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The trick to mosaic smuggling is to bring it into the country one tessara at a time, and reassemble it.
First, some of the most audacious thefts in criminal history revolve around arts and antiquities. Second because of the way the art market works, it is almost uniquely set up to facilitate money laundering and the international movement of diry money.
An example I like to give is a hypothetical 5", 12 lb. Ball of iron.
In and of itself, scrap metal value is about 4-5 bucks.
Throw a wooden base on it, sell it as a "cannonball paperweight," and you can probably get $20 for it.
Put a piece of paper next to it that verifies it is a cannonball recovered from the Gettysburg battlefield and you can probably get $100-200 for it.
Put convincing, authenticated paperwork with it that establishes it as the cannonball that took of General Sickles' leg at Gettysburg, and you're likely looking at 6 figures from a museum or seven figures from the right private collector. All for the sam cannonball.
Moreover, that $4-5 scrap metal and that million dollar provenance can be pretty easily transported across borders separately with minimal inspection or question, qnd paired up again at the destination.
What customs inspector would give a single tessar from a mosaic much thought?