The best way to store unused metal clay is to keep it in an airtight container at room temperature. Metal clay can dry out quickly when exposed to air, leading to a loss of its workability. Using a zip-lock bag inside a sealed container helps to create an airtight environment. It is also important to store metal clay away from direct sunlight and extreme temperatures. If you have opened a package of metal clay, you might want to wrap the unused portion in plastic wrap before placing it in the container to further prevent air exposure. It is advisable to keep it in a cool, dry place to maintain its consistency and usability for future projects. For any specific recommendations or product details, checking the current web page of Metalclays.com may provide additional insights.