24-pounder Coehorn Mortar 

Army Heritage and Education Center

HISTORIC OVERVIEW

After Treatment 2: Coehorn MortarThe Coehorn mortar was developed by a Dutch military officer named Baron van Manno Coehoorn and was first utilized in 1692 and continued in service, with modifications, well into the late 19th century. The design utilized a copper alloy gun tube secured to a wooden bed by two bolted iron straps. The tube was often set at a 45 degree angle and “vertically” fired a projectile at a range determined by the amount of propellant and the weight of projectile fired. The mortar weighed approximately 300lbs and could be lugged by two men or moved rapidly by four into unprepared locations. This particular mortar was manufactured by the Ames Manufacturing Co. of Chicopee, Massachusetts. The Ames Company, founded in the early 1800s, was originally contracted to the US Government for the production of swords; however, by 1837 the company was producing cast bronze canons.

 

PRE-TREATMENT CONDITION

Before Treatment: Coehorn MortarThe Coehorn Mortar was in unstable condition due to extensive fungal deterioration of the wooden bed which had caused a significant loss of material. The primary areas of loss were found on the proper right side of the wooden base and extended throughout the underside of the bed. The proper right front corner of the wooden bed had been repaired with a section of tin sheeting that had been nailed into remaining areas of sound wood; however, the weight of the mortar had caused the metal to compress; causing the eventual failure of the old repair. The iron plates which secured the mortar tube to the bed were loose, again due to the loss of wood on the underside of the base. The bed which had been varnished and painted various times throughout its use and display was now actively flaking revealing the raw wooden substrate in several large areas. The bronze tube appeared to be in stable condition with the exception of minor abrasions, scratches, and the deteriorated cellulose nitrate lacquer coating which was flaking and scaling from the barrel. All surfaces were covered with a heavy layer of dust and grime.

 

TREATMENT

Microscopic Cross-sectional Analysis of Paint SampleThe mortar was documented and digitally photographed prior to disassembly. Treatment of the mortar then focused upon the two primary components: the gun tube and the wooden bed or base. The conservation of the gun tube required the removal of the deteriorated synthetic lacquer coating using a proprietary stripping gel. After application, the tube was wrapped in a thin membrane of polyethylene sheeting to prevent the rapid evaporation of the solvent. Once the lacquer was removed, the tube was rinsed with mineral spirits; any residual lacquer was removed using cotton swabs wetted with acetone. The surface of the tube was then carefully heated and hot waxed using a blend of microcrystalline waxes. Once cool, the tube received an additional two applications of cold wax, buffed between and after application.

 

The base, once separated from the gun tube, was again photographed and paint samples were taken for microscopic cross-sectional analysis to determine the earliest paint color and long history of having been repainted. The sheet metal repairs and all iron hardware components were removed to facilitate further treatment. The areas of deteriorated wood were then consolidated using polyvinyl butyrol in ethanol in increasing concentrations using brushes and hypodermic syringes. Because of the weight of mortar tube it was determined that the base would require solid weight bearing inserts or fills. U.S. Army staff required that the fills separate from the base and easily removed. This was accomplished by placing the base into a protective vacuum compression bag, constructing a custom built form, and then drawing a vacuum to pull the plastic membrane into the areas of wood loss. A mixture of catalyzed polyester resin and glass microspheres was then poured into those cavities. Upon curing, the apparatus was disassembled and the polyester inserts were modified to fit perfectly and accept all hardware. A mold was made from the undamaged proper left side of the base using RTV silicone mold-making materials and techniques to create a panel which would help support the mortar and visually reintegrate the areas of loss. The interior of the cast panel was covered with a sheet of barrier material (Marvelseal) and then attached using the original hardware components. Minor cracks and seams were filled with special mixture of pigmented microcrystalline waxes. The inserts and repairs were then in-painted using a reversible palette. Prior to reassembly all iron components were mechanically cleaned, chemically stabilized, and given a protective lacquer coating. The tube was then reinstalled on the stabilized base or bed. After treatment, the Coehorn Mortar was packed in a custom fabricated storage crate and returned to the Army Heritage and Education Center.

 


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