Resin

• 1. Introduction

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  Resin is one of the oldest materials humans have ever put to use. Deep in the late Middle Stone Age, people in Southern Africa were pressing plant resin into service as an adhesive, binding stone tools to handles long before any written record existed. Frankincense and myrrh were prized in ancient Egypt and later documented by Theophrastus in ancient Greece and Pliny the Elder in ancient Rome. The word itself traveled through French and Latin before arriving in English, tracing back to the Greek rhētínē, meaning the resin of the pine, a term of unknown and probably non-Indo-European origin.

  At its core, resin is a solid or highly viscous liquid that can be converted into a polymer. It pours out of trees as a defense mechanism, and humans have been intercepting that chemistry for thousands of years. What drives a plant to produce it, what forms it takes across the natural world, and how it became the foundation for varnishes, perfumes, adhesives, and modern synthetic materials are the questions this documentary sets out to answer.

• 2. A Plant's Defense

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  Plants secrete resins in direct response to injury. When bark is damaged, the flow begins, and with it comes a package of chemistry aimed squarely at the plant's enemies. Resins repel herbivores, insects, and pathogens. The volatile phenolic compounds within them do something more targeted still: they can attract the predators of insects that are attacking the plant, effectively calling in reinforcements.

  Most plant resins are built from terpenes, a broad class of organic compounds. The specific building blocks include alpha-pinene, beta-pinene, 3-carene, and sabinene. Limonene and terpinolene are monocyclic terpenes also found in the mix. Smaller amounts of tricyclic sesquiterpenes appear too, among them longifolene, caryophyllene, and cadinene. Some resins carry a high proportion of resin acids alongside these terpenes.

  Not all resins are alike in their volatility. Rosins, a subset, are less volatile than typical plant resins and consist of diterpenes among other compounds. The distinction matters for how each type behaves when processed or applied.

• 3. A World of Examples

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  Amber is fossil resin, also called resinite, preserved from coniferous and other tree species over geological time. Copal, kauri gum, and dammar may also be found as subfossil deposits, meaning they are preserved but younger than true amber. A practical test separates the two: subfossil copal becomes tacky when a drop of acetone or chloroform touches its surface, while genuine fossil amber does not.

  African copal and the kauri gum of New Zealand are both procured in a semi-fossil condition. Kauri gum comes from Agathis australis trees. Dragon's blood resin flows from Dracaena species, the dragon trees. Frankincense is harvested from Boswellia sacra. Myrrh comes from shrubs of the genus Commiphora. Mastic resin is extracted from the mastic tree Pistacia lentiscus. Sandarac resin comes from Tetraclinis articulata, which is the national tree of Malta.

  Not all resins originate in plants. Shellac is an insect-derived resin. Lac, obtained from Kerria lacca, serves as sealing wax in India and as a component of lacquerware in Sri Lanka. Asphaltite and Utah resin are petroleum bitumens, placing them in a category of their own.

• 4. Rosin and Its Chemistry

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  Rosin is what remains of a plant resin after the volatile terpenes have been removed through distillation. The result is a transparent or translucent mass with a vitreous, glassy fracture and a faintly yellow or brown colour. It carries little to no odour, or at most a slight turpentine smell. Rosin is insoluble in water but dissolves readily in alcohol, essential oils, ether, and hot fatty oils. When heated, it softens and melts; when burned, it produces a bright but smoky flame.

  The chemistry inside rosin centers on resin acids, which are oxidized terpenes. These acids dissolve in alkalis to form resin soaps, and the acids themselves are regenerated when treated with acids again. Specific resin acids identified in rosin include abietic acid, also called sylvic acid, with the molecular formula C20H30O2; plicatic acid, which is found in cedar; and pimaric acid, also carrying the formula C20H30O2, a constituent of galipot resin. Abietic acid can be extracted from rosin using hot alcohol.

  Rosin comes primarily from pines and other conifers. An unusual variation of resin production appears in certain Central and South American species of Dalechampia and Clusia, which produce resins not as stem secretions but as pollination rewards. Some stingless bee species collect these resins for nest construction. Honey bees use propolis, a substance consisting largely of resins gathered from poplars and conifers, to seal small gaps in their hives.

• 5. Ancient Uses and Modern Forms

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  The oldest known use of plant resin is adhesive. Southern African people in the late Middle Stone Age used it to haft stone tools, binding cutting edges to handles. The same stickiness that served a hunter thousands of years ago also drew ancient Egyptians to frankincense and myrrh, substances required for certain religious rites and prized as incense.

  Hard transparent resins, such as the copals, dammars, mastic, and sandarac, found their main application in varnishes and adhesives. Softer, odoriferous oleo-resins including frankincense, elemi, turpentine, and copaiba moved toward food and incense uses. Some crossed into flavor: the resin of the Aleppo pine is used to flavor retsina, a Greek resinated wine. Natural resins also became raw materials for perfumery; the balsams of Peru and tolu, elemi, styrax, and certain turpentines all serve as perfume ingredients.

  The meaning of the word resin expanded considerably in the modern era. It now applies to nearly any liquid component that sets into a hard lacquer or enamel-like finish, with nail polish cited as one example. Casting resins and synthetic resins, including epoxy resin, carry the name too. Bisphenol A diglycidyl ether is one such synthetic resin, converted to epoxy glue when a hardener is added. Silicones are often prepared from silicone resins through room temperature vulcanization. Alkyd resins, used in paints and varnishes, harden by exposure to oxygen in the air.