The Blue Wool Scale – Hemingway & Wright

THE BLUE WOOL SCALE, HIBISCUS MATERIALS AND THE LIGHT FASTNESS OF INK

blue wool scale

The Blue Wool Scale measures and calibrates the permanence of colouring dyes.

Traditionally this test was developed for the textile industry, but it was later adopted by the printing industry as a measure of lightfastness for ink colourants.

Light fastness tests, using the Blue Wool scale as a reference, are carried out using a Xenon Arc lamp as a light source. Light from the Xenon lamp has the nearest artificial wavelength distribution to that of the sun.

The Blue Wool scale, with a range of 1 to 8, uses samples of wool dyed with 8 different blue pigments each of which fade after different exposure times: 1 being the least resistant, and 8 being the highest.

The test simply compares which blue pigment fades at the same time as the ink sample being tested. The ink is then allocated that number on the Blue Wool Scale.

Generally, the time the ink takes to fade usually depends where in the world you are. A dull climate will be very different to a desert exposure in Africa. In addition, the film-weight of the ink applied can also make a difference, as does the degree of white pigment within a given formulation.

Average values for time, as measured in the UK, for full strength ink is as follows :

Blue Wool 1 – A few days up to a week in dull conditions. A few days down to a few hours in full sun. Very Low resistance.
Blue Wool 2 – About twice as good as 1. Very Low resistance.
Blue Wool 3 – 4/6 weeks. Low resistance.
Blue Wool 4 – Up to 3 months. Moderate resistance.
Blue Wool 5 – Up to 6 months. Good resistance.
Blue Wool 6 – Over 6 months. High resistance.
Blue Wool 7 – Excellent performance, only a limited number of pigments give this level of light fastness, even at full strength.
Blue Wool 8 – Exceptional – virtually no fade.

Direct comparisons of the Blue Wool Scale to the light fastness required to meet BS 5609 do not exist.

Labels passing BS5609 have had light fastness values of Blue Wool 6+.

At Hibiscus we pride ourselves on the quality of our materials and the printing inks we use are of vital importance in the stability and durability of our labels.

All of our inks, for BS5609 standard labels, conform to Blue Wool Scale 7, or above. This ensures that your label details will not fade under UV light and will remain legible under the harshest conditions and environments.