Don't be surprised if you get some foaming: the combination of wood ashes and animal fats are the basic ingredients in the soap making process. It is possible that the grease won't float to the surface (and pig mats won't work) because any saponification that occurs will act as a surfactant, keeping the grease in suspension.

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The ashes, as it has been said, can cause pH to go up so you might need to add some acid and physically filter them out. and the best way to get out the fat is skim off what you can and then use an enzyme as Chem Geek suggested. IF there is a problem woith persitant combined chloramnes possibly the use of a strong oxidizer like sodium percarbonate might be in order. If nothing else it can make any debris that has sunk to the bottom rise to the surface for easy skimming.
