Use a mild acidic solution such as vinegar or a cleaner containing a mild acid (e.g. phosphoric acid). Most of the calcium and lime deposit removers on the market are primarily a mild acid. Use gloves and some form of eye protection. The gloves protect the hands from becoming dry. Do not spray on the solution: wipe it on with a damp sponge. The most important instruction (and the least followed) is to let the chemical do the work. If the buildup is considerable, it may take multiple applications. DO NOT use anything on the glass other than damp sponge or soft cloth. This procedure should remove all of the calcium buildup without scratching the glass.