In the Genesis Ark story, Noah uses pitch to make the Ark watertight.
Genesis 6:14 tells Noah to cover the Ark inside and outside with pitch. Pitch is a waterproofing substance made from a sticky, tar-like material, traditionally derived from wood, coal, or petroleum depending on the historical method. The biblical instruction focuses on sealing the vessel against water.
This detail is separate from the Ark’s building material. Genesis identifies the construction wood as gopher wood, while pitch serves as the protective coating. Confusing those two materials produces an incorrect answer to this question.
The text does not provide a modern chemical formula or specify exactly how the pitch was produced. It does, however, clearly assign the substance a practical function: closing the seams and coating the surfaces so the vessel can float during the Flood.