Ferrous Metal Pinning
Many monuments constructed historically included metal pining to fasten together their sections. The Romans, sometimes, tied together masonry with bronze clamps to strengthen the joints, and prevent a structural failure. So when multiple piece monuments, became more popular then the typical colonial gravestone during the middle of the 1800, metal was again employed.
Unfortunately, many of the pins used to strengthen the multiple piece cemetery monuments, were formed out of ferrous metals. Some monument companies used bronze or brass as pinning stock. These have tended to stand the test of time, and are often in fine condition. But the pinning formed from metal that rusts, has caused extensive problems.
Even when pinning was placed in a lead sleeve, the expansion through rusting and oxidation was often more then the lead could absorb.
You can find monuments with problems attributed to ferrous pinning in almost every older graveyard. Sometimes greenish streaking is visible on the base around the area of the pinning. Very advanced pin corrosion can sometimes lead to the splitting of the base or die. In the most extreme cases damage which had begun with a rusting ferrous metal pin, can lead to a complete failure of the structure, in this case a monument. This problem is also encountered in older masonry buildings.
When resetting a monument with ferrous metal pins, if possible they should always be extracted. This often proves easier said then done. They tend to bind to the weakened stone in such a way, that their removal may prove to be extremely problematic. It may be necessary to drill a series of holes around the perimeter of the pin to remove it. This may cause new damage and is very time consuming.
I have encountered pins that while removing, have literally split the stone in pieces at the pinning point. Sometimes the only option is to cut off the pin where it enters the stone. This is less then an ideal solution because the pin remaining may continue to rust and expand. Painting the exposed end with a rust inhibitor, may help to limit the future expansion of the remaining piece of pinning.