Lightfastness Rating - What It Means

The lightfastness or permanence of a pigment is its resistance to change on exposure to light. This depends on the chemical nature of the pigment, its concentration, and the medium in which it is employed — oil, watercolour, acrylic.

Permanence is especially important in painting because the length of time a pigment retains its original colour value determines the life expectancy of the work of art.

The pigment must be lightfast. Pigments must also be chemically and physically stable and must not break down or change in any way as they age So you can see that it is important to know your pigments!

As an aid to artists, our comprehensive Oil and Acrylic Colour Charts list not only the colour name, but also individual pigment lightfastness rating, transparency and opacity details for every colour.

There is now a worldwide standard for permanence. The independent ASTM (American Society for Testing and Materials standard) was established and ratings from this organisation represent the most standardized classification in use for artists’ materials. ASTM classification monitors quality, permanence and health guidelines. These pigment ratings were established in 1984.

To help you identify paints that are lightfast, look for the official ASTM rating information on individual paint tube labels. You will also find full ASTM information on each brands Colour Chart.

The lightfastness ratings are:
ASTM I — Excellent Lightfastness
ASTM II — Very Good Lightfastness
ASTM III — Not Sufficiently Lightfast to be used in artists’ paints

NOTE: The ASTM ratings were done many years ago by Henry Levison (the inventor of Liquitex) during his retirement and very little updating has been done since. As a consequence we have some colours without ASTM ratings, but which do have BS ratings. We therefore also give the B/S (British Standard) woolscale ratings, which have been used to evaluate a larger number of pigments, and which give more specific information noting pigments at full strength, mid tint (with white added) and pale tint, (more white added).

The BS rating for pigment lightfastness goes from 8 which is absolutely lightfast to lower numbers which are progressively less so.

It also shows that any colour in mass tone is more lightfast than when tinted with white: 1/3 ISD (International Standard Depth) is about ½ to 1/3 strength and the lowest is 1/25 ISD which is a very pale tint.

The information is more accurate than the ASTM ratings, because the pale tints at 1/25 ISD level were never tested by ASTM and it is in the palest tints that unreliable colours fade most.

Our stringent controls mean you can have total confidence in the permanence of every pigment used in all of our professional artist’ paints.

For more information on ASTM ratings please visit www.astm.org