jar, pot
坩 centers on a container made of earthenware: a jar or pot.
坩 combines 土 (earth, clay) with 甘, which likely contributes a phonetic element. The character refers to an earthenware container.
The left side 土 is earth or clay, and the right side 甘 looks like a container with a lid. Picture a clay jar or pot made from earth.
For カン, imagine a can made of clay: can -> カン, and the clay jar is just like a can.