What is "really" an art authentication
What an art expert should provide in art authentication, the 7 steps 

Seen on the web about art authentication, errors to avoid part 1

How to be 100% sure the painting you buy is "really" authentic.

Is an art authentication "really" indisputable?

An art authentication research made a customer unhappy but...

an art authentication file includes:
-Justification of authenticity by comparison with other works by the artist
-Reestablish the works in the "catalogue raisonne". 
-Comparison with other works made by the artist at the same time 
-Comparison with other artists in the same period or movement 
-Research about the historic, provenance, 
-Reestablish historic of the painting 
-Graphology analysis of handwriting ( not always)
-Signature analysis On-site analysis of painting 
-Eventually study of the pigments 
-Study of the support 
-Study of the manner 
-Wood lamp test 
-Infrared test  ( not always)
Conclusion: presentation of the file to a recognized expert