What the Kauri-Butanol Value Measures

When a technical data sheet rates a solvent’s “strength,” the number it usually means is the Kauri-Butanol value, or KB value. It is a simple, standardized way to say how aggressively a solvent dissolves things, and it explains at a glance why xylol (xylene) bites harder than mineral spirits but sits below the harshest solvents.

The test behind the number is refreshingly physical. Kauri gum, a natural resin, is dissolved in butanol to make a standard solution. Solvent is then added until the resin can no longer stay dissolved and the mixture turns cloudy. A strong solvent can take a lot before that happens, giving a high KB value; a weak one clouds early, giving a low one. The result is a single figure that lets you rank solvents on one scale.

Roughly where common solvents fall:

Read this as a guide, not a promise. A higher KB value means a solvent is generally more aggressive and better at cutting oils, resins, and heavy coatings, which is why xylene handles jobs mineral spirits cannot. It says nothing about evaporation rate, odor, flammability, or toxicity, all of which matter just as much when you choose a solvent. KB value answers one question well, how strong is it, and leaves the rest to the other lines on the data sheet.