Steps to clean a bronze ground marker
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Clean a Bronze Marker
All actions taken based on advise of this web site are done so at the sole risk of the person conducting the cleaning.  No responsibility is assumed for any damage to a monument as a result of the advise given on this web site.
Evaluation of a bronze marker and warnings on the pitfalls of cleaning
Bronze markers are made of bronze metal.  Bronze is a mixture of copper and tin in about 9 to 1 ratio.  Bronze is a relative hard metal but is very brittle.  A bronze monument can be easily broken with only a minimal amount of force.  Bronze was probably the first wide spread composition metals with its common use beginning around 2.500 BC. A bronze marker is made by casting the base monument. At this point, the design and lettering are cast into the monument.  The monument is them trimmed by hand, some areas polished and given the background paint.  The monument’s lettering is them surfaced so they are flat and polished.  Finally, a protective coat is applied to the whole monument.  The protective coat is designed to slow the oxidation of the monument.  At this point the bronze portion of the monument is complete.  The monument is them attached to a granite base.  The granite base provides the support for the bronze. The aging process of a bronze monument occurs in two stages.  The first stage occurs as the protective coating begins to ware.  Although many advances have been made to improve the longevity of the protective costing, over time it will ware off.  The second stage occurs once the protective coating has worn off.  The copper in the bronze begins to oxidization.  This is similar to rust on a piece of steel.  Oxygen combines with the copper to form copper oxide.  As opposed to rust on steel, the oxidization does not harm the bronze.  In fact, the oxidation forms a protective layer.  The oxidation can take a dark brown to green appearance based on other chemicals which can be present as the oxidation occurs. 
When cleaning a bronze monument, you really have three areas to clean.  First you have the granite base.  Second you have the polished details which are primarily the lettering. And finally you have the bronze monuments background and decorative elements.  Each surface has its own issues to evaluate the cleaning challenges and solutions to those challenges.  Just to restate the obvious.  Cleaning a bronze monument is tricky and you are most likely not to get good results.  Use these suggestions at your own risk.  I strongly suggest getting a professional to help you out. Again, the first step in cleaning any monument is to evaluate the condition of the monument.  Here are the steps to follow when evaluating the monument: 1) Look and see if either the granite base or the bronze monument is cracked or broken.  If so, locate a qualified monument dealer in the area to assist you with the monument.  A cracked base can be replaced easily at a minimum cost.  Replacing the base is important as it is the support for the bronze monument.  Poor support will result in the bronze ultimately breaking.  If the bronze is broken or cracked, the monument may be repairable.  In many cases, the monument cannot be repaired and will have to be replaced.  DO NOT try to deal with this type of problem alone.  Get professional help.  Both a monument dealer and a bronze manufacturer can aid in the repair process. 2) If the granite is just dirty or stained, this is by far the simplest with which clean.  You should just refer to the section on how to clean a stone monument on this web site.  I have a couple of additional comments though on cleaning granite.  Many of the chemicals which you might use to clean the granite will react with the bronze.  DO NOT use bleach or any household cleaner with a high or low ph.  These will damage the bronze.  Our recommended cleaner, D/2 is chemically neutral and will not affect the bronze. 3) If the bronze is simply dirty, cleaning it is again simple.  I recommend using a mild dishwashing soap.  Dawn dishwashing liquid seems to work very well.  Use a lot of water and a soft brush.  Less soap is better as it is easier to clean off.  Be sure your rinse the monument well.  Any soapy deposits left will result in a dirty residue building up on the monument. 4) Now comes one of the tricky conditions for you to handle, tarnished letters.  No matter what you do, overtime, the letters will tarnish.  The tarnish is called patina and is considered by many to be attractive.  But if you just insist on polished letters, here are the steps to polish the letters.  The best product is called “Brasso”.  It is a brass and bronze polishing agent.  It is abrasive, so it will scratch the lettering somewhat, but will remove the patina.  Use the Brasso sparingly and only on the top of the letters.  The Brasso will remove the background paint, so keep it off as much as possible.  Once you polish the letters you will need to seal them.  If you just leave them as is, they will tarnish in a matter of weeks.  To reseal the letters, you can use a spray can of clear lacquer.  You will need to seal the whole monument so it appears consistent.  Be sure you have cleaned off the Brasso and the whole monument is free of dirt and grime.  The other option, and the better of the choices, is to remove the monument and have it professionally refinished by one of the monument manufacturers.  A professional job is the best way to ensure that the monument is restored to its original condition. 5) In the paint on the background is peeling, you can repaint it.  Several of the companies which manufacture bronze monuments sell kits to repaint the background.  I have used the kits in the past and it is tricky to get good results.  DO NOT use some spray paint you get from the hardware store.  This is not the paint used on these bronzes and it will not work or last.  Once again, you have the option of a professional refinishing.
Cleaning a Bronze Monument