The microstructure is the assembly of microscale crystals and inclusions in a material with characteristic features like grain dimensions, morphology and composition. This microstructure is the result of specific steps of the production process. Consequently, by studying the features of the two Geistingen axes, this process can be reconstructed.
3.2.1 Dendritesnext section
The axes from Nijmegen and Tongeren both exhibit a microstructure (fig. 5) that contains tree-like structures with a rounded outline, called dendrites, which is typical for cast structures (Allen et al. 1970). The dendrites are intact and the intradendritic pores are spherical, suggesting that no mechanical working of the upper part of the axes has taken place during the production process of the axes.
Figure 5 Microstructures of AC20 (top) and BH76 (bottom), Scanning Electron Micrographs. The distance indicated as d2 is the secondary dendrite arm spacing.
The dendritic phase in axe AC20 is the α-phase of copper, with tin and nickel in solid solution. In axe BH76 the dendritic α-phase contains antimony, arsenic and nickel in a solid solution with copper.
Dendrites consist of main (or primary) branches which at a certain stage of solidification develop secondary side-branches. The number density of these secondary arms, quantified by the secondary dendrite arm spacing (d2, see fig. 4), depends on the thermal and compositional conditions during cooling. Dendrites in copper alloys typically form in the temperature range 800-500 °C, in which both the undercooling below the melting temperature and the atomic mobility are sufficiently high to form the solid-state microstructure (Porter & Easterling 2001). The cooling rate in this temperature range therefore governs the characteristics of the resulting microstructure. This means that d2 provides an indication for the average cooling rate that was applied and d2 can thus be used to determine the cooling rate during the production of the Geistingen axes. The values for d2 in this research have been determined using detailed scanning electron micrographs of the microstructure (fig. 5). For the AC20 axe, the average secondary dendrite arm spacing is determined as 9±1 µm and for BH76 it is 14±2 µm. The relation of this spacing with the cooling rate can be expressed in a general empirical form as (Miettinen 2001, Kumoto et al. 2002, Frame & Vandiver 2008):
where d2 is the secondary dendrite arm spacing in µm, α a factor dependent on the alloy composition, R the cooling rate in °C/s and n a constant with an empirical value of 0.3. Using the average calculated values α = 30 µm °Cn/sn for AC20 and 39 µm °Cn/sn for BH76 (using equations for α from Frame & Vandiver 2008), the average cooling rate is calculated to be approximately 39 °C/s for the Nijmegen axe and 22 °C/s for the Tongeren axe. The relative uncertainty in R is estimated at 30%. Since for both axes the temperature range of 800-500 °C is the range at which the secondary dendrite arm spacing evolves, this result implies that most features of the microstructure are formed in approximately 10 seconds, which is only a part of the time in which the whole axe is solidified.
Several types of inclusions can be found in the microstructures of the Geistingen axes.
Small silver and lead-antimony particles are sporadically seen throughout the structure of AC20. The most plausible explanation for their presence is that they are remnants of the ore since their presence is not uncommon in copper ores (Lindgren 1933). The melting point of pure silver (962 °C) is higher than that of bronze with a composition like AC20 (~950 °C) and of BH76 (~800 °C), and the silver particles thus remain solid in the liquid bronze. Lead has a very poor solid solubility in copper. Both cases result in these elements (either solid silver or liquid lead) being dragged along with the solidification front in the melt subsequently to be found in the interdendritic phase between the solid dendrites.
Both AC20 and BH76 display another distinct type of inclusion, namely copper-sulphide particles. Almost all of these particles in the two axes contain the same elements but their concentration differs: copper, sulphur, iron, oxygen and sometimes also tin, nickel and antimony. The majority of the particles can be identified as Cu2 S with the aforementioned elements in solution. A difference seen between the two axes lies in the morphology of these inclusions (see fig. 5). They are spherical and star-shaped in AC20, while BH76 only contains star-shaped particles. Substructures have been qualitatively identified in the spherical particles, but further research is needed to provide more information about the origin of these structures. The different inclusion morphologies indicate that for both axes the temperature of the melt has been around the melting temperature (1130 °C) of copper-sulphide. The spherical particles have been completely molten and solidified into spheres, while the star-shaped particles represent particles of which only the outside has been molten. This implies that the temperature of the melt of both axes ranges between 1100 °C and 1150 °C, and the absence of spherical Cu2 S-inclusions in BH76 indicates a somewhat lower temperature than for AC20. For both axes the applied temperature is higher than the melting temperatures of the main phases.
The copper-sulphide particles solidify during cooling at a higher temperature than the bronze does and can therefore end up between the dendrites, within the interdendritic phase as described above. This type of inclusion represents an important clue to the production of Geistingen axes since they can be identified as matte, a by-product of the smelting of copper-sulphide ore. It is therefore assumed that the presence of silver, lead-antimony and matte particles with their different morphologies can be attributed to the imperfect smelting of sulphidous copper ore, used for the initial melt that eventually formed the Geistingen axes after re-melting.