Dhokra casting is an elaborate process. Black soil from the riverbank is mixed with rice husk and kneaded with water. The core figure or mould is made from this. On drying, it is covered with a second layer of cow dung mixed with clay. Resin collected from saal tree is, then, heated in a clay pot till it becomes liquid to which some mustard oil is also added and allowed to boil. The boiling liquid is, then, strained  through a cloth, collected and kept in a metal vessel over water. As a result the resin solidifies but remains soft and malleable. It is, then, taken apart in small pieces, heated slightly over low burning coal and stretched into fine threads or coils. Such threads are joined together to form strips. The dried clay form is, then, overlaid with these resin strips or coils and all decorative details and eyes, nose, etc., are added to the figures. The clay form is, then, covered with layers — first of fine clay, then, with a mixture of clay and cow dung, and finally, with clay obtained from ant hills mixed with rice husk. A receptacle is, then, made from the same clay and fixed to the lower portion of the image. On the other side, a cup filled with metal pieces is sealed with the clay-rice husk mixture. For firing in the furnace, saal wood or its coal is preferred as fuel. The cup, containing metal, is placed in the bottom with clay moulds over it, and covered with firewood and potshards. Air is blown continuously into the furnace for about 2 to 3 hours till the metal turns into a molten state. The moulds are, then, taken out with a pair of tongs, turned upside down, given a brisk shake and the metal is poured through the receptacle. The molten metal flows exactly into the space once occupied by resin, which would by now have evaporated. The moulds are allowed to cool and the clay layer is hammered away to reveal the metal image.

What is the preferred fuel for firing in the furnace during Dhokra casting?

Saal wood or its coal

Answer: Saal wood or its coal
The passage mentions that for firing in the furnace, saal wood or its coal is preferred as fuel.

Dhokra casting is an elaborate process. Black soil from the riverbank is mixed with rice husk and kneaded with water. The core figure or mould is made from this. On drying, it is covered with a second layer of cow dung mixed with clay. Resin collected from saal tree is, then, heated in a clay pot till it becomes liquid to which some mustard oil is also added and allowed to boil. The boiling liquid is, then, strained  through a cloth, collected and kept in a metal vessel over water. As a result the resin solidifies but remains soft and malleable. It is, then, taken apart in small pieces, heated slightly over low burning coal and stretched into fine threads or coils. Such threads are joined together to form strips. The dried clay form is, then, overlaid with these resin strips or coils and all decorative details and eyes, nose, etc., are added to the figures. The clay form is, then, covered with layers — first of fine clay, then, with a mixture of clay and cow dung, and finally, with clay obtained from ant hills mixed with rice husk. A receptacle is, then, made from the same clay and fixed to the lower portion of the image. On the other side, a cup filled with metal pieces is sealed with the clay-rice husk mixture. For firing in the furnace, saal wood or its coal is preferred as fuel. The cup, containing metal, is placed in the bottom with clay moulds over it, and covered with firewood and potshards. Air is blown continuously into the furnace for about 2 to 3 hours till the metal turns into a molten state. The moulds are, then, taken out with a pair of tongs, turned upside down, given a brisk shake and the metal is poured through the receptacle. The molten metal flows exactly into the space once occupied by resin, which would by now have evaporated. The moulds are allowed to cool and the clay layer is hammered away to reveal the metal image.