December 19, 2012
Metal Objects Conservation....continued
In a previous post I showed how I clean up corrosion products (rust) and other material from metal objects that were found in soil under moist conditions. I mentioned that I consider this as a two step process in that once cleaned (Step One) these objects should be stabilised (Step Two) to prevent further deterioration due to the possibility of high relative humidity. Of course, the cleaning process has the side effect of exposing more of the metal surface to moisture and oxygen thereby promoting the accumulation of corrosion products (rust). I use conservation grade wax to coat the objects to provide a barrier to contact with moisture and oxygen combination thus ensuring that they will last a considerable time into the future. Not all objects would be treated this way but certainly most smaller objects can be. Particularly those with high iron content since they are susceptible to rusting. It is not practical to do this with larger objects as you will see by the techniques I use. I prefer to use Renaissance Micro-crystalline Wax since it is pure and safe to use on most objects plus the added benefit of being developed by the British Museum for use on a wide range of historical objects such as furniture, leather, ivory, onyx, marble, metals, and so on. For non-metals, it is typically applied as a thin layer and then buffed with a soft cloth. For metals, it is often used over top of an anti-corrosion undercoating. For our purposes in a small, community museum setting it is not always affordable to purchase anti-corrosion undercoating so simply applying a layer of this wax will serve to protect the object at minimal cost. I use heat sparingly applied to the wax and object to help smooth it into a thin, uniform layer.
|Back of padlock with tools and materials used to apply wax.|
Before heating, I apply a thin layer of wax to the object with a small, coarse bristled brush. I then apply minimum heat via a hair dryer or heat gun to melt the wax and brush it again to remove lumps and ensure low spots and all the surface is covered in a thin layer. The heat source must be carefully applied to ensure that it is not too hot to handle. In the photograph to the right you can see the tools and materials I used to treat the padlock. I placed the object on a wooden cutting board as a neutral material which would not readily conduct the heat. I used a paper towel (above the brush) to clean excess wax off the brush. I find that in a short time the object becomes quite warm to the touch. Keep in mind that most metals expand when heated and contract when cooled so it is best if the objects to be treated are comprised mostly of iron rather than an alloy since different metals expand and contract at different rates. Alloys for the most part would be handled differently.
In the photographs below I show the railway padlock I worked on with the wax applied before it was melted and then after it was melted so you can see the difference. The brush is the right tool to use since it aids in getting right into all the depressions and allows for us to smooth out the excess wax around all the bumps, cracks, edges and depressions.
|Padlock with wax heated and smoothed|
|Padlock with wax applied before heat|
The edges, rivets, depressions, lettering, and contrasting colours are all still visible after the wax is applied and smoothed by applying heat. The object on the right is now protected from moisture, oxygen, oils from handling, pollution and so on. It can be safely packaged, stored away, and maintained in this condition for many, many years.
Conservation Tips: Treating simple metal objects can be done with minimal time and cost as shown above. Reversing the process is relatively simple by carefully reheating the object and wiping off the wax with a soft cloth. I suggest wearing gloves such as oven mitts that will protect your hands from the heat applied and using a thin board as a working base. As with most objects it is always a good idea to consult with a trained conservator beforehand who can assess the object, determine the best treatment, and provide an estimate of the cost. The above treatment has been done successfully on railway spikes, a railway valve wheel, a padlock and these archaeological objects: hand made nails, a key, a short length of chain, a shoe buckle.