|387,400 per hour|
2,600 x Mithril bars (4,024,800)
You can make money by using Superheat Item on mithril ore and coal, turning it into mithril bars. It's highly recommended to get a coal bag, which costs 4,000 dungeoneering tokens and will hold 81 coal. This way you can make 21 bars per inventory, compared to the five without the bag.
At a bank, with your nature runes and your coal bag in your inventory, withdraw 81 pieces of coal and fill your bag (you can do this while still in the bank screen; just right click the bag and select "Fill Coal bag"). Then withdraw 21 mithril ores and fill the rest of your inventory with coal.
Place the Superheat Item spell on your action bar (key "1" is recommended) and repeatedly press the keyboard shortcut to cast the spell while clicking on the last ore in your inventory. There is no need to withdraw coal from the bag, as the spell automatically takes coal from the bag.
Bank the bars, hit "Esc" to exit the bank screen quickly, and repeat.
You can make the process much faster by making a bank preset. First, withdraw all of your nature runes, your coal bag, 21 mithril ores, and 3 coal. Make this preset 1. Now you can hit "1" while at the bank and it will automatically remove any mithril bars and fill your inventory with the required coal and mithril ore. If hitting "1" also exits the bank screen, you can click the banker again, right-click the coal bag, click "Fill Coal bag", then hit the "Esc" key to exit the bank screen.
A way to make the process even faster is to move your Backpack so that the last mithril ore falls right underneath where the "Fill Coal bag" appears in your bank inventory. This way, after clicking "Fill Coal bag" and hitting "Esc", you don't have to move your cursor to start superheating. Simply hit the action bar key for Superheat Item and click the ore.
Make sure to position the screen so that the banker is close to the edge of the Backpack so that you don't have to move the cursor very far to bank your bars (you probably have to zoom all the way in to get a good position).
This is an incredibly click-intensive activity. To superheat 2600 bars in an hour requires undivided attention.
It should be noted that Grand Exchange has a buy limit of 25,000 per 4 hours for coal, so it will limit the method.
It is strongly recommended to check the live prices on the Grand Exchange before making large investments in any method.
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