How do you get old sludge cooking oil off of the bottom of a cast iron skillet?

My husband has just relocated his cast iron skillet, but it looks like it was never cleaned the last time he cooked in it. He has put dawn detergent and water in it and boiled it but it is still just a thick sludge that won't budge. Is this skillet hopeless or will a little bit of elbow grease help? Any other ';tricks'; would be greatly appreciated.How do you get old sludge cooking oil off of the bottom of a cast iron skillet?
Put it in the oven and turn the oven on and it will burn it off. Then you will have to season it after you do this with a spray of oil and then wiping it of with a paper towel.How do you get old sludge cooking oil off of the bottom of a cast iron skillet?
The most effective way to strip off even years of the most hardened crusty burnt-on grease from cast iron is to pour a mixture of water and lye into it and let it sit overnight. Lye (sodium hydroxide) is more commonly known as Crystal Drano and is usually used to unclog plumbing. Do be careful with this chemical as it's highly caustic, and when handling it make sure you wear long rubber gloves and a pair of safety goggles. Keep in mind, this treatment will also remove the seasoning from the pan and goes right down to the bare metal, so you will need to re-season the cast iron if you want it to be non-stick..