exceptionally rare 1850s massive milwaukee mansion residential entrance door mercury doorknob set with tapered shanks

set of hard to find 1850s american "mercury" or silver glass doorknob containing a cast brass tapered stepped footed shank. contrary to belief, these glass doorknobs never contained the toxic metal mercury. instead, the knobs was coated on the interior with silver nitrate, then affixed to the shank. these doorknobs enjoyed widespread popularity primarily in northeast cities, along with stately mansions of the midwest between the 1850's - 1870's. the footed shank contains a nicely aged finish. likely fabricated by the nashua lock co., nashua, new hampshire. priced for the set.