[joedoh]

its acid, it attacks/eats the surface. the part hasnt shrunk, its just had an equal amount taken off every surface, including inside the engraving.

which is actually an interesting way to think of shrinking something I guess, a method that changes the mass at the same time as the size (which IN REVERSE is why we have acrobatic flying quadcopters but not quadcopters that can carry humans, changing the size up changes the mass up too)

but it would only work for homogeneous materals though, something that had different material as the acid eroded it would not have the same result, if for example you put your finger in there for a week you might lose the epidermis and maybe the dermis too! it woulndt look like a smaller finger.