It all depends on the technique you are using and what you want to achieve.
If you are using a deep penetrative carbon technique like sagger, barrel or pit firing, ELEMENTS AIR will be sufficient to protect your ceramics pieces in giving it that final coating after firing. With these techniques, the burning carbon penetrate relatively deep into the ceramic piece because of the length that it is exposed to the carbon process. Elements Air also leaves no noticeable residue or "look" on the surface. The ceramics surface looks completely natural after application and curing time. If this is what you need for your ceramics pieces, please see the section on " WHAT IS ELEMENTS AIR AND HOW DOES IT WORK"
If you are using a shallow penetrative carbon technique on functional ceramic ware like Obvara, or marks done with horse hair or other hair and feathers etc that need extra protection against wear and tear, then you need to use ELEMENTS EARTH. With these techniques the fired clay body is typically not exposed to the burning carbon for a long time, and as a result does not penetrate deep into the clay. As I use high fired stoneware (1200 deg celsius) I  found that the Obvara signature started to wash out after a while. This started the search for a durable food safe sealer that I could use to protect my functional pieces. Elements Earth can leave a soft matt finish after the drying and curing time on the clay body, depending on how much you apply.  If you use these types of alternative firing techniques, and want to be sure that your ceramic pieces are adequately protected, please see the section "WHAT IS ELEMENTS EARTH AND HOW DOES IT WORK"  on the Elements Earth FAQ page.