An alchemist, a library and golden stars: the scientific work that has revealed the secret of the Alhambra's purple color

Some areas of the Alhambra have purple spots.

And it was not known why, or what had caused them, or through what process.

But two scientists from the University of Granada, Carolina Cardell and Isabel Guerra, have revealed it.

His investigation, the result of which has been published by the journal

, has all the elements of a good detective story: an apparently unsolvable mystery in which the protagonist is gold, a chance discovery in a library in Florence, and even the permanent presence on the case of a medieval alchemist, Cassius.

And behind all this science, a lot of science.

It all started in the nineties.

“Then we identified purple stains in the Alhambra that were not any added pigment and we did not know what they were due to.

So we started to investigate”, recalls Carolina Cardell, professor at the Department of Mineralogy and Petrology at the University of Granada.

“Shortly after we went to a conference in Florence and spent an afternoon in a library.

And there the light went on for us because we found information about what we had to look for: the purple color could be due to gold particles.”

Searching for those gold particles was not difficult.

Isabel Guerra, head of the scientific instrumentation center at the University of Granada, is an expert in the use of the electron microscope and went to it to trace the gold particles in the purple spots: “There they were.

I call them

because gold shines a lot under the microscope.

I remember that day perfectly, I will never forget it”.

They had taken a big step in his work: he already knew that those purple spots were gold particles.

But that in science is very little.

The important thing was to know by what process these particles had been produced.

“Gold is a very noble material, it is unalterable.

That is precisely what gives it so much value”, explains Guerra.

And yet, something had altered the gold of the Alhambra.

“At some point we thought to make public that the spots were gold particles, but that we did not know how they had formed.

After all, the Alhambra is hundreds of years old and it was very difficult to find an explanation.

But we are both very stubborn, very tenacious, so we continue to investigate.

Detail of the plasterwork of the Alhambra.UGR

The purple stains of the Alhambra are not everywhere, only in some of the golden areas of the monument that are outside or exposed to humidity.

And not on all the golden surfaces in those areas.

"The type of gilding in which we found the stains is the one in which a very thin sheet of gold was applied to a sheet of tin," explains Cardell.

“In other words, we start from a bimetallic structure.

It was done this way because the gold sheets were very thin, about 2 microns thick [a millimeter is a thousand microns, a hair is 150 microns] and the tin foil gives the gold more shine and improves handling.”

There is also another fact that is also key in this story: Granada is 50 kilometers in a straight line from the Mediterranean.

"And that means that the Alhambra is exposed to marine aerosols that contain chlorides," says Carolina Cardell.

The researchers discovered that, due to humidity, the tin sheets had oxidized and the by-products of this oxidation had begun to come out through the pores and fissures of the gold sheets and had partially covered the noble metal.

They had taken another step, but not the definitive one because neither of these compounds can alter gold to form the particles that cause the purple stains.

“What happened”, continues Isabel Guerra, “is that these by-products did not completely cover the gold sheets;

there were areas that were covered and others that were not.”

And that meant that there were areas of the sheets that were still exposed to oxygen and others that had ceased to be so because they were covered.

This fact caused the formation of some electrochemical microcells, which they call “differential aeration nanocells”, because some are exposed to oxygen and others are not, “and these nanocells can dissolve gold”, concludes Guerra.

The two researchers had always had what is known as Cassius purple in their heads.

"Gold particles are widely used in microbiology," says Isabel Guerra.

"For example, the first pregnancy tests used them and that's why the result looked pink."

There is a great industrial technology dedicated to producing gold nanoparticles and its historical antecedent is the work of a medieval alchemist named Cassius.

He discovered that gold only dissolves if you add hydrochloric and nitric acid to it and then it forms a yellowish solution.

“If you add tin oxide to that yellowish solution, you have a purple precipitate made up of tiny gold particles known as Cassius purple, which was a highly coveted pigment in the Middle Ages,” he adds.

When Isabel Guerra and Carolina Cardell reached that point, they already had it all: they had found the explanation for the purple stains in the Alhambra, which are due to a mixture of elements: the tin sheets behind the gold sheets, the humidity of the environment and the aerosols that the wind brings from the Mediterranean had been able to dissolve the gold in the sheets.

"Gold nanospheres are one color or another depending on their size," explains Cardell.

“The 30 nanometer ones are reddish in color and as their size increases they acquire shades that go from light blue to purple or brown.

Most of the ones in the Alhambra are 70 nanometers, and that's why they're purple."

The discovery that the purple stains of the Alhambra are nothing more than dissolved gold has practical applications, as Carolina Cardell explains: “On the one hand, it has advanced basic research on gold alteration processes, but it also provides clues to that conservators and restorers can choose the most advanced intervention methodologies.

The different heads of conservation of the Alhambra are enthusiastic about the information that we have given them”.

And adds Isabel Guerra: “Once we have trained our eyes to see those bruises, we have observed that they are in many other places.

For example, we have seen them in a monument in Toledo and in another in Seville”.

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