On the nails used to stretch a canvas

[Valerit]

Considering the "expensive" paintings in the collection of my friend, I saw that the center is a breakthrough of the paint layer and ground (and in part of the canvas), which at first glance be mistaken for craquelure. I am interested in the causes that could lead to this result. I began to examine the nails, which must withstand a load! What was my surprise when they pulled out with no effort! And the painting dates from the first quarter of the 20th century. Cap on the outside was rusty. Itself was a nail straight from the tin. Canvas not overtighten. I began to check the paint ... It does not dry out for 80 years!
Here is a story with a simple nail ....
Of course it all started with doubts about the authorship.