Two paintings made around 1660 with contrasting ultramarine blues: “The Milkmaid” by Johannes Vermeer on the left and “The woman scouring a metal basin” by Jan Steen on the right.
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© Rijks Museum (Amsterdam)
Not all coloring matters - pigments and dyes - resist the same way over time, according to a study published by our partner The Conversation.
Ultramarine blue is, for example, the only pigment victim of a photo-oxidation phenomenon which sometimes leads to its "bleaching".
The analysis of this phenomenon was carried out by Anne Michelin, lecturer at the National Museum of Natural History (MNHN).
Who has never thought that their children's drawings deserve to be preserved as works of art for generations to come?
And to wonder what would become of them, in what state they would be found in a few decades, or even in a few centuries?
We can imagine that machine-erasable markers or water-based paints are not the most durable materials.
But even by providing the palette of the great masters of times gone by, would we be able to keep the colors and luminosities of these paintings for a long time?
Some paints, like oil paint, are known to last longer.
But not all colors are spared - this is the case, for example, with ultramarine blue.
Colorant or pigment?
Ultramarine blue pigment, insoluble © Marco Almbauer / Wikimedia, CC BY-SA
Not all coloring materials - pigments and dyes - resist the same over time.
A
dye
is a chemical species soluble in the binder used;
in the past, it was most often of plant or animal origin.
It will often turn pale in the sun.
We all remember family photographs that have now become blue or yellow, depending on the resistance of the yellow, cyan and magenta dyes used.
One could then prefer the oil paint for which a pigment, that is to say small solid particles, insoluble in the medium which it colors and often of mineral origin, is dispersed in a binder, the linseed oil.
The resistance of the pigments over time is indeed often better.
Ultramarine blue in history
Lapis lazuli from Afghanistan (blue) and pyrite (yellow) © Hannes Grobe / Wikimedia, CC BY-SA
Ultramarine blue was first extracted from lapis lazuli from distant lands, notably Afghanistan, hence its name “ultramarine”.
The process is long and complex, so much so that at certain times, the price of ultramarine blue was higher than gold.
This also explains why its use was reserved for prestigious religious works, particularly in the Middle Ages.
At the beginning of the 19th century, with the development of chemical synthesis, attempts were made to produce ultramarine blue synthetically.
A challenge finally achieved in 1826 by J.-B. Guimet, which allowed a generalized use of this pigment.
This synthetic blue has also been used by artists - Ingres for example tested this pigment at the time of its creation, Yves Klein mixed it in 1960 with a binder to create the famous "International Klein Blue" - as for applications of the daily life (paints, printing inks, blueing of paper and linen, etc.).
However, this highly sought-after pigment is the victim of a specific degradation phenomenon: "ultramarine blue disease (or bleaching)".
Many works are victims when others are totally spared.
How can a pigment lose its color?
In order to be able to explain how the color of a pigment can disappear, we must first understand where it comes from.
We now know that the color of ultramarine blue does not come from a transition metal (iron, copper, cobalt, etc.), as is often the case in pigments (azurite is an example of a copper-based blue), but sulfur which is present as a free radical anion trapped in an aluminosilicate cage.
This particular chemical configuration of the sulfur atoms allows absorption in the red - which explains the blue color of the pigment.
Unlike smalt, which has been known since the 17th century to fade in a few years, ultramarine blue is considered to be a stable pigment.
In fact, the sulfur is here “stabilized” in its aluminosilicate cage and can only escape from it at high temperature.
At room temperature, only the destruction of the aluminosilicate can explain the loss of color, and this is what happens in the presence of acid.
But acidic environments, or other types of aggressive environments are not often encountered in museum conditions, and therefore cannot explain all cases of alteration of ultramarine blue paints.
Ultramarine blue aged under controlled conditions, in the laboratory © Anne Michelin
In fact, neither heat nor humidity is necessary for bleaching ultramarine blue.
Controlled aging experiments, in which the environmental conditions to which the samples are subjected are chosen, however, show that light is a decisive factor in bleaching.
In addition, oddly, if oil paints (or for certain other binders) whiten under the effect of light, this is not the case for compressed ultramarine blue pigment pellets, without binder.
This simple experiment allows us to question the starting point: have we really lost the color of the pigment?
The answer is no: the pigment itself is not impacted.
If it's not the pigment, then it's what surrounds it
In our experiments where the illumination is controlled, we can clearly see that the binder is in a sorry state: there is not much left of the very smooth film from the start after 1,500 hours in the limelight.
The film is entirely fragmented and leaves room for a very significant roughness, responsible for the whitening.
Roughness of the pigment in its binder observed by a scanning electron microscope, before bleaching at the top and afterwards at the bottom © Anne Michelin
The observed change in color is therefore due to a physical and more precisely optical phenomenon.
The strong roughness here gives rise to an increase in the diffuse reflection which is here the main cause of the whitening: the multiple grains act like many mirror facets which send significantly more signals back to the viewer's eye.
So it gives us an impression of loss of color, but in reality the paint just appears lighter.
Exposure to light seems in fact to be at the origin of a "photo-oxidation" mechanism: ultraviolet rays interact with the binder to produce very reactive chemical species, "free radicals".
These, via chain reactions, are responsible for the massive oxidation of the binder and chain cuts (breaks) in the polymer.
The binder film, very weakened, ends up fragmenting, leaving the pigment grains visible, resulting in a rough final appearance.
As proof, if we cut off the ultraviolet radiation or if we add an antioxidant to the initial mixture, bleaching no longer occurs.
The phenomenon of photo-oxidation of binders is a classic mechanism.
What is surprising here is that this phenomenon only occurs in the presence of ultramarine blue.
With other pigments, nothing happens.
The catalytic effect of ultramarine blue
There is therefore a catalytic effect linked to this particular pigment.
It can be assumed that a specific interaction at the level of the aluminosilicate cages occurs.
Indeed, this catalytic effect is well known for other zeolites which are themselves uncolored (“zeolites” group together minerals with a microporous crystalline aluminosilicate structure - ultramarine blue is one of them) and have long been exploited in the industrial world. , but this effect has not been studied specifically for ultramarine blue.
Our “Painting” file
If the explanation for the bleaching of ultramarine blue is now known, many questions remain: why are some paints spared?
Are there pigments that neutralize the catalytic effect of ultramarine blue?
Is there a threshold below which the effect is not perceptible?
Is there a difference between natural and synthetic ultramarine blue?
A team now claims that the former would have a catalytic effect more than four times greater than the synthetic pigment.
What is really going on in the aluminosilicate cage?
Knowing the bleaching mechanism allows us to better restore the works
For old paintings, it is possible to restore them by reforming a continuous film: by filling in the gaps and cracks, the color will reappear.
For modern paints one should consider a formulation containing antioxidants to delay degradation - provided the binder does not need radical processes to dry, which is the case with oil paints.
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This analysis was written by Anne Michelin, lecturer at the National Museum of Natural History (MNHN).
The original article was published on The Conversation website.
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