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Monument Preservation 102: Ferrous Metal Pins

Stone monuments and mausoleums vary in style. They also vary in construction method. The methods vary from dry stacking, which is the literal stacking of stones with no mortar or bonding agents; stacked with a bonding agent; or the stones are stacked with a bonding agent and are pinned together for reinforcement.

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How are stones held together with pins? Pins are used to reinforce joints at key points in the structure or they are used as a repair for broken tombstones. In earlier repairs and construction, metal pins were used. Unfortunately these pins were mostly ferrous; which means they were made, at least in part, of iron. If moisture reaches these iron pins they begin to rust. When rusting occurs, it expands the metal causing pressure and the stone will split.

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This takes time and varies based on the stone, the environmental conditions where the monument is located, and the condition of the joint bonding agent.
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The finer grain of marble, with the smaller overall pieces of aggregate seem to corrode more quickly with the ferrous metal pins.
Today, when any repairs are made ferrous metal pins are removed. Current restoration practices call for stainless steel, aluminum, plastic, or fiberglass pins. At Oakland, we have only used the stainless steel or aluminum pins.

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