Blue Wool Scale and Grey Scale: How to Rate Light Fastness Accurately

Blue wool scale and Grey scale are both color assessment tools used to measure color change by color fastness. Grey Scale, with a range of 1-5, is used to measure changes in color due to washing or environmental exposure. Blue wool Scale, with a scale of 1-8, is particularly made for Color Fastness to Light.

Grey Scale vs Blue wool Scale — Grey Scale vs Blue Wool Scale

Contents hide

1 Decoding the Blue Wool Scale (The Secret of Grades 1-8)

1.1 The Logarithmic Rule

1.2 Practical Example

2 The Photochromism Trap: Avoiding False Failures

2.1 The Phenomenon

3 ISO vs AATCC Rating Systems

4 Blue Wool Stability and Batch Consistency

5 The Hidden Variable

6 Conclusion

7 Related Resources

Decoding the Blue Wool Scale (The Secret of Grades 1-8)

In the textile industry, the blue wool scale measures the color intensity and dye persistence of dyes. It is used for color-fastness to light and weather testing to indicate the level and effects of exposure after tests on the light fastness tester.

Blue wool scale is made of wool due to its high resistance and low humidity resistance, which greatly affects color fastness to light. It is an eight-point scale, with 1 being the lowest level of light fastness and 8 the highest.

The Logarithmic Rule

The Blue Wool scale is designed using a logarithmic progression. This design principle is simple. Every successive scale takes twice as long to show the same degree of fading. If a sample takes 20 hours to reach level 3 of the blue scale, it will take 40 hours to reach level 4, and then 80 hours to reach level 5, and so on. This logarithmic progression is useful in several contexts where the fading of life spans over several months or years.

Practical Example

The major factor in Light fastness is UV (Ultraviolet) radiation. The UV radiation intensity differs from place to place. In the textile lab, these conditions are artificially created by instruments such as the Xenon Arc Tester.

During this test, humidity and temperature are controlled, and fabric samples are exposed to UV rays through a series of filters to replicate natural sunlight. For standardization and comparison, the blue wool scale is used. If a colorant has a rating of “BW6”, it will have the same degree of fadedness as level 6 of the blue wool scale.

The Photochromism Trap: Avoiding False Failures

Photochromism is a reversible transformation that results in the temporary fading of color upon exposure to light. These dyes are photosensitive but revert to their original color when UV rays are removed. So, to avoid this phenomenon in results, it is advised that the samples should be placed in the conditioning room according to the testing standard, so that they will be recovered before grading.

The Phenomenon

In this process, dye molecules are affected by exposure to light. These photosensitive dye molecules, upon exposure, are excited from their ground state to a metastable state. This triggers a photolytic reaction that damages molecular structures, leading to fading. Therefore, shade depth is inversely proportional to the effect of light. UV rays affect these molecules in every exposure. Dye’s molecular structure and its binding with other molecules also play a great role in color fastness to light.

Sample and Blue wool For Color Fastness to light — Sample and Blue wool for Color Fastness to light

For example, if a sample contains 500 molecules of colorants and UV rays affect 50 molecules, then 10% of the shade is lost. But if a sample contains 1000 molecules of colorants, and UV rays affect 50 molecules, then only 5% of the shade is lost.

ISO vs AATCC Rating Systems

ISO and AATCC both use a Xenon Arc lamp that emits UV rays resembling sunlight. Both standards have the same 1-8 blue wool scale for evaluation. But both standards use different test parameters and units for results.

ISO 105 B02 has 4 test options: A1, A2, A3, and B. Each option has different humidity and temperature levels. Most commonly used is option A2. The test results are reported on the blue wool scale.

AATCC 16 has 6 testing sub-methods, i.e., Option 1, 2, 3, 4, 5, and 6. Each option allows evaluation in different light sources and conditions. Most commonly used is option 3 with the Xenon Arc lamp. It has continuous light and Black Panel option. It resembles ISO 105 B02 A2. It measures the test results in AFU units (American Fading Units).

Blue Wool Stability and Batch Consistency

Blue wool scale is made from two types of wool batches. One has poor lightfastness, and the other has excellent lightfastness. Both wool batches are mixed with different proportions to give a range from grade 1 to 8.

To ensure consistent test results, batches are tested, and manufacturers provide correction factors. Buyers can check their batch number listed on the certificate of conformity. The batch number relates to the wool batch used in the batch formulation.

The Hidden Variable

Always keep in mind that grey-scale assessment is subjective; i.e., a lab technician or observer must be professional. He must have extensive experience comparing test samples to reference swatches.

Temperature, humidity, variable light intensity, and other environmental factors also affect test results. A change in UV wavelength can also affect test results.

Conclusion

For Textile lab and technicians, the Blue wool scale has a recommended lifespan of almost 2 years if stored properly. For optimum performance, always store them in a cool, dark place and avoid exposure to sunlight. The ideal place to store these standards is in a dark room, in their original packaging.

For enhancing the fabric color fastness properties, special UV finishes and coatings are applied to the fabric. UV stabilizers can also be added during dyeing. These finishes absorb UV radiation and protect dye molecules from damage, retaining their color.