A Tale of Two Props: Conservation of Wooden WWI Propellers

The National Air and Space Museum’s conservation team is conserving thousands of artifacts to be displayed in new and reimagined galleries as part of an ongoing multi-year renovation project. The transformation of the Museum’s National Mall building has provided opportunities for each object to be examined, documented, cleaned, and stabilized prior to being installed in new galleries. While some milestone objects are returning to display, others have been chosen to be exhibited downtown for the first time. In the new World War I in the Air gallery, two such artifacts are wooden propellers manufactured for the American aviation production program.  

When the propellers arrived in the conservation department, it was evident that they both appeared to have their original finish present, which is rare for artifacts of this type and age. However, each artifact displayed distinct condition issues that needed to be addressed and required very different treatment approaches.

American Propeller and Manufacturing Co. Propeller

One of the artifacts in question is a gray painted propeller built by the American Propeller and Manufacturing Co. for the American Liberty engine. When the propeller arrived in the conservation laboratory, there was a significant layer of dust and grime on all its surfaces. The paint towards the center of the artifact exhibited deterioration inherent for these materials such as flaking and loss.  The propeller is constructed of multiple layers of wood which are laminated together, and, as the wood ages, it swells and shrinks in relationship to changes in temperature and relative humidity. While the wood changes size, the oil paint film on top of the wood remains relatively stiff, resulting in cracking and lifting of the paint film as seen on the surface of this artifact. A significant quantity of paint was missing, and numerous remaining paint flakes were lifting from the surface and almost completely detached. Additionally, exposure to high relative humidity or moisture caused the surface of the metal tips to develop a consistent layer of corrosion overall.

The treatment required careful consideration of both the wood substrate and paint film. The phases of the treatment process would need to be sequentially integrated with the removal of dirt trapped in the surfaces. The priority of treatment was to stabilize the fragile paint surface to prevent any further loss. To consolidate the paint, a conservation grade adhesive was fed under the paint surface using a very small brush. Where the lifting paint was flexible, it was laid back down onto the wood surface. Where the paint was too brittle and firmly lifting from the surface, heat was gently applied to the paint surface with a tacking iron through a barrier material that prevented the paint surface from getting too hot or burning. This heat gently relaxed the paint film and allowed it to lay properly on the wood substrate.

Once the paint layer was stable, it was time to reduce the layer of grime that covered the surface. The object was vacuumed with a HEPA filtered vacuum and soft brush and then cleaned with a vulcanized rubber soot sponge. The final streaky gray layer of grime was reduced using a dilute chelator to reveal the lighter gray surface. Chelators help clean the surface by binding to metallic ions in the grime and making it easier to remove the grime with water. The metal surfaces were very evenly corroded and appeared stable. The metal was surface cleaned and then coated with a wax to prevent future corrosion.

Following stabilization and cleaning, the aesthetic appearance of the artifact was addressed. In consultation with Museum curator Jeremy Kinney, it was established that the ethical goal was for the artifact to appear complete, but to retain any damage that provides evidence of age and historic use. Once that was determined, it was time to begin the process of aesthetic compensation. Conservators attempt to use reversible treatment techniques when possible to allow for easy removal in the future without causing damage to the original artifact components. A synthetic resin barrier layer was applied to the exposed wood in areas of loss to allow for easy reversal of the fills in the future if needed. The synthetic resin can be removed with a solvent which will not solubilize the adhesive used to secure the paint, so the paint consolidation will not be reversed if the fills are removed. After applying the barrier layer, we next had to fill the shallow losses so that the fills were the same thickness as the original paint layer. This creates a smooth uniform surface and makes the fills disappear better. Once the fills were leveled, they were toned with paint to match the aged appearance of the original paint.

B.L. Marble Chair Co. Propeller

The second propeller prepared for display was a wooden propeller built by the B.L. Marble Chair Co. Rather than being painted overall, this artifact features areas of wood that were painted and other areas that had a clear finish coating.

Like the American Propeller Co. propeller, when this propeller arrived in the laboratory, it had dust and grime covering the surfaces. The original decals were so dirty that they blended in with the surrounding paint.  It was also evident that the clear finish had deteriorated with age. The surface featured raised black bumps scattered overall which produced a darkened “alligatored” appearance.

We conducted scientific analysis to determine the composition of the original coating. This helped us understand why it looked the way it did and how to safely treat the artifact. Analysis with ultraviolet light and Fourier Transform Infrared Spectroscopy (FTIR) revealed the presence of shellac and linseed oil on the surface. It indicated that, as the coating aged, these two coating components separated, likely due to changes in temperature and relative humidity. The linseed oil pooled into small islands which then darkened, while the shellac component remained smoother and clearer, producing the alligatoring effect seen overall.

While the coating had a visually inappropriate appearance, a large percentage of the coating was still intact and properly protecting the surface. While stripping the coating and applying a new finish would remove the alligatoring in the quickest manner, the authentic naturally aged appearance of the wood would also be lost. We devised an ethical conservation approach that would preserve the original coating while restoring it closer to its originally intended appearance. The small black spots were reduced mechanically using a combination of scalpels and extremely fine sanding materials. Only the black surface crust was removed, leaving the underlying coating surface intact. The painted surfaces were cleaned to remove grime. Through photographic records it was possible to determine that some staining on the paint was not related to its original use, as it had occurred after the propeller entered the collection. Thus, this staining could ethically be removed. Metal on the propeller tips was surface cleaned and then waxed to prevent future corrosion.

These treatments expanded beyond the typical preservation-only approach employed by most conservators and included a significant amount of restoration. This approach was chosen as it displays a more representative appearance of how the propellers would have looked during use. The restraint with which the restoration was done reflects the ethical standards of modern conservation and restoration by ensuring the artifacts are structurally stable but also retain the signs of natural aging and original use. Be sure to keep an eye out for these exciting artifacts in our new World War I gallery opening when the Transformation project is completed!