Colophane

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Rosin is not a fat. It is what is left after turpentine is distilled off pine oleoresin, and it is roughly nine parts resin acids — abietic and its isomers, C20H30O2 — which are single-carboxyl FREE acids. They neutralise lye directly, one hydroxide per molecule, with no ester to hydrolyse first. That is why IS 553's acid number (155 min for the pale types) and its saponification number (160 min) are almost the same figure, where a triglyceride's acid number is near zero and its saponification number near 200. So rosin soap is a real soap, sodium abietate, and it is what the 19th-century books mean by yellow soap. Six of the transcribed Thomssen and Watt recipes call for it by weight alongside the fats. It lathers freely, it is tacky and soft on its own, and the books keep it under about a quarter of the charge. What this record gives the calculator is a FLOOR, not a value. IS 553 specifies saponification number as a minimum per type — 165 for extra pale, 160 for pale and medium and dark — and sets no ceiling, because a standard's job is to say what a buyer is entitled to. WW is a pale-type grade (Gardner colour 7), so 160 is the figure here. For scale at the other end: pure abietic acid is C20H30O2, molar mass 302.45, one carboxyl, so 56106 ÷ 302.45 = 185.5 mg KOH/g, and a rosin that was entirely resin acid could not exceed that. A real WW grade sits between the two, and this record uses the floor, which under-doses the lye slightly and leaves the surplus as superfat. That is the safe direction to be wrong in. No fatty-acid profile is given because rosin has none — resin acids are diterpenes and none of them is in this schema's key list. That means the calculator can work out rosin's lye but not its contribution to hardness, cleansing or conditioning, and a blend containing it will report those qualities over the fats alone. The standard's other requirements, for reference: softening point 70 C min (a softening point, not a melting point, which is why the melting-point field is null), relative density 1.052–1.082 at 27/27 C for the pale type, volatile matter 2.0 % max, ash 0.2 % max, matter insoluble in toluene 0.40 % max.

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