How we read a Blue Wool test

Every colour on this site carries a Blue Wool number from 1 to 8. It's on the Chart, on every product page, and on the pan itself. Most watercolour brands either don't publish it or bury it in a technical PDF nobody reads before buying. Here's what the number actually measures, and why we'd rather show a 6 than hide it.

What the scale is

The Blue Wool scale is a lightfastness standard: eight strips of wool dyed to fade at known, doubling rates under controlled light exposure. A pigment is exposed alongside the strips, and whichever strip fades to match it sets the rating. Blue Wool 1 fades fastest, Blue Wool 8 barely fades at all inside a human lifetime of normal display. Each step up roughly doubles the light exposure required to produce visible fading, so the difference between a 6 and an 8 is not small.

It's a comparative test, not a guess. Two labs running it correctly on the same pigment sample should land on the same number, which is part of why we trust it over marketing language like "permanent" that has no defined threshold behind it.

Where the range actually lands

Sixteen of eighteen colours in the range test at Blue Wool 7 or 8, which is the band generally considered safe for permanent display, framed and lit, without special glass. Two don't: Beitou Sulphur and Quinacridone Rose test at 7, still in the safe band but with less headroom. Hansa Yellow Light is the outlier at Blue Wool 6, meaning it will show visible fading over years in direct light faster than the rest of the range.

We didn't reformulate Hansa Yellow Light to hide that, and we didn't drop it from the range. It's a genuinely useful clean yellow for work that isn't going to hang in a sunlit room for a decade, and the honest move is publishing the number and letting a painter decide, not making that decision for them by omission.

What this doesn't tell you

Blue Wool measures fade under exposure, not chemical stability, staining behaviour, or granulation, which is why the Chart lists it alongside those properties rather than instead of them. A colour can be Blue Wool 8 and still stain a mixing well permanently, or granulate unpredictably in a wet wash. It's one number in a fuller reading, and it's the one number we think is least defensible to leave off a label.