| Blind Pinning or Concealed Repairs
Excerpted from CRM Bulletin, Vol. 12, No. 2, 1989 (A technical
bulletin regarding cultural resources management published
by the National Park Service)
Blind pinning or concealed repairs may be made to marble
gravestones if there is a clean break between two sections.
The repair involves drilling holes and inserting nylon reinforcing
pins or rods into each fragment. Epoxy is used to adhere
the fragments.
Blind pinning should only be undertaken on stone that
is in sound condition. EXTREMELY IMPORTANT: Only skilled
craftsmen should attempt this type of repair.
Drilling Holes: The number of holes will vary with
the project, but generally they are drilled in a staggered
pattern or far enough away from one another to avoid splitting
the stone. The diameter should be about 1/8" greater
than the diameter of the pin. Depth of the hole should be
half the length of the pin. Pin sizes will vary depending
upon the job, however a small pin, approximately 1/8" -
1/4" in diameter by 1" - 2" in length will
suffice.
Pins: Non-corrosive pins should be used. Thermoplastic
rods, such as nylon, have coefficients of expansion similar
to epoxy and are an excellent choice. Pins that are threaded
or grooved will provide a good surface for adhesion.
Adhesive: Two epoxies are used in this type of repair.
The adhesive used to hold the pins should be a high-strength
(high modulus) epoxy used for structural repairs and setting
of bolts. A more flexible adhesive epoxy, a low modulus compound,
should be used to bind the two fragments together.
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