Effect of light on paint

Couple of months ago I started this small experiment to test how light affects various pigments and mixtures of paint that we commonly use in painting. I am interested in some particular pigments since although they are very popular and were traditionally used, however now we know that these are in fact not as stable as one thinks and hence they tend to disappear with time especially on exposure to light. The technical term used in labelling on tubes is "lightfast" usually graded (numbered with an ASTM prefix or starred) with 1 being the best hence meaning that that pigment is very resistant to light even if exposed to sunlight. As an artist I am always trying to use the best and most stable pigments available on the market, preferably from brands that use the minimum amount of fillers and driers, since both of these affect the stability of the paint as time passes. My objective is always to give the best quality artwork that lasts for long time to my clients. Besides that I also want that the hours that I spend working on a painting will be worth it.

The experiment is going to take at least a year and today I am reporting some of the preliminary observations I did after 5 months from the start. The paints that I used were: lead/zinc white - PW1/PW4 (Old Holland), Ultramarine Blue - PB29 (Michael Harding), Cobalt Blue light - PB28 (Norma), Pyrrole red - PR254 (Langridge), Quinacridone Magenta - PR122 (Langridge) and Rose Madder Genuine - NR9 (Michael Harding).

I prepared some colour swatches on cotton canvas after first mixing the colour and preparing tints to around mid-value 5 on the Munsell scale on a glass palette. I prepared three replicates of each colour, one for each lighting condition. 3 lighting conditions were used: (1) normal room light conditions (similar to same condition a painting is normally hanged); (2) No light stored in a dark cupboard; (3) Exposed to sunlight in a window. I used these 3 lighting conditions because I wanted to test the performance of the pigment in conditions as close to the real situation as possible. So although manufacturers test the lightfastness by exposing the pigments to light however normally paintings are kept hanged on walls in doors and are not exposed to the lighting conditions used for testing. Also one needs to remember that each pigment might behave the same in oils and acrylics and differently than watercolour, so one needs to look at the lightfastness according to medium.

After 5 months I took them out to perform the first observational analysis by first photographing them and then analyzing the photos using a photography software to measure the intensities of the values and the three primary colours of light known as the RGB system (Red, Green and Blue). Using the software I used the colour picker function taking readings from various spots across each coloured squares and then calculating an average value. I took readings from areas having a similar value, using the values to normalise the results. I then calculated ratios of red/blue to check for colour shifts in the mixtures. A common mix of grays used in landscapes is that using either ultramarine blue or cobalt blue with one of each of the above red paints toned to a mid-tone value.

What I observed? So far the results didn't show any shift in colour between the conditions for the mass tones. Mass tone refers to the colour as it comes out from the tube. A control consisting of cadmium yellow light (PY35) known to be excellent lightfast did not showed any changes.
Some slight changes were observed in the mid-tone of Rose Madder Genuine and Cobalt blue exposed to sunlight. It was expected that Rose Madder will show a difference as it is known not to be excellent lightfast, however cobalt blue was a very surprising finding.
Both mixtures of cobalt and ultramarine blue with rose madder show a decline in red colour with ratios 0.58 vs 0.45, respectively, between dark and sun exposed. There were no changes for magenta however a slight change was observed for pyrrole when exposed to sunlight which I need to follow up with more testing on this pigment.

The pigments were returned to their original conditions for further testing within couple of more months. From my observations so far it seems that magenta is the best cool red to choose. I will need to compare the pyrrole red as a cool-warm red with permanent orange PO73 that can be used to give grayer hues when mixed with ultramarine blue.