Gums and Resins
Read about Gums and Resins in the Standard Cyclopedia of Horticulture
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Gums and Resins. Of the many thousands of vegetable substances falling under the denomination of gums and resins, but a comparatively small number are of such general importance as to warrant mention here. Their economic value depends upon the physical and chemical properties possessed by them and the abundance in which they are produced. A gum, in the ordinary use of the word, is a substance of a more or less sticky nature or which was at one time of a sticky or plastic consistency The chemist, however, restricts the term gum to certain products having very definite properties, classifying other plant exudations according to their properties and grouping them as resins, gum resins, balsams, and the like. A gum in this restricted sense, is a substance which dissolves or softens in cold water, forming a mucilage, or at least a liquid of gelatinous consistency, and when held in a flame only chars with an odor of burnt sugar. It is insoluble in 60 per cent alcohol, oil of turpentine, benzene or fatty oils. A resin, on the other hand, is a lustrous vegetable substance resembling a gum but which neither dissolves nor softens in cold water and which burns with a bright smoky flame, giving off an aromatic odor. It is more or less soluble in alcohol, oil of turpentine, benzene or warm oils. Gums are related, chemically, to cellulose and are not secretion products, as was formerly supposed, but are formed directly from the plant tissues by a breaking down of the cells themselves; sometimes this is a perfectly normal process but very often it may be considered pathological and is the result of bacterial action. Humidity appears to be the principal controlling factor in the production of gum. Volatile or essential oils are secreted by the cells of many plants of widely divergent relationships and are often characteristic of certain families, like those to which belong the mints and the pines. The resins are oxidation products derived from certain of the volatile oils and thus may indirectly be considered products of secretion. Some plants yield only gum, others only resin, while others again may yield both, in which case the gum and resin may come from different parts of the same plant or may be exuded as an emulsion or mixture. In many plants the resins occur dissolved in volatile oil and though exuded in a liquid state soon become solid through the evaporation of more or less of the volatile oil. Some resins contain aromatic acids and others consist of certain liquid organic compounds of these acids in which is dissolved a solid resin; such products are called balsams. The camphors constitute another group of oxidation products derived from the volatile oils. Uses of gums and resins. Soluble gums, the most typical of which is gum arabic, are used for a great number of purposes in the arts; the varieties having the least color, highest adhesive power and viscosity being the most valuable. They find application in confectionery and pharmacy, in sizing and finishing textile fabrics and paper, m calico printing and dyeing, and in the manufacture of fine water-colors, ink, mucilage, and so on. The gums which are more or less insoluble in water, but which swell with it to form a mucilage, as for example traga- canth, are used as thickening agents in calico-printing and in pharmacy, and for pastes, pills and colored crayons. Resins are applied to a great variety of industrial purposes but probably the most important of these is the manufacture of varnishes and lacquers. For this purpose they may be roughly divided into two classes: (1) those which after melting can be combined with Unseed oil and turpentine to form an "oil varnish," and (2) those which dissolve more or less in alcohol, oil of turpentine or other volatile solvents to form "spirit varnishes." The important resins of the first class are amber and the copals; while those of the second class include rosin or colophony, benzoin, dammar, sandarac, mastic and elemi. Aside from varnish-making, certain resins are employed in medicine and pharmacy, for incense, and in the manufacture of soap, and the like. The gums and resins of greatest economic importance. The following list of the plant exudations which are of the greatest economic importance and which comprises one or more typical examples of each of the groups already mentioned may be arranged as follows: True gums Gum arabic Tragacanth True resins Amber Copal Dammar Sandarac Mastic Rosin Balsams Balsam peru Storax Gum-resins Gamboge Myrrh Olibanum Galbanum Oleo-resins Turpentine Canada balsam Copaiba Elemi Camphors Camphor (Common or Laurel Camphor) Menthol Gum arabic.—The name still generally applied to the most important gum produced in northern Africa and which has been an article of commerce since the first century of the Christian era. It was shipped from Egypt to Arabia and then thence to Europe and was therefore called "gum arabic." At present the gum is usually known as "Sudan," "Kordofan" or "Senegal" gum, depending upon the region from whence it is shipped to market. The best gum is produced by the gray-barked acacia tree, Acacia Senegal, and is collected both from wild or unowned trees and from gardens of- acacia trees which are private property. In the gardens the gum is obtained by making incisions in the principal branches of the trees while from the wild trees the naturally exuded gum is collected. Inferior varieties are collected from Acacia senegal, chiefly from the forests of the Blue Nile, and from Acacia arabica, A. stenocarpa and A. albida in Senegal. The gum is cleaned from pieces of bark and other debris before leaving Africa but the bulk of the product is exported without grading and is sorted in Europe, principally at Trieste and Bordeaux. Tragacanth.—The most important of the so-called insoluble gums, and the only one regularly found in commerce, is obtained from several species of small shrubs of the genus Astragalus, found in Asia Minor. Syria, Armenia, Kurdistan and Persia. It is produced chiefly by the following species: Astragalus adscendens, A. gummifer, A. pycnocladus, A. kurdicus and A. stromatodes. In order to obtain the greatest quantity of gum, the shrubs are stripped of their leaves in July or August and short incisions or slits are made in the trunks. The gum flows out, forming flat ribbon-like or vermiform pieces depending upon the shape and size of the incision, and is dry enough for gathering in three or four days. Smyrna is an important market for gum tragacanth and it is there sorted into various qualities for the European market. Amber.—A fossil resin found principally on the shores of the Baltic Sea. The larger and finer pieces are used for jewelry, beads, trinkets, mouthpieces for pipes and cigar-holders, while the smaller pieces and the waste from carving or turning are used for varnish. Copals.—The term "copal" is now used commercially to designate a group of widely distributed hard resins of high melting point having the common property of being capable of being used for the manufacture of oil varnishes. Aside from amber, Zanzibar, or true copal was the first resin used for this purpose, hence arose the custom of terming as "copal," each new resin which was discovered to be useful for this purpose and distinguishing it from others by prefixing its port of shipment or other geographical name. Copals are obtained in round tears, nodules or flat pieces of varying degrees of hardness, either from living trees (recent or raw copals) or dug from the earth at spots, occupied centuries before by trees long since disappeared (fossil or ripe copals). The most important are yielded by the following plants: Zanzibar copal, Hymen-sea Hornemanniana; Sierra Leone copal, Copaifera Guibourtiana; Gold Coast or Accra copal, Cyanothyrsus Ogea; Niger copal, Daniella oblonga; Kauri copal, Agathis (Dammara) australis; Manila or East Indian copal, Agathis (Dammara) orientalis; West Indian or Demerara copal, Hymemea courbaril. Dammar.—Dammar is the Malay term for all gums and resins which exude from trees and solidify upon exposure to the air, but as used commercially it designates a group of varnish resins obtained from Indian or East Indian trees belonging to the Diplerocarpaceae and Burseraceae and thus does not include the resins from the genus Agathis (or Dammara,) which are known as copals. Indian dammar, Shorea robusta; white dammar, Vateria indica; black dammar, Canarium strictum; rock dammar, Hopea odorata. Sandarac.—The hard brittle resins produced by several species of coniferous trees in North Africa and Australia. Mogadore sandarac, yielded by a small cypress, Thuya articulata, common on the southern slopes of the Atlas Mountains, is shipped principally from the port of Mogadore, Morocco. Australian sandarac is the product of several species of cypress pines, especially the Murray pine, CaUitris verrucosa and the red or black pine, Callitris calcarala. Mastic.—A soft yellow resin obtained in brittle, yellowish, glassy, rounded drops from Pistacia lentiscus, a small tree indigenous to Asia Minor and the Greek Archipelago but cultivated on the island of Chios. Used for varnishing paintings, for incense and as a tooth cement. Common rosin or colophony.—This is the solid residue obtained as a by-product in the distillation of oil of turpentine from crude turpentine. For a list of the most important sources, see Turpentine, below. Rosin is used for cheap furniture varnishes, in the sizing of paper, as a flux for solder, as a coating for the inside of casks, and in the manufacture of laundry- soap. It is the source for rosin-oil and rosin-spirit, which are produced by the destructive distillation of rosin. The first is used in the production of lubricants, printing inks and paints, while the second is a substitute for oil of turpentine. Gamboge.—A hard brittle yellow gum-resin, composed of a variable mixture of gum and resin, and produced by several species of Garcmia, especially G. Hanburyi of Siam and Indo-China and G. morella of India and Ceylon. It is used to color golden lacquers, as a water- color pigment and in medicine as a drastic purgative. Myrrh.—A fragrant gum-resin obtained in Arabia and northeastern Africa from a burseraceous tree Balsamodendron myrrha. It is used in medicine and for dental preparations. Olibanum or frankincense.—A fragrant gum-resin obtained from the stem of several species of Boswellia, especially B. carterii, native to northeastern Africa and the southern coast of Arabia. Its principal use is for the incense used in the Roman Catholic and Greek churches. Galbanum.—A strong-smelling, yellowish brown gum-resin exuded from the stem of certain species of Ferula, especially F. galbaniflua and F. rubricaulis, natives of Persia. It is mentioned by the earliest writers on medicine and was an ingredient of the incense used in the worship of the ancient Israelites. It is now used only to a small extent in medicine. Turpentine.—The crude oleo-resin obtained by tapping any one of several species of coniferous trees native to North America, Europe and northern Asia. The most important varieties are yielded by the following species: American turpentine from the long- leaf pine, Pinus palustris, and the Cuban pine, Pinus heterophylla; French turpentine from the maritime or cluster pine, Pinus maritima; Russian turpentine from the Scotch or Swedish pine, Pinus sylvestris; and Indian turpentine from the Indian blue pine, Pinus exeelsa, the Himalayan long-leaf pine, Pinus langifolia and the Burma pine, Pinus khasya. From these oleo-resins there is distilled oil of turpentine, leaving behind rosin or colophony. Venice turpentine, from the common larch, Larix europaea, is about the consistency of clear honey, and is used in fixing colors, enamel painting and firing and in medicine. Canada balsam.—A thick, yellow, transparent, liquid oleo-resin obtained in the northern United States and Canada from the balsam fir, Abies balsamea. On account of its great capacity for refracting light it is used to cement lenses and for mounting objects for the microscope. It is also used in medicine. Copaiba balsam.—A thick, transparent, brownish, liquid oleo-resin obtained from several species of leguminous trees belonging to the genus Copaifera and native to northern South America. It is used in medicine as an antiseptic and stimulant. Elemi.—More or less soft resins yielded by burseraceous trees belonging to the genera Protium and Canarium. Manila elemi from Canarium luzonicum is a fragrant resin used for toughening varnishes. Balsam Peru.—A dark brown molasses-like liquid balsam obtained in Salvador and Guatemala from the stem of a leguminous tree, Myroxylon pereirae. It is used in medicine, perfumery and chocolate manufacture. Styrax or storax.—A thick, grayish, sticky, liquid balsam obtained in Asia Minor from the oriental sweet gum, Liquidambar orientalis. It is used in perfumery, pharmacy and in medicine. Common or laurel camphor.—A white, crystalline, pungent substance obtained by distilling with steam the twigs and chips of the camphor tree, Cinnamomum Camphora, native to China and Japan and cultivated in the southern United States. It is used in medicine and for the manufacture of celluloid, lacquers and smokeless powders. Menthol or peppermint camphor.—The principal constituent of oil of peppermint, the essential oil of Menlha piperita, from which it can be crystallized by chilling. It resembles common camphor but has a strong peppermint odor. Used in medicine and perfumery.
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References
- Standard Cyclopedia of Horticulture, by L. H. Bailey, MacMillan Co., 1963
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