The Clay forge is a metal smelting furnace on the island of Neitiznot. It is very close to the bank, located next to a clay oven. It requires completion of the Fremennik Trials, and partial completion of The Fremennik Isles to access. This is one of the closest forge-to-bank locations in the game, if not actually the closest. It is tiny in comparison to the regualar forge which is something along the lines of a 3x4 object, with the clay forge being 1x1. It is a fully functional forge, a fact that many people don't recognise.
This forge is a common location to find players grinding slowly along to level 99 smithing by smelting thousands upon thousands of gold ore and using the bonuses of the goldsmithing gauntlets. It is chosen mainly because it is far closer to the nearby bank than any other options in the game. It is also a one-click bank, which is a bonus. The clay forge is nearly as close as the Varrock anvils are to their own local bank, which makes it a valuable asset despite its low recognition.
The primary reason it is underused is the fact that it is located on Neitiznot. The quests required to reach this island are not simple. In addition to the quest requirements, Neitiznot is also rather remote. Despite the fact there is a bank and rather well rounded town and surrounding areas, the Fremennik area as a whole is not especially quick to travel to. Many players choose to simply stick with their own familiar forges, or superheating, rather than bother to make the journey to the island simply to use this forge.