What not to clean or not clean, as more mistakes are made in this area. A Rule thumb is that if you are not sure, don't do it! No! Halt! Stop! Danger! What took nature hundreds or thousands of years to create can be destroyed by in a moment. What you might think of as "dirt" is known to the collector as "patina" and adds to the value and beauty of antique and ancient items. How can you tell the difference between "dirt" and "patina"? This is a bit of a fuzzy area, but I would say in general patina cannot be removed with water, or by soft brushing, but dirt can. DO NOT REMOVE the patina! You can feel free to contact me with questions if in doubt. So what is this patina? Patinas are chemical compounds formed on the surface of metals. Many patinas form naturally, by weathering. Figuratively, "patina" can refer to any fading, darkening or other signs of age on any material. It is natural and/or unavoidable. The chemical process by which a patina forms is called "patination," and a work of art coated by a patina is said to be "patinated." For instance a statue or object which has been handled a lot can be worn down and acted upon by the oils in the human hand, imparting a smooth (as imposed to encrusted) patina. Patina often adds value, shows authenticity and should not be removed. An exception would be a heavily encrusted object which one planned to keep forever, or exhibit in a museum and which would be enhanced by getting down to original material. In my opinion many, many objects in museums (even world-class ones) are over-cleaned or over-restored or both.. The Bottom Line: Leave it a bit dirty rather than risk damage or over-cleaning!
Let's continue with some basic tips. 
- warm water is the best and most gentle cleanser. Use caution, however on delicate porous materials: cloth, paper, wood, silk, etc, or anything really delicate or fragile. Clean an itsy-bitsy, teeny-tiny, little area first. If it looks like it might damage the item stop!
- Lemon Juice is also a good and fairly mild cleaning agent for stickers, etc. Don't use lemon juice on porous materials in general.
- For truly delicate objects the best you may be able to do is to brush the item with a soft brush and clean whatever dust and dirt you can by gently dusting the object. It is better to have a somewhat dirty object than to destroy it of course!
- If you are more certain of the objects solidity you may proceed to using warm water with a little bit of dish soap. A 1% solution should be pretty safe. I use this for most pottery, even ancient pottery, for example.
- The next level would be warm water, soap and scrubbing from gentle to vigorous.
- Finally harsher methods such as solvents and scraping can be used. I will only touch on this briefly as the potential for damage is high.
- Professionals can also use lasers, obviously we won't be talking about that!
- Porcelain ceramic and modern glass: this is really the easiest. Just hand clean as you would a normal dish, but do so by hand and gently. Use a 1% soap solution. a toothbrush should do the trick for stubborn bits. Try lemon juice if this does not work. Porcelain is amazingly hard and if it has encrustations on it (old sticker material, etc.) you can remove it with a razor blade. the blade must be held perpendicular to the object. push the blade only as hard as you need to do to remove the offending substance. Don't push too hard or you will leave a black mark which you will not be able to remove. You can do the same for glass and mirrors, provided they are not heavily damaged.
- Hard Stone (granite, marble, etc): more or less the same as above, but DO NOT scrape it with metal as metal is harder than stone in most cases and will scratch it. If you must scrape, use plastic or wood.
- steel, iron, aluminum modern metal; this is usually pretty easy to clean using the 1% solution and a soft brush (toothbrush should be fine here too). Iron (and non-stainless steel) are susceptible to rust. Light rust and encrustation can be safely removed with mild chemicals. I recommend "Never-Dull" and nothing else. If you use it, read the label first. Heavy rust can be removed with steel wool, but this may leave scratches if the metal is soft. This is a personal decision. I would, for example use steel wool on an old wagon wheel, but never, ever on an item like an antique sword.
- Wood, leather: I would avoid soap if at all possible, as it may penetrate the item and have unintended consequences. Use a damp cloth and a lot of patience. Use butchers wax in a can and a fine paintbrush to work the wax into the item after you clean it. Then finish up with a soft cloth. It is amazing what wax can do for wood and leather! Don't use pledge or other spray waxes, they have to many other "ingredients". Keep it simple.
- Bronze: Dust it, wax it, and forget about it. Don't do anything else or you will probably be sorry. Don't come crying to me if you ignore this sage advice. do not remove the patina!!!
- Anything ancient or really valuable: Dust it with a fine bristle paint brush and leave it alone. Anything more should be left to professionals. If you must clean it contact me and I will see if I can help.
- Paper, cloth, silk, etc.: These materials are exceedingly delicate, however a damp (almost dry) automotive quality cloth can be used to very, very, gingerly remove surface dirt. Test a teeny-tiny non-visible area first for fastness.
