Specifically, only a thin layer of bitumen was found, which was used in its day to waterproof the container. They've recently protected the weak layer with clay and gypsum, taken it to the lab and made a 3D model.
In addition, thanks to the bitumen, they have dated the container for 4,000 years and have been able to know what material was made, since inside the layer there have been retained remains of rods, wood and palm leaves.