Short answer
They are ranks, not lifetimes. The Blue Wool scale grades a colourant 1 (very fugitive) to 8 (excellent) by which of eight standard dyed wools fades as much as it does under the same light; the ASTM scale used for artists' paints runs from I (excellent) to V (very poor), with I corresponding to Blue Wool 7–8. Each Blue Wool step is roughly two to three times more resistant than the one below, so the scale spans a very wide range of exposure.
Colourant labels use three related systems. The Blue Wool scale, from the ISO 105 textile fastness family, ranks light fastness 1–8 by comparison with eight reference wool strips dyed with blue dyes of graded stability. The grey scale, 1–5, rates how large a colour change or staining is, and is used for wash, rub and other fastness tests as well as for judging fading. The ASTM categories I–V, used on many artists' paints in North America, group colourants by the colour change measured after a specified exposure. They are linked but not interchangeable: a common correspondence places ASTM I with Blue Wool 7–8, II with 6, III with 4–5, IV with 2–3 and V with 1.
The Canadian Conservation Institute describes each blue wool as about two to three times as sensitive to light as the next one up. The scale is therefore geometric: Blue Wool 8 needs very roughly a hundred to two thousand times the exposure that fades Blue Wool 1 by the same amount. That is why the difference between a 6 and a 7 matters so much more than it looks, and why a rating is best read as the order of magnitude of light a colour can take. CCI groups the grades into its museum sensitivity categories: 1–3 high, 4–6 medium and 7–8 or above low, each with a typical number of years to a just noticeable fade at 50 lux.
A rating is tied to the test: its light source, its humidity and temperature, the concentration of colourant and, for paints, the binder and whether the colour was tested at full strength or as a tint. Many pigments fade faster when let down with white than at full strength, because there is less of them to lose. A rating does not cover darkening, yellowing of the binder, or chemical reactions with other pigments, and it cannot say how long a picture will last on a particular wall — that depends on the light it receives. The Colour Index warns that products under one pigment name can differ in fastness, so a rating belongs to a product, not to a pigment name.
For work meant to be displayed for decades, conservative practice is to choose colourants rated Blue Wool 7–8 or ASTM I, accept ASTM II or Blue Wool 6 with care, and avoid anything lower for display. Treat ratings from different systems as approximate equivalents, and prefer a rating that states the standard used. Where a label gives only a star or 'permanence' code, check how the maker defines it. And remember the rating assumes indoor, glazed or typical display conditions; direct sunlight is many times more intense, so even a high rating can show change within a few years in a sunny window. The colour-ageing pillar explains how the tests themselves are run.
| Blue Wool grade | ASTM category (approx.) | CCI sensitivity category | Exposure relative to Blue Wool 1 | Years to just noticeable fade at 50 lux (category range) |
|---|---|---|---|---|
| 1 | V | High sensitivity | 1 (reference) | 1.5–20 years |
| 2 | IV | High sensitivity | 2–3× | 1.5–20 years |
| 3 | IV | High sensitivity | 4–9× | 1.5–20 years |
| 4 | III | Medium sensitivity | 8–27× | 20–700 years |
| 5 | III | Medium sensitivity | 16–81× | 20–700 years |
| 6 | II | Medium sensitivity | 32–243× | 20–700 years |
| 7 | I | Low sensitivity | 64–729× | 300–7,000 years |
| 8 | I | Low sensitivity | 128–2,187× | 300–7,000 years |
Why: Ratings describe relative resistance under test conditions; direct sun delivers far more light and UV than indoor display.
Fix: Hang work out of direct sunlight or behind UV-filtering glazing, and choose Blue Wool 7–8 / ASTM I colours for display.
Why: Fastness belongs to the product — grade, concentration, binder — not just the pigment name.
Fix: Look for the maker's own tested rating, not only the pigment code.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactModerate evidence
The Blue Wool scale rates lightfastness 1–8, the grey scale 1–5, and the ASTM scale I–V; ASTM I corresponds to Blue Wool 7–8 and ASTM V to Blue Wool 1.
Source: Lightfastness
FactStrong evidence
Each ISO blue wool is about two to three times as sensitive to light as the next; CCI defines high sensitivity as blue wool 1–3, medium as 4–6 and low as 7–8 or higher.
Source: Agent of deterioration: light, ultraviolet and infrared
Colourwise analysisModerate evidence
On CCI's two-to-three-fold spacing, a Blue Wool 8 colourant tolerates very roughly 128 to 2,187 times the exposure that produces the same fade in Blue Wool 1.
Based on: Colourwise calculation: 2⁷ and 3⁷ from CCI's stated factor of two to three per blue wool step; see the table.
Caveat: The step factor is approximate and varies along the scale.
Source: Agent of deterioration: light, ultraviolet and infrared
FactStrong evidence
Products listed under the same Colour Index Generic Name may differ in fastness properties.
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.