360 Layers of Perception – CAA 2007

Hans-Joachim Mucha – Hans-Georg Bartel – Jens Dolata

Finding Roman Brickyards in Germania Superior by Model-Based Cluster Analysis of Archaeometric Data

Abstract: Chemical analysis of ancient ceramics and of other archaeologically important materials has been used frequently to support archaeological research. Often the dimensionality of the measurements has been high. Therefore, multivariate statistical techniques such as cluster analysis have to be applied. The aim of the present paper is to give a review of the research on bricks and tiles from Roman military brick- yards in Germania Superior and to present the main results obtained by multivariate statistical analysis. In particular, new adaptive cluster analysis methods and modified model-based clustering are applied on archaeometric data (Mu c h a / Ba r t e l / Do l a t a 2002; 2003a; 2005b; in press; Ba r t e l / Do l a t a / Mu c h a 2000; 2003). The main result was the discovery of military brickyards that were not known when the project began about ten years ago. Recently, they have been discovered by the application of these multivariate sta- tistical analysis models. Newly developed visualization methods support and facilitate the interpretation of both the data set and the results of grouping. This means archaeologists can easily identify a new finding of a Roman brick or tile by comparing its chemical fingerprint with those from the detected provenances.

Introduction period of operation was from the middle of the first century AD until the end of the fourth century. About 1000 Roman stamped bricks and tiles from Archaeologists are most interested in the location the Upper Rhine area were the objects under of brickyards and in the chronology of the pro- investigation by methods of chemical and statisti- duction-marks, which are found on the building- cal analysis. Using both archaeological information material. and the results of mineralogy and chemistry allowed archaeologists to develop a complex model of brick- At present we have a long history of chemical and tile-making in the Roman period. In south-west and statistical analysis of Roman brick and tile Germany a few large brickyards existed, of which making in Germania Superior (Do l a t a 1996; 1998; the operating authority was the Roman army. The 1999; 2000; 2001; Do l a t a / We rr 1999; We rr 1998;

Fig. 1. (left) Staatliche Museen Berlin Preußischer Kulturbesitz (Antikensammlung) TC 5778a: later (brick) with stamp of legio XXII, findspot: Neuwied-Niederbieber, provenance: ‘not yet known 1’. (right) Landesmuseum Mainz ZS 1581: tegula (tile) with stamp of legio XXII Primigenia Pia Fidelis, findspot: Mainz, provenance: Frankfurt-Nied. Analysing Ancient Economies and Social Relations 361

Ba r t e l / D o l a t a / M u c h a 2000; 2000 / 2001; 2001; Model-based Cluster Analysis in a Nutshell 2002; 2003; Do l a t a / M u c h a / Ba r t e l 2001; 2003a; 2003b; 2004; 2006; 2007; Mu c h a / Ba r t e l / D o l a t a The aim of clustering (grouping, unsupervised 2002; 2003a; 2003b; 2005a; 2005b; in press; Mu c h a classification) based on chemical components was e t a l . 2006; Sw a r t e t a l . 2004). both to confirm supposed sites of brickyards and to find places of those ones that are not yet identi- The data of ancient coarse ceramics from Ger- fied. Clustering was accompanied by multivariate mania Superior used here is described by 19 vari- graphical presentations of the objects and the clus- ables: nine oxides (measured in mass-%: SiO2, TiO2, ters found (see the references above and especially

Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O) and ten (Ba r t e l / Do l a t a / Mu c h a 2002) and (Do l a t a / Mu- trace elements (measured in ppm: V, Cr, Ni, Zn, Rb, c h a / Ba r t e l 2003b). Moreover, clustering was fol- Sr, Y, Zr, Nb, Ba). Besides the different scales of the lowed by validation of (a) the number of clusters, (b) variables, often problems with outliers and with the stability (reproducibility) of each cluster, and (c) long-tailed (skew) distributions of the variables in the reliability of the class membership of each object the archaeometric data were addressed, see recent- to its cluster (see for example Mu c h a / Ba r t e l / Do l a - ly (Ba x t e r 2006). t a in press; Do l a t a / Mu c h a / Ba r t e l 2007). Fig. 1 shows two objects from our data base that Generally, the final goal of cluster analysis is to were provided with a stamp. Consequently they find meaningful and stable clusters that can be re- appear to be very valuable documents. produced to a high degree. When done well, clus- tering techniques can support scientists of various The chemical components of the bricks and tiles research areas in their search for hypothesis. Details were measured by Gerwulf Schneider at the Free on cluster analysis can be found in numerous papers University Berlin using the X-ray fluorescence (Ev e r i t t 1980; Sp ä t h 1985; Mu c h a 1992; Ba r t e l 1996; analysis (XRF, concerning this method see for in- Pa p a g e o r g i o u e t a l . 2001). In this instance we present stance Le u t e 1987). At the start a total of 613 Roman some basic formulae based on distance measures bricks and tiles were analyzed. However, there is that were also used in the following practical prob- an ongoing research process. Day-to-day new find- lem of the identification of new findings from the ings can be reported. For example, quite substan- clusters found. tial new findings come from Boppard and Mainz. Let a sample of I independent observations Current details on both the history and the ongoing (objects) be given in the space RJ and denote by X research can be obtained from the web site http:// = (xij) the corresponding data matrix consisting of I www.ziegelforschung.de. rows and J columns (variables), where the element

xij provides a value for the jth variable describing

Fig. 2. (left) Bivariate density based on the first two principal components. (right) Several cuts of the density and the underlying objects projected as points bringing together the PCA-plot of points and the continuous density plot. 362 Layers of Perception – CAA 2007

Fig. 5. Graphical presentation of the identification of brick H239 to the provenance Straßburg-Königshofen using chemical profiles. Fig. 3. Localization of the Roman theater in Mainz. In the case of adaptive cluster analysis used here the ith object. Here the objects are archaeologi- the criterion above is modified by considering adap- cal findings. Further more, let C = {x1, …, xi, …, xI} tive weights of variables in the definition of the dis- denote the finite set of the I objects. Alternatively tance measure (Mu c h a /Ba r t e l /Do l a t a 2005a). The written shortly as C = {1, …, i, …, I}. squared weighted Euclidean distance Following the definition of the starting point of cluster analysis, then let us formalize the simplest

(elementary) solution to the clustering problem is such an adaptive distance, where sj is the pooled with a fixed number of clustersK : a partition {C1, …, standard deviation of the variable j (Mu c h a /Ba r t e l /

CK} of C, where every pair of two subsets (clusters) Do l a t a 2002).

Ck and Cl has an empty intersection, and where the union of all K clusters give the total set C. In the simplest kind of model-based Gaussian Roman Bricks and Tiles Classified cluster analysis the sum of within-clusters sum of squares criterion has to be minimized. This criterion Successful applications of simple model-based can be formulated as Gaussian clustering of Roman bricks and tiles have already been reported (Mu c h a / Ba r t e l / Do l a t a 2002). In this case new adaptive distances were ap- plied for finding provenances of production of mil- by using the squared Euclidean distance itary brickyards. As a result the following locations of military brickyards are identified: Frankfurt- Nied, Groß-Krotzenburg, Rheinzabern, Straßburg- between two objects i and l. Here nk is the number of Königshofen, Worms (initially ‘not yet known 2’) objects in cluster Ck. and two with respect to their provenience not yet

Fig. 4. (left) A brick from the Lothary brickyard from Mainz (19th century). (right) A tile with a stamp of the legio XII that was recently found in Boppard (Rhine). Analysing Ancient Economies and Social Relations 363 known ones. In Fig. 2 the ‘mountains’ (clusters) of Provenance Identification of New Findings the bivariate density estimation and several cuts of this density are shown. This estimation is based on As demonstrated above, the assignment (identifica- the first two components of the principal compo- tion) of provenance to new findings is an important nent analysis (PCA) of 613 objects. The PCA is a task in archaeometry. Beside identification of large projection technique that, hopefully, makes visible sets of objects, often individual objects have to be as- the essentials of the data in two dimensions (for signed to known brickyards or in general to identi- details see Gr e e n a c r e 1984; Ja c k s o n 1991; Mu c h a fied locations of production of any kind of artifacts. 1992). The quality of projection into two principal An identification of such objects can be based on the components is high (80% of variance). The bivari- same distance measures that are used in cluster anal- ate density looks like a main data body with two ysis. We recommend the K-nearest-neighbor tech- arms. Some of the clusters are compact ones; others nique (Ba r t e l / Do l a t a / Mu c h a 2004). are not clearly isolated from one another. The right Fig. 5 gives an additional graphical insight when arm consists of three compact clusters. In opposi- comparing chemical profiles (fingerprints). For sim- tion, the ridge at the left hand side shows neither plicity reason here the comparison is restricted to clear peaks nor clear separation into clusters. three profiles only: the pooled profile of members Usually the locations of findings are geographi- of the clusters Straßburg-Königshofen and Groß- cally different from the locations of military brick- Krotzenburg, and the chemical fingerprint of the yards. They all have in common that they are locat- finding H239 with unknown location of produc- ed nearby rivers. The transport was done by cargo tion. The numerical distance values are: d (H239, ships. Straßburg-Königshofen) = 0.37 << d (H239, Groß- Krotzenburg) = 4.13. Therefore, H239 is assigned to Straßburg-Königshofen by the nearest-neighbor Substantial New Findings in Mainz and rule. By visual inspection of Fig. 5 the profile of H239 Boppard looks much more similar to Straßburg-Königshofen.

During the excavation of a Roman theater in Mainz (Fig. 3) many bricks and tiles were found. The chem- Summary ical compositions of 70 objects were measured. The question arose: Was there a Roman military brick- During the past dozen years, a complex model of yard in Mainz? In order to answer this question a history and relations of Roman brick and tile pro- comparison with bricks from Worms, Rheinzabern, duction in south-west Germany has been devel- Frankfurt-Nied, and from the modern brickyard of oped by archaeologists. Clustering techniques are Christian Lothary was done. The latter was located able to support the archaeologists in their search in Mainz. Thirty bricks from this brickyard (Fig. 4) for hypothesis. The cluster analysis referred to were analyzed. here comes with validated results and outstanding The multivariate statistical comparison based on multivariate graphics of objects and clusters. The the chemical composition delivered the mathemati- graphics are visual methods for obtaining a better cally confirmed result that without any doubt the understanding of the statistical results obtained by objects from the theater were produced in Worms the archaeologists. (Do l a t a / Mu c h a / Ba r t e l 2006; Mu c h a e t a l . 2006). During an important excavation in Boppard, 43 additional bricks and tiles were discovered References (Fig. 4). It could be shown that these objects can be assigned to Worms with high probability (Mu c h a / Ba r t e l 1996 Ba r t e l / D o l a t a 2005a). Additionally the archaeo- H. Ba r t e l , Mathematische Methoden in der Chemie logical knowledge about the military brickyard in (Heidelberg 1996). Worms and its importance in the Rhine area could Ba r t e l / D o l a t a / Mu c h a 2000 be improved. H. Ba r t e l / J. D o l a t a / H. M u c h a , Klassifikation gestempelter römischer Ziegel aus Obergermanien. Archäometrie und Denkmalpflege, 2000, 86–88. 364 Layers of Perception – CAA 2007

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