Unfortunately, on the Internet, methods for cleaning gilt bronze (called ormolu in France) are at about the same stage as medicine was in the mid 19th century. Every crackpot has a nostrum guaranteed to produced marvelous results, while typically doing more harm than good.
In my early years I fell afoul of some of these, and I will carry the shame of damaging the gilding on 200 year old bronzes to my grave. Fortunately, I now know that there is one method of cleaning gilt bronze that is usually effective, and will not damage the gilding in any way.
The image shown above is a part from a Restauration period chandelier with the right half cleaned. The original gilt bronze was of excellent quality. The dark half of the bronze shows the condition of all gilded bronze elements when I purchased it. It was covered with a dark brown finish that was hard like shellac; it was actually a combination of cigarette smoke and grease accumulated over 180 years of use. So, here is my cleaning method:
1. Stick with water, but make it boiling hot. The best solution would be a steam dispenser, but not many people have those. I just use boiling water from the kitchen hot water dispenser. Don't worry about getting the water too hot - gold melts at approximately 2000 degrees Fahrenheit so it will not even soften at 212°. Cold water will not work as you are trying to cut grease and tar. When cleaning multiple items, keep reheating the water.
2. Soak the gilt Bronze for approximately 10 minutes. A shorter soak will not be as effective. Carefully take it out and place on a wet cloth in the sink and squirt dish washing soap all over it.
3. Scrub with a tooth brush until clean.
4. Dry it completely. Not just its surface, but in all cracks and mounting holes. I typically dry with a rag and then place on the rack in my gas oven (only pilot on) for 30 minutes. Doing a good job drying is very important. if not dried completely, corrosion could occur to the bronze or later to the mounting screws. The low humidity interior of the oven insures all moisture is removed from the crevices in the bronze work. To protect exposed (ungilded) spots on the bronze, wax with a good floor wax.
If this cleaning method does not work, obviously you have a real problem. This is because you are dealing with damage, not dirt. Typically, the gilding has worn off and what you have is oxidized bronze. In my opinion it is better to just live with it rather than resort to harsher cleaners that will give it an artificial or "new" look. For mercy's sake, please don't use brass cleaner. Ammonia is also very high risk - it will strip off thin gold plate in the blink of an eye.
Based upon one reader's feedback, I wanted to add another cautionary note. This reader informed me that he used the method above to clean gilt bronze and it caused it to become dull. I have given this some thought. Of course, hot water has no effect on gold, or we would all take our wedding rings off to shower. So what caused the gilt bronzes to lose their sheen? Were they another metal other than gold on bronze? Still no metal should have tarnished after just a few minutes in water. After some thought, I think it may be the bronzes were not gilded, but instead varnished (a golden shellac for instance). This was a common practise even in the 18th century. If varnished bronzes were soaked, the varnish would dissolve completely in the water. Fortunately, the reapplication of varnish will return them to the same condition as they were before.
As a final note, you must also be careful when having someone repair a piece of gilded bronze, such a broken arm on a candelabra. The repairer MUST NOT attempt to weld the bronze parts together. The high temperature weld will permanently remove all gilding and blacken the bronze for two or three inches in each direction.