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I have a bit of a minor mystery that I'm hoping you can help me with.
I have been testing a lot of different paints from various brands when mixed with walnut oils to a more fluid texture and stored in the dark in a box in a wardrobe at room temperature.
This was to find the paints that stay open the longest without the use of clove oil, or other techniques.
As part of this I tested 9 different paints of either PB15 or PG7 (and in one case, a mix of both). From these tests I concluded that Phthalo pigments are indeed fast dryers as all of them except one totally dried in 2-8 days.
The one exception was a tube of Royal Talens Van Gogh Phthalo Green (PG7). This was still open for a few weeks when I had to stop the test and go on holiday. When I got back I thought I'd test all the colours to see if any were still open after all this time.
To my astonishment the paint was still open after 62 days! I then took the paint sample out of the dark and left it in a normal lit room for at least 3 weeks, after which it dried. The smell of the paint is the same as Linseed oil, the texture upon drying is firm (not soft) and the colour the same as other PG7 paints.
I tested this again with both pure paint and paint mixed with wallnut oil, again it stays open even for a few weeks exposed to the light.
I can only think that this paint has been exposed to some kind of anti-oxidant such as it used to preserve cooking oils, as I can't think of another reason why it stays open so long.
I would like a long drying PG7 paint, but I'm concerned about the resulting paint film integrity with using this paint.
I have emailed Royal Talens, but no response yet.
I was hoping you have some thoughts about what might be happening here?
Move this whole section up, swapping places with the section above it.
I have sent your question to our representative at Talens.
Thank you Brian. I didn't know you had a representative from Royal Talens here. :)
I'm not aware of any antioxidant preservatives or anti-rancidifiers in use for oil paints, but it could be some brands are using a proprietary ingredient to increase shelf life. While I don't know the Talens formulas, I think it's possible that the long drying time may be due to the presence of catalytic siccatives (driers) in the other brands, and/or the type and proportion of stabilizers in the Van Gogh color. It's often necessary to add something like stearates with colors like Phthalo Green to achieve a workable paste, and the resuting paint, having a high oil content, may dry more slowly. This effect could be more pronounced if pigment load is lower in one sample compared to the equivalent color in other brands, especially if additional drier has not been added.
Thank you Matthew, that is interesting. I did wonder about dryers and have seen some paints dry much quicker than on other brands. But I haven't seen a pigment dry so fast in every brand except one and that one dries so very slowly.
I also tested their PB15 Phthalo Blue, and that dried in 2 days. So I would have thought they used a similar formula as PG7..?
There is also the possibility that the pigment itself has been reformulated or obtained from a different source. Art materials manufacturers represent only a small percentage of pigment consumers, and when a large source (sometimes the only global source) makes a change, it's not going to be for the express benefit of artistic paint makers (though we might get lucky). But, one can't assume that one phthalo color will have the same formula as others, because each pigment might require different preparation.
Well I assumed the copper component was acting as a drying agent. Isn't Phthalocyanine actually classed as a lake pigment which are normally slow drying? I am aware of PB16 which is the metal-free version, but more of a turquoise colour. I don't know if that dries any slower or not though.
I know Sarah Sands did testing of drying rates for various colours (including Phthalo Green) in a JustPaint article and the drying rates in their testing seemed consistently around 6 days regardless of the light levels:
I'm wondering whether I should buy another tube and see if this also takes such a long time to dry.
I cannot answer specifics about the Talens’ paint but I do
have to weigh in on general pigment nomenclature. Lake are dyes struck on a
substrate during precipitation. Generally, this was done by extracting a dye,
usually in an alkaline aqueous solution of potassium of sodium carbonate and precipitating
aluminum hydrate by the addition of alum. There are a few variants on this but
the above was the general procedure. In the above process, the dye is
coordinated on the aluminum hydrate and creates a water insoluble lake pigment.
Phthalos are not made in this process at all and are an organo-metallic complex
that is precipitated in a completely different manner. The copper usually does
act as a dryer making an oil paint that dries quicker than would be expected of
pure organic pigments, some of which are actually anti-oxidative.
Ah, thanks Brian for correcting my misunderstanding :)
Just an update:
Royal Talens have been in touch and want to find out what is happening with this paint. They have said they don't use anti-oxidants or MEKO in their paints, so this shouldn't be the cause of the slow drying. I have given them the batch number so they can check that tube.
I also have bought another tube (with a different batch number) which so far is wet on a palette after 8 days, so seems to be showing the same behaviour.
I'll let you know if I hear anything more.