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An Observation on the Corinth Diatretum

All About Glass

A great deal of attention has been directed toward understanding how Roman vasa diatreta were made.1 Many of those who have handled the objects are convinced that they were made by deep cutting and undercutting heavy-walled blanks. Others have proposed explanations which require that the posts joining the cage structure to the cup wall be, in effect, separate pieces of glass which were fused to the wall and finished by grinding. The shape of fragment 6 of the Corinth Cup2 suggested a simple experiment which would tell whether the post and wall were cut from one piece of glass or whether they were once two separate pieces which had been fused together.

One edge of the thin wall of the fragment passed immediately adjacent to the base of the post. By polishing this side of the post a cross-section was prepared which revealed a clear view of the interior of the region where the post and wall meet. When this region was examined under a microscope it became obvious that the post and wall were made from one continuous piece of glass. The glass there is completely free of striations or discontinuities of any sort. This structure definitely was not made by fusing together two pieces of glass since this would have left a visible joint. That such joints are visible was confirmed by several experiments in which rods of glass were fused to flat surfaces of glass of identical composition. Invariably such joints showed pronounced striations, fields of minute bubbles, or planes of refractive index discontinuities when viewed under the microscope. Even when a piece of rod was broken and the fractured surfaces fused together again, a joint was clearly visible.

Although the actual microscopic examination of the diatretum fragment, especially through a stereomicroscope, leaves no doubt that the structure is one continuous piece of glass, the author found it impossible to obtain a photograph which recorded the evidence convincingly. The fragment has so many fractured surfaces and is so deeply pitted in places, that the curved fillet where the post and wall meet catches many reflections under any lighting condition. The result was that in all of the photographs taken through the polished surface, multiple reflections were caught by the fillet (on the reverse side) and produced images which could be mistaken in the photographs for refractive discontinuities. Nevertheless, a few photographs are shown here which may help to explain the logic behind the experiment.

The conclusion that can be drawn from this experiment is that this post and the wall of this vessel were cut from one continuous piece of glass. The results rule out the possibility that this structure could have been made by any technique that required joining a separate piece of glass to the wall. It is reasonable to assume that the same is true of other posts, and that therefore the Corinth Cup was indeed cut from a heavy-walled blank. Of course, this observation does not entirely answer the question of how these cups were made, but it provides one useful bit of evidence for scholars interested in this problem.

The results of spectrographic chemical analyses of the Corinth Cup, the Athens Cup and the cup in the Benaki Museum3 are given in Table 1. The analyses were virtually non-destructive, consuming only 4.2 milligrams of each glass.4 These preliminary data are presented here to complement Dr. Weinberg's discussion. A full interpretation of these results will be included in the next issue of the Journal.

Table 1: Analyses of Cage Cups.
  Corinth Cup
Corinth Cup
Athens Cup
Corinth Cup
Benaki Cup
SiO2* ~70 ~67 ~70 ~71 ~69
Na2O 16-18 16-18 16-18 16-18 16-18
CaO 5.7 5.3 7.6 5.3 6.0
K2O 0.2 ** 0.2 0.4 **
MgO 1.1 1.1 0.8 0.8 1.1
Al2O3 2.7 3.1 1.6 1.5 2.6
Fe2O3 ~1.0 ~1.0 ~0.7 ~1.3 ~0.8
MnO2 0.08 0.03 0.01 0.20 0.20
Sb2O5 0.38 0.32 0.27 0.21 0.23
PbO <0.01 0.9 <0.01 0.9 0.6
SnO2 <0.01 0.08 0.01 <0.01 <0.01
TiO2 0.7 0.6 0.4 0.1 0.7
CuO <0.01 1.7 <0.01 0.08 <0.01
CoO <0.01 <0.01 <0.01 0.2 <0.01
* By difference, allowing approximately 0.8 for non metals, 17.5 for Na2O).
** Insufficient sample.
The following oxides were not detected at concentrations in excess of the limits shown:
  NiO 0.01   BaO 0.1  
  Cr2O3 0.02   SrO 0.1  
  V2O5 0.01   P2O5 1.0  

This article was published in the Journal of Glass Studies, Vol. 6 (1964), 56–58.

1. The most recent contributions are reviewed by D.B. Harden in "The Rothschild Lycurgus Cup: Addenda and Corrigenda," Journal of Glass Studies, Vol. V. 1963, pp. 8–17. To these should be added, Josef Röder, "Die Diatretglasscherbe N 6211 des Römisch-Germanischen Museums Köln," Kölner Jahrhuch für Vor-und Frühgeschichte, Vol. 6, 1962/63, pp. 98–106.

2. G. Weinberg, "Vasa Diatreta in Greece," Journal of Glass Studies, v. 6, pp. 47–55.

3. G. Weinberg, op. cit., pp. 47–55.

4. These analyses were carried out by Dr. R.H. Bell and Mr. C.A. Jedlicka of Lucius Pitkin, Inc., New York, N.Y.

Published on July 22, 2013