After the large gaps between the corners of the straw bales have been stuffing with dry straw, the walls need to be stuffed with light clay straw and shaped with a lancelot tool to create a relatively flat surface on which to apply the plaster.
The light clay straw fills any smaller gaps between the bales as well as filling any pockets on the surface of the bale walls. This includes the channels cut in the bale walls for the electrical wiring and the I-beams. The walls need to be as flat and solid as possible to decrease the amount of plaster that needs to be used.
Plaster is more expense and labour intensive to make than the light clay straw is. Light clay straw is used for shaping, and the plaster is used for finishing the walls.