From MineScape Wiki
A Mithril bar is a bar of refined Mithril ore. It can be created from level 50 Smithing skill by using a furnace with 1 Mithril Ore and 4 pieces of Coal Ore, providing 30 experience. A mithril bar can be smithed on an anvil by using the Smithing skill to create Mithril weapons and armour, granting 50 Smithing experience per bar. A list of mithril items that can be smithed from Mithril Bars and the Smithing levels required can be found here.
A Mithril Bar is usually known to be considered the 4th tier mineral bar to be used in the world of Minescape.