I have seen several students, all located in different area of the country with either the wrong metal or impure metal. When you are enameling for fine jewelry it is most common to select fine silver as your base metal. For several reasons, sparkle and clarity of the enamels, and second the enamel fuses to the fine silver.  With hobby style resources available for purchasing metal and the introduction of new metals such as Argentium silver it is easy to get the wrong product. Before you begin working with any silver, it is not that easy to tell the difference. But once you have began your project, maybe four layers of enamels later you start seeing the bubbles and discoloration of impure metals.  I highly recommend to buy all your metal from one reliable supplier, and only one. Then you know exactly were you purchased it and can hold them accountable if it is fine silver. But more than likely, if it is from a notable refinery they will not have a problem of recognizing metals.