Spotted some cast metal markers in the cemetery in Houghton, Michigan, last week. Two were definitely zinc (aka "white bronze"), but the third, which is about 30 years older than the two zinc markers, is much heavier metal. No visible rust, but I think it's iron.
The Houghton cemetery is an interesting one. It's right on the edge of the Michigan Tech campus -- students cut through there constantly, shortcutting from Daniel Heights (aka married students housing) to the Student Development Complex, which houses the Athletic Department (PE classes and sports events) and health clinic. I can remember walking through myself on the way to PE classes in the 1980s and pausing to read headstones dating back to the 1840s. Several professors at Tech use the cemetery as a resource for teaching human ecology, sociology, and history. The cemetery is quite well maintained and is still active. I don't know about the National Register status of the entire cemetery, but it does contain at least one family mausoleum that's listed. There's Michigan SHPO plaque next to the door noting that the structure is historically significant.
6 months ago