Friday, June 16, 2017

Fade Test for Gouache

A little more than nine months ago, the Shinhan company of Korea sent me a set of their Pass Design watercolor/gouache hybrid paints to try out. 


I gave a preliminary review of the brand's working properties after a painting a pharmacy using a limited palette of colors. 

I also painted a set of test swatches. The top half of each swatch is pure paint from the tube. The bottom half is tinted with some titanium white. 


I then cut each set of swatches in half. The left half of the swatches went into an envelope in the basement, where they remained dark and cool for more than ten months. 

I put the other half of the strips into a south-facing window, where they received full sunlight. I re-united the two halves, with the exposed half on the right. If there's a color change from the left side of the swatches to the right, it's because of the difference in exposure.

The overall impression is how stable and lightfast most of the colors are, such as 880 (Raw Umber) and 885 (Burnt Sienna) below. With those lightfast colors, you would be safe framing and displaying your gouache painting even near a sunny window inside your house.

Pigment, lightfastness, chemical descr., and pigment (CI) identification
808 Bright Red ** Rhodamine 6G, Monoazo, BR1, PR48:1
810 Vermilion Hue * Rhodamine 6G, Monoazo, PO16, PR48:3
811 Orange ** Diazopyrazolone Arylide Yellow G, PO13, PY1
861 Red Violet * Zanthine Rhodamine 6G Lake, PR81, BV11
868 Shell Pink ** Diazopyrazolone Naphthol AS, Titanium Dioxide, PR 48:3, PO16, PW6
865 Opera * Tetrachlorozincate, Rhodamine 6G Lake, BV11:1, BR1
880 Raw Umber *** Synthetic Iron Oxide, PR101
882 Light Red *** Synthetic Iron Oxide, Disazo Pyrazolone, PR101, PR60:1
885 Burnt Sienna *** Synthetic Iron Oxide, PR 101
However, there are some colors, particularly those in the pink and red range, that suffered significantly from light exposure. I was surprised that the fading was worse in the bottom half, where the pigments were tinted with white. Even some famously stable pigments, such as Light Red (882 above), faded when tinted with white. 

Fortunately, the manufacturer gives detailed information about every pigment, and the pigments that suffered the worst light damage for the most part had low lightfastness ratings ( one star * in a range up to ****).

There are only 8 one-star rated pigments in the set of 48 colors. Those fugitive pigments would be better used in design applications or sketchbooks where they won't be exposed to light. 
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5 comments:

Eve said...

Thanks for this update! I recently bought this set and this information couldn't come at a better time.
Sorry for the newcomer question, but have you done a similar test with W&N's gouache? I noticed that they seem to use a lot more less-lightfast pigments in their gouaches, compared to the watercolors.

Roberto Quintana said...

Very Good!-
I am surprised by the more dramatic fading of the tints as well. I suppose one explanation would be that the tints contain less pigment % in the mixture and so the de-saturation of the total mix is more apparent (since the white remains constant). I have seen this in outdoor murals where in a mixture of colors, say a brown-mix, will shift over time as the more fugitive reds or yellows will fade out of the mixture faster than the blues. I also wonder if the white might somehow accelerate the effect of the light in the mix as the white reflects or scatters the light into the colored pigment? -RQ

David Stay said...

I wonder if the bleaching is being accelerated by the titanium dioxide in the titanium white. TiO_2 is often used as a catalyst for photobleaching. Here is a paper I found after a quick search which is looking at the photobleaching ability of TiO_2 on various organic dyes.
https://www.ncbi.nlm.nih.gov/pubmed/11838435

James Gurney said...

Roberto and David, thanks for that. Interesting hypotheses. I guess one way to test them would be to do a transparent tint of all the colors next to equivalent tints done with titanium dixoide and zinc oxide to see of those two whites behave differently.

Dmitry Kharitonov said...

For a long time wanna try this paints, and going to buy them individually around 10-15pcs, can anyone suggest which color to take?
thought about buying white, indigo, yellow ochre, perm. yellow, light red, ultramarine, alizarin, vermilion, carmine, prussian blue, cobalt blue hue ++
But not sure if i'm not missing something
Thanks in advance