An art authentication or C.O.A. is a certificate made by the solely recognized authenticator for an artist.
A solely recognized authenticator is an expert that certifies a painting is authentic or isn't. Major auction houses recognize this expert all over the world.
Usually, this expert is somebody who knew the artist. Sometimes it is a family member like for Picasso; the expert is Claude Picasso, or an individual or an organization that worked for many years on the study of an old master. For example, Julius Held was the unanimous recognized expert for Peter Paul Rubens artworks. Today this privilege comes to the Rubenianum in Antwerpen Belgium.
The authentication of the authorship of an old master painting is complicated, especially for artists like Rembrandt, Raphael, etc. The approval of several foremost experts will be required, which makes it very difficult to obtain and takes a very long time. Today if someone wants to obtain authentication for a Raphael painting, it will take a minimum of 5 years and will be very expensive.
What these authentication experts request:
A/ photos
3 black and white photos 5" x 6" or similar
3 color photos 5" x 6" or similar
An Ektachrome transparency was requested, but with the digital technology we have today, they like to receive very large digital photos of the artwork on a USB file.
Needless to say that the photos are supposed to be made by a professional photographer.
B/ documents
1/ the complete explanation of the provenance of the painting. The provenance is the way an owner came into possession of an artwork. This document must be verifiable. It is not enough to say I received the painting from Picasso himself New Year's eve 1957. You need to have documents of justification like photos etc.
2/ make a condition report: showing the restoration done to the artwork, the actual state of conservation,
3/ make photos of the back of the artwork and detailed photos of all annotations made to the canvas of the frame or on the back of the artwork. Photos of patches of restorations etc.
4/ eventually a bill of sale from a gallery or an individual from whom the artwork was bought.
An expert should do the work:
1/ to create the file
2/ to represent you to the authenticator.
An experienced expert will create the file described above.
But also a file including but not restricted to:
Justification of authenticity by comparison with other works by the artist.
Make a comparison with other works made by the artist in the same period.
Reestablish the works in the "catalogue raisonne."
Make a research about the history, the provenance, of the artwork.
Reestablish the history of the painting.
Graphology analysis of handwriting.
Signature analysis.
Have an onsite analysis of the painting if necessary.
Make eventually study of the pigments.
Make a study of the support, canvas, panel, board.
Make a study of the manner, the technique.
Wood lamp test.
Comparison with other artists of the same period
etc.
Last but not least: The expert will represent the owner to the authenticator expert.
Especially with French artists and French authenticators, I advise having someone to represent you. In France, an authenticator has the authority to seize a painting he believes is a copy or a fake, and let destroy it under legal supervision.
The authenticator can't seize the artwork if the owner is not present at the meeting.