According to Genesis, Noah used pitch to seal the Ark and keep it watertight. Genesis 6:14 instructs him to cover the vessel inside and outside with pitch. The same verse connects the coating with the ark’s construction from wood.
Pitch is a general term for a thick, waterproofing substance, often produced from petroleum, coal, or wood-based materials depending on the historical setting. The biblical text does not specify the exact chemical recipe Noah used. Its practical role is clear: sealing the wooden structure against water.
Pitch was widely used in ancient boatbuilding and waterproofing, so the instruction fits a recognizable construction practice. It is different from the ark’s building material, which Genesis calls gopher wood. Another common mix-up is to answer “wood” when the question asks what was applied as the sealant.