Scope, concepts and role
In order to support the platform’s educational role, a set of related-to-the-discipline terms were defined and classified for the development of the semantically linked thesaurus. ‘Polygnosis’ platform contains domain concepts linked through a network of well-defined relationships and a rich set of terms identifying these concepts through its thesaurus, which is accessed and maintained via the thesaurus management system ‘THeMaS’. It concerns an application specific thesaurus that aims to support diagnosis and conservation decision making, as well as, analyse in depth and map out the concepts of this multidisciplinary domain, through organizing them by a faceted classification. Therefore, it provides global subdivision of concepts through Broader-Narrower Term Hierarchy, as well as, it sets concepts and terms into context, relating concepts to terms, and giving definitions. As a result, it offers related concepts the user might not has thought initially, thus help him explore and clarify the information need and find useful related information. Specifically, ‘Polygnosis’ thesaurus conceptual structure and data interconnections are better interpreted below in Fig. 4.
Thesaurus construction methodology
Design process and challenges
The main challenge that we had to deal with when designing the thesaurus, was matching a user question against a semantic network at the categorical level since generalizations and specializations of concepts are unrestricted. The different scientific communities have different perceptions of the same terms and concepts that were to be incorporated in the ‘Polygnosis’ thesaurus. Thus, it was crucial to avoid the logical errors and idiosyncratic decisions, which may lead to inconsistencies and consequently would require backward-incompatible restructuring of the classification system and its application. Consequently, the demand for objectivity in designing and establishing the top-concepts of the thesaurus leads us to look for formal rules that would enable ranking the specific terms on the basis of typical criteria. As a result, we have turned our attention to the ontology of CIDOC CRM Model [14].
Accordingly, an ontology-driven faceted analysis method was used for the definition of the top-level concepts that consist of the backbone for organizing its knowledge. It is critical to point out that it has not been used to define any of the terminology appearing on the thesaurus, but it has rather explained the logic of the documented terms. Due to its structure and formalism, this ontology has assured the ability to integrate terminology from different relevant scientific sources and it has enabled semantic interoperability.
The methodology followed for the construction of ‘Polygnosis’ thesaurus include the following main steps:
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Data collection and analysis: collection and processing of related scientific sources, terminology and thesauri.
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Thesaurus structure: (i) formation of semantic categories and facets of the thesaurus according to the main concepts of ‘Polygnosis’, (ii) building of hierarchies and finally (iii) formation of the semantic relationships between terms.
Below follows a more detailed description of the above mentioned methodological steps.
Collection and data analysis
Like most knowledge organization systems, the thesaurus is the result of processing documents and information resources. The first step is the collection and processing of a set of “content objects”Footnote 5 [20], which will be the primary material used for producing the subsequent homogenized indexing system [6].
The documented scientific terminology in ‘Polygnosis’ thesaurus is part of the expert languages of the disciplines of art conservation, technical art history, optics and lasers, among others. Thus, the process of selecting terms for inclusion in ‘Polygnosis’ thesaurus involved the consulting of various related sources. Each of the studied source, either describes experimental procedures performed for the study of heritage materials (e.g. spectroscopic material analysis), or includes terminology referring to the condition state, degradation processes, pathology, features and components of a heritage object (e.g. detachment, pentimento) or terminology regarding the object itself (e.g. acrylic paintings, murals).
Primarily, existing analysis/condition reports from past campaigns that were conducted by the ‘Photonic for Cultural Heritage’ laboratory of IESL-FORTH were studied in order to identify the terms and concepts that we aim to document in the thesaurus. The ‘Ariadne’ conservation documentation system’s conceptual model [21] has also assisted in the identification of the conservation steps in our analysis/condition reports. In parallel, laser system’s user manuals were analysed in order to collect technical terms regarding the equipment and the digital documentation data.
‘Polygnosis’ thesaurus has been based on the experience, structure and content of CREBITEL thesaurus [16]. CREBITEL thesaurus facets include “Material”, “Evidence of Technique, Mark and Trace”, “Alteration”, “Intervention” and “Investigation Methods”. Therefore, both thesauri share common scopes, notions and concepts regarding the examination and conservation methods of artworks, their evidences and the resulting data.
Furthermore, many terms were taken and defined from other thesauri and vocabularies such as the AATFootnote 6 [22], NARCISSEFootnote 7 [23, 24], CRISTALFootnote 8 [25] and EwaGlosFootnote 9 [26], which actually played a verification role in the final terms’ selection. Finally, a high number of books, related journal articles, condition reports, etc. were studied in order to localize relevant terminology to be included in ‘Polygnosis’ thesaurus. The source(s) of each thesaurus term is/are documented as a reference on each term’s identity card in the Thesaurus Management System (TMS) ‘THeMaS’.
Thesaurus structure
After having collected a great number of terms, we had to deal with the challenge of organising them based on their content and meaning, in other words their intension. The process and method followed in designing and building our thesaurus is the same as the one adopted for the implementation of the backbone thesaurus of DARIAH EUFootnote 10 backbone thesaurus (ΒΒΤ) [27]. This method exploits all the advantages offered by categorical semantics, in order to define the intentional propertiesFootnote 11 of the general concepts under which we can subsume more specific terms.
Thereafter the categorisation of terms/concepts, we defined our facets and hierarchies through identifying the intentional properties of the concepts to be classified [27, 29]. The latter properties provide the necessary and sufficient conditions for a term to belong to a category and not to be replaced without loss of meaning. It is through the intentional properties of concepts that we can identify hierarchical relationships that will lead to broader categories (facets), which will be used for the classification of the terms [27].
Within this framework, four extensible facets relevant to the content of the platform were defined. These facets together with their scope notes are presented below:
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“Material Objects”: This Facet comprises types of things with a physical substance that constitute complete units and have a relatively stable form with identifiable boundaries in at least one dimension. Such units can be natural or man-made (with regard to the origin), simple or complex (with regard to composition) or consist of parts. In this latter case it is possible that the parts are either distinct and independent from the unit of which they are a part (e.g. a cave on a mountain) or that they have to be defined with reference to the sum of the parts (e.g. chess-chessmen). The definition of this facet is based on BBT [28].
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“Investigation Methods”: This Facet comprises systematic procedures designed to detect, identify and demonstrate the qualities and characteristics of an object. These methods allow the assessment of the object’s condition state, the study of its structure, material, manufacturing technology, the nature and extent of its damage, as well as, the estimation of the deterioration factors. Often, the aim of these methods is to serve the development of the required conservation methodology and the determination of the type and extent of the necessary treatment. For example, spectroscopic methods, imaging methods etc.
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“Identifiable Features”: This facet consists of features that are inextricably linked with the objects on which they are found without being themselves autonomous objects. These features result from either forced or impulsive actions or procedures. They can be defined by physical characteristics (geometry, colour, etc.), and can be identified and determined by investigation methods. For example, a trace from a previous intervention, a signature, a sign, saturation of colours in a mural due to the existence of moisture etc.
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“Data”: This facet includes digital material that provides information relevant to the documentation of the object (material or information object) and the processes that take place during all stages from the acquisition of primary information, by recording or digitization, to the production of secondary and tertiary information, through studies, interventions, presentations, exhibitions and publications. This facet includes terms like interferogram, infrared reflected image, crack map etc.
The mentioned facets are organized in hierarchies according to the IsA relation which dictates that every subsumed term must belong to the same inherent category as its broader concept [30]. Using the IsA relation as the criterion for building hierarchies ensures that consistency is maintained since all narrower terms must possess all the fundamental properties attributed to the broader concepts of the hierarchy into which they are subsumed [29].
Apart from the hierarchies created by the IsA relation, terms are also linked through the associative relationship which covers associations between pairs of concepts that are not related hierarchically.
Based on this relationship, we achieved to relate an object with:
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The investigation methods that can most effectively examine it.
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The features that can be detected and identified on this object.
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The data resulting from the examination method applications.
All the above-mentioned relationships are reciprocal between all the associated terms. In this manner, a term acts as an entry node for viewing and understanding a large part of the knowledge in this area.
Even though ‘Polygnosis’ covers a wide range of concepts related to the study of cultural heritage by laser technologies, it still remains a specific application-based thesaurus. Although, DARIAH backbone thesaurus (BBT) consists of a coherent overarching thesaurus for the humanities [27], a metathesaurus under which ‘Polygnosis’ thesaurus can be aligned.
Polygnosis uses BBT’s ‘Material Object’, which is common in the field of conservation/restoration of cultural heritage, archaeology, and history of art. The other three ‘Polygnosis’ facets: ‘Investigation Methods’, ‘Identifiable Features’, and ‘Data’ can be aligned to BBT’s hierarchies and facets [27], as follows:
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‘Investigation Methods’ (Polygnosis) can be aligned to ‘Methods’ (BBT’s hierarchy) under ‘Conceptual Objects’ (BBT’s Facet). The latter ‘Polygnosis’ facet according to the CIDOC CRM can also be classified under ‘Conceptual Objects’ (E28) class. ‘Conceptual Objects’ comprise non-material, man-made products and information, supported by the use of technical devices that may exist on more than one particular carrier at the same time [14].
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‘Identifiable features’ (Polygnosis) can be aligned to ‘Physical Features’ (BBT’s hierarchy) under ‘Material Objects’ (BBT’s Facet). This ‘Polygnosis’ facet includes the specific features that might be revealed and identified by the investigation methods, and therefore, it is important that they comprise of an independent facet.
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‘Data’ (Polygnosis) can be aligned to ‘Information Objects’ (BBT’s hierarchy) and ‘Propositional Objects’ (BBT’s hierarchy) under ‘Conceptual Objects’ (BBT’s Facet).
To sum up, the mentioned four facets of ‘Polygnosis’ thesaurus represent the basic conceptual schema of the domain of diagnostic processes and investigation methods on material cultural heritage objects.