For conservators working with iron, rust is always a consideration. In museum collections or in our homes, we easily encounter rusty fences, grates, car parts, artwork and collectibles. Iron oxide (commonly called rust) results from the natural oxidation of iron by oxygen. Some metals, like bronze, can form a protective layer of corrosion products called a “patina.” However, iron corrosion will continue to negatively affect an object until treated. Rust is destructive, not protective.
To combat this destruction, conservators utilize a few different methods. For artifacts not containing heavy chlorine-induced corrosion (objects not heavy in salts or near salt water), rust conversion is a reliable avenue for protection. Rust converters, most often containing tannic acid or phosphoric acid, react with the iron oxides to form a stable layer on the exterior of the treated object. There are many commercially available rust converters that are accessible to both the professional conservator and concerned homeowner. However all rust converters do not function equally well, especially over a long period of time.
With these concerns in mind, NCPTT’s Materials Research Program undertakes a new study this summer centering on the effective treatment of rust. In this comparative study, we will investigate which product most completely converts rust and which product performs best over a period of extended exposure. We want to determine which rust converters are both effective and long-lasting.
The experiment follows this basic structure: (1) Document physical condition, (2) Analyze chemical condition, (3) Artificially weather.
To document the physical condition of the samples, we photograph our rusted sheet metal samples at various points in the experiment. Additionally, a laser profilometer records a three dimensional map of the sample surface and any changes in that surface during the experiment.
We use a number of analytical techniques in our analysis of the chemical condition. First, a Fourier-Transform Infrared Spectrometer (FT-IR) helps us characterize the chemical structure of the primary compounds in the surface of the sample. After treatment and as the weathering progresses, the FT-IR allows our research team to chart the chemical changes occurring in the surface of the sample. We also quantitatively measure the color, gloss, and coating thickness using a colorimeter, a glossmeter and a magnetic induction instrument (respectively).
Finally, we simulate natural exposure of our samples through the UV exposure and condensation cycles of an artificial weathering instrument (called a QUV weatherometer). The artificial weathering admittedly differs in certain ways from natural weathering. However, this process provides a controlled environment in which to begin studying the long-term efficacy of the rust converters. The samples are periodically removed during the weathering process so that the condition can be documented and chemical analyses can be performed.
When the study concludes, we hope to have the scientific data to compare the performance of commercially available rust converters based on their efficacy and durability. This information can then be used in the effective treatment of rusty objects—whether that item will be displayed in a museum or on a back porch.