"Report ... On the Action of Light on Water Colours (1888) by the chemist Walter Russell and the amateur painter Capt. William de W. Abney."
The Blue Wool Scale
The Blue Wool Scale measures and calibrates the permanence of colouring dyes. Traditionally this test was developed for the textiles industry but it has now been adopted by the printing industry as a measure of "lightfastness" of ink colourants and also within the polymer industry for measurement of pigment & colour stability (lightfastness). Lightfastness is the chemical stability of the pigment or dye under long exposure to light.
Note: this should not be confused with Permanence or fastness which refers to the chemical stability of the pigment in relation to any chemical or environmental factor, including light, heat, water, acids, alkalis, or mold. For example, the pigment ultramarine blue is extremely lightfast, but it will fade if brushed with a dilute acid.
The Ultraviolet (UV) radiation in light is responsible for the ink fading and hence the change in the blue wool. Hence the blue wool scale has been widely adopted as a standard during UV exposure or UV weathering tests.
The normal procedure is to take two identical dye or pigment samples. One is placed in the dark as the control and the other is placed in the equivalent of sunlight for a 3 month period. The amount of fading is then measured by comparison to the original colour and a rating between 0 and 8 is awarded. Zero denotes extremely poor colour fastness whilst a rating of eight is deemed not to have altered from the original and thus credited as being lightfast. Most apparel will have a light fastness of 4 whereas most furnishings will have a light fastness of 6 and most polymer materials for outdoor use would require a light fastness of 7 or above.
Blue wool textile fading cards or kits typically consist of 8 swatches of blue wool dyed to various levels. They consist of eight strips of wool mounted side by side on a small card; each strip or reference is colored with a blue dye that fades after exposure to a known amount of light. The dyes have been chosen so that each reference takes about two to three times longer to begin fading as the next lower reference in the scale. (Under normal solar testing conditions, reference 1, the least permanent, will begin to fade in 3 hours to 3 days, depending on geographic location, season, cloud cover and humidity; reference 3 will fade in 5 days to 2 weeks; reference 6 in 6 to 16 weeks; and reference 8, the most permanent, in 6 to 15 months.) These scales are used for paint lightfastness testing under international standard ISO 105-B, and are also used by gallery curators to measure the accumulated amount of light received by museum displays of paintings, textiles or photographic prints. The blue wool scale cards willl normally be used in conjunction with grey scale cards in order to assess the degree of change.
(top) unexposed;
(bottom) exposed to sunlight
for 800+ hours
This table below gives one version of the eight blue wool lightfastness levels, from 1 (fugitive) to 8 (extremely lightfast), with the amount of light exposure required to produce a color change at each level and the approximate match between the eight blue wool and five ASTM lightfastness categories. This table is a guide for the selection of artist paints only and should not be used to extrapolate to other products.
Blue wool / ASTM lightfastness standards |
||
A |
B |
Comments |
8 |
900 |
I. Excellent lightfastness. Blue wool 7-8. The pigment will remain unchanged for more than 100 years of light exposure with proper mounting and display. |
7 |
300 |
|
6 |
100 |
II. Very good lightfastness. Blue wool 6. The pigment will remain unchanged for 50 to 100 years of light exposure with proper mounting and display. |
5 |
32 |
III. Fair lightfastness (Impermanent). Blue wool 4-5. The pigment will remain unchanged for 15 to 50 years with proper mounting and display. |
4 |
10 |
|
3 |
3.6 |
IV. Poor lightfastness (Fugitive). Blue wool 2-3. The pigment begins to fade in 2 to 15 years, even with proper mounting and display. |
2 |
1.3 |
|
1 |
0.4 |
V. Very poor lightfastness (Fugitive). Blue wool 1. The pigment begins to fade in 2 years or less of light exposure, even with proper mounting and display. |
A : Blue wool reference strip |
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Grey Scale for Change in Colour
The grey scale consists of nine pairs of non-glossy neutral grey coloured chips, which illustrate the perceived colour differences. These are used along side the blue wool samples to determine the corresponding fastness rating (as described in ISO 105-A02).
The problem with the blue wool scale and the grey scale is that it is a subjective test i.e. it depends on an experienced operator comparing changes in test swatches versus reference swatches. Hence it can lead to some variability. Secondly, significant changes on the blue wool scale are not particularly onerous with respect to outdoor performance of most moulded or fabricated products. Whilst it is a useful reference for printed medias and the textile industry, products that require significant outdoor exposure will require a rating significantly greater than the maximum of 8.
Most modern testing now uses colourimeters or spectrophotometers to accurately measure any colour change or colour differences and report these as a Delta Eab reading. At Materials Technology we will typically use the colourimeter measurement method using the L*A*B* colour space model along with Delta Eab readings. A full explanation and interpretation of this technique is supplied with our reports as standard.
A brief summary of the LAB colour space is also provide below. The diagram to the right demonstrates the L, A and B coordinate systen.
The L*a*b colour space system defines the colour of an object as a 3 dimensional coordinate within a colour sphere. The L coordinate represents lightness and runs from 100 (complete lightness) to 0 (complete darkness), a is the red direction –a is the green direction, +b is the yellow direction and –b is the blue direction. L*a*b readings taken are an average over a 2mm spot size in each case under standardised lighting conditions.
Delta E* represents the difference in each of the L, a and b readings compared to the initial readings and hence show the significance of change in each element. In quoting overall colour difference values it is common to produce a single value referred to as Delta E*ab, this is produced using Pythagoras theorem and gives an absolute value for the size of the colour difference (magnitude) , but not the direction of change.
A value of about 1 is just about perceptible to the human eye under normal lighting.