Why was Moses' ark coated with slime and pitch?
The bulrush ark was daubed or caulked with slime and pitch to seal the rectangular box and make it a place of safety for Moses, echoing Noah's ark. The Hebrew word for pitch also translates as atonement, which is in the blood, so the coating pictured safety and atonement.
Les explains that an ark in Hebrew is simply a rectangular box, the same word later translated as coffin and used for the ark of the covenant. Moses' mother placed him in a box of bulrushes and daubed or caulked it with slime and pitch. Just as God gave Noah instructions at the flood, so here the same kind of instruction appears. Les notes that the Hebrew word translated pitch is also translated atonement, and atonement is in the blood. Thus the little box became a place of safety for Moses, and God was in all of it.