First photograph shows a cracking pattern caused by the painting touching the cross bar. This happens because the painting is loose and/or pressure is put on the front of the painting causing it to press against the cross bar, and/or could be the cross bar is not spaced far away enough from the painting.
Second photograph shows a vertical line that goes just inside and parallel to the edges of the painting, that may or may not be visible on all four sides. Giving the above example the cracking caused by the cross bar. This cracking appears because of the contact with the inside edge of the stretcher bar, as you can see from the photograph this cracking can lead to flacking.
Third photograph shows diagonal cracking that will form in each conner, this cracking pattern is very common, and has been recreated in artificial aging tests which have confirmed that they form due to the expanding of the canvass and the ability of the stretcher bars to hold the painting properly.
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That’s useful information about the cracking on art. Now that I know, I can see what you are talking about on my art collection. Keep writing! Great and informative blog.