When the player has a prayer urn in the inventory while killing an NPC that drops this type of ashes, they will automatically be transferred into the urn. Each impious ashes fills an impious urn by 4%, an accursed urn by 1.28% or an infernal urn by 0.26%. A full urn will yield 120, 375 or 1,875 Prayer experience, depending on type, effectively increasing the output per ashes to 4.8 experience. It should be noted that the ashes cannot be put into the urn manually. Prior to the release of impious and accursed ashes, F2P players could only train Prayer by burying bones. Lower levelled players may consider this as a small source of income as the imps that drop these ashes are easy to kill. You can find many imps near the volcano in Karamja. You can get there by using the boat in Port Sarim.
40 impious ashes are a possible prize from Treasure Hunter. When choosing the option to convert this prize to coins, players receive 1000 coins during the Double Cash period, implying that normally they would be converted to 500 coins, which is 3,820 less than selling them on the Grand Exchange.