Looking forward to reading your blog post, Koen!
With both of these examples, we can have no definitive proof that the artists intended to paint a hidden face, though I agree that the Giotto example is very clearly meant to be a face, once you notice it, and the discovery is now widely known and agreed on. This painted "cloud face" is of course, very different from the phenomenon of people thinking they can see certain shapes hidden in
real clouds or natural features of the environment--that's definitely pareidolia, but it's another thing to
recognize a hidden image intentionally created by an artist's hand.
Part of recognizing that the hidden image is intentional can be supported by the context, which seems to be the case with Giotto's clouds--the devil's face isn't just random, it is placed next to angels, so it would add a symbolic or ironic meaning to juxtapose it in this unexpected place.
The "devil face" apparently hidden in Lorenzetti's hanged man's clothing is less widely known and agreed on, but it would have some good reasons to fit with the context, alluding perhaps to the influence of the devil connected to the man's crimes, and a similar contrast between a force of evil and its opposing virtues of good, the angelic winged Security. I hadn't thought of coattails being related with the appearance of the devil, as R. Sale mentioned, but that's an interesting connection too.
I think there is a good case for these two examples based on context. With the VM and f17 v, context is of course the difficult part
But I think if one is open to the possibility of a symbolic reading of some of the VM's plant designs, there could be more to discover about context than what meets the eye.
Your examples of marginal illustrations of face profiles are interesting too. I might put those in a slightly different category though, since they are immediately recognizable as faces, even if they're in an unexpected place. Whereas the other type is a more optical effect, created by the artist with the intention of cleverly hiding something or conveying two meanings at once. These are especially interesting since they are an example of the optical illusion of an You are not allowed to view links.
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Login to view. or something known as multistable perception. (Like the "Rubin's vase" or the "duck/rabbit" illusions.) Still, it's helpful to look for all types of potentially related imagery.