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A comprehensive lightfastness test
2019/08/09
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This section presents complete instructions for lightfastness testing using the method that I believe gives the best overall results for watercolor paints.
My method is not exactly the standard (industry accepted) tests described by the American Society for Testing and Materials International, so I present my method in contrast to the ASTM recommended tests and my concerns with them. Before we get to that, I need to explain two important issues, measuring light exposure and preparing paint samples. Finally we get to a list of the required materials for the test and my recommended procedures for conducting lightfastness tests.
Measuring Light Exposure. Because sunlight intensity changes dramatically across the spring and fall equinoxes, and clear weather, longer days and more vertical sunlight produce quicker test results, the most effective test interval is between April 15 to September 15. But how do you determine the accumulated energy of light exposure?
The first method is to use a sample of alizarin crimson, exposed to light in the same way as the other paints you test (as described below). An obsolete 19th century pigment, alizarin crimson is too impermanent for artistic use, but it does make a useful exposure meter when painted out as a sample paint swatch.
OK, but at what dilution? For reasons explained below, I suggest a 1:12 or dilute concentration — about 1/4 teaspoon paint dissolved 1 tablespoon water, brushed out with a saturated 1" acrylic flat brush to produce a homogenous color.
When this swatch begins to fade, you have reached approximately one half the light exposure that any artist's grade pigment should withstand without visible color change. During the months of April to September in most USA locations, with clear weather, this will happen in four to six weeks. Then expose your test samples for twice the number of days it takes the alizarin crimson tint to fail. If the alizarin begins to fade after six weeks, expose the rest of your samples for 12 weeks. Don't count foggy or heavily overcast days; light overcast or partly cloudy days should be included. Paints that remain unchanged after this period are minimally adequate as art materials. To be really confident in your materials, they should remain unchanged for four times the alizarin standard — that is, four to six months of daily sunlight exposure.
The other and better solution is to purchase a blue wool scale designed to measure light exposure. This is especially handy for testing year round, testing across extended periods of variable weather, and testing cumulative indoor light levels (for example, the impact of gallery lighting on art displays). The brilliant blue reference strip is the most fugitive (level 1), the dull, dark reference strip the most permanent (level 8).
Golden Paints no longer makes their very handy Lightfastness Test System, but you can still purchase individual blue scales (textile fading cards) from TALAS in New York. The blue scales cost about US$10 each, less in quantities of 10 or more.
For instructions on how to perform the blue wool tests, you can buy a reprint of the ASTM's Standard Practice for the Visual Determination of the Lightfastness of Art Materials By the User (D5398-97) for US$25, if you can't find the encyclopedic ASTM standards volumes in your local city or university library. Or you can use the excellent lightfastness test instructions in Mark Gottsegen's painting handbook (pp. 128-31).
In general, as soon as a significant change occurs in the exposed area of a blue wool reference, then the light exposure has reached that level of the blue wool scale. All paints that change after that point, but before the next reference changes, have that level of lightfastness. That is, every paint starts with a lightfastness of 1, and remains at 1 even after blue wool reference 1 changes. When reference 2 changes, all paints that have not yet changed acquire a blue wool rating of 2, as do all paints that change before reference 3 changes — and so on up the scale. When reference 8 changes the test is complete, and all paints still unchanged receive a rating of 8. (Use this table to translate the blue wool levels into conventional lightfastness ratings.)
"A significant change" means an unmistakable, definite discoloration: it must be clearly visible when viewed through a 5mm x 40mm (3/16" x 1-5/8") rectangular window cut in a piece of medium gray card or construction paper. Buy a sheet of medium gray construction paper or art board and make one of these cards for yourself; you will need it for the tests described later.
This window equalizes the visible area of the blue wool scale and paint samples, and reduces the area enough to hide the slight dulling of color that usually occurs before the sample actually fades.
Finally, the samples must be viewed under bright indirect (indoor) daylight or under an artificial light source of color temperature between 5000K to 7500K and a CRI of 89 or higher. This essentially means samples must be viewed under four 150 watt tungsten halogen bulbs or a photographer's incandescent flood lamp, or a 200 watt metal halide bulb of 5400K or higher. It explicitly excludes all fluorescent, sodium and mercury lights: these will make some color changes very difficult to see.
Preparing Paint Samples. Lightfastness results are greatly affected by the density or thickness of the test sample, the fillers or extenders added to the pigment in the paint, and the paint vehicle. You've already determined the vehicle and the amount of fillers or extenders in the paint by your choice of watercolor paint brands. So the only issue is the required concentration or dilution of the paint used to prepare the test samples.
Here I disagree with the ASTM standards, which stipulate (ASTM D5607-99, p.3) a liquid draw down sample over Whatman white filter paper, at a dilution that produces approximately a 40% reflectance in the most absorbing (darkest) part of the paint's reflectance curve. That is, the point of minimum reflectance in the reflectance curve must be about 40%; all other parts of the curve will have an equal or higher reflectance. This gives a homogeneous color at a specific paint density, but in most paints the result is a fairly thin color appearance.
How thin? Well, a lamp black paint has a roughly flat reflectance profile at around 5%, so a minimum reflectance of 40% means a total reflectance of 40% across the entire spectrum, which in the reflectance to lightness curve equals a CIELAB lightness of about L* = 69 (a Munsell value step 7), which is a light gray.
We can translate this lightness into an approximate dilution recipe if we consider the paint's value range, which is provided for all watercolor paints in the guide to watercolor pigments. The value range of a black watercolor is around 75, and a lightness of 69 defines a value range of 28, which is roughly 40% of the masstone value range for black.
Since black is the darkest paint, we can conclude that every other paint prepared to the same specification must have a lighter value than L* = 70. Inspection of pigment reflectance curves (linked from the spectrum icon in the notes to major pigments in the guide to watercolor pigments) indicates that nearly all paints have a minimum reflectance of 15% or less at some point in the curve. So we can apply the 40% ratio to the value range of a paint to determine the dilution of that paint necessary for a lightfastness sample.
However, we are really only interested to prepare an accurate exposure sample of alizarin crimson (PR83). So we first look up its value range across all the watercolor paints tested, which is about 55. Then the light exposure samples described above must have a value range of 22 (that is, 40% of 55), or a CIELAB lightness of about 76. The samples below show these value differences; print the image to compare with the lightness of your own paints.
Most watercolor painters will have the immediate reaction that these are strongly diluted paints. And while watercolors are used in diluted concentrations, they are also commonly used at nearly full strength. The ASTM standard is not representative of common painting practice.
The wisdom of experience is that diluted paints are less lightfast than concentrated paints, so measuring the diluted paint is a more rigorous or conservative test of paint permanence. But this is not always true. Some pigments are prone to darken in moderate to thick concentrations and are actually more durable in dilute applications. As I've documented for several common pigments (PBr7, PR101, PR108, PY35, PY42 and PY184), discolorations appear in concentrated samples of paints that appear "totally lightfast" as diluted layers in my own tests. So the better way must be to test paints in both concentrated and diluted samples. Surprises do happen!
Here is how to get the necessary paint variation. Charge a flat acrylic brush in the paint mixture, wick it gently against the side of the paint well, and paint half the length of the test swatch with a single, even stroke of the brush. Partly rinse the brush, shake out excess liquid, then continue the stroke to the full 3" length, using only the water remaining in the brush. The first brushstroke with concentrated paint should vary from a wet masstone to a thinning, nearly dry middle color, while the second stroke with the moist, rinsed brush should dilute the end of the stroke into a tint as light valued as the diluted samples shown above.
When dried, the swatch should have a dense, almost full strength concentration of paint at the "masstone" end, and a tint as light as the samples above at the opposite end. You want the stroke to vary from masstone to tint across its length, but to have even color across its width at every point. Wick up excess paint with a clean, thirsty brush, if necessary, to minimize backruns or blotching at the "tint" end of the swatch. (See samples at right.)
If this is too difficult, try painting the top third of the swatch in the masstone mixture, then pull the stroke down the rest of the way with a brush moderately charged with clear water.
An alternative procedure is the draw down sample. With a brush, prewet the lower half of the swatch area with clear water to a moist to satin wetness, then place a small bead of paint on the paper at the top of the swatch area. Set the edge of a putty knife or large palette knife on the paper just above the bead, hold the blade at a 60° angle to the paper, and press into the paper until the blade is slightly flexed. Then pull the knife sharply down and across the paint bead to the bottom of the swatch area. This smears the paint across the paper in an even, thin coat: the prewetted area should hold more paint to give a darker color. The drawdown method is not good for testing the complete range of paint dilutions, but it is very quick and easy to do, and extremely consistent once you get the hang of it.
ASTM Recommended Tests. Once you've prepared paint samples and have a blue wool scale, you can do an ASTM standard test. Both tests require a viewing card to control visual examination of the paint samples, and samples mounted or framed under glass and light protective strips of aluminum tape, as described for the quick & dirty lightfastness test.
There are two versions of the ASTM procedures, one for general use and the other for technical use. First, here is the method recommended for general use by artists:
There is evidently some concern about the validity of the references 1 to 5 on the blue wool scale, because the more rigorous method for using the blue wool scales, as described by the ASTM in their Standard Practice for the Visual Determination of the Lightfastness of Art Materials by Art Technologists (D5383-02(2003)), collapses these references into larger categories and relies on the apparent contrast, rather than the timing of the color changes, to define lightfastness levels. This more rigorous procedure is as follows:
Judging the amount of contrast or change in two different colors is somewhat subjective. The ASTM attempts to address this problem by having three individuals with normal color vision separately make these judgments, then averaging their ratings to obtain the final result.
I have less confidence in this approach, because it is in fact difficult to compare the amount of contrast in any light valued yellow or orange paint sample, or any color applied as a tint, with the contrast in the dark valued blue wool references, which are 70 units away from white on the CIELAB L* scale.
Another problem is that paints change in different ways across time. There is variety in the color change itself. Most paints seem to fade and become duller in both masstone and tint, but there are many exceptions. Chinese white and convenience paints mixed with it will become whiter and more opaque. Impure synthetic inorganic paints, especially yellow cadmiums, will gray or blacken in masstone but change relatively little in tints. Aureolin grays in masstone and fades in tints. In paints that gray or darken, the color change is in the opposite direction from the blue wool scale.
It is much easier to identify the time at which any change first occurs in the paint samples or reference scales. But there is also variety in the rate of change. Durable synthetic organic pigments, such as the quinacridones, pyrroles or dioxazines, will fade very slowly but steadily; others, such as alizarin crimson, will hold strong for a while and then rapidly collapse; still others, such as prussian blue, will show slight fading in the first few days, then remain completely unchanged thereafter.
The ASTM method of evaluating visible contrast after a specific exposure time attempts to get around these complications, but consistent visual evaluation of color changes is still difficult to do.
The method I used in the 2004 paint tests, and the method I suggest you use too, is to review the samples at regular intervals and note the number of days (weeks) of exposure at which each blue wool reference first begins to fade, as well as the time at which each paint sample begins to fade or darken. When the exposure has reached BWS 6 and higher, you (and two painting colleagues) can make the contrast matching judgments between paints and the blue wool reference strips. Using both time and contrast, you make a better judgment of color change in the paints.
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