Bronzecasting

Abstract

Bronzecasting

This article presents the results of experiments with bronzecasting in Iron Age context, carried out in connection with the reconstruction of the Langå Waggon, a Funen version of the Dejbjerg waggon type. The experiments have been carried out by Fyns Stiftsmuseum and Historisk Værksted, Næsby. The processes employed have been based on written medieval sources (1) and archaeological material illuminating the prehistoric casting technique. This comprises crucibles, mould parts, parts of bellows, casting waste and the actual casting pits where melting and casting have taken place.

For the moulds, loam has been used, in order to obtain a material which is stable in shape. The proportions depend on the grain sizes of the materials used. In the present instance, the blend is c. 60 % sand/gravel, c. 35 % clay and c. 5 % organic material. It is desirable that there be as large a quantity of quartz (4), the heat-resistant part, as possible, although clay is of course necessary to bind the quartz grains together (5). The moulds consist of several parts, the parting faces serving as "air vents".

In order to use a loam mould for bronze-casting, it has to be fired, or it will shatter during casting. Likewise the temperature of the mould at the moment of casting is critical (700°C) (Fig. 5). During the firing, all oxygen in the crucible is consumed, and the body is therefore reduction-fired and black. The black colour is thus a result of the firing and not of the heat from the bronze at the moment of casting. The outside is open to oxygen during the cooling and is therefore yellow or red.

Melting proved to occur best in a pit, c. 20 cm in diameter and c. 20 cm deep (16). During melting, concavo-convex pieces were formed on the sides of the hole, the concave side becoming vitreous (17) (Fig. 10).

The metal and crucibles were in the present experiment both of commercial origin. These crucibles of fireproof clay could withstand only 3-5 melts however, before they broke. Prehistoric crucibles of non-fireproof clay could therefore only have endured fewer melts before breaking. Prehistoric crucibles often exhibit a sintered outer surface where the temperature has been so high that the clay has become vitrified. These sintering zones could, but do not necessarily, indicate repairs, but do show how many times the crucibles have been used for melting.

It has proved absolutely necessary to employ double bellows in order to maintain a continuous stream of air, since the bronze cannot otherwise melt and achieve casting temperature. It is also vital that all joins in the assembly shown (Fig. 1 and 2) be tight.

In front of the tuyére, the heat rises vertically, so the vitrification of the front end of the tuyére shows that it was placed somewhat at a slant, with the front end down. The vitrified clay will after a time fall off and occur as "sintered clay" (27) (Fig. 15 and 16).

When these conditions were present, it took on average c. 15 minutes to melt 250-300 g bronze. The rate of cooling for these small amounts of bronze is very high, however, so casting has to be completed within 3-5 seconds (30) after the crucible is removed from the fire.

One major problem is how to judge when the melt is hot enough. This can be learned only by comparing the results of a number of castings with the conditions present at the corresponding meltings.

Estimation of the temperature of the mould is slightly easier. The mould must be red hot, i.e. over 700°C (31). According to Indian, medieval tradition (32), the colour of the ingate should be that of "marrow flowers" and a similar rule of thumb might have been used in prehistoric Europe.

In addition to the results presented here, a further unexpected result was obtained: it proved impossible for one man to perform a casting. The rate of cooling of the system was too high, so two men were required to carry out successful casting by the method described.

Many persons have helped with the experiments, in word and deed. We thank them all for their assistance, without which the trials could not have been carried out. Especial thanks are due to civil engineer V. F. Buchwald, Danmarks Tekniske Højskole.

Bjarne Lønborg