The focus of this essay is on an apparent lack of consistency that has been going on for decades. ASTM (The American Society for Testing and Materials International) formed a subcommittee many years ago for the purpose of creating standards for art materials. One of the subcommittee’s crowning achievements was to create the underlying foundation for United States federal health and safety regulations related to the labeling of art materials. The label: “Conforms to ASTM D-4236,” is found on all art materials sold legally in the US. The other is the standard D4303 for testing art materials for light fastness.
While the title of this entry may indicate that I am about to commence on a rant about metric verses imperial units, that is not the case. Despite the fact that in 1999, NASA lost a Mars orbiter because an engineer used imperial units as opposed to metric units. With a loss of $125 million dollar spacecraft, I bet that engineer responsible is no longer working in the space industry. Nobody is suffering the same fate in the art materials world despite disparities in labeling.
I want to concentrate on why the art materials industry maintains so many different and, in some cases, conflicting data on light fastness test results. The light fastness test developed by ASTM for art materials is very rigorous. All forms of artificial, accelerated testing has issues because the act of accelerating and amplifying the test process has the potential to produce physical/chemical aberrations that would not appear if test samples were aged slowly and naturally. Unfortunately, none of us has 50 to 100 years to wait for a real-time test result. However, the chemical makeup and a history of fading performance gives us many indicators of how a pigment will behave when exposed to light energy.
So, we have some level of assurance that when testing pigments, much of what we know that happens to colors in the real world is reflected in accelerated test results as well. Pigments we know that are unwavering perform as expected and pigments that have fugitive properties are revealed in accelerated testing.
OK, so now you should presume that every manufacturer testing pigments would publish the same results. Unfortunately, that is far from the truth. It appears that manufacturers all have different “yardsticks” or “meter sticks” (for our metric colleagues,) when it comes to reporting light fastness results.
Let’s examine how a chemical family of pigments known to fade substantially is reported by various manufacturers. The pigment in question is Alizarin Crimson and related pigments that use a natural extraction or artificial laboratory born mixture to obtain the color of alizarin. Its chemical name is 1:2 dihydroxyanthraquinone and unless specified, today it is synthetically derived rather than produced naturally from the perennial, Rubia Tinctorum, the plant that traditionally was the traditional source of this red dye colorant. Madder, it is one of the colors that goes through the process if laking in order to bind the dye to a substrate so that it can be used as a pigment.
When examining the light fastness ratings manufacturers report on Alizarin Crimson, we are confronted with the first major hurdle. Does everyone use the same measurement and reporting system? Unfortunately, not at all. ASTM developed a well thought out, objective measurable system using a spectrophotometer and ranking of pigments into ranges based on the amount of fading that occurs after light exposure. The system involves use of Delta E calculations for determining the amount of change that occurs in an exposed sample when compared to an unexposed sample. The ASTM ratings use Roman Numerals from one (I) being the best to five (V) being the most fugitive.
If you reference Wikipedia as a resource for research this subject, the author of this entry misconstrued the rating system and even went as far as to call the ASTM organization the “American Standard Test Measure.” I never heard that name bestowed on ASTM.
The chart in the Wikipedia entry displaying the exposure time for testing and longevity ratings are simply wrong. So, there is a yet another source of confusion.
Many artists simply want to know, how long will a pigment continue without showing signs of fading. They gain solace when they find a chart that says a pigment will remain unchanged for 100 years. But that prognostication does not take into account the environment that the artwork will be in and the light levels where a work is displayed. It does not consider binder degradation or even how different binders and additives promote or inhibit fading.
In addition, some critics of light fastness testing upon first learning about the test method, express concern that the test environment is wrong. They quickly say, “Paintings are not hung outside (referring to the Arizona outdoor test) so the results can’t be valid. My reply is that we are not testing a painting and its related components. ASTM D4303 was designed with great care to focus on testing the strength of a pigment to withstand light energy exposure and either remain unchanged or fade. Using an outdoor sun test is considered the “gold standard” in the weathering industry. We are comforted by the fact that many pigments remain unchanged after sun exposure. This is not surprising because many pigments are marketed for use in harsh applications where high heat or light exposure is the rule.
The Blue Wool Issues… to be continued in the next posting…
The Syntax of Color