[Mark Crabtree]

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"We realize when glazed over, for instance on the 'Mona Lisa,' that he managed to place layers as thin as one or two micrometers, which means one or two thousandths of a millimeter," Walter told EuroNews. "By super-imposing the layers very progressively and slowly, he managed to create the effect he was seeking."
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I don't know the answer to your question (or even really understand what your question is), but did want to comment on the portion quoted above.

It surprises me that they would say that Leonardo "managed to place layers as thin as one or two micrometers". Surely if that is the current thickness (as is indicated in the story), wouldn't they have been significantly thicker 500 years ago?