Lost Wax Cast

The Cast of decorated models: the Vognserup Enge artefacts.

The cire perdue technique :

The cire perdue technique, also called lost-wax technique, ‘Wachsauschmelzverfahren’ or commonly known as lost-form casting, is a method used for the casting of single pieces, based on the almost complete formation of the artefacts in a wax model (Fig.2.025). This technique is especially useful for creating complex shapes, shapes with undercuts, and rich ornamentation. The main component of the malleable material to create the models is beeswax. However, since pure beeswax is very sticky and probably the stamp or other tools (like the hands) would have adhered to the wax and would thus have destroyed the surfaceit is inferable that various additives were added to the wax. Several attempts have been made to describe those mixtures, for example Jackson states that the wax was a mixture of beeswax with mineral and vegetable oils (Jackson 1972: 24; also Armbruster 2000: 75; Grassmann 1983; Büll 1977). Born suggests a mixture of wax, talc, rosin and grease (Born and Hansen 2001: 182), and also Hunt argues for an incorporation of talc, chalk or rosin (Hunt 1980).

(Figure based on Wübbenhorst and Engels 1989)

The Vognserup Enge artefacts and the first evidence of the casting of decorated wax models.

On many artefacts studied within my work, clear traces of a production in a soft, deformable material could be documented. Additionally, it was also possible to provide clear evidence of the cast of completely decorated models. On some belt discs and tutuli from the hoard in Vognserup Enge, the cleaning techniques of the farmers who found the hoard (Frost 2011: 8) have left stunning traces. On the surface of one tutulus and one belt disc fine dendritic (tree-like) structures are visible, also in the grooves of the decoration.

Dendritic structures (the evidence for a as-cast metal) visible on the surface of the belt disc VM1680KJ (Nationalmuseum Denmark) highlighted due to a chemical reaction on the surface which probably happened while using acid fluids for cleaning, this otherwise invisible microstructure on the surface is an
irrefutable proof for the casting of fully decorated models via cire perdue. Picture: H. Nørgaard, citation: Nørgaard 2018, 101.

Such a tree-like structure (dendritic) develops due to the segregation of the metals copper and tin, contained in the melt, during slow cooling. This structure is a definite proof for the casting of a decorated model. Within plastic deformation, as indicated by a working of the disc or adding the decoration with punches, the microstructure of the metal would be subjected to a conversion, and, instead of dendritic cast structure, crystal grains would be visible with strain lines. This otherwise invisible microstructure on the surface is an irrefutable proof for the casting of fully decorated models via cire perdue. Also, the remaining bronzes of the Vognserup deposit show clear signs of pure castings when examined closely. Several discs display a very fine cast skin, which also appears unchanged in the indentations of the decoration, and others show the difference of reworked decorative lines next to those with a still intact cast skin.

Difference of reworked decorative lines and cast decoration on the large belt plate from Vognserup. Picture: H. Nørgaard, citation: Nørgaard 2018, 102.

The application of decorative elements in the model:

Furthermore, the application of the humps already in the model could be detected on the belt disc VM 1680KL and a further disc belonging to the Molzen hoard in Lower Saxony (ML 242:84 g). The first disc shows that the hump was pressed into the model as a large depression, roughly in the same way as one might imagine in metal. After casting, the weak indentations were then reworked with metal punches.

This reworking is seen particularly clearly on the second disc from Molzen. The weak depression has a fairly porous casting skin, while the impression left by the punch, which was used for the deepening of the hump, is clearly distinguishable and partly caused stress cracks on the front. Thus, there are, while examining the ornaments closely, various evidences for the application of the decoration already to the wax model. In the same way, also, distinct traces of post-casting reworking can be detected.

Casting eyelets made in the model: the use of organic cotter in model-making

The material examined showed multiple direct references to the moulds, their construction and the tools used. Thus, on the small belt discs from Appel, Harburg in Lower Saxony (LMN 4769, 4770 and 4771) the use of a presumably bony or wooden cotter could be documented, which was incorporated into the model in order to stabilise the loop and secure the perforation during casting. On the figures the rounded edge of the auxiliary, and at the edges the accumulated material, is clearly visible. A similar implement was used to model the eyelet of disc NM 25789a from the Vellinge hoard.

A small disc found in Molzen shows a cotter that was struck too far into the model and thus damaged it. The finished artefact thus has a perforated spike. On the basis of this error, the use of a cotter of organic material can almost be assured, since a similar ceramic placeholder would not have left so sharp-edged a perforation as seen on the Molzen disc.

Another method frequently documented uses a rod-like implement that pierced a modelled torus, and was probably fixed in the mould in a similar way as the cotter and served during casting as the core.

In contrast to the belt discs or tutuli with solid spike, the small discs with hollow spike often hid the remains of core materials in the spike´s interior. Particularly well preserved are these predominant ceramic cores of the belt discs from the Svenstrup hoard (Fig. 2.037-38). Analogously, rough-cast surfaces and undercuts in the hollow spike can be regarded as traces of poorly crafted ceramic cores and their irregular surface pattern, similarly documented for the round tutuli from Vendsyssel on Zealand.

All pictures are taken by Heide Nørgaard and can be used in presentations and articles provided the original author and source are credited, in this case Nørgaard 2018, Bronze Age Metalwork.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: