Resin

Resin is not sap. Sap is the tree's circulation — water and dissolved sugar moving through the phloem — and it is watery, sweet and does not harden. Resin is a defence: a viscous mixture of terpenes secreted into a wound, which flows over the damage, traps whatever is trying to get in, and sets hard as the volatile fraction evaporates.

That distinction is worth holding onto because it explains almost everything else. Resin is antimicrobial and insecticidal because it evolved to be. It hardens in air because the terpenes that make it flow are the ones that leave. And it preserves whatever it engulfs with a fidelity nothing else in nature matches, because the same chemistry that stops bacteria attacking the tree stops them attacking anything caught in it.

Fresh oleoresin is roughly a third turpentine and two-thirds rosin, and the two behave as opposites.

Turpentine is the volatile fraction — mostly α- and β-pinene, small molecules that evaporate. It is what makes fresh resin runny and what makes a pine forest smell of pine.

Rosin, or colophony, is the involatile remainder: mostly abietic acid and its relatives, large enough not to evaporate and irregular enough not to crystallise. That irregularity is why rosin is a glass rather than a solid with a melting point — it softens over a range, around 70 to 80 °C, and shatters conchoidally when struck, exactly as amber does.

Rosin's usefulness comes from a single unusual combination: it is sticky under slow load and brittle under fast load. A violin bow works because rosin grips the string as the bow draws and releases as the string springs back, thousands of times a second. Nothing synthetic has replaced it.

Tapped, in a practice essentially unchanged since antiquity: a strip of bark is removed from a living pine, the tree responds by flooding the wound, and the oleoresin is collected in a cup below. A pine will yield for decades without being killed, which is what separates naval stores from timber as a use of the same forest.

Most rosin now arrives a different way. Tall oil rosin is recovered from the black liquor of kraft pulping — a by-product of making paper, and a substantial one — and gum rosin from tapping is the minority, concentrated in China, Brazil and Indonesia.

Frankincense and myrrh are the same operation on different trees, Boswellia and Commiphora, and they were traded across Arabia for the same reason spices were: the tree grows in very few places and the material does not spoil.

Rosin is a flux, and that is its largest technical use: it removes the oxide film from copper at soldering temperature and lets the solder wet the metal, which is why the smell of soldering is the smell of pine. Rosin-cored solder wire carries it in the middle of the wire so it arrives exactly where it is needed.

Then adhesives and tackifiers, printing inks, paper sizing, chewing gum base, and the block of rosin every violinist, gymnast and baseball pitcher owns for the same reason.

Turpentine was the paint and varnish solvent for three centuries and has largely been displaced by petroleum distillates, which are cheaper and less likely to sensitise the skin.

The historical use is the one the trade is named for. Naval stores — pitch, tar, turpentine and rosin — waterproofed the hulls and rigging of every wooden fleet, and were strategic materials in exactly the sense that oil is now.

Resin is among the first materials anybody engineered rather than merely used. Birch bark tar, made by heating bark without air, was hafting stone tools in Europe at least 200,000 years ago — which makes it a manufactured adhesive older than our species, and one that required controlled pyrolysis to produce.

Egyptian embalming used resin heavily, for its antimicrobial behaviour as much as its scent. Greek and Roman ships were sealed with pine pitch, and retsina is a survival of the era when wine was stored in resin-lined vessels and eventually acquired a taste for it.

The naval stores trade shaped the American southeast: longleaf pine forests were tapped so hard for so long that the industry, the landscape and North Carolina's nickname all come out of it.

Resin is not shellac and not lacquer, and the words are used interchangeably by people selling all three.

Shellac is secreted by an insect, the lac bug, not by a tree. Lacquer in the East Asian sense is the sap of Toxicodendron vernicifluum, which polymerises by enzymatic oxidation rather than by evaporation and is chemically related to poison ivy — genuine lacquerware can give a serious rash while it is curing.

And 'resin' in a hardware shop now usually means epoxy or polyester, which are synthetic thermosets sharing the name and nothing else.