Why would you think this is a forgery?
It has all the elements of a classic forgery process. First, get a historically correct frame so the history is legitimate by dating. This is fairly easy to do because the art in of itself is not devalued if you put brand new framing on a historical or any painting. The frame is merely the vessel to what is valuable. Kind of like a Porsche 911 is a 911 regardless if stored in a luxury condo garage with 24/7 security or parked in the ghetto. So you can toss out the frame for relevancy.
The next part is to look at the image. Another stage in forgery is to copy elements of well-known paintings of the artist and copy those elements, yet add new elements to look authentic. In this case we can see the near identical nose, lips, eyebrows of Mona and Jesus as well as even the hair treatment. The dress of the Jesus is fairly similar with translucent layering and the fine embroidering. The difference really is in the background. In this case a solid black background vs. an intricate kind of fantasyland background of the Mona Lisa. If you were going to copy, the easiest solution for variation is the black background common in other medieval artists. But did Leonardo use black backgrounds in his other paintings? Let's take a look.
In this case, we find that Leonardo did use a dark, but not solid black background. So the fingers look like a good match, yet the background doesn't appear to be what Leonardo preferred. So either it was painted this way as a one-off or maybe filled in afterwards. Returning to the eyes, you can see that Leonardo always paints the figure looking at the viewer while the Jesus Christ figure is kind of looking cross-eyed. So the eyes execution is fairly poor. Now if this were merely doing a local priest or nobleman, then maybe the eyes would look this way. Or did the painter just not have the talent to copy the eyes well?
Another tell tale sign is the shoulders of the Christ figure are short. If you compare to John the Baptist, the right shoulder to neck is about twice the Jesus figure. Did he paint Jesus cross-eyed and with incorrect shoulder length? You have to remember that Leonardo was considered to be of High Renaissance period where perfection in the body proportion reigned supreme.
If we look at the expression of the mouth, we see that in both John the Baptist and Mona Lisa, both figures have a mischievous look of either hidden glee or unknown thoughts. Looking at the Jesus figure the expression is kind of dead on arrival look or stoned off his ass. This would suggest another painter did the Jesus figure that just could not master the excellent hidden expression of Leonardo's figure.
Have there been other famous forgeries in the techniques you suggest?
Another famous forger was Han van Meegeren. He copied Vermeer paintings by buying old frames to date the painting correctly. He would paint mixing raw pigments into linseed oil and bakelite. Then he would bake the paintings slowly to get the correct crackeling effect to show the "age" of the painting. He managed to sell even to high-end Nazis like Goering fake Vermeers that Goering swore were original. After the war, people thought he was an art dealer selling off Jewish paintings to the Nazis, so he confessed to being a forger to not get hung as a war criminal.
But hasn't the new Leonardo been certified for six years?
Yes, this is correct, but keep in mind the verifiers. The verifiers don't want to be fooled, but at the same time would love to have the reputation enhancement of "finding" a new Leonardo. The incentive for the auction house is obviously $50 million in commission for the real deal vs. zero for a fake. It has been proven that 100s of works have been sold by Christies, Sotheby's or other high end auction houses that were fakes. In Jan 2017, "St. Jerome" was said to be fake sold by Sotheby's for $842,000. In October 2016, "An Unknown Man" portrait fake was sold by Soethby's for 8.4 million euros. The list goes on and on.
Are they other artists copied?
Above we see Jackson Pollock, whose style seems easy to copy with drip painting. Any artist with an easy to duplicate style is a prime example of forgery. Miro comes to mind.
Why do so many of the fakes land up at the big auction houses? Don't they offer a refund of some sort?
Is there a worse offering than an auction house for risk of fakes?
ALL SALES ARE FINAL