Ultramarine Blue, some thoughts

Recently I learned that ultramarine blue can degrade from even mild acids, called ultramarine sickness. I tried to avoid it for a few weeks, but the colour is just too darn good. Ultramarine blue in a blue-violet pigment chemically similar to the ancient blue pigment called lapis lazuli. Being a blue-violet pigment that it absorbs virtually all green light, and most of the red light leaving only an intense blue-violet. The retina sees colours with three different cones that actually work together to see the full rainbow. The cone that specializes in detecting blue light is the S cone, named after short wavelengths of blue light. Ultramarine blue also reflects a small amount of red light, which stimulates the L cone (long red wavelength) a tiny bit, which creates a blue-violet sensation. For comparison, the magenta squares in the top left and bottom right reflect a little blue light, but more red light, creating violet-magenta in your retina because of increased L cone activity. I also included a square of cyan (blue-green) above the signature box, it is the same blue I use to paint skies, but here it appears green when surrounded by a sea of blue. I love looking up close at this painting, I want to make a giant 40 inch painting of ultramarine blue, and just stare at it to give my S cones some exercise. 8 x10" acid free paper pad, watercolour. June 2020

Another reason ultramarine blue is so good, is that it is the perfect  complementary colour for iron oxide earth colours like burnt sienna, in this case iron oxide red PR101. They neutralize into interesting greys when mixed, and can produce near black at mass tone. The little design on the right is meant to symbolize how much these two colours intertwine. 1.5 x 11" cold press, watercolour, June 2020

What's the catch? Any form of ultramarine blue including the natural mineral forms are susceptible to mild acids even vinegar or apparently air pollution. You would think with a name like ULTRA MARINE, it would be tougher than that!! I have french ultramarine from winsor and newton in a giant tube (that's how much I used this colour, I bought the 37 mL size), and recently bought a slightly different version from The Stoneground Paint Company in pan form, along with some ultramarine pink a derivative that looks the shade of bubblegum. To test it again, I painted out carbon black, ultramarine pink, ultramarine blue, french ultramarine blue, manganese violet, and iron oxide red (from left to right in the scan). On the left of each paint out I applied a small drop of concentrated vinegar, on the right just water. Clearly, the three ultramarine paints (#2, 3, 4) lost their colour from the vinegar exposure, while the other colours were unaffected. Manganese violet was slightly disturbed but it was seen in the water group too, meaning that the paint was easily lifted by the capillary action of evaporating water. Carbon black and iron oxide were indestructible of course.

What to make of this? Clearly watercolour paintings were not meant to be cleaned with vinegar or water. They are supposed to be protected behind UV coated glass and sealed at the back. I can protect it from vinegar, but what about air pollution. Sadly, I'm going to phase out ultramarine blue, luckily these days there are good options like phthalo blue which comes in red and sapphire shades.