PB27 PRUSSIAN BLUE. Fugitive pigment often mislabeled as lightfast in watercolors. Unfortunately PB27 always fades in sun, but recovers in shade
. Manufacturers who are aware of the issue may mark it as stable (LF I /BW8/max star lightfastness depending on the country) due to it's ability to regain color after
being removed from light. In my opinion, paint should not need to be taken off the wall for a nap in a dark corner for it to be considered lightfast. I avoid this pigment if planning to sell a painting or display it in a gallery, as a buyer is likely to hang it in a well-lit room. It is stable in indoor artificial light or museum low-light conditions. Artwork made with Prussian Blue will fade from nearby window lighting, which can become noticeable in rooms with large windows or in locations closer to the equator.
"BUT Kim, --- brand of Prussian Blue is lightfast!"
I'm sorry for the bad news, but it's chemically impossible for PB27 to be totally stable. It is produced by oxidation of ferrous ferrocyanide salts. Sun bleaches the FeIII
(Iron) which requires time to re-oxidize away from light. Prussian has a reaction to UV light on a chemical level, which is only reversible over time in darkness. You don't have to take my word for it though, feel free to do your own testing. For a quick test just cut a swatch card in half and tape one side to the window facing outward (be sure it's not UV-blocking glass, and it's a direction that gets the most sunlight) for 1 month. Compare the two sides of your swatch immediately upon removal from the window. You can also do some "Prussian Lightfast?" research on Google search, Dick Blick has a pigment info tab for each tube of paint
, and Handprint mentions that because this pigment can range anywhere from nearly stable to totally fugitive that you should always run lightfast tests for it. (I assure you, that I am not
the only one to come to this conclusion.) Winsor and Newton is one of the only major manufacturers to politely include a note about this on their color charts:
I advise caution about trusting company or ASTM lightfast ratings, for more than just Prussian Blue, because some testing methods offer limited information. They do not always correctly distinguish between masstone and diluted (some pigments have been proven to fade in lower % tints). They may test mixes with white paint, which is not the same as just diluting with water as in watercolor painting. It's also important to note that sometimes a larger company who supplies pigment powder to the paint manufacturer supplies them with a lightfast rating which the paint maker may not double check for accuracy.
Each Prussian Blue watercolor I've tested faded with sunlight exposure within 1-3 months. In comparison, extremely fugitive paints (such as opera neon pink) fade at about the same rate, with some brands of Opera outlasting Prussian. Totally lightfast colors can stay in direct sunlight for several years without signs of fading. PB27 is an interesting and unique pigment though, if you put it in a shady drawer for a while the color will almost regain full strength. The only variance between brands is how MUCH it will recover AFTER fading. Prussian Blue takes time to become darker again in the shade. Some brands can make a nearly-full recovery in several weeks to months. The variance may be due to the differences in pigment source, how long the UV exposure lasted, and any protection from binder additives. It is likely that the original rating came from some one who collected results and let them sit in the shade before comparing them, or decided that since it was able to recover it was OK. To me this defies the definition of a "lightfast" rating.
PB29 Ultramarine Blue: Also known as French Ultramarine, Lapis Lazuli (genuine or hue), Cobalt Blue (hue) or Permanent Blue. Non-hazardous (low to no toxicity in humans). Typically lightfast (LFI / BW8) but care must be taken to use acid-free paper, as sulfur-based pigments will discolor when exposed to acids. Ultramarine Blue is a replacement for the chemically identical Lapis Lazuli (a semi-precious stone). Daniel Smith still offers Lapis Lazuli genuine, the naturally occurring version of this pigment used in many famous paintings throughout history. It used to be reserved for the very wealthy, until chemist Jean Baptiste Guimet came up with a formula to make it synthetically in France during the 1820s. This took a surprising number of ingredients including sulfur, kaolin clay, silica, charcoal, sodium carbonate and sulfate. The ratio and heat used to create the material, and the exposure to oxygen, results in a variety of deep blue colors.
Ultramarine comes in multiple shades such as "deep" and "GS" (green shade) and when further exposed to heat and ammonium chloride (Salammoniac) it turns slightly violet. It is then renamed Ultramarine Violet PV15. Winsor and Newton have made a color called "Smalt" without the word hue. Smalt is incredibly misleading to those seeking historical pigments, as it is actually made from Ultramarine Violet PV15 and NOT genuine Smalt PB32. When you add hydrochloric acid, Ultramarine turns into a delicate pale pink called Ultramarine Pink PR259. Be aware that all Ultramarines are generally regarded as lightfast. However, "Ultramarine Green" PG24
which may be found as a raw material pigment powder, handmade watercolor or from the company "Kremer Pigment" is not lightfast.
Mixes: An incredibly useful color for mixing and often included in both beginner and professional watercolor sets. Ultramarine Blue can be mixed with PBr7 to create an ideal neutral gray. Another popular mix is with PV19, where it makes deep royal purples. Daniel Smith's mixes have reliable texture effects in wet washes, as the PB29 granulates and separates from the PV19.