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How to clean art bronze sculpture

Posted by Andrew Silla on

The care and cleaning of bronze sculpture is a complicated subject and one that can be daunting to collectors of sculpture.  There are many nuances to conservation of these pieces and it is a topic we field questions on regularly. Below are some of the most common questions we receive:

What is bronze rot and what steps should I take to avoid it?

This is one of the most important questions. Bronze rot is a reaction between chlorine and the metal that results in a white powdery residue that is an absolute nightmare to deal with. It is generally brought on by an ill-advised cleaning of the sculpture using non-purified water, such as would be found in public utilities, where even the slightest trace of chlorine left on the surface of the bronze will create a long term chain reaction. The steps we recommend to avoid introducing this to your sculpture are:

  1. Always use purified water with zero chlorine content
  2. Try to never submerge the bronze or allow water to get into the joints; water should only be used sparingly and in areas where all moisture can easily and completely be removed from the sculpture
  3. Make sure your sculpture is always sealed against moisture, generally with beeswax or a beeswax/carnauba mix
  4. In the event that you had to submerge the sculpture in order to properly clean it, moisture must be completely dried from the sculpture. A fan, space heater, hair dryer or even use of an oven set on low can help release the moisture from the deep recesses of a lost-wax work, where moisture may have found its way through the body and into a remote appendage of the figure.

How do I safely clean the dust and grime from my bronze sculpture?

First you need to find a clean and well lit working space to work on the sculpture. Most bronze sculptures remain in beautiful original condition until they are moved, cleaned or handled - a poorly planned work space can change a dirty sculpture into a broken sculpture very quickly. Next, dust the sculpture using a soft toothbrush with masking tape around the plastic parts to prevent scraping. A wooden toothpick can be very helpful in cleaning cracks and crevices - and never use metal or any hard substance that could scrape the surface! We prefer to use the cheapest brands of paper towels for cleaning, like Bounty Basic, as they have a very slight abrasiveness to the material that helps remove grime. This is time consuming if the sculpture is in particularly bad shape, but it is imperative to take time and remove all dirt, as it will turn slightly green and hazy under wax if the surface is not perfectly clean.

Warning, do not attempt to remove all "color" from the sculpture! There are several types of patina, the most popular being acid, acid-pigment, painted/polychrome and in some cases colored waxes. Often there may be a mixture of these, the artist using colored wax to achieve some transition or effect in the sculpture. We have seen so many scenarios where a client has used a toothpick or some other object to remove "grime" from crevices only to realize that they actually removed a blackened wax covering the golden hue of bronze beneath.

After the bronze has been satisfactorily polished and cleaned of all dirt, grime and residue, clean it with a lint-free cotton cloth to remove any remaining dust.

What is the best wax to seal my bronze sculpture with?

There are many dozens of very good waxes, so much of this has to do with preference. We enjoy working with Liberon’s Black Bison wax in Neutral (available here at Amazon). It is a nice mix of beeswax and carnauba suspended in mineral spirits and applies very evenly.

How do you apply wax to a bronze sculpture to achieve professional results?

Not unlike polishing a pair of shoes, less is more when waxing a sculpture. I prefer to take a square of soft cotton, like used undershirts or a sock, and gather just a touch of wax on the tip and work it around until it is very thin. Then repeat until the sculpture is covered. I’ve tried other methods, like using a bit of wax inside cheesecloth, but I find that the “dip and rub” method allows me the most control. Q-tips can be used for hard to reach areas (be sure to get Q-tips with a paper column and a full head of cotton versus the cheaper plastic variety). A part of the waxing process is actually a cleaning process, so don’t be surprised to see a dirty brown color coming off on your rag as you wax the bronze. The mineral spirits in the wax will cut the old wax on the sculpture and remove the dirt, replacing it with the new fresher wax. Be careful though not to get overly aggressive with “removing old wax”, as this wax may have some aspect of color for the sculpture that you were not considering. After the entire sculpture has been sealed, it will begin to haze over - allow it to sit for 20 minutes so that the wax can harden. It should be in a relatively cool area, as it will not harden if left in the sun or other hot space. Then buff it using a clean lint-free cotton cloth until every bit of excess wax is removed from the surface and it glows brilliantly.

What kind of maintenance do I need to do on my sculpture?

First, environmental maintenance is very important. Bronze reacts with moisture, even when protected with wax, so maintaining a consistent low humidity is crucial. We like to strive for under 50% relative humidity, though in some climates this may be impractical. Lower is better. Next, they should be kept in a place where sunlight won’t alter the patina - this may be achieved with UV filtering on windows or by simply placing them out of the sunlight. Finally, the sculpture should be regularly dusted - this can be very fast as long as it is somewhat regular, perhaps every six months or so. If dust is allowed to settle for long periods of time on the wax, it can become a part of the wax and form a film that is a project to remove. If a sculpture is maintained with these simple principles, the sculpture will last in its original condition for an indefinite period of time without need for further waxing or cleaning.

What about storing bronze sculpture? Is there anything that should be considered?

The most important consideration is what is touching the surface of the bronze. Is it being stored in bubble wrap and boxed? If so, you will want to make sure the plastic is not in direct contact with the bronze - it would be wise to instead wrap it in wax paper prior to wrapping it in bubble wrap, fabric, foam, etc. Over time, anything with a texture can make impressions on the wax surface of the sculpture - particularly if there is heat or moisture ever present in the air. We have seen some pretty depressing scenarios where the beautiful surface is marred by checker marks from a mover’s blanket the piece was wrapped in - and these are very difficult to remove without professional assistance. The sculpture should then be double boxed with three inches between the sculpture and the inner carton, then three inches on all sides between the inner carton and the outer carton. The innermost carton with the sculpture should have some moisture removal packets tossed in with it to keep humidity to an absolute minimum.