Question:
I have purchased an old soda and acid fire extinguisher. It is made of copper and bronze. Can anyone recommend a way to safely clean it?

Answer: Your statement is too broad to be totally accurate. Lacquering brass/copper does not necessarily lower it's value, in fact with beds, it increases the price I could get for them. (I am assuming you are equating value with price.) Only the true purists, like me(and you?) would want one unlacquered. A fire extinquisher value/price would usually be enhanced by polishing and then lacquering. Most people want the "bright and shiny" look and will pay more to get it. This also applies to the old copper wash boilers. Also, it is a little hard to devalue a piece that is not now nor ever will be an antique. If you are having a piece relacquered, there is usually a very minimal charge for removing any old lacquer. (in the midwest USA). I don't know who you deal with that charges you 3 times the cost of applying it, but you may want to shop around. It is also very simple to remove it yourself. Bronze of course is a totally different story, and rarely, if ever would it be appropriate to lacquer it. (I refrain from the use of the word "never" because someone else, who knows a lot more about it than I do, may know of an instance where it is done.) I would guess the same would apply to silver. You are so right about cleaning(not polishing) bronze very carefully. (although I can't say I would worry too much about the bronze on a fire extinguisher A damp soft cloth is usually all that should be used.