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.

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.

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.


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?

17 thoughts on “Effects of water hardness on watercolour paints

  1. I always read your posts with great interest. I appreciate all the test trials you do which make life easier for us all. Thank you very much for sharing and happy painting!


  2. I just love the info you share in this post. Thank you. I will be showing this to my watercolor friends and students. It never occurred to me that the water could have so much to do with granulation. I have been fortunate to live where the town/city water is soft.

    Perhaps the M.Graham is more susceptable to the minerals in the water because it has honey in the formula. The honey makes the paints dry a bit slower and acts as a natural anti-mold agent.

    Liked by 1 person

  3. Very interesting! I wonder if is has any effect on ultramarine blue which I avoid because of the granulating? I don’t know what the chemcal reaction in the process is, but I know for a fact that in textile dyeing, red colours get dull if you use hard water, so for that we always use rainwater. I live in Denmark in an area with somewhat hard water (dH 12 – 18), so I will try testing some of my colours, which are not M. Graham. Including ultramarine…


    1. Ultramarine is a granulating pigment no matter what water is used. I also don’t love the effect, and tend to avoid it, but my paintouts of ultramarine look very similar to the expected behaviour even when using hard water. You can get Ultramarine Finest from from Schmicke, which is the same pigment PB29, but ground very fine, which reduces the appearance of granulation significantly.


    1. Hehe. You can get lime powder (calcium hydroxyde) very cheap at the hardware store. Sprinkle liberally into your paintwater and over your pans, and you’ll have a fairly accurate approximation of the water I get to work with 😛

      Liked by 1 person

  4. Hi, I use filtered water as I have slightly hard water. It certainly saved my coffee machine and kettle so I reckon it’s dealing with any calcium buildup. I use Daniel Smith and was advised to use them with distilled water but the filtered stuff is just fine for me.
    P. S. I also used to stay in Kitchener many years ago when I went to school there but now live in Scotland.


    1. If you’re happy with your water and paint behaviour there’s absolutely no reason to change to using something else. Dissolved minerals in water may react with some paints, but if nothing in your water is reacting with the paints you use, or if it is and you like the effect, by all means, use the local water.

      I’m curious about what kind of filter you use? The little carbon filters don’t seem to have much of any effect on my local water (we use one at the studio for drinking water, but the kettle and glasses still get very crusty). We run a water softener at home, which definitely does work, but outputs saltwater (it just replaces calcium/magnesium ions with sodium), which would have it’s own range of effects on paint. The last common kind of filtration I’m aware of is reverse osmosis, which does in fact remove most dissolved alkali. In the next few months, I will be getting an RO system installed at home for my paint and drinking water (we’ll keep the sodium softener for showers and toilets and such). Reverse osmosis systems are about a third the price of distillers and more durable, while achieving very similar results.


  5. Yep, it’s probably a question of scale (pun intended). Brita filters do remove some lime scale but not enough in my case. I guess if I replaced my Brita filter more frequently it might be more effective, but I think buying distilled water for my paints might be cheaper/less frustrating until I get an RO system.


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