Wednesday, May 22, 2013

Tech Talk: Lightfastness tests – 5 months


full view of my test card –
sun-exposed half on left


center panel compares full-strength color –
sun-exposed paints to left of center


center panel compares tints (diluted washes) –
sun-exposed paints to right of center


Back in January of this year, I began a lightfastness test of these Mission Gold watercolors after receiving some samples of the paints. Because they are new to the market, there were no published lightfastness tests, and many of the pigments used in the manufacturing of these paints were poorly rated in the resources I have.  (See my previous posts here (introduction), here (at 2 weeks) and here (at 6 weeks).)  I put half of the sample sheet in a folder made of acid-free matboard and stashed it in my flat files.  My sun-exposed half of the test sheet started out with several hours of winter sun each day, but at this point, the sun is blocked most of the time by trees.  I've now moved the sample to a spot that gets sun for about 4-5 hours most mornings and will continue the test for about six more weeks.  However, there are such clear changes at this point that I would be remiss in not sharing the results.

I took these photos in full sun yesterday and have done a bit of digital cutting and pasting on the second and third images so you can more easily see the comparison of the full-strength pigments and the tints. (Note that the sun-exposed pigments are on the left side of the full-strength sample and on the right side of the tint sample.)

I should point out that pigments ranked as lightfast on some lists may indeed be lightfast in other media, but unreliable in watercolor paints. As I understand it, this is due to the binding agents used -- oil and acrylic paint bases being more protective of the pigments than the gum arabic used for watercolor.

As noted in earlier posts, the full-strength Permanent Yellow Light (PY17) has darkened with exposure and it has also faded significantly in the diluted wash. Quite a predictable outcome, since the pigment is known to be fugitive.

Yellow Orange (PY65 and PO13) is composed of a very reliable yellow pigment, but also contains the same unreliable orange pigment as Orange (PO13). Both show darkening at full strength and fading as a diluted wash, although it's more pronounced in the Orange sample.

The Permanent Red (PR112) is only slightly changed at either strength, but Permanent Rose (PR122 and PR209) is quite faded both at full strength and as a tint. A bit of a surprise, actually, since Permanent Rose is a mix of two supposedly reliable quinacridone pigments. [added note: There is some disagreement on the lightfastness of PR122, with Michael Wilcox labelling it as unreliable.] The Rose Madder (PR83:1) is actually alizarin crimson, a pigment which has been replaced in most professional-quality product lines by a more lightfast substitute. This paint changed only slightly at full strength, but is noticeably faded as a tint.

Permanent Violet (PV3:1) is anything but. It began showing signs of fading by the second week of sun exposure and is now seriously changed both at full strength and in the tint. This comes as no surprise, because the pigment is a known bad actor.

All three blue paints in my sample -- Ultramarine Deep (PB29 and PV12), Prussian Blue (PB27), and Peacock Blue (PB15:3 and PG7) -- seem quite reliable, despite the inclusion of a mystery violet pigment in the Ultramarine Deep. I can find no information in any of my resources on it, but it doesn't seem to affect the color stability.

Viridian (PG7) is actually phthalo green, which makes it very stable, but also a very strongly staining paint. Sap Green (PG36, PBr25, and PY17) includes the same yellow pigment as Permanent Yellow Light, which means it shares the same problems -- darkening at full strength and very noticeable fading in the tint.

Burnt Sienna (PR101) was a definite suprise -- fading badly across the board. Not a true burnt sienna, it is composed of a red pigment that is supposed to be very lightfast, but this sample didn't live up to the reputation of the pigment. 

VanDyke Brown (PBr9) is made of a pigment that does not appear in any of my resources, but is similar to one that is rated fugitive/unreliable. It faded particularly badly as a tint -- in both samples!! I had to go back to check my photos at 6 weeks to verify that there had been a noticeable difference in the exposed and non-exposed tints at that time. I therefore have to conclude that simply exposing this paint to air will cause it to deteriorate at tint strength.

So I'm sorry to say, but my advice would be to avoid Mission Gold paints as they are currently formulated -- unless you're planning to channel Picasso's Blue Period or keep your work in a drawer.

6 comments:

  1. Thanks for sharing your lightfast experiment - I could see right away, any with orange pigment had trouble in the sun and that is where one gets that POP of color in paintings.

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    1. There are a number of pigments that are unreliable in these paints, but the ones that are the brightest seem to be the most fugitive in many cases.

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  2. Thanks for posting this Chris. I've seen those paints and wondered about the quality.

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    1. You're welcome, Nancy. Quality is a concern with any paint -- sadly, pretty packaging and marketing don't always guarantee a reliable product. But starting with reliable pigments is a must -- something they have failed to do.

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  3. Always a good thing to do. I have color charts dating back 20 years and some (even in a closed drawer) have certain colors that have faded!

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    1. Hi R -- great to hear from you!! It's always a surprise to see what happens with color tests -- from the shockingly fugitive to the unexpectedly lightfast. I'd be interested to hear what paints you used to make the charts and which colors faded even without light exposure.

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