Learn how to construct schematically a stratigraphy of a corrosion form / structure

The MiCorr application enables the user to digitally construct stratigraphies that he / she documents during the visual observation of corrosion forms and local probing of artefacts. If sampling of the corrosion structure is allowed, this non-invasive approach can be supplemented by observing it on a cross-section.

Fig. 1: Visual observation of an artefact under a binocular microscope, credit Jeanneret.

By observing the artefacts surface under a binocular microscope, the conservation professional tries to understand the stratigraphies of the corrosion forms (Fig. 1).

The conservation professional makes a first schematic representation of the stratigraphy of strata observed, which he / she corrects after having observed several other areas of the surface of the object. In this drawing, he / she gathers together all the corrosion structures on the object. This conceptual approach is the first step in the descriptive methodology of corrosion structures developed by R. Bertholon (Bertholon 2000). 

Fig. 2: Schematic representation of  corrosion structures observed on an artefact deduced from visual observation and local probing. Selection of a corrosion structure (red square), credit Rotzetta.

Digital construction of the stratigraphy of the corrosion structure

The user clicks on the Search button of the lower menu on the MiCorr homepage. The window below opens (Fig. 3). He / she has then to click on the By stratigraphy representation search tool.

Fig. 3: Screenshot of MiCorr Search tools.

A stratum is added by clicking on “+ Add a stratum” button (Fig. 4). A code with its number is added automatically and highlighted (with the orange rectangular shape on the left of the stratum) to specify the type of stratum: M (Metal), CM (corroded metal), CP (corrosion product), NMM (non metallic material), S (soil), D (deposit), SV (structural voids), etc. The fields of the characteristics (morphology, texture, microstructure, composition, interface) and sub-characteristics of the strata are provided in parallel. They can be filled once a stratum is created or at the end of the construction of the corrosion strata structure (see below). Each characteristic and sub-characteristic is explained by moving the mouse on its name.

Fig. 4: Screenshot of the Search tool by Stratigraphy representation showing the corrosion strata structure on the right (only “M1” stratum is currently visible) and the characteristics / sub-characteristics on the left.

Any new stratum added of the corrosion strata structure is appearing below the previous one (Fig. 5). 

Fig. 5: Screenshot of the Search tool by Stratigraphy representation showing “M1” and “CP1” strata.

Once the corrosion strata structure is completed, the user has to re-locate each stratum versus the other using the arrows included next to each code (Fig. 6). When more than one stratum of a type is considered their number progresses from the exterior to the interior of the stratigraphy.

Fig. 6: Screenshot of the Search tool by Stratigraphy representation showing the corrosion strata structure corresponding to the stratigraphy of Fig. 2 cross-section. The thickness field of each stratum has been filled to visualise correctly the structure (otherwise it would be out of the screen).

Building the stratigraphy representative of the corrosion form observed

The next step is to characterize each stratum (Fig. 7). It is an essential step: each field has to be filled to make the comparison with the existing case studies / corrosion forms of MiCorr database more efficient. To fill in the characteristics / sub-characteristics of each stratum, the user places himself / herself in the position of an observer of the artefact observing it in cross-section from the outermost strata to the innermost.

Sub-characteristic fields should be completed when the information is available (otherwise the space should be left blank). The general principle is to consider the structure as a whole and to fill the sub-characteristics of one stratum versus the other. Usually the approach is as follows:

• Insert a stratum M and start filling the sub-characterististics of the morphology characteristic: if there is a large amount of metal left, select the following fields: shape (parallelepipedal if the metal is a sheet, etc.), thickness (medium), direction (longitudinal, in case of a flat object), width (large), continuity (continuous). If not, adjust these sub-characteristics  according to the situation. Continue with the other characteristics / sub-characteristics.
• Pursue with a stratum CP and fill the sub-characteristics of the different characteristics using the same approach.
• In case a CM stratum has to be considered between M and CP strata, insert it only at that stage. Since CM is considered as intermediate stratum between stratum M below and stratum CP above (the ratio M/CP has to be selected from the composition tab), some characteristics do not need to be filled (texture, interface). New sub-characteristics might be added though (microstructures, additional element(s) in M and CP).

Sub-characteristics which are made visible on the corrosion strata structure are indicated by an open eye. Those which are not made visible are indicated by a striped eye.

Before constructing your first stratigraphy of a corrosion form of the artefact under study, it is recommended to look at some of the stratigraphies of the artefacts files on MiCorr database. For this, go back to the search tools on the homepage and select the search tool by keywords (Fig. 3). The following window opens (Fig. 7).

Fig. 7: Screenshot of the Search tool by keywords.

Filters are provided and make the appropriate selection and click on the Search database button. A window opens with Results. Clicking on one of them will open the artefact file. By scrolling down the page, you will find the stratigraphic representation of the corrosion structure of the artefact using the MiCorr application. Clicking on it opens the search tool by stratigraphy representation of this artefact. 

Comparing a new corrosion form to MiCorr database

Once the stratigraphy has been completed, the user can question the MiCorr application to find case study(ies) / corrosion form(s) similar to the corrosion form newly built by clicking on the “Find similar” button (Fig. 8). 

Fig. 8: Screenshot of the Search tool by Stratigraphy representation showing the stratigraphy corresponding to the corrosion structure of Fig. 2 once each stratum has been characterized.

A new window opens, which gives proposals of stratigraphies from the MiCorr database that are the closest to the stratigraphy that has just been constructed (Fig. 9). The table provided gives for each proposed stratigraphy the matching score and information on the corresponding artefact.

Fig. 9 Screenshot of the list of stratigraphies’ proposals from the MiCorr database that are the closest to the stratigraphy under construction.

By clicking on each digital stratigraphy and the additional information, the user has access to the characteristics of the stratigraphy and the artefact file of the corresponding artefact.

Saving a stratigraphy

The user needs to register to save his / her stratigraphy(ies) under his / her profile.

Export a stratigraphy

To export his / her stratigraphy, the user clicks on Export to PNG. The following window opens and after right clicking on the page, the stratum can be saved in the format indicated below.

References

- Bertholon, R. 2001. Characterization and location of the original surface of corroded archaeological objects. Surface Engineering, 17 (3): 241-245.

- Bertholon, R. 2000. La limite de la surface d'origine des objets métalliques archéologiques, caractérisation, localisation et approche des mécanismes de conservation. PhD dissertation, University Paris 1 Panthéon-Sorbonne, France. Corrosion doctors: http://www.corrosion-doctors.org/pict-type.htm (accessed 10.19.2015).