Experimental Procedure

A series of samples were prepared in order to investigate some of the alterations detected among the analysed lithic assemblages. Chert samples were collected from the same sedimentary depositional environment of most of the archaeological specimens, which outcrops are located in the Central Ebro Valley. This raw material, of a dark-brown coloration, is characterized by a cryptocrystalline matrix, occasionally banded (called Liesegang rings), with the presence of charophyte stems and oogonies (Mangado et al. 2007).

Analysis of chert samples through X-ray Powder Diffraction (XRD) indicates a relevant percentage of no-silica materials, mostly carbonate elements (Tab. 2.3.1)

Among the chert samples I used both fresh, unused samples, and some fragments of previously used instruments. This latter elements are characterized by the presence on their surfaces of use-wear polishes derived form the work of plant/wood materials. My aim was to prove whether acid or alkaline environments also affected micropolishes or not, and if yes to which extent.

In this preliminary experiment I only selected those traces that derive from the work of

Fig. 2.3.1. Example of alterations detected on the archaeological assemblages. a) Stereoscopic image of white-yellow patina. 8x magnification; b) Stereoscopic image of white patina spots on the edge of a tool. 15X magnification; c) Moderate lustre. See how the rounding mainly affects the highest point of the topography, while depressions and craters remain unaltered. 400X magnification; d) Strong lustre. See how the rounding mainly affects the highest point of the topography, while depressions and craters remain unaltered. 400X magnification; e) Altered plant polish. Silica gel is partially dissolved, characterized by craters and roughness. 200X magnification; f) Unspecific polish of unknown origin.

200X magnification.

vegetal substance, such as fresh grasses or wood, as they are the most represented in the archaeological contexts in study. However, in the future this type of experimentation should be extended to other class of polishes/traces.

Two different classes of experiments were realized to reproduce both 1) mechanical and 2) chemical post-depositional surface alterations.

1. Mechanical alterations are mainly produced on chert surfaces by soil physical forces.

The intensity of those forces depends on a great number of factors, as the specific soil structure, the amount of water in soil, the amount and the type of organic matter, the presence of specific oxides, the type of vegetation, the effect of other natural agents as wind, ice, snow, etc. For the moment, this experimental program will be focused on the replication of the effects of soil mechanical forces on lithic surfaces, through the employment of a tumbling machine. Similar experiments, even if they are a simplistic approximation of the natural processes, have been already realized by use-wear analysts (Plisson 1985; Levi-Sala 1986; Plisson & Mauger 1988; Clemente 1997a) and represent a useful instrument to reproduce some of the types of post-depositional modifications.

I decided to replicate a ‘tumbler experimentation’ as previous studies were realized on different types of chert and soils. To approximate at best the conditions of the analysed archaeological contexts, I used the same type sediment that was deposited in the archaeological layers, mainly characterized by loess with the diffuse presence of gravels and charcoals. No water has been added to the sediment, as I was interested in isolating mechanical stress from potential chemical agents. Nevertheless, I am consciousness that water has a great archaeological relevance as it surely affected the deposit.

Ten different samples of chert, both unused and used samples, were put in the machine for different time intervals, from 8 hours up to 90 hours of tumbling. The used machine is a QT-6 tumbler with 2.7 kg of capacity and a tumbling speed of about 50 rpm. Those parameters are similar to those utilized by Levi-Sala (1986) in her experimental program.

After tumbling, all the samples have been cleaned in an ultrasonic tank with a 30% H2O2

solution for 15 minutes, in order to remove any possible greasy lustre. A proper cleaning of the experimental tools is fundamental for the analysis and the comparison of the micro-wears.

Tab. 2.3.1. Composition of the chert samples obtained through X-ray Powder Diffraction (XRD). Laboratory ALS Global (Canada). Courtesy of the Lithotecha of Siliceous Rocks of Catalonia (LitoCAT), of the Milà i Fontanals Institució (CSIC-IMF). Modified from Mazzucco et al. (2013a).

2. Chemical alterations are all those types of modifications that involve a changing in the chemical composition of the chert samples. There are many possible factors in archaeological soils capable of producing a variation in the chemical composition of the flint artifacts. Among those, many authors have pointed out the influence of soil pH in chert alteration and use-wear conservation (Plisson 1985; Gijn 1986; Burroni et al. 2002).

Both alkaline and acid environments are usual in archaeological layers. Alkaline condition are common for shell-rich soils, for pyroclastic layers of volcanic ash and for layers rich of ashes from ancient hearths, especially where wood, peat and/or cow-dug had been used as a fuel (Braadbaart et al. 2009). Otherwise, soil acidity is often caused by high rainfalls, plant root activities, soil bacteria and decomposition processes of organic matter (Sparks 2002).

There are few experimental works that tried to replicate systematically the actions of chemical agents on archaeological artifacts. I mainly followed the method proposed by Plisson (1983) and Plisson and Mauger (1988). To reproduce effects of both alkaline and acid environments, four different solutions were prepared. In order to maximize the reaction both strongly alkaline and ultra-acid attacks were tested: 1 Mol and 0.5 Mol of NaOH and 12 Mol and 0.5 Mol of Hcl solutions. Samples were immerged in the solutions and stored in laboratory conditions, for time intervals of 6 hours at a temperature of 60˚C. After reaction pieces have been washed with water. A thin section of each altered sample has been realized in order to evaluate the extension of the reaction.

However, to check the reproducibility of the experiments under conditions comparable with the archaeological depositional environment, two weaker solutions where tested. One acid solution of 0.1 Mol CH3COOH and an alkaline solution made of natural wood pine ashes. I utilized wood ashes as they are a distinctive and common element of many archaeological layers. Moreover, ash naturally contains all the elements, like calcium (~30%

of the ash), potassium (~15%), sodium ions (~1%) and other metal ions as magnesium (~7%), necessary to produce the alkali-silica reaction (Misra et al. 1993).

2.3.1.2. Results

2.3.1.2.1. Mechanical Alterations

These experiments confirm the observations already made by other scholars, suggesting that soils forces are capable of producing strong modifications on the chert samples.

Resulting wear mainly consist of a pronounced rounding of all the micro-topographic high points of the surfaces. A first rounding appears already after 15-25 hours of tumbling, especially on the edges and on the ridges of the tool (Fig. 2.3.2, a-b). Already after 30-40 hours almost the entire surface is affected, even if the polishing is still not pronounced (Fig.

2.3.2, c). After 50-60 hours the topography already appears completely smoothed (Fig. 2.3.2, d). Under the microscope rounded spots have a rough appearance, characterized by holes and craters (Fig. 2.3.2, e). Only the highest points of the surface appear affected, while craters still preserve the raw material natural colour and brightness. At this stage of development, flint appears shiny even at the naked eye, with a bright lustre completely covering the tool.

Tumbling machine also produced on chert surfaces a number of ‘bright spots’ (Levi-Sala 1986). Those spots are never extensive and limited only to the edges and ridges of the tools (Fig. 2.3.2, f). Elongated striations were almost absents, maybe depending on the low percentage of gravel present in the sediment.

In literature has been proposed various models for the formation of this type of wears.

Both chemical (Hurst & Kelly 1961; Rottländer 1975; Howard 2002) and mechanical

Fig. 2.3.2. Mechanical alterations. a) Edge rounding produced by tumbling machine after 25 hours of motion.

400X magnification; b) Slight lustre caused by 25 hours of tumbling. 400X magnification. c) Surface rounding caused by 40 hours of tumbling. 400X magnification; d) Pronounced surface rounding caused by 60 hours of tumbling. 400X magnification; e) Edge rounding produced by 60 hours of tumbling. See the rough appearance of the polish. 200X magnification; f) Isolated friction spot produced after 50 hours of tumbling. 200X magnification.

processes (Levi-Sala 1986) have been indicated as responsible for their formation. However is possible that with the various terms ‘gloss patination’, ‘glossy sheen’, ‘glazing’; ‘soil sheen’,

‘surface sheen’, etc., I are referring to a number of different phenomena of widely varying origin.

In this experimentation these types of wears seem to have been produced mainly by mechanical forces, even if probably there are other minor agents involved that I am not capable to detect.

2.3.1.2.2. Chemical Alterations

Observed reactions can be divided in two main groups, acid and alkaline solutions.

Acid solutions. Acids react with chert impurities as carbonates, oxides, clays and organic matter located among the quartz and chalcedony grains. At low pH, carbonates solubility grew (Limbrey 1975; Luedtke 1992), thus, acids attack calcite elements, for example remains of bioclasts (radiolars, bivalves, foraminifers) and other carbonate elements incorporated within silica matrix. In this case, acids (Hcl or CH3COOH) when invade the chert porous

Fig. 2.3.3. Thin section of chert sample altered by acid solution. a) chert sample. 1. Unaltered. 2. Altered; b) White-yellow patina produced by acid solution. 2X magnification. c) Acid alteration affects carbonate components; 1. Carbonate elements, naturally of a beige-brown colour, when dissolved appears of a darker coloration; 2. The aspect of the microcrystalline silica in the background appears unchanged. 2X magnification; d) Detail of the same thin section; 1. Dissolved carbonate elements. 2. Unaltered silica.

10X magnification.

Fig. 2.3.4. Thin section of alkaline altered chert. a) Chert sample: 1. Unaltered. 2. Altered; b) Fully developed white-grey patina. 2X magnification; c-d) White spots produced by alkaline solution. 2X magnification; e) Bands of altered SiO2. 2X magnification; f) Detail of the same thin section. 10X magnification. 1.

Unaltered macro quartz; 2. Unaltered carbonate elements; 3. Partially dissolved chalcedony; 4. Dissolved silica layer;

matrix, reacts with calcium oxide (CaO) producing compounds as calcium chloride, or calcium acetate, and water. This reaction produce a white-yellow patina over the surfaces, as the chert turns of a clearer colour than the natural tone (Fig. 2.3.3, a-b). Reaction times depend on a number of factors, as the concentration of the acid solution, the temperature, the amount of calcite elements and their surface area, etc. However, a light patina on the flint surface appears after about 30 minutes with a 3 Mol Hcl solution. In thin section is possible to appreciate that, while SiO2 and micro quartz grains remain unvaried, carbonates are dissolved. Altered areas appear darker than the natural colour (brown-beige) of calcite (Fig.

2.3.3, c-d).

Use-wears derived from plant/wood materials do not show any modification after the immersion in acid solutions. These type of traces are probably mainly constituted by layers of amorphous silica (Anderson-Gerfaud 1981; Fullagar 1991) and, thus, they are scarcely affected to acid aggression, at least for the concentrations and time intervals experimented.

Alkaline solutions. Alkalis react with SiO2 matrix. The alkali-silica reaction (ASR) is well studied in material sciences. It occurs between alkaline compounds and cryptocrystalline or amorphous silica. More in details it requires the presence of hydroxyls (OH), alkali metals (K, Na, Mg), calcium ions (Ca) and water. Simplifying, the hydroxyls ions provoke the destruction of the atomic bonds of the siliceous compounds, then the alkali ions, reacting with the new soluble silica complexes, Si(OH)4, form a silica-gel that exchanges ions with calcium elements, gradually solidify upon them (Prezzi et al. 1997). This reaction only occurs between alkali and reactive siliceous aggregate. Opal, which has a very disorder structure, is the most reactive form of silica; chalcedony and microcrystalline quartz have an intermediate reactivity, while quartz α, having an ordered structure, is scarcely reactive (Knauth 1994, Bulteel et al. 2004). The precipitation of the silica gel the lithic surfaces, starts when the solution in which the chert is immersed becomes saturated with respect to silica, leading to the formation of a white-grey patina (Hurst & Kelly 1961). This type of patina at first is visible only in some isolated spots, then attacks the edges and finally expands to the entire surface (Fig. 2.3.4, a-d). The rapidity of reactions varies on the concentration of the solution.

A first patina appears already after 24h with 0.5 Mol NaoH solution. However, it is interesting that also in a solution of pine ashes – with concentrations similar to those that could be encountered in an archaeological layer – a slight patina gradually covered the surface after 270h of immersion at 60˚C, forming and increasing over time (Fig. 2.3.4, b-d).

In the thin section is possible to see how the alteration principally attacks areas of mayor structural inhomogeneity in which starts the dissolution of amorphous silica. Alkaline alteration mainly affects microcrystalline and fibrous silica (Fig. 2.3.4, e-f), while do not affect microcrystalline silica (Fig. 2.3.4, f).

Use-wears derived from plant/wood materials also suffer strong alterations as a consequence of the immersion in alkaline solution at 40˚C for different time intervals (Fig.

2.3.6). The dissolution of the amorphous silica deposited on the lithic surface appears extremely rapid also in this condition. When a fragment employed on plant or wood materials is immerged in an ‘ash solution’ at natural temperature I observe a gradual but increasing dissolution, already after 24 hours of immersion at 40˚C. After 270 hours of immersion however, I observe certain stabilization of the wears and also after 320 hours the polishes did not seem to lose much more material.

Even if only preliminary, this experimentation appears extremely interesting, not only in relation to issues of wear conservation, but more in general in relation to polishes composition and formation processes.

Fig. 2.3.5. Chemical alteration of use-wear polishes: a) Polish produced by the cutting of fresh grasses, after 25’.

400X magnification; b) The same polish after the immersion in a ash solution (pH 12) for 72 hours at 40ºC, 400X magnification; c) Same as before, after 168 hours; d) Same as before, after 240 hours; e) Polish produced by the scraping of dry pine-wood, after 20’, 400X magnification; f) The same polish after the immersion in a ash solution (pH 12) for 48 hours at 40ºC, 400x magnification; g) Same as before, after 120 hours; h) Same as before, after 144 hours.

a) b)

c) d)

e) f)

g) h)

Without entering in the debated of about the polish formation process, I would like to point out some relevant reflection. Following the idea of Kamminga (1979), I believe that polishes are created in a different way depending on the type and chemical composition of the contact materials. For instance, polishes created by plants are mainly an additive process, involving the deposition of a layer of amorphous silica gel, while other categories of traces are mainly abrasive, as for example hide-working. Tribological model both for additive and abrasive wear has been recently proposed also for various contact materials (Astruc et al.

2003).

If one looks to the use-wears employed in this experimentation, one can notes some relevant differences. In the first case I immerged in the ash solution a fragment of a tool employed for cutting fresh grasses. The original trace was a very smooth and fluid polish with domed topography and closed texture (Fig. 2.3.5, a). After 240h, alkaline alteration led to a gradual solution of the silica layer deposited in the surface, with an evident opening of the texture and losing of the smooth aspect. Resulting polish has a rougher aspect and the bright ‘polished’ spots are now mainly concentrated on the highest point of the surface (Fig.

2.3.4, b-d). The abrasive component in this case seems almost uninfluential or, anyway, covers a minor role.

The second trace that I analysed, is a polish produced by the scraping of a dry wood (Pine). The original trace was characterized by the presence of striations, however also spots of bright polish were visible on the very edge of the tool (Fig. 2.3.5, e). With the immersion in the alkaline solution the ‘additive’ component is gradually lost, with the dissolution of the silica gel, while the abrasive elements (i.e strias) are now more clearly visible on the surface, suggesting an abrasion of microcrystalline matrix of the chert itself (Fig. 2.3.5, f-h). In this case, the pine-wood trace appears to be produced by both components, additive and abrasive.