No, I'm not sure. I was able to break it into bits with my fingernail, and it still seems more like a very soft metal than anything else. Here's a photo of it with some of the bits beside it. I notice that parts of the design are missing on the portion where the substance was, but I don't see damage indicating that the print came off with it.
I'm new to the stamp world, but it seems to me that there are also a number of printing errors on the stamp (which I believe is Scott 632 if I measured correctly); e.g., there is a gap in the top of the E in UNITED, and the designs in and around the curlicues above the 1's are noticeably different.