direct to Bombay. The trade is chiefly in the hands of Banians, and the great emporium for the drug is Bombay. A certain portion is shipped through the straits of Bab-el-Mandeb to Jidda,—Von Kremer[546] says to the value of £12,000 annually. The quantity exported from Bombay in the year 1872-73 was 25,100 cwt., of which 17,446 cwt. were shipped to the United Kingdom, and 6,184 cwt. to China.[547]
Uses—As a medicine olibanum is nearly obsolete, at least in Britain. The great consumption of the drug is for the incense used in the Roman Catholic and Greek Churches.