1. Introduction
Building archaeology tries to reconstruct the history of existing buildings, using direct observations of the building themselves. The archaeologists identify and analyse the following main data: materials, building techniques, continuousness and gaps, demolition tracks, the way a specific element sticks to the next one, etc. as significant traces of the passage of the monument throughout the different historical periods.
Building archaeology is nowadays a well defined research field even if it is on the border line between different disciplines. It was born, in fact, inside the digging archaeology, but it developed autonomously throughout the studies of the elevated structures. This discipline must be compared, for what regards its results, with the instruments and methods of historians and critics of architecture; it works, both on the academic and professional field, within architectural conservation, studying the same objects; it uses scientific and technological instruments to describe, to analyse and to date materials and architectural structures.
Knowledge comes together with respect; the aim, here, it is to conserve the existing cultural asset of historical buildings for the future. For this reason the analytical techniques are non-destructive or micro-destructive[1] and this is a kind of ethical issue. This is a big difference between building and excavations archaeology: in this latter case it is required to dig and somehow destroy the target objects in order to find them and have them visible; but a building already shows itself, its own surfaces, and materials, building techniques, stratigraphy may be studied with no impact (or minimal impact) on the object itself.
Also in the building archaeology, in any case, destruction could seem to be a better way to obtain a deep and sure knowledge of the "real history" of the monument. But destruction inevitably would erase a great part of the monument itself and what we could gain on the pure field of the analyses would be definitely paid on the field of the conservation. In fact, we have not a need of simple physical conservation but a deeper need to ensure chances of "more" knowledge for the future. Knowledge does not require destruction. It is rather necessary to imagine and experiment new non-destructive methods of investigating ancient architecture.
Not everything is visible in an existing building, but a lot of opportunities are open for the researcher who knows what has to be observed in it and how. Objects are dumb for the one who doesn't know how to question them; but they answer several answers to the one that approaches them with knowledge sake.
One of the archaeological question is: "WHEN?". Establishing dates is not the final target, but it is necessary for many other things:
- to obtain meaningful and thorough description of the development of the building;
- to examine every data inside its own historical context;
- to find explanations to the "WHY?" question, under the historical point of view;
- to better understand degradation processes in act or concluded.
The archaeologists have thus tried to elaborate the maximum range of possible analytical instruments, non destructive and of easy application, to obtain relative and absolute datings. We could of course ask: "why" the maximum range? Because in this way, if we can not apply one specific instrument, another one could be applied instead. Moreover, many dating methods supply only probabilistic results, signed by significant mistake ranges. The comparison between different results, derived from various analytical methods, will give us answers characterised by a wider truthfulness. Dates are therefore searched for, using a combination of several dating tools, where each one of them may be applied in different situations or to different elements, so they compensate each other for errors.
Many non-destructive tools have been developed or tuned-up since seventies. In Genoa (Italy) innovative experiences have been carried-on by ISCUM[2], and since late eighties by the "Laboratorio di Archeologia dell'Architettura" (Faculty of Architecture of University of Genoa). This research efforts made the tools described below available in Genoa and in its region (Liguria), while other groups are now elaborating them for different regions, so that several analytical and non destructive methods for the archaeological studies of ancient architecture are now employable in a very wide territory.
2. Building stratigraphy
"Stratigraphy" is the main "reading" tool imported from digging archaeology. Its target is to reconstruct the sequence of constructive and destructive actions that produced the handwork how it exists today. The representation technique and the comprehension of this sequence are based on the "stratigraphical unit" (S.U.) concept, i.e. a part of the building that no doubt was built as a unit, that is the result of a single building action. During the whole history of ancient buildings this process went on adding (positive S.U.) or deleting (negative S.U.) new parts to the construction; some of them are now intact, other are ruined, other disappeared. Nowadays, observation allows identification of S.U., and the analysis of the "points of contact" between them brings layering identification, that is an important key to reconstruct their temporal superimposition sequence.
This analysis process can only give relative dates, not absolute; however this is very useful, because it produces a stable framework where to integrate absolute dates obtained by other techniques. The same dating information alone, out of any stratigraphic context, may lead to erroneous interpretation instead.
When parts of the building remains hidden (think about a plaster layer that hides the internal structure of a wall), stratigraphy may become more difficult; however other techniques may help in this case.
The stratigraphic method has a long history, but correctives and improvements are still needed and searched for, both on concepts and on formal techniques. A debate is now open, with experiments in didactics and applied research, on such topics as:
- is it correct to define an opening or scaffolding hole to be a "negative S.U."?
- is it correct to deal with "S.U. at large", for examples S.U. that are hidden or not existing anymore nowadays? and is it expedient to insert them in the stratigraphic sequence?
- is it useful to represent the decay and impairment in the form of "transformation S.U."?
3. Chrono-typology
Chrono-typology is a dating method, based on different architectural features during historical periods. The concept is to identify particular constructive elements associating them with the period when they were used.
The core of the method is the reference database, that records the meaningful instances of constructive elements, associated with an appropriate date period, obtained by other dating methods; the period boundary is marked by the oldest and most recent occurrences of the element in an homogeneous territory.
The whole classification process, which is essential to create an useful database, includes the following steps:
- definition of the geographic context for the research, that has to be homogeneous
- systematic and complete filing and indexing of a large number of items of the chosen architectural element, having the date obtained from other sources;
- classification of distinctive features of the element itself;
- identification of the group of instances that share the same distinctive features;
- identification of the date period for the group itself;
The result of this process is the reference database, that can be used later for dating by comparison other similar elements.
For example if architectural elements of some kind happen to be dated in a unique definite range of years, obtained by inscriptions or documents or by other methods, then similar elements may be dated to the same period. The process requires a large number of samples, in order to minimize errors and polarization throughout a statistic elaboration of data. The database applies to an homogeneous geographical and cultural area. Differences in culture and lack of communication (e.g. difference between town and country) may bring as result difference in date ranges for the same element type.
For any single element, distinctive features may include materials, construction techniques, shapes or dimensions. It has to be noted that chronotypology applies to element types, not on the whole building. Data become meaningful when a large production of the same common-use element is identified, i.e. the single-piece craftwork of an artist is almost useless for this purpose.
The following chrono-typology databases have been developed for Genoa and its neighborhood:
- chrono-typology for characteristic architectural elements, focused on formal aspects (e.g. portals, facades decorations, floorings, etc.)
- mortar chrono-typology, based on mineralogical and petrographic composition;
- wall building techniques chrono-typology, based on materials, manufacturing and weaving technology.
The Lunigiana[3] portals database counts up to a thousand elements, each one dated at a specific year by the presence of epigraphs. This amount is certainly sufficient for the definition of reference chrono-types, and finally to date every other portal in the same territory. Moreover the same database could support reliable comparison with the portals of other countryside regions in northern Italy (Liguria, some Alps regions).
Mortar database allows dating of a single sample with tolerances of one to four centuries, depending on the inert contained inside. Here the reference samples count up to 1700 items. Now the extension of the database, to the whole Liguria and southern Piemonte, is in progress (1500 more samples), searching for new chrono-types.
The wall-building techniques database contains a large number of items for the Liguria region, and allows a reasonably reliable chrono-typological date in this context. On a larger scale, at northern Italy level, substantial homogeneity in techniques and temporal periods have been recognized[4].
However wall-building techniques are a very complex issue, and they strongly depend upon economical, cultural, technical factors. For this reason results have to be taken with great caution.
4. Measure-chronology
All the measure-chronologies are kind of chrono-typology where the main dating features taken into account are the dimensional characters of the elements.
The most important form of measure-chronology is the "bricks measure-chronology", a technique that uses bricks dimensions to obtain a chronological date for bricks and, indirectly, for the parts of the building constructed with them; typical tolerance values of this kind of dating method, is generally "twenty years".
The wall bricks measure-chronology curve defined for Genoa, including also the ancient Genoa Republic territory, ranges from the XIIth to the XIXth century, and in better cases allows dating with a five years tolerance. A flooring bricks curve is also been developed. Special curves have been traced for sub-regional territories whith specific political and economical history: Savona, Albenga and Finale. Other curves hare been realized for adjoining lands: Alessandria, Asti. More than seven thousands dating operations have been realized by measure-chronology.
A more difficult case is represented by "stones measure-chronology". The result tolerance in this case may be in the 1 to 2 centuries range, but physical dimensions alone seem not to be enough for reliable dating, and need to be integrated with the wall-building techniques chrono-typology or other tools. A more interesting case is the study of the black and white marble bands on ancient facades, a typical medieval feature of this region.
5. Dendro-chronology
This dating tool is widely known, it allows to identify the date of a wood constructive element reading the sequence of ring thickness values of the tree section, by comparison with reference curves, specialized both on the climatic region and on the different species of woods.
A dendro-chronology curve for chestnut has been developed in Liguria, extending from the beginning of the XVIth century, with some uncertainty on the initial period.
Conifers dating is possible by comparison with curves of other regions (Provence, Austria, Central Alps), integrating them with reliable reference sequences of woods installed in Liguria since XIVth century; the same thing happens for oak-trees.
6. Plastered buildings configuration analysis
A new research is now in progress, within the University of Genoa, to rationalize a chronology for building conformation analysis, both general structure and details. This tools is targeted mainly to plaster-covered buildings, where it is more difficult to apply building stratigraphy. This new tool allows a real archaeological analysis also in these more complex cases, with its own tolerance level.
7. "Indirect sources archaeology"
Another research is in progress at D.S.A., Sciences for Architecture Dpt., University of Genoa on the comparative reading of technical archive documents (building contracts, specifications and tenders, materials trading, etc.) and the material datum of the ancient building reality. Seven hundreds documents have already been collected and indexed. The target is an "historical building glossary", describing the building objects and elements. Indirect sources archaeology should also be a dating tool, reporting chrono-typological keys obtained by written and iconographic sources: first and last quotation of a given material, an architectural element, a technique. Another issue is the comparison between intentions and practical results of the ancient builders, taking into account the operating practice reported in documents.
8. Conclusions
The related use of all the above mentioned dating methods and instruments (dendrochronology, chronotypology, measure-chronology of bricks and stones), compared with non destructive stratigraphic inquires and "configuration analyse" allow to reach a sort of "objective history" of architecture. This particular kind of history enriches our knowledge and gives useful information for a more careful and efficient conservation of cultural and architectural heritage.
An objective history of architectural artifacts can in fact be obtained correlating relative and absolute dating tools, integrated where necessary with other dating methods and scientific analyses (thermoluminescence, radiocarbon, thermography, etc.).
References
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