Casting is the creation of a required shape by filling a mould with a liquid which hardens and acquires the shape. In the case of bronze, the most common means of casting is called cira perdu or “lost wax”. This means that the shape is formed with wax. The wax is then surrounded with liquid clay or ceramic shell to make a mold. A pouring hole is left open at the top. The mold is then heated to harden the clay and burn out the wax. Bronze is fired to a temperature of 1100 degrees C approx when it becomes liquid and is then poured into the mold. If everything has gone to plan the shape should be in bronze inside when the mold is broken open. In the case of the horns this system requires a large amount of preparation, good luck and a lot of welding and clean up afterwards.
In the Bronze Age, however, a two part clay mold with a central core was used to make horns. Professor Peter Holmes’ study on original instruments concludes that the mold was formed around a solid shape in two halves and a core of clay was then positioned between and held in place with bronze pins or chaplets. The clay had to be fired before casting and this meant that the mold would shrink by 10%. Thus the shape of the horn had to be artificially altered to take this into account. He concludes that in some instances no finishing work was necessary after the horns were poured. A mold line can still be seen on instruments where the two clay halves did not absolutely match up, very much as one might see the mold line on a milk bottle today.
Many techniques and devices were employed to achieve such excellence which are lost in time but through exploration and trial and error these casting methods are being relearned.
Reference : Boolybrien The Personal Hoard of a Bronze Age Musician Simon O’Dwyer: Appendix : Notes on the Methods Used in the Reproduction of the Boolybrien Hoard. Holger C. Lönze. Page 15.. (page 157 within volume).