Cadmiums in oil paintings, conservation.

​Hi –
You are correct that over the last 10-15 years there have been various articles about specific paintings where Cadmium Yellow has undergone changes in color. While the conservation studies are continuing, cases of cadmium degradation so far appear limited to only a very small percentage of the cadmium yellows produced between 1880-1920. On its own, this limitation to such a narrow band of time strongly points to the problems being connected to early manufacturing processes. This would include the presence of initial reactants that were not fully removed, impurities, and the lack of calcination as a final step. This last point is particularly important, since early manufacturers feared that hearting cadmium yellow could cause it to darken. However, that was only true if carried out in the presence of oxygen, while calcining in an inert atmosphere – as was later done – produced a much more stable, harder, and permanent crystalline structure. Without this step the cadmium yellow remained in a more reactive and vulnerable amorphous form, which in turn made it more susceptible to degradation from humidity and light. The lack of calcination would also help explain why these problems are limited to cadmium yellow. All cadmium colors start out as yellow cadmium sulfide, and to make oranges and reds, the cadmium yellow needs to be heated to a very high temperatures along with other metals meant to alter the crystal lattice. Calcination is therefore always needed to make these other shades. Thus it is likely not a coincidence that the introduction of cadmium reds and oranges around 1920, and the adoption of calcination for cadmium yellows as well, coincides with the end of the problematic 1880-1920 period where we find the cases of yellow degradation. 
For some additional information along these lines we would recommend the following, publically accessible conservation article:SR-FTIR imaging of the altered cadmium sulfide yellow paints in Henri Matisse’s Le Bonheur de vivre (1905–6) – examination of visually distinct degradation regions

http://pubs.rsc.org/en/content/articlehtml/2013/an/c3an00892d
and the section on Cadmium Yellows, Oranges and Reds in Artists’ Pigments A Handbook of their History and Characteristics, Vol 1, published by the National Gallery and available for download:
https://www.nga.gov/research/publications/pdf-library/artists-pigments-vol-1.html
As for the continued concern about even modern, well-made cadmiums used outside, keep in mind that the conditions needed to trigger the process of cadmium sulfate to oxidize to cadmium sulfide are unique to outdoor environments. Sunlight contains thousands of times the amount of UV and spectral energy compared to artificial lighting, and in particular contains the very high energy, destructive UV-B, which window glass filters out. And of course, there would also need to be a sustained exposure to very high humidity or moisture at the same time. These simply are not conditions that would be expected inside a house or museum. And indeed, environmental conditions do not appear to be the main cause of the few cases examined. 
Lastly, as to why manufacturers continue to include it as one of the most permanent choices, the fact remains that other than in outdoor settings – and cases of early manufacturing – the cadmiums have continued to do well in accelerated and natural aging tests under indoor conditions. That said, there are certainly alternative yellows one can use – such as the benzimidazolones and bismuth vanadates – that have so far been proven to be equally durable, although often not as opaque.
We hope this is helpful.

​Yes everything that Sarah said 🙂 I would simply add however that some of these studies that focus on problematic issues relating to Cadmiums found on paintings dating to the turn of the century (or a bit before) make things sound a bit “doom and gloom.” While calcination, temperature, etc., etc. can all certainly play a role in the stability/instability of the resulting pigment, things that have NOT been adequately addressed is a) how fairly invasive restoration procedures may have played a role (lots of water and heat involved with glue/paste linings that can adversely affect certain pigments) b) how the binder may have played a role (we recently published on the Matisse painting cited above and in our research we found that during this period Matisse was also playing around with distemper and casein…it is not altogether impossible that he may have used a few colors bound in these mediums on the Barnes painting as we could not detect ANY fatty acids markers that are commonly associated with drying oils in the CdS paint samples collected from the canvas using ToFSIMS which is rather odd to say the least) and c) the role that pollution may have also played (city air was far more detrimental to the surfaces of paintings during the 19th and early 20th century). I would finally state that Brian Baade and I attempted to replicate the dreaded discoloration that can potentially occur in problematic CdS yellow pigments that were prepared using the “wet process” method under the guidance of Dr. Jennifer Mass. We sent a series of paints out to the GCI to be artifically aged and they came back perfectly fine…so again lots of questions there that remain unanswered. I would say you are FINE to continue painting with Cd pigments…as long as you take proper health and safety and disposal steps of course.

Hi Carlos –

I was wondering what happened as I actually did respond to your email to us at Golden about two weeks ago. So when I saw your questions posted here I thought perhaps you never received our email – and sure enough, in checking my records just now, the reply was accidentally sent to someone else. Aye! Anyway, it was essentially the exact wording that I gave above -and I so glad you took it upon yourself to ask the question again, as otherwise, I would have never discovered my mistake!