Lightfastness

Lightfastness

Light fastness should be defined in terms of the entire paint, not only for the pigments itself. Binders impart an additional degree of protection for the pigment. Other components such as the presence of water or chemicals, for example, in the atmosphere or in the color system itself will also affect the lightfastness.

Mixtures with other pigments can also affect the light-fastness:

  • Titanium dioxide, for example, promotes the loss of light in real-organic pigments. Therefore, high mixing ratios of titanium dioxide can result in poorer light fastness.
  • Iron in turn can improve the light fastness of organic pigments, due to the fact that it is an effective absorber of UV light.

In order to compare the light fastness of pigments internationally the "Blue Wool / ASTM Light Fastness Standard" has been established. The "Blue Wool Scale" has its origin in the textile industry. ASTM D-4303 is the "standard of the American Society for Testing and Materials".

The following table gives an overview of the eight blue woolscale units 0 (extremely poor light fastness) to 8 (extremely light-fast), and the approximate correspondence between the blue wool scale standard and the five ASTM lightfastness categories.

The units of the "Blue Wool Scale" as well, and the ASTM are used for marking the lightfastness of oil paints.
The following table is a guide for the selection of artists' colors in terms of their light fastness.

Blue wool  ASTM lightfastness standards
8 I. Excellent light fastness
Blue wool 7-8.
The pigment stays unchanged with more than 100 years exposure time.
Suitable for artists' colors
7  
6 II. Very good light fastness
Blue wool 6.
The pigment remains unchanged at an exposure time between 50 and 100 years.
Suitable for artists' colors
5 III. Average light fastness (inpermanent)
Blue wool 4-5
The pigment remains unchanged for 15 to 50 years.
Can be used as an artist in full tone color or in protection against light.
4  
3 IV. Schlechte Lichtechtheit
Blue wool 2-3
The pigment begins to bleach after 2 to 15 years.
Not suitable for artists' colors.
2  
1 V. Sehr schlechte Lichtechtheit (Flüchtig)
Blue wool 1.
The pigment begins to bleach after about 2 years.
Not suitable for artists' colors.

... Lilly Artist Oils are characterized by the means of the ASTM standard.
Two exceptions (PR83.1 and PY3) are in our range off colors with lightfastness II (7-8).
Both pigments were included due to their history and common usage.