[Essentials] Blue Wool Standards – How to Use

• July 08, 2020

Blue Wool Standards: Purpose

Blue Wools are used for light fastness and weathering testing, to measure and calibrate the permanence of coloured materials, and have been used for many years to indicate the end of test exposure and for grading purposes. Testing a material’s reaction to light and moisture is one of the most important, yet traditionally difficult, areas of colour fastness testing, and it’s an area we receive many support requests for, from our customers.

Blue Wool Standards: Your questions answered

Below, we highlight some of the most common questions and challenges with using Blue Wools:

What is the fading time for ISO & AATCC Blue Wools?

There are two types of Blue Wool References: “ISO” and “AATCC”, and we’re often asked about Fading Time for both types. This is an important question because there are eight Blue Wool Standard fabrics (referenced 1-8) which have known, yet varied, resistance to light fading – each has approximately twice as resistant to light fading as the preceding fabric.

The Blue Wool Standards range from 1-8, starting with number 1 which fade quickly, through to number 8 which has a good resistance to fading and fades much slower. During light fastness testing, the samples are exposed to an intense artificial light generated by a Xenon arc lamp, which goes through a series of filters to ensure that its spectrum closely matches that of natural daylight. whilst the humidity and temperature of the test atmosphere are controlled.

Then, a small sample of material, together with the Blue Wool Standards are concurrently exposed to the high intensity light. Following this, the two exposure times are used to calculate if fading has progressed steadily, or at a different rate from the longer-term exposure.

ISO Blue Wools

The ISO blue wools are each dyed with a different dye giving rise to a light fastness scale which is an approximate geometric progression, ranging from 1 to 8. The full range of ISO blue references is currently available.

The typical conditions for ISO 105-B02 are:

• 42 W/m2 (300 – 400 nm)*
• 50°C BST
• 40% effective humidity (use red azoic fabric to determine).

*(The SolarSens used with TruFade has one light sensor, referred to as the 300 – 400 nanometre broadband sensor).

ISO Blue Wools Fading Times

• BW1 fades to grey scale 4 in approx. 6 hours
• BW4 fades to grey scale 4 in approx. 48 hours
• BW5 fades to grey scale 4 in approx. 80 hours

Some of the ISO methods use a ‘first break’ and a ‘second break’. If the second break is required then fading is continued further until greyscale 3 is achieved (this takes the same time again as it did to get to greyscale 4).

AATCC Blue Wools

The AATCC blue wools are produced from two dyed wool batches: one batch has a very low light fastness dye, and the other a very high light fastness. The two batches are then blended together in different proportions to give the range L2 to L9.

Currently, due to problems sourcing the dyestuff in the wider supply chain for health and safety reasons, the L9 (high light fastness) cannot be produced. As a result, only L2 is currently available.

The typical conditions for AATCC 16.3  are:

• 48 W/m2 (300 – 400nm)
• 63°C BPT
• 30% relative humidity.

AATCC Blue Wools Fading Times

L2 fades to grey scale 2-3 in 20 AFU (American Fading Units), which is approximately 20 ± 2 hours under the above conditions.

How many hours of daylight exposure does each Blue Wool Reference correlate with?

Blue Wool References are not correlated against actual daylight exposure. This is because real conditions vary. It is not just the light that needs taking into consideration, but the temperature and humidity also. As an example, we can say that if the irradiance is controlled at 42W/m² (300 – 400nm):

• Blue Wool 5 will fade to a grey scale 4 in approximately 80 hours
• Blue Wool 6 will fade to a grey scale 4 in approximately 160 hours

“Blue Wool References are designed to fade in a geometric progression. Each successive reference taking twice as long to fade to the same grade as its predecessor, e.g. a fabric with a colour fastness to light of 6 is twice as good as a fabric which has colour fastness to light of 5.”

Sara Williams, Technical Specialist, James Heal