Deep Hole Drilling for Museum Conservation, Restoration and Display Fabrication: Bronze Sculpture Core Pin Holes, Stretcher Bar Mortises, and Armature Tube Bores

A conservator restoring a 19th-century bronze equestrian statue removed a corroded iron core pin (25 mm x 400 mm) using a portable magnetic drill with a 20 mm carbide-tipped drill (Vc = 30 m/min, f = 0.04 mm/rev, laser-guided alignment). The pin was hollowed through its full 400 mm length and removed in pieces. The 25 mm bore was tapped M24 and a 316L stainless steel replacement pin was installed.

Bronze Sculpture Core Pin Removal Drilling ​

Museum conservation drilling uses deep hole drilling techniques for removal of corroded metal core pins from bronze sculptures, without damaging the surrounding bronze. The iron core pins used in historical bronze casting (wrought iron or mild steel, 10-50 mm diameter, 200-1000 mm long) rust over time, expanding and cracking the bronze. The Getty Conservation Institute's guidelines for bronze sculpture repair specify that corroded core pins should be removed by drilling (not by acid dissolution, which can damage the bronze).

The drilling procedure: (1) locate the core pin position by radiography; (2) mount a portable magnetic drill press with a custom contoured base plate padded with felt; (3) align the drill axis using a laser guide; (4) drill through the full pin length at low speed to prevent heat buildup; (5) remove the collapsed pin fragments; (6) clean the bore by solvent flushing; (7) install a threaded 316L stainless steel replacement pin with corrosion-inhibiting compound.

Stretcher Bar Mortise Drilling and Taxidermy Armature Bores ​

Painting stretcher bar mortise drilling: large-format painting stretchers (the wooden or aluminium frames that hold canvas taut) use mortise-and-tenon corner joints with corner keys. The mortise is a 10-20 mm wide x 40-80 mm deep slot drilled or routed into the stretcher bar end. For aluminium stretcher bars, the mortise is gun-drilled using a carbide drill on a CNC machining centre.

Taxidermy armature tube bores are drilled in stainless steel or brass tubes that form the internal skeleton of taxidermy mounts. The tubes must accommodate the mounting rod that attaches the specimen to the display base. The bores are gun-drilled through the full length of the armature tube (100-500 mm) using a carbide gun drill.

Quality Control for Museum Drilling ​

Frequently Asked Questions ​

Why are corroded iron core pins drilled out rather than dissolved with acid? ​

The Getty Conservation Institute recommends drilling over acid dissolution because acids can penetrate the bronze microstructure through porosity and micro-cracks, causing the bronze to become brittle or discoloured. Acid can also react with the bronze alloy constituents (tin, zinc, lead), selectively leaching them from the surface and creating a patina that does not match the original. Drilling, by contrast, is a purely mechanical process that removes only the iron pin material without chemically affecting the surrounding bronze. The drill is guided by radiography to ensure it stays within the iron pin and does not contact the bronze.

How is the drill aligned with the core pin axis in a bronze sculpture? ​

The drill is aligned with the core pin axis using a combination of radiography and laser guidance. First, X-ray images are taken from two perpendicular angles to determine the pin's position, diameter, length, and axis angle relative to the bronze surface. A laser guide is then mounted on the drill press and aligned so that the laser beam is centred on the pin cross-section as seen in the radiographs. The drill press is adjusted until the laser beam remains centred on the pin at both the entry point and the estimated far end of the pin. This two-point alignment ensures the drill axis matches the pin axis within 1-2 degrees.

What type of replacement pin material is used after the corroded pin is removed? ​

The replacement pin is typically made from 316L stainless steel, which provides high corrosion resistance (molybdenum content prevents pitting corrosion in humid museum environments) and mechanical strength comparable to the original wrought iron. The replacement pin is threaded (M10-M30, depending on the original pin diameter) and coated with a corrosion-inhibiting compound such as microcrystalline wax or a polyurethane sealant before insertion. The thread allows the pin to be screwed into the tapped bore in the bronze, providing a mechanical lock that does not rely on adhesives.

Can painting stretcher bar mortises be drilled after the frame is assembled? ​

Stretcher bar mortises are always drilled before frame assembly, while the individual stretcher bars are still separate components. The mortise is drilled into the end of each bar using a gun drill or a router with a jig. After drilling, the corner keys (wedges) are inserted into the mortises, and the four bars are assembled into a rectangular frame. The keys are then tapped inward to tension the canvas. Drilling mortises in an assembled frame would be impractical because the drill would need to access the inside corners of the frame.

How are display mount blind bores drilled for seismic-safe artifact installation? ​

Display mount blind bores are drilled into the base of an artifact (or into the display pedestal) to accept a locating pin that prevents the artifact from sliding off during an earthquake. The bore is drilled using a portable magnetic drill press or a hand drill with a depth stop, using a carbide-tipped twist drill. The bore depth is controlled by the depth stop, which is set to leave a minimum 5 mm wall thickness between the bore floor and the artifact surface. The bore position is marked using a template transferred from the display pedestal, ensuring that the locating pin aligns with the mating hole in the pedestal.

Data are based on published research and industry experience as of 2026. Always consult your conservator or museum specialist for application-specific parameters.