Don't Forget: Copper Tarnishes. Your mugs will naturally develop a beautiful patina with use over time.  However, if you would like to keep your mugs looking brand new, you will need to take time to care for them. Polishing is necessary to keep the mugs looking new.  You can use citric acid as a polish (from fresh lime or lemon juice) or you can purchase Barkeeper's Friend. Either product will work nicely to keep your Moscow Mule Mugs looking new.MANUFACTURER'S DISCLAIMER   This product is made of copper and in order to ensure your enjoyment of this product, please note the following:   1. Copper is a highly conductive metal and is extremely sensitive to temperature. It is not recommended that you subject this product to extreme hot or cold temperatures.   2. Copper absorbs material easily and therefore contents stored in, or consumed from this product may over time lose or change their taste. Thus, we do not recommend that you use this product for long-term storage of food or liquid contents.   Currently available information suggests that there is no confirmed adverse health risks associated with food or beverage consumption from a copper container. If you believe you have allergies or other health conditions which may be triggered or worsened by consuming food or beverages from this product, you should consult a licensed health care professional, and follow their advice.   This product is sold as is and Paykoc Imports expressly disclaims all warranties of merchantability or fitness for a particular purpose. Use of this product is solely at your own risk. Paykoc Imports assumes no liability or responsibility for any adverse impact that use of this product may have on the user's health or on the quality of taste of liquids or foods stored in or consumed from this product.