Comparative stratigraphy as an exact method of dating works of art has its beginnings deep in the past and ivas used, as in archeology, wherever attention was paid to the place of deposition of works of art or historical strata of alterations place upon them.

Sometime at the beginning of this century extensiv excavations and restoration work was carried out in the Roman basilica of Santa Maria Antiqua and in other places in Rome. From the rubble and underground tombs they brought to the light of day extensive and numerous remnants of early medievalpaintings, often with one layer upon the other. By separating layer from layer, analyzing them and making mutual comparisons it gradually became possible to determine individual phases of time and to reconstruct the development of the Roman School of painting from the 5th to the 11th century. The result of this pioneering work ivas summed up by Grüneisen in the book St Maria Antiqua, published in Rome in 1911 1. Later work proceeded in similar manner in Dura Europas 2. The stratigraphie method was developed in the framework of Christian archeology; in contact with classical archeology the methods of stratigraphy on the terrain were applied 3. More recent examples include the work of Kazimír Michalovsky at Pharas in the sixties, a large part of which is shown in the National Museum in Warsaw 4.

All the examples cited refer to the sphere of mural paintings, where the stratigraphie method is clearest and most easily comparable. It is obvious even to the layman that each layer of plaster or coat of lime represents a certain period of time and that it dates the laver below it relatively. If the re search into strata is carried out with sufficient thoroughness, the record of the sequence of layers of plaster or lime dates not only the possible find of mural paintings but even the architecture itself. The stratigraphie record sets aside those parts of the architecture that are additions to the original building, e.g. various enlargements, additional parts etc. 5


Dating Possibilities of Overpaintings
Restorer's reports often include data on overpaintings, their colours (quality), thickness and time sequence. Rarely is sufficient attention devoted to them although they can be a primary source for stipulating dating ante quem. Such data do not always give forth witness immediately but careful records and later confrontation with other facts can acquire basic value as verification.

A classical example can be a group of statues that apparently are compact and identical in style with only insignificant differences in style. When later coats are gradually removed it is, however, shown that the original layer of polychromy of one statue is the overpainting of a second. Result: the second statue is older than the first, the group was made up secondarily as shown in table A The number of stratus can be widened, in a more numerous group there may arise more complex relationships, hut the basis remains the sa me: by a method of elimination of the record of stratus the relatively oldest origin of one or other statue can be stipulated, possibly their sequence in time. This is a current way of recognizing later additions – hands, parts of the draperies, the pedestal, etc. The restorer very often finds such relationships in a stratified composition. The fragment of a group of the Titrée Maries at the Tomb in the Museum in Polička can serve as illustration of the simple relationship of strata indicated, ne number of stratus found might have con finned the preliminary dating of the sculpture to the end of the 17th century. But on one of them – Magdalen an older origin was stated. The light wax layer of ochre colour forming the first coat of the garment of Mary forms overpainting on Magdalen, covering even parts later adapted by re-carving. In folk sculpture, in particular, where written records usually are absent, consistent stratigrafie research is often the only possibility for orientation as to the time of origin.

Methodology
We use the term stratigraphic to concur with terminology used in archeology. It is now being used in work on mural paintings. The archeologist uses stratigraphy as a record of the individual cultural layers in a given locality and regards the sequence as an exact proof of time sequence. The art restorer can use a similar record of the placing of individual strata of paint (over-painting) and can stratigraphically date a given artifact. For an archeologist rubble, chippings, alluvial deposits, loess represent relative time strata as cultural layers, and their deposition gives indication of relative time sequence. For a restorer and for art historians who gain information from him these traces of time sequence are represented by the individual repairs to an artifact carried out at different periods of time. They take the form of renewal of coats of paint (or different overpainting), new varnishing, mastic, additions, carving alterations. The data on the sequence of layers of polychromy are of equal relative value as the archeologist's stratigraphic record. Not always is there something to base their interpretation on: it requires long and exacting comparison to which physical and chemical methods of research can contribute, and not always is it possible to find possibilities of comparison. As in archeology, the basis may be one single fixed point, a historical record relating to one of the stratas, or the identity or similarity of one stratum to another, for which such a historical finn point does not exist. And as was said at the beginning, there is no alternative but to make a record of the sequence and quality of each layer for interpretation at some future date.

The stratigraphie method of dating an artifact comprises two phases:

a) the record
b) the interpretation




The record is not always sufficient, and we should not be content even with the best. Insufficient capacity for carrying out chemical and other analyses should not discourage us from amassing material (samples) for future use. Records are kept of the sequence of strata, any striking marks of different quality (be they in colour scheme, thickness of application or different solubility) and it is of great help to draw (record) the individual strata and their interdependence into a graph (e.g. carnation, characteristic samples taken of the draperies and other surfaces in the case of sculpture). The more precise the record, the more exact can be the resulting interpretation. The interpretation comprises, in the first place, simple comparison of the composition of strata between individual parts (the body –» addition, sculptural group –» older or newer figure, architecture –» additional part), on which parts added later can be very precisely eliminated The following result is achieved by comparison with historical data. Should these data be indirect ones (which is most frequent), the result has the value of hypothesis with more or less probability. If the data are direct ones, definite dating can he given. The importance of a precise record of strata rests in the fact that most often historical records refer to repairs of a given artifact (i.e. one of the intermediate layers), and for that reason the original strata can be dated with relative likelihood. This, in fact, represents the current value of restorer's reports for the terminus ante quem.




Values of high probability (which archeology normally has at its disposal) can be achieved even without direct historical records, by extensive comparative operations, in particular when dealing with a larger set of artifacts-e.g. furniture with works of art for which outline reports exist (records of repairs, change of ownership, destruction in wartime, etc.). These extensive comparative operations will, in future, be carried out on the basis of physical and chemical analyses even for artifacts that are more remote in place and time. So far there are no conditions for this. A prerequisite is systematic collection and concentration of records and samples.

In the interpretation of stratigraphy mechanical addition and elimination must be excluded, it can happen that a sculpture of older origin has fewer strata than a later statue that was more often painted. In other words, the number of strata is not a reliable proof of relative age unless upheld by the identity of the technical structure. Identity here means full agreement of one stratum of a given artifact with another stratum of a second artifact. And again, as in archeology, all interpretations involve dangerous drawbacks that can be avoided only by consistent search and linkage of identity. This relationship is shown graphically on table B. On the left side is the normal composition of layers in polychromy of artifacts Ą, B, C. Using mechanical comparison of the number of strata object A might seem the youngest. But using the rules of identity we reach a different stratification in time drawn in the right-hand part of the table: the horizons lead from the oldest at the bottom to the youngest at the top, i.e. identical with the position of the strata. The resulting interpretation is the following: the oldest in time are objects A and C. In the second phase object C was over-painted and object B was added to the group. The third phase left its marks only on object B. The fourth phase brought the first (and only) overpainting of object Ą while object C was left in the state of phase 2 and remained so until phase 5 when it was belatedly likewise over-painted.

  



Two Maries (Magdalene on the right) from Polička, Municipal Museum and Gallery

The example chosen is an extreme one. In normal practice we can encounter such a situation on very rare occasions. Most often repairs involve all parts of a given group, and then the relations in time are far clearer. But there are cases when not all time strata are kept, that all gaps disappear if the older coat is scraped off during repairs or is washed off, leading to accidental linkage that will be explained below. Sometimes a group of 2-3 strata belong to one repair, one phase of time, or, as the archeologists say, to one horizon. Associated strata, as one might call this one-phase configuration, usually contain a base anda coat of paint, sometimes a glaze and their mutual comparison with individual parts of the unit is more reliable in view of the multiplied clarity. From a technical and artistic point of view these associated strata tend to be of higher quality (reflexive optical construction) and appear most often in older phases. Later over-painting tends to be compact without transparency. The chalk interlayer in later phases usually fulfils merely a function of ground and mastic without sufficient optical use. The method involves an element of checking. It has become a rule that by removing subsequent overpainting a block of the removed strata is left for analysis as orientation and a dating feature not dependent on the availability of restorer's reports.


Sequence, Conjunction, Record

The time horizons in table B are marked with dotted lines. These time horizons are imaginary lines separating the stages of individual repairs and they are immensely important for the distinction succession (sequence) and con tact (conjunction) of strata.

A mere conjunctions of strata can occur in the cases cited when part of the historical measures were removed before newer coats were applied. Every restorer knows from experience that all layers are not to be found on every place. The chemist who tests a given section of an accidentally chosen spot of the painting or polychromy tests only the local conjunction, which might lead to a mistake. Only the restorer that, in the process of work, tests the whole work or group of artifacts can reach an overall interpretation on the basis of his observations, i.e. can give a real description of the sequence of strata. The same applies to the description of painting technique, i. e. from our point of view in the interpretation of one time horizon. The carnation of one syncopic and reflexive composition can serve as example. Samples that were taken from various places show different sequence and composition of paint layers - along the edges were seen reliefs of shadow, in lit up parts coats of white were applied. The patterns differ in conjunction. Sequence has to be explained comprehensively.

The sequence of repairs of larger sets (e.g. altars, epitaphs) is that of interpretation and description. More than 40 samples taken from an epitaph in Most (Northern Bohemia) did not give a single complete sequence of strata. Sequences are looked for also during research into layers of plaster, for in architecture the application of later strata is often preceded by the removal of older layers. Simple conjunction is often confusing. But in seeking identities a very precise relative chronology can be attained. The research into mural decorations of the chateau at Holešov brought rich material for this method. On the basis of numerous probes, their mathematical and graphic record and final evaluation it became possible to date some of the finds with relative precision, some strata could be dated directly, and, at the same time, certain mistaken dating was corrected after having been used in literature. The result is the table that depicts the time horizons and their expression in individual rooms of the castle.

This method was earlier applied in research into the furnishings of the Church of John the Baptist at Velký Bor (Klatovy district). It had as result explanations for the colour scheme of the furniture in seven basic time phases, which simultaneously reflected certain changes in aesthetic attitude to the colour setting. On the side altar of the Virgin Mary there were identified remnants of an originally larger high altar, dating from the Renaissance, which included in its centre a Gothic statue of the Madonna from the early 15th century. In the extension of the later high altar were identified two Renaissance heads of angels. These examples from my own experience are representative of many other investigations that restorers have in recent decades carried out and which – withcertain variationsin method- regularly extend the limits of our knowledge of technical development further afield.


Possibilities of Interpretation in Future
Precise terminology and precise data are essential for the interpretation of our partial discoveries to be adopted and used. Research during each restoration brings facts, either direct or as analogy to a  work of dating. Some can be exact and irreversible, others can serve as a point of departure (supporting point) today or in the distant future. The record needs to be precise to a maximum. That is the purpose of the Archives of Historical Art Technology that gathers, classifies and publishes study material. Our field is at present in the state that archeology found itself in the initial phase of development when scholars were working with a variety of methods and used different terms to refer to identical facts. We need to record the results of our work, make terminology more precise; like the archeologists we need to »draw the pots« and place them in the archives. We must reckon on the results of our work being used in twenty or even fifty years. Archeologists know no other perspective. And thanks to that patience and to careful records of any and all facts and observations archeology has attained its present syntheses.

The precision of records facilitates better communication with art histarians whose interpretation is no longer possible without technological analysis. As is clear from what has been said, the search for the date of origin of a work of art is not the most fundamental matter. It is more an imaginary unattainable line towards which individual operations are directed: it is a moving force that helps refine the method and make the knowledge of historic truth more precise.

But from exprience we know very well that such concurrence cannot be achieved immediately and that our practical work is likely to be accompanied by open problems. But there is no other way out if we do not wish to remain on grounds of mutually hidden empirical knowledge. It means constant discussion in an open atmosphere, with the right to risk, the right to mistakes that can later be withdrawn. For as our knowledge advances, with the aid of numerous series of comparative analyses the scientific media we use will gradually become more sophisticated There can be little doubt that we shall see the individual historical horizons far more clearly in future.

  author
Antonín Novák, AHVT A 015 (T.G.)

 

1 W. de Grüneisen, Sainte Marie Antique, Rome 1911

2 Marcel Aubert, in: Rostovtzeff-Brown-Welles: The Excavations at Dura Europos (Preliminary Report of the 7th and 8th Seasons of the Work – 1933/4 and 1934/5), London-Oxford-Leipzig-Prague 1939

3 The pioneering work of Professor Cibulka should be recalled, the work of the Prague Seminary of Christian Archaeology of the Faculty of Arts of Charles University, the Kondakov Institute, etc. A large pan of the libraries of these institutions has survived thanks to the Department of Art History at Charles University. Worth mentioning also is an »analytical method« of conservation, especially in architecture, favoured not only during the First Czechoslovak Republic, but earlier in the case of St Vitus Cathedral. In the work ofV. Birnbaum (reedition cited above.) an exact application of archeological methodology of its time (St George's Basilica) was explained.

4 K. Michalowski, op. cit. But characteristics of the paintings in the catala gue lacks restorers' analyses – the description of the stratified structure, which is a necessary further stage of the method developed.

5 The method of a thorough record of strata was applied with exceptionalresults in restorer's research on houses on the square at Jihlava (no. 39, 47, 50-51, 66-67), carried out in 1981-89 in collaboration with the Regional Centre for the Care of Ancient Monuments in Brno.