Gobierno Corporativo de la Función de Riesgos

The technological assessment of slags is often an obscure and complicated field. There are many studies that claim to reveal various technical aspects of the smithing and smelting process by determining slag compositions and plotting the results on a phase diagram of a similar chemical system (Rehren et al 2007, Charlton et al 2010, Sharp & Mittwede 2011).

An example of this approach is that of Rehren et al (2007, 212), where two compositional and thermal optima are identified on a corrected ternary diagram plot of FeO-SiO2-Al2O3. These optima positions indicate the lowest melting points of a slag at each given composition, and are described as follows; “These temperature minima are also areas which maximise slag fluidity relative to energy inputs. They are optimal engineering solutions for bloomery smelting and may reflect iron-making behaviour within specific socioeconomic contexts.” However it appears significantly more probable that both the raw ore composition and the idiosyncratic routine of the smelting process itself exhibit the greatest influence over composition, regardless of other potential factors, which results in very similar plot positions for this general process, making the suggested plot patterns behaviourally independent.

Many of these studies will often attempt to estimate a metalsmith's skill level and methodological choices based on calculated optimum iron slagging conditions as determined using isotherm temperatures and eutectic compositions. These conditions supposedly reveal details of process control and efficiency by examining how close to these calculated optima the collected results lie.

However, the relevancy of these findings is questionable, as the slags produced would never have completely achieved equilibrium conditions. Also the predicted solidus-liquidus transition temperatures and eutectic optima often cited in these studies could never be truly realised in a bloomery furnace, and therefore any information elucidated using these methods will not be representative of the skills and choices of the metalsmith but rather the potential slag composition types, based on absolute isotherm values and the ore's composition.

In addition to this, the phase diagram solidus-liquidus transition isotherm values will not typically reflect the actual operating temperatures of the furnace and, as the viscosity of the newly forming slag within a furnace is inversely proportional to temperature, the slag that is developed must be at a significantly higher temperature than these predicted values to produce a sufficiently low viscosity, free-flowing slag-melt that will effectively separate from the bloom. There is no current method of assessing how much hotter than the predicted isotherm value a slag was at the time of formation.

Even though predicted phase diagrams cannot provide information regarding absolute thermodynamic conditions due to equilibrium deficiency, they do give insight in to the chemical systems involved, and potential compounds and solid solutions that can be formed, this method may also be able to predict the dominant bulk composition type. This information may be used to predict phase composition to some extent, and to examine relationships between individual samples according to their bulk compositions. The usefulness of phase diagrams should be more pronounced for smelting slags than for smithing slags as smelting slags were once fully molten and closer to a

quasi-equilibrium state.

Smelting slags derive their bulk chemical composition predominantly from their ores, as such, the composition of the iron ore itself will have the greatest influence on the resulting slag plot-positions on a phase diagram, and will have very little to do with any technical choices or decisions of the metalsmith, unless fluxes were involved. As such these aspects cannot be used to predict or infer technical choices, furnace temperatures, or intent of optimum efficiency. The only significant aspects of choice to a metalsmith were those of fuel and ore selection, which they would probably have very little control over. Another potential aspect of choice would be charge ratios, that is, the quantity of ore to fuel, however, this too would to some degree be dependent on the quality of the ore and of the fuel, and on the technical knowledge of the metalsmith.

Additional factors influencing the compositional variability of slags and therefore the plot positions on a phase diagram include the presence of minor and trace elements introduced from the charcoal fuel ash, minor and trace elements introduced from the use of fluxes, and the furnace design and construction itself.

Fuel ash can potentially contribute a number of elements to slags, particularly calcium, and often potassium and phosphorus, however this contribution is typically relatively small and insufficient to have a great affect on the overall slag composition (Crew 2000).

Calcium minerals, particularly carbonates, may have been used as fluxing agents in iron smelting processes to slag silicate gangue minerals, thereby reducing the loss of iron as fayalite. It is currently unclear to whether this was practised to any extent in early bloomery furnaces, as iron ores are considered to be self fluxing, negating the necessity of an additional fluxing compound (Rostoker & Bronson 1990). Also the addition of excess calcium minerals to a charge can lead to

the undesirable side effect of a frozen furnace, where slag and bloom do not separate due to the formation of a very high viscosity immobile slag (Tylecote 1962). Calcium minerals, particularly calcium silicates, may also have been used to some extent as smithing fluxes, which can melt and produce a protective crust on a metals surface at lower temperatures than silica only fluxes (Rosenqvist 1983 ).

Silica fluxes may have occasionally been used in smelting processes and were extensively used in smithing processes, typically in the form of crushed silica minerals or siliceous sand. In smithing practices the crushed silica was applied directly to the metal's surface to reduce the potential for surface oxidation and unwanted decarburisation during hot working and welding (Serneels & Perret 2003). Silica fluxes may also have been employed in smelting processes to promote slag formation in silicate poor iron ores, or iron ores high in other non-silica gangue minerals (Charlton et al 2012).

The addition of silica minerals to a smelting charge provides a sink for otherwise immobile oxides, which can combine with the silica to form a liquid slag.

The design of the furnace itself will also influence the slag composition to some extent. The composition of the furnace walls or furnace lining will often contribute both minor and trace elements to any contacting slag. Iron rich slags can be particularly corrosive to aluminosilicate furnace linings resulting in elements including silicon and aluminium being incorporated into the slag in greater quantities.

For a slag to incorporate additional elements from sources such as the furnace lining and fuel ash, intimate contact between these sources and the slag are necessary. And the longer this intimate contact is maintained, whilst the furnace it at operating temperature, the greater the quantity of additional elements that will be incorporated into the contacting slag. Due to low mixing and diffusion potentials within the slag melt, this may have very little influence on the overall bulk

composition of the slag, with the elemental exchange limited only to the contacting regions. The uptake of elements such as aluminium, magnesium, titanium, silicon and calcium in the contacting slag will also result in a localised increase in slag viscosity, causing this altered slag layer to adhere to the contact surfaces forming a deposition layer, whilst the less effected, lower viscosity slag flows over its surface and away. This results in the formation of a distinct ceramic-rich slag, which is composed primarily of molten glassy furnace lining, characterised by high silicon and aluminium, and low iron (Blakelock et al 2009, Crew 2000). As well as contact time and high temperatures, the chemical composition of the bulk slag itself will have a distinct affect on the extent to which the slag can incorporate elements from it's surroundings, with slag basicity controlling whether the slag will incorporate either basic or acidic oxides (Moore 1990).

Both the size and shape of the furnace will determine the charge volume and its spatial distribution.

The size, shape, wall thickness, and any openings in the furnace will also determine its insulating properties and gaseous exchange potential. These design factors will influence efficiency, achievable temperatures and redox potentials.

Generally speaking high quality and well prepared ores will result in a high quality metal product and an efficient extraction, whereas poor quality ores and/or poorly prepared ores will result in both an inefficient recovery and a poor quality metal product. How important these aspects of ore quality and ore preparation were to a metalsmith depended entirely on a number of factors, these include the intended use of the metal product, the availability of ore, and the availability of fuel. In situations where ore and fuel were plentiful, an less efficient process resulting in only a partial recovery of metal would be acceptable. And in situations where a lower quality metal product would suffice, a poorer quality ore or a less well controlled smelting process would often be acceptable.

A commonly attested belief is that the quantity of both iron oxide and iron silicate present in smelting slags can be used as an indicator of extraction process efficiency (Bachmann 1982, Fells 1983, McDonnell 1986, McDonnell 1991, Selskienė 2007, Humphris et al 2009). These particular aspects are not necessarily an indicator of extraction efficiency, but rather they are related to the ore composition, the ore-to-fuel ratio, and the addition of any fluxes. However, the total quantities of free iron oxide present in slag can be used as an indicator of the overall efficiency of the process, which can be used to infer the level of technological understanding (McDonnell 1991), with little or no free iron oxide being indicative of an efficient process, and large volumes of free iron oxide being indicative of an inefficient process. The quantity of dissolved iron oxide in smelting slag is also involved in viscosity modification and may have deliberately been encouraged to form, to reduce the free-flowing temperature of the slag melt.

The cooling rates of slag can potentially be estimated by examining crystal sizes and shapes, this is particularly the case for fayalite (Donaldson 1976). This method could reveal aspects such as whether the slag cooled in-situ in the furnace. For slag fragments with no particular morphological attributes, this method could be used to distinguish between tap slags, which cooled relatively quickly outside the furnace, and furnace bottom slags which typically cooled much more slowly inside the furnace. If slag cakes are left in-situ within a furnace during multiple smelting events, it is possible that the microstructure of the slag will change with the heating-cooling cycles, forming new phases and crystal systems.