In my opinion, it is impossible to have a 100% certitude that a painting that comes with a C.O.A. is authentic.
Unless you bought a painting directly from an artist, and you have a certificate of authenticity made by him, and a bill of sale or a paper showing the artwork is a gift to you from the artist,
If you buy an artwork with a Certificate of authenticity:
1- have this certificate updated at the time you buy or re-sell the painting
a/ an expert may change his opinion about a painting. I saw a Parisian expert for Eugene Boudin changing his opinion about a painting he authenticated 30 years earlier and that he described with photo in his catalogue raisonne.
b/ Mr. X has made the certificate you have, but today Mr. X past away and is replaced by Mr. Y.
Mr. Y may or may not renew the certificate of authenticity. Many paintings by Andre Lhote with C.O.A. made by Jean Grouin, are no longer authentic in the opinion of Mrs. Berman- Martin, solely recognized expert for Lhote today.
c/ you need to be sure that the painting has the appropriate certificate.
For example, a painting by Amedeo Modigliani needs to have a certificate of authenticity made by Ceroni. Only 337 paintings have been admitted as authentic by Mr. Ceroni. The problem is our expert past away. You have other very important experts on Modigliani's work, Parisot, Restellini, and others. If you have a Modigliani artwork with the C.O.A. made by Christian Parisot, who is a very charming man, and a great historian, you will have it very difficult to have it sold because he was accused of making fake Modigliani works etc ..see links here under. Restellini (ex-secretary of Christian Parisot !) abandoned the writing of the catalog raisonne he started since he received death threats. Yes, fine art has his" Dark side" very dark...
2- Is the certificate of authenticity made by a gallery 100 % reliable?
a/ if a gallery makes the certificate of very high standing, they will have the certificate of authenticity made by the solely recognized expert. Often these galleries were exhibiting the artist in the past. The galleries are of course, reliable, but I will always ask the original certificate. Scandals made surface recently and especially one of the oldest galleries in the U.S.A., the Knoedler Gallery in NY: http://www.nytimes.com/2015/10/12/arts/design/lawsuits-in-knoedler-forgery-case-are-set-for-trial.html
b/ if the C.O.A. is made by a gallery but is not considered as one of the top 25 galleries in the world, consider that this certificate is completely useless.
You always should request the original updated certificate by the solely recognized authenticator and submit your buy to the contingency of providing this certificate if they don't want to have the sale canceled.
