I was startled to read a recent article which included a quote from the American professional conservator Mary T Baker, who said that the Mona Lisa will outlast NASA Apollo space suits. She said this because the Apollo space suits in museums are deteriorating as their plastics break down. Meanwhile the Mona Lisa has seen several centuries, and being well cared for, will see several more.
Oil paints are made from nature's natural polymer whereas acrylic paints are made from artificial plastic-like polymers. In both cases the polymers have been compared to strings of spaghetti which when applied in layers of paint are joined to each other at many points, creating a criss-crossed network of polymers which make a strong and durable film.
This polymerisation and cross linking can go on for decades in oil paintings as the vegetable-oil based paints slowly polymerise. Oil paint starts with smaller molecules but these form longer chains of polymers, which are proving to be at least as long lasting as their acrylic counterparts. Over very long periods of time the extensive cross linking can cause oil paintings to become brittle.
Acrylic paints are polymerised at manufacture into much longer chains of polymers. These do not cross link as much as oil paints and a different drying process causes them to form films.
Acrylic polymers are steadily broken down by natural forces which act inside and from outside of their chemistry. Heat and ultraviolet light are major offenders. The structure of the polymers is long chains of carbon, hydrogen, oxygen and other atoms which especially in the case of oxygen can be destabilised by ultraviolet light which creates loose oxygen atoms that then further destabilise the other polymers.
When acrylic paints were commercially introduced in the mid 1950s, extravagant claims were made about their portended longevity and in that modern age of wonder, space travel, steel and concrete the claims seemed believable. Are they believable now?
Mark Golden co-founded the company that manufactures the respected Golden Acrylics paint brand. He points out that as acrylic paints have only been around for approximately 70 years, no one can really know how long they will last. He further states that acrylic paintings created in the 1950s are showing no more signs of ageing than oil paintings created at the same time. His view is that there is no reason to assume that works of art, created with artist quality acrylic paints, will deteriorate quicker than oil paintings.
From the evidence available Golden's assertion seems to be true. A few scientists who have investigated also back up his claims that good quality acrylic paints are no less reliable than oils, and in some ways more reliable.
It is known for certain, as explained above, that oil paint films eventually will certainly become brittle and crack. No one can definitely say if and when this will happen to acrylics. When comparing acrylics with deteriorating plastic items in museums it is important to understand that they are not the same.
Acrylic paint does not suffer from the same problems as materials such as mass-produced PVC and the plastic in the aforementioned Apollo space suits. Some ill-informed articles on the Internet have confused the plethora of materials that are 'plastics' with acrylics.
Furthermore the manufacturers of high quality acrylic paints have made progress in improving their paints and mediums to ensure archival quality.
There have been some stories in the media about deterioration in acrylic paintings by postwar artists such as Andy Warhol and Mark Rothko. On further investigation it seems that the problems of these paintings is not due to the paints used but rather the way they were used, and the tendency of acrylic paintings to accumulate more dirt than oils, as the paint film remains porous and soft for longer.
To conclude there is currently no evidence that acrylic paints are more prone to deterioration than oils. There is also a general lack of evidence due the the relatively recent availability of acrylic paints when compared to the centuries long history of oil painting as a medium.
We may never know in our lifetimes the answer to the question 'Do works painted in oils last longer than those painted in acrylics'.