Player made by heating a Raw swamp paste, a mixture of flour and swamp tar, on a fire. Found randomly in Shades of Mort'ton chests. Shop prices are controlled by the Grand Exchange.
Used most often for fixing the holes in the boat when you trawler fish. Also used for rebuilding the Mort'ton temple. Small usage in the Sea Slug quest and other quests.
Notes:
The cheapest way (free) to get flour for swamp paste is to bring a pot to the Sinclair Mansion (Murder Mystery quest house) and click the barrel of flour in the kitchen.