Stone and bronze murtis are the work of sculptors trained in a specific craft lineage, historically following proportional canons laid out in texts called shilpa shastras that specify the measurements of a deity's limbs, ornaments and posture rather than leaving them to individual artistic choice.
Regional materials and casting
Certain stones are prized regionally, the fine grained black chlorite of Odisha among them, while South Indian bronze work, the Chola period images being the best known historical example, uses panchaloha, a traditional five metal alloy, cast by the lost wax method in workshops that have passed the technique down through specific artisan families for generations. Clay murtis made for festival use, the large Durga and Ganesh images of Bengal above all, are built up over a straw and bamboo frame by artisan communities such as the potters of Kumartuli in Kolkata, working to a strict seasonal deadline rather than the slower pace of stone or bronze work.
What happens when the image is retired
A stone or bronze murti installed in a temple is expected to last indefinitely and is not casually retired, but a clay festival image has a built in end, it is ritually immersed in a river or the sea, visarjan, at the close of the festival, a planned and celebrated act of return rather than a disposal of something gone wrong, and it is a different fate entirely from the durable image meant to remain in the sanctum for generations.