You may have noticed in museums that a lot of paintings have a little yellowing or seem a bit darker than the composition might call for. This is actually quite normal and today we’re going to talk a little about why oil paintings turn black. Studies have been done on this and it is believed to be an issue with mercury in these old paints. Vermillion is notorious for it, as it has sulfur and mercury in it’s composition, and it’s believed that over time chemical reactions end up producing a fairly pure mercury that darkens up these old painting.
There is another way that paintings darken as well that can be fixed by an item that you’ve got at home. Stay with me and read on and we’ll talk a little more about why paintings darken to satisfy your curiosity on the subject!
Why do old paintings look dark?
One of the most common reasons that paintings look dark has to do with too much exposure to the air. Varnish doesn’t last forever and when oil paints get exposed to too much air for too long then they are also being exposed to trace elements of hydrogen sulfide. This causes a painting to undergo a slow change that will definitely darken it.
What happens is that the lead oxide (which makes up your white) in the paint undergoes a slow conversion to lead sulfide (which is, incidentally, black). The process is extremely slow, but it inevitably leads to a paint darkening very slowly over time.
Now, the neat thing is, this process can be reversed by means of hydrogen peroxide. It requires that you basically ‘dip’ the painting into a diluted solution of the peroxide for a bit of time and this causes the lead sulfide to oxidize. Once that happens, it converts into a white lead sulfate.
It’s some pretty amazing chemistry and also a great reason why you should only get a painting restored by a professional. They really know a lot of amazing tricks when it comes to fixing old work that you never would have thought of in a million years!
Do all paintings get darker over time?
Oils, yes, they’ll eventually get enough exposure to air over time to darken, though modern paintings are not going to have as much likelihood of darkening as the older paints. As we’d mentioned with the vermillion, those older paintings end up with mercury content although now that we understand what is occurring then there is a greater likelihood of stopping the process.
Acrylic paints, on the other hand, will pretty much last forever. This is because they are made with what amounts to plastic particles and pigment in a suspension. When the paint dries, you end up with a somewhat ‘rubbery’ plastic that scientists believe will probably last twice as long as oils.
With oil paints, you’re more likely to see cracking than darkening and they will also yellow if they are stored in the dark for too long. The hairline cracks usually add a bit of character and restoration pros usually leave those alone. The yellowing is not a problem either if it comes from the painting being stored in the dark, rather than a chemical reaction, as exposure to light quickly reverses that sort of yellowing.
Does oil paint change color over time?
Oil paints can change color for a number of reasons. The darkening that I talked about earlier is one such example. Yellowing is another common reason that oil paints change color over time. In the case of yellowing, I’d mentioned that one cause is a painting being stored or painted in low light. The cause of this has to do with color molecules known as chromophores, which are produced as the paints dry, although prolonged exposure to bright light actually reverses the effect in such cases.
Other reasons why paintings change color are a little more mundane. For instance, the slow collection of dust and dirt that can end up binding to the surface of the painting are one of the most common reasons for color issues. In some cases, it causes more light to be absorbed into color pigments and this too can change the color of the painting.
While they can last hundreds of years, time takes a toll on everything. Thankfully, restoration professionals have a lot of tricks up their sleeve so we can keep enjoying the classics!
Some final words
Today I’ve talked about why oil paintings turn black so that you can get some idea why those paintings in the museum look a little bit on the somber side. Reasons why paintings darken or otherwise change color include mercury content in paint, lead oxide conversion, too little or too much light, and good old-fashioned dust and grime. While there are many more ways that this might occur, these are the most common.