If you have regularly handled or touched the bark or cones of pine, spruce or larch, you know about the fragrant "sticky" resin they copiously ooze. That resin is contained in ducts or blisters that run through the bark and wood and diminish in size and number as they enter roots and needles. Hemlocks, true cedars and firs have resin mainly restricted to the bark.
Wound trauma to a tree can stimulate the production of "traumatic resin canals" that help in containing the injury and help in healing any resulting infection. Resin-ladened blisters contained in the conifer secretes the light liquid which immediately loses oils to evaporation and forms a heavy solid scab. It is interesting to note that this reaction to trauma by a tree is used in the manufacturing process of certain commercial resins and essential oils by stimulating resin flow by inflicting a purposeful injury or bark irritation (see tapping below).
The production of resin is very common in nature, but only a few plant families can be considered of commercial importance to resin collectors. These important resin producing plants include the Anacardiaceae (gum mastic), Burseraceae (incense tree), Hammamelidaceae (witch-hazel), Leguminosae, and Pinaceae (pine, spruce, fir, true cedar).
How Resins are Formed, Collected and a Little History
Resins are formed as a product of the oxidation process of a tree's escaping essential oils - also called volatile oils, ethereal oils or aetherolea. As I have mentioned the resin is usually stored in ducts or blisters and frequently oozes out through the bark to harden when exposed to air. These resins, as well as being critical to a tree's health, can be commercially valuable when collected or "tapped".
Resinous concoctions have been used for millennia in the form of waterproof and protective coatings made by the Ancients. Varnished objects have been found in Egyptian tombs and the use of lacquer in the practice of their arts has been used in China and Japan for centuries. The Greeks and Romans were familiar with many of the same resinous materials that we use today.
It is the ability of tree resins to harden as essential oils evaporate that makes them necessary to the production of commercial varnishes. These resins are readily dissolvable in solvents like alcohol or petroleum, surfaces are painted with the solutions and as the solvents and oils evaporate, a thin waterproof layer of resin remains.
Tapping is usually necessary in order to obtain a sufficient amount to be of commercial value but can also be extracted during the processing of a tree species for another product - pine resins and oils can be collected during the paper pulping process. Commercial hard resins are also frequently mined and extracted from ancient fossil materials like copal and amber for varnish. It is important to understand that resins, unlike gums, are insoluble in water, but they are easily dissolved in ether, alcohol and other solvents and used in many products.
Other Resin-based Products
Hard transparent resins, like copals, dammars, mastic and sandarac, are mainly used for varnishes and adhesives. The softer odoriferous oleo-resins like frankincense, elemi, turpentine, copaiba and the gum resins containing essential oils (ammoniacum, asafoetida, gamboge, myrrh, and scammony) are more often used for therapeutic purposes and incense.Resin, Kraft or pine soap (one trade name is "Pine Sol") is made by reacting resin acids in wood with sodium hydroxide. Kraft soap is a byproduct of the Kraft process for manufacturing wood pulp and used as a super strength cleaner for heavy soiled and greasy cleaning jobs.
Resin in the form of "rosin" is applied to the bows of string instruments because of its ability to add friction to bow hairs to increase sound quality. It is used similarly in sports to provide tack to grip bats and balls. Ballet dancers may apply crushed resin to their shoes to increase grip on a slippery floor.