Las bases del Concurso Algunas sugerencias

From a personal aspect I learned much from undertaking the experiment, having previously only smelted copper and produced copper alloys on a small scale in laboratory conditions. The most notable experience was being able to differentiate between good and bad ore by touch. The other experience of ore processing with the quantity of dust produced during crushing. Even after processing only the 11kg used in this smelt much of my exposed skin was stained as can be seen below.

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Figure 4.5.2 Showing ore processing set up.

The ore sorting was done during crushing, working right to left selecting ore pieces from the roasted pile, crushing on the anvil and then sweeping it to the left. Rejected pieces were thrown away to the top right of the image above, creating a small scatter of material while the majority was processed, sieved into the bucket and the separated larger fraction re-processed. This generated a large quantity of dust which can be seen covering the anvil, and surrounding surfaces.

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Figure 4.5.3 Showing large piece of ore (left).

The figure above (figure 4.5.3) illustrates the nature of the roasted ore. This larger piece shows the heterogeneous nature of the ore and host rock, and the significant fracturing that occurs with roasting. The best ore was found to be the bright rusty-red coloured portions with the darker surface, seen on the top left of the piece, while the other iron bearing regions were not as suitable due to an increased proportion of host rock.

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Figure 4.5.4 Showing furnace in use, red 1 indicating location of sealed tapping arch.

The furnace was powered by a two directional vacuum cleaner. This greatly reduced the physical effort required to provide a draught to the smelt while also ensuring that the supply of oxygen was consistent. Previous bellows-powered experiments conducted at Rievaulx Abbey have demonstrated that fluctuating draughts from bellows operated by different individuals result in less reliable reaction conditions which alter the rate of iron reduction (McDonnell pers. comm. 2014).

The tapping arch was sealed with a removable block and covered by a mixture of charcoal and sand. This allowed for faster opening of the furnace and subsequent slag and bloom extraction.

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Figure 4.5.5 Showing opened tapping arch.

Upon opening the furnace there was no initial slag flow. The slag was contained within the furnace behind the quantity of charcoal visible in the above image. The first flow of slag fell between the two blocks and was significantly paler in colour than the slag from previous smelts. The rest of the slag remained within the furnace and had to be pried out along with the bloom, as discussed in section 5.4. The bloom itself had adhered to the left of the furnace close to the tuyere. The blooms from previous smelts in this furnace were also found in similar positions, although the bloom from this event had caused more damage to the interior wall than had occurred in previous smelts. Due to time constraints the bloom was placed in water to cool before sampling.

123 4.6 Products

Two samples of the bloom were cut using an angle grinder and then subjected to metallographic and electron microprobe analyses to determine the predominant compositions present. The following composite images (figures 4.6.1 and 4.6.2) show the metallographic samples in the etched condition following etching with nital.

Figure 4.6.1 Composite image showing metallographic sample 1 in the etched condition scale bar 1mm.

124 As can be seen in figure 4.6.1 above, there are regions of limited carburisation and regions exhibiting no carburisation with a pale colour. This is diagnostic of the presence of phosphorus as has been observed by Hall (2008, 2012). The limited carburisation close to the surface of the bloom, from which both of these samples were taken, demonstrates the impact upon carbon diffusion that the presence of phosphorus has. The pores which are visible, as irregular black spaces, in both samples also posed a problem with etchant and alcohol retention, which is responsible for some of the dark yellow colouration in close proximity to these hollow spaces.

Hardness testing was also conducted upon the sample shown in figure 4.6.2 to provide supplementary data to further clarify the interpretation of the optical analysis. While the dark colouration of the carburised regions is diagnostic of carbon steel and the pale colour, and extremely large grains are indicative of phosphoric iron. The subsequent hardness testing of these different regions helps to confirm the initial interpretation as well as providing recognisable locations for EMPA study from which to gather chemical composition data for definitive proof.

Figure 4.6.2 Composite image showing metallographic sample 2 in the etched condition scale bar 1mm.

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Figure 4.6.3 showing compositional image with slag inclusion within bloom sample 1 top right and linear porosity visible top right indicative of phosphorus presence.

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Figure 4.6.4 showing compositional image of bloom sample 2 showing large pores and linear porosity visible bottom left indicative of phosphorus presence.

Figure 4.6.5 showing compositional image of feathered fayalite within the first flow slag produced during the experiment.

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Figure 4.6.6 showing compositional image with no free wüstite in pale coloured smelting slag.

Figure 4.6.7 showing compositional image of slag from the previous smelt conducted in the same furnace with the same ore. Iron rich skin visible top right.

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Figure 4.6.8 compositional image of slag from the previous smelt exhibiting severe micro-porosity.

Figure 4.6.9 compositional image of archaeological slag from Tisbury showing well developed fayalite laths and small quantities of wüstite visible centre.

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Figure 4.6.10 compositional image of archaeological slag from Tisbury showing close up of fine wüstite dendrites.