the porcelain covered crowns are usually precious metal inside - gold, platinum, palladium, and sometimes nickel(pay more and avoid that nightmare.) the problem is, the tooth they are capping has probably had a filling in it already. the dentist will simply use the filling as part of the underlying material if that material is needed for the base. so there is a chance of having mercury under the crown but not part of it. my bridge and one crown were done before i became mercury savvy. i have no idea what is under there. i will have to find out some day when it is available to me to do so. unfortunately, that quadrant is some of the best work in my mouth!