[ihidaya]

I work for the Zeitoun Institute (http://www.zeitoun.ca).  We are involved in preservation of middle-eastern Christianity, and handle a lot of various objects from ancient vellum manuscripts to antique bronze crosses.

Several weeks ago, my office received a box of ancient bronze crosses from the Middle East, and they were tightly packed in newspaper, then placed inside a styrofoam box, and duct taped together.  When I opened the outer plastic, the styrofoam was wet on the inside, and the crosses were wet also.  The styrofoam had broken down, and there were small pieces of styrofoam everywhere.  The styrofoam began foaming up on contact with the crosses, and left white corosion on the surface on the crosses.

Our usual method of cleaning these crosses is by soaking them in flour/water/vinegar/salt paste for 15-30 minutes, and brushing them with toothbrushes.  However after doing this process on several of the crosses, I have found that it will not remove the white substance caused by the styrofoam.

I am not a chemist, so I don't know even what causes stryrofoam to react in that manner.  Can anyone tell me if it is possible to remove the white plaque caused by the styrofoam, without damaging the crosses?

[mike]

Styrofoam is a tradename and I guess it is what people usually refer to as polystyrene, or the polymer of styrene which is an aromatic compound. Styrene is phenylethene, it is an oily liquid, while polystyrene is a solid a room temperature. I am not sure what the liquid is or what the corrosion is caused by. It could be that the polymer is not polystyrene. It could be that the liquid is from another source other than the polymer.

[mdlhvn]

These links can help u, I hope
http://en.wikipedia.org/wiki/Styrofoam

From my point of view, u can try petroleum washing the white plaque on the surface of crosses.

[constant thinker]

Something else may have spilt on the wrapping/package that caused the adverse reaction.

[Bakegaku]

From my point of view, u can try petroleum washing the white plaque on the surface of crosses.

I agree with that suggestion.  Given the fact that using water, salt, flour, and vinegar (all polar substances), and that the white substance is somehow related to polystyrene, a nonpolar substance should be used. 

I also hear acetone is good at dissolving many plastics, so you might try that out, too.  Observe safety precautions, as in all situations.