In general, liquids tend to absorb UHF RF energy while metals tend to reflect it.
Liquids: As stated above, liquids absorb UHF RF energy. What is important to note is that the liquid does not have to actually be in liquid form. If the item has the ability to hold or attract moisture, you will have to take that into consideration. For example, a large paper manufacturer found out that because paper has a high moisture content, their large rolls of paper proved more challenging than expected.
Metals: In most cases, metals reflect UHF RF energy. This can be a positive or a negative. In some cases, the metal may reflect the signal in random and unwanted directions. If done correctly, it can help reflect the signal back to the antenna thereby improving performance. There are many items that have metal content that are not easily apparent. For example, the shiny packaging you see on grocery items is metallic.
Therefore, it is important to test your RFID solution thoroughly in your environment with your items as liquids (or high moisture content) and metals (or mineral compounds) will definitely affect the UHF RF energy. Neither of these should stop you from implementing RFID technology. With proper system design and engineering, the issues can be addressed and overcome.
