To flatten a hollow horn, it must first be opened by a cross cut (figure IV.4) at the point where the side of the hollow section becomes thicker. The cut is made not across the entire width of the horn but only up to the inner surface of the hollow section. A longitudinal cut is then made (figure IV.5) along the entire length of the hollow section.

Figure IV.4: Cross cut

Figure IV.5: Longitudinal cut

The hollow section is heated on both sides of the longitudinal cut. The opposite side of the hollow section must remain straight and rigid when the horn is opened. When the heated section of the walls has softened, the horn must be placed on a post fixed firmly in the ground or on a spindle held in a vice and pushed, as shown in figure IV.6. The hollow section will now open easily. Care must be taken to see that the horn is hot enough to be malleable as the hollow section will otherwise crack and be unusable.

Figure IV.6: Opening the hollow section