A piece is considered solid brass if the material is pure brass … it can be a heavy cast candlestick, the hollow arms of a chandelier, or a sheet of brass encasing a steel post as found in many vintage beds. Solid brass will always polish to its original beauty, although an old lacquer may need to be removed first. Brass plated items are usually made of steel or white metal (zinc) to which the brass has been applied via electroplating. Even though lacquer is usually applied to protect the plating, brass plating is exceedingly thin and will deteriorate over a period of time. Brass plated pieces can sometimes be polished successfully (once the lacquer is carefully removed), but if the plating is deteriorated the piece will probably need to be replated.
The first way to test an item is with a fairly strong magnet and many times, a refrigerator magnet will do. Solid brass is not magnetic. If the magnet sticks, the item is likely steel or cast iron, with a brass plating. If the magnet does not stick, you can test further by scratching a hidden area with a sharp tool. If you see a shiny yellow scratch, the item is likely solid brass. If you see a silvery scratch, your piece is likely white metal (zinc). Iron, steel, and white metal can all be replated, in which case a lacquer is always applied to protect the plating.