Effects of water hardness on watercolour paints

For the past couple of years,  my tap water has been curdling my paints.

It all started with a tube of Quinacridone Rust (PO48)  from M. Graham.   I was attracted to this paint partially because the pigment is listed as non-granulating,  and I fell in love with the colour, but ever since I got it, the paint has had a tendency to clump and curdle when mixed with water in the palette or dropped into a wash, resulting in an incredibly pronounced granulation pattern.  This pattern becomes even more pronounced when Quin. Rust is mixed with another dark valued, non granulating paint such as a phthalo.

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Colour mixing experiments with Quin Rust (PO48) and Phthalo Turquoise (PB16)

I searched the internet extensively, and could find no other reports of quinacridone rust granulating like this.  For months I was actually convinced I had received a dud tube, but avoided investigating further, because I really adore the colour and transparency of the tube I own.  Instead, I simply developed strategies for mitigating the crazy clumping and curdling.

Then, a few months ago, I purchased two more quinacridone paints by M. Graham – Quinacridone Rose (PV19)  and Neutral Tint (PV19 + PG7).  Again, all of the pigments are listed as non-granulating, but both of these paints showed the same clumping/curdling behaviour (albeit to a lesser extent).

Now, one dud tube would be easy to explain, but 3 dud tubes purchased at different times would reflect very poorly on the manufacturer.  And yet, still I could find no reports of similar experiences online.  I felt like I was going crazy.

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PV19 Quinacridone Rose (M. Graham) on palette with KW water

Finally, I reached out to other botanical artists and asked around.  Someone suggested I should paint with distilled water, as acidity and mineral in tap water can react with some paints.  Suddenly it clicked.   I live in Kitchener-Waterloo, known to have the hardest water in all of North America.

Water hardness refers to the amount of dissolved calcium and magnesium in the water.  Water tends to be harder when drawn as groundwater rather than from lakes or streams.  Water hardness leads to limescale deposits in pipes and fixtures, and premature degradation of water heaters in pipes.  In my area, most households (including my own) have a home water softener, which functions by replacing the dissolved calcium/magnesium with sodium.  We shower and do our dishes with saltwater, which is not drinkable, and have a hard-water tap direct from the city for drinking water. At my art studio, a few blocks away, we do not have a softener.  Thus, my paintings in the past few years have all been done with limewater, and perhaps occasionally saltwater.

In order to test the effects of the tap water on my paints, I purchased a large jug of distilled water at the grocery store, and painted out samples wet-in-wet and wet-on-dry on identical paper using hard water vs. distilled water.

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The difference between the two samples was dramatic.  However, there was still a little bit of texture visible in some of the paintouts even when using distilled brushwater.  I attribute this to mineral build-ups in the pans themselves, as well on the brush and the plates I use as palettes, which of course get washed in tap water.  I even tried a sample using fresh tube paints, which was even cleaner, but still contained a little bit of granulation.

So, from now on, I will be painting using distilled water.  My paints are much posher than me, and will get fancier drinking water than I do.  Eventually, I will have a reverse osmosis system installed for the drinking water line at home, although likely not at the studio, which is a rental shared with other artists.  I will however be able to fill up a bottle at home and take it to the studio.  However, I can’t imagine going through the trouble of only washing my palettes and brushes with distilled/RO water, so some hardness effects will still be seen.

I am somewhat torn about manufacturers.  I love the brightness and saturation of M. Graham paints, but they seem to be much more prone to this kind of curdling. Since running this test, I did look through my mixing charts and found a few other paints where some minor unexpected texture, but nothing remotely close to what is seen in these three paints.

My quinacridone pigments from other manufacturers do not show nearly as much clumping.  My PV19 Permanent Rose from Winsor Newton is unfortunately not as bold and red as the M. Graham version, but it shows only the faintest amount of texture even in hard water.  My PR122 Purple Magenta and PV55 Quinacridone Purple from Schmincke and PO49 Quin. Gold from Daniel Smith are also quinacridone pigments, but paint out completely smooth.  Given that in most cases I prefer smooth, non-granulating paint, and that my paint will no doubt continue to come into contact with some dissolved calcium and magnesium, would I be better served switching to different manufacturers for quinacridones (and any other paint that may be similarly affected)?

Does anyone know what the chemical reaction occuring might be in these cases?