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Rosemarie Lierke

Antike Glastechnologie / Ancient Glass Technology


Following the traces of the Amphora

                     Translation of “Auf den Spuren der Amphora”, in Restaurierung und Archäologie Vol. 2 (2009) 67-80
(with some minor additions or corrections). 


The majestic Amphora of the Antikensammlung in Berlin with its hight of 59.6cm is the largest ancient glass vessel known (fig. 1). It was obtained by a collector in Nikolajeff, the ancient Olbia at the shores of the Black Sea, and it belongs to the museum since 1913. For stylistic reasons its making is dated to the time of 120-80 BC [1]. It is still debated how it was made [2]. The manufacturing traces of the original are the only dependable criteria to decide whether our ideas of its making could be realistic. These traces therefore are in the center of our considerations.    

    Fig. 1: The great Amphora of Berlin, c. 120-80 BC, H. 59.6cm, Antikensammlung SMPK Berlin, Inv. No. 30219,154.


An investigation of the Amphora

In the year 1976, the Amphora was entrusted to the restoration workshop of the Römisch-Germanische Zentral- museum in Mainz to repair the damages which were caused by its storage during the war. At this occasion, the restorator Hilmar Staude published a detailed description of this vessel [3]. From his description we take the following facts which are important to get an idea about the making of the glass body. 

The Amphora is made from an excellent, bubblefree glass material, with only little corrosion. The present surface is „silky, smooth and dense“. The original surface is partly preserved. On the in- and outside, the vessel shows the rotary traces which are so typical for ancient glasses (fig. 2). The wall thickness of the bottom part varies extremely from 1.5 – 15 mm. The neck shows strong deviations from an exact roundness. 

     Fig. 2: Detail of the neck with the rotary traces and the sleeve

     Fig. 3: One-sided narrowed neck above the slightly slanting sleeve

For several reasons, one must agree with Staude that a manufacture of this vessel by blowing or by cutting from a full blank can be precluded. The same applies for pouring into a two part mold which was made after a wax model. Staude is convinced that the vessel was made as a molded blank by using a so far unknown technique. In accordance with the ideas of his time, he concluded from the existence of the rotary traces that the blank was ground and polished inside and out. “For this purpose a device was used which may have looked like a lathe, or perhaps like a potter’s wheel.” The following descriptions after Staude are all influenced by the idea of a final abrasive treatment like finishing by grinding on a lathe or on a potter’s wheel.

The clearly visible one-sided narrowing of the neck above the sleeve (fig. 3) Staude explains by a shift of the axis of turning. In his opinion, the upper part of the vessel was fixed once with its top and once with its bottom to the lathe in order to enable each time the grinding of the part which was covered before by the clamping device. At this change assumedly occurred a shift of the axis of turning. At the bottom part he even assumed such a shift twice since the foot is shifted against the vessel and the cavity on the underside of the foot in his opinion was carved out “excentrically and slanting”. The shifting assumedly became counteracted in the area between body and foot – that means by the stem. Staude gives no hint, how this actually was realized.  

Inspired by the contribution of E. Marianne Stern about the great footed bowl in the Wolf collection in Stuttgart [4], Gertrud Platz-Horster in her detailed paper about the dating and provenience of the Amphora treats such questions too [5]. She proposes a complicated manufacturing concept, involving three reheatings of the Amphora components which partly have cut grooves. Among these components she assumes for the neck a „full or roughly hollowed glass cylinder“ of which the cavity had to be drilled out (and polished!) to comply with the smooth and shiny cylindrical interior of the original neck.


Own observations

In the beginning of the nineties of last century, Dr. Platz-Horster and the conservationist Barbara Niemeyer kindly offered to me the opportunity to see the inside of the Amphora while the museum case was cleaned. To my surprise, I discovered that indeed the inside of the Amphora neck is not interrupted by a seam or a sign of an attachment at the hight of the sleeve. The neck on the inside really is absolutely smooth [6]. This means especially, there is on the inside of the neck no step or narrowing. The vertical outline of the neck inside follows the ideal outline of the vessel, except for the upper end of the neck. Here the curvatur on the inside is more abrupt than the curvature on the outside. Because of the different curvature, the wall thickness reaches here up to 20mm. On the outside however, the one-sided narrowing of the neck above the sleeve forms a step around part of the neck circumference (fig. 3). This narrowing is caused solely by a reduction of the wall thickness from the outside. The decorative sleeve helps effectively to camouflage the otherwise aesthetically disturbing sight of this step.

Even on the photographs is visible (fig. 2 - 4), that the sleeve is somewhat slantingly applied on top of the rotary traces which assumedly were generated by grinding the vessel on a lathe. The sleeve obviously was not cut, it must have been applied with hot glass (as noticed also by Dr. Platz-Horster, see note 1). However, a sleeve of hot glass can only be applied around the neck of a vessel while this vessel is hot too. A vessel which assumedly was ground all around must be reheated to 500°C or more after the cold abrasive treatment. Doubts therefore are raised whether the rotary traces really are grinding marks. The grinding could have caused stress in the glass, and this – upon reheating - could lead to cracks initiated by the numerous grinding-related injuries of the surface. A still greater risk of any reheating is the danger of involuntary deformations of the vessel shape because of the partly extremely varying wall-thickness. A successful reheating finally would at least partially remelt the sharply engraved traces and make the surface smooth (fire polish).

     Fig. 4: The top part of the Amphora with cut grooves on the shoulder

The decorative cut grooves on the vessel shoulder are slanting against the assumed grinding marks with an angle of c. 5° (fig. 4) – a further astonishing observation. If the vessel really would have been ground all around on a lathe or on a potter’s wheel causing the rotary traces, the decorative cut grooves should run around exactly parallel to the assumed grinding marks.

The idea of the “lathe-cut” ancient glasses was in the past so firmly established that in 1976 it was not questioned – not even by an experienced restorator like Hilmar Staude. Still today, it is treated as factum in some important publications [7] - and it was the inspiration for the reproduction of the Amphora which was recently presented [8]. However, there are repeatedly contradictions – as in the case of the Amphora – which raise doubts that the typical rotary traces of ancient glasses truly are grinding marks. It seems necessary to treat this problem with additional arguments once more, even while the basic considerations have been published several times [9]. The tenacious error about the typical traces of ancient vessels has caused a tremendeous confusion and prevents the appreciation of the logic development of ancient glass working.  


The rotary traces of ancient glasses 

Rotary traces in agreement with their appearance are often called scratches. However, most often they are erroneously explained to be grinding or polishing marks. They appear since about the middle of the first millennium BC, especially on open vessels. They are typical, for instance for the big hellenistic footed bowls [10] – the close relatives of the Amphora – but also for other luxury vessels: skyphoi, gold glasses, goldband glasses (fig. 5), or reticella bowls [11]. In addition they are typically found on the so called ‚ceramic related vessels’ of the early Roman Imperial time, and as a rule on the inside of ribbed bowls, or even on the inside of hollow vessels, for instance on cameo glasses, including the famous Portland Vase (fig. 6) [12]. Why grinding or polishing marks could be found on the inside of hollow vessels is hard to explain, but the scratches were indeed tried to be explained by grinding. Such an explanation naturally fails in view of the scratches on the inside of a narrow-necked bottle (fig. 7a, b) [13]. How and why should a narrow necked bottle be ground on the inside [14]? There must be another explanation for the typical rotary traces.

    Fig. 5: Goldband pyxis with rotary scratches, c. 50AD, H. 5.2cm, Corning Museum of Glass 55.1.2 a,b.

    Fig. 6: The Portland Vase, c. 25 BC, H.24.8cm, British Museum London GR 1945,9-27.1.

    Fig. 7: Cameo fragment c. 10 BC-10 AD, H. c. 8.6cm. Martin von Wagner Museum Würzburg Inv. No. H1725. The inside shows the rotary scratches. The onset of the neck testifies that the fragment belongs to a narrow-necked vessel

    Fig. 8 a: The Auldjo Jug, M. 1st c.AD, H. 22.8cm, British Museum London GR 1840.12-15.41 and 1869.2-16.1

    Fig. 8b: The bottom of the Auldjo Jug, Dm 7.8cm

     Fig. 9: Fragment of a vessel bottom with rotary scratches and a fused-on glass thread on it’s inside, Dm 7.1cm, Museum Kunstpalast/Glasmuseum Hentrich Düsseldorf 1990-412. The shallow round depressions are traces of withering.

    Fig. 10: Detail of the inside of the fragment from fig. 9.

The misfortune of an ancient glass worker illustrates the problem. This unknown master fashioned a glass vessel with a bottom which looked just like the bottom of the Auldjo Jug (fig. 8a, b). The Auldjo Jug is the second famous cameo vessel of the British Museum, and the almost identical bottom is preserved as fragment in the Museum Kunstpalast/Glasmuseum Hentrich in Düsseldorf (fig. 9). This fragment shows – hardly visible in reality because of the identical color – a tiny glass thread molten across the rotary traces [15].

A thin hot glass thread would not fuse to a cold surface. It would immediately become stiff upon contact and fall off. The surface with the traces therefore must have been hot, roughly at a highly viscous molten state. The thread assumedly was created during the making of the vessel, perhaps while a handle was applied and the superfluous hot glass was drawn off, or while a contamination of the hot glass was removed with pincers and likewise a thread was drawn out. The thread fell onto the hot surface with the traces and became fused to it. 

In the restoration workshop of the RGZM microscopic photographs were made which proved (fig. 10) that the rotary traces indeed proceeded under the thread – the traces therefore must have existed before the thread was fused to the surface, they were not caused by grinding [16]. The original vessel cannot have been ground after the thread was fused to it since otherwise the thread would not exist any more. Analyses confirmed that fragment and thread belong together with the „most high probability” [17], meaning the thread which was fused to the surface consisted of the same glass material. 

Who still doubts that the typical traces of ancient vessels are not grinding marks should consider: today nobody really would get the idea that glass vessels – as a standard production procedure (!) – are finished from a blank by grinding on a lathe. But today glass blanks without internal stress, as well as smoothly turning grinding instruments are available, greatly diminishing the risk of cracking. This risk must have been rather insurmountable 2000 or more years ago.


The possible cause of the rotary traces 

If the rotary traces are no grinding marks, just what are they in reality? To say it in advance, they obviously are exactly what many scientists call them spontaneously and without prejudice: they are scratches. As a rule, they are remarkably sharp scratches in a shiny surface (comp. fig. 2). Real grinding marks look somewhat different. A surface which has to be ground is usually not smooth or even – because this normally is the reason why a glass needs grinding. For grinding a surface, a more or less coarse grinding wheel or grinding powder in a suspension is used. Their action leaves a diffusely mat or – on a turning lathe – a stricly parallel striated roughness, in any case no separate sharp scratches. Polishing generates an increasingly shiny surface which may still show dullness or roughness in deeper spots, but no scratches with sharp edges.  

    Fig. 11: Ribbed bowl fragment, inside with rotary scratches, 1st c. AD, H. 7,2cm, Archäologisches Museum Frankfurt/Main a24292

    Fig. 12: Horseshoe chatter marks of ‘cold’’ scratches

    Fig. 13: Chatter marks of ‘hot’ scratches

    Fig. 14: Detail of the ribbed bowl fragment of fig. 11 with chatter marks of ‘hot’ scratches

The fragment of a ribbed bowl (fig. 11) shows the typical traces on its partly originally  preserved surface and – different from the fragment of fig. 9 – it shows only slight corrosion. The ribbed bowl fragment was kindly made available for an investigation in the Schott AG Mainz. It was discovered that the typical traces of this example were „hot scratches“, that means scratches generated during the hot manufacturing process in the comparably cool and solidified thin skin of the hot glass. This is a possible manufacturing ‚defect’ which was detected and firstly described 1976 in the Schott AG, Mainz [19]. Hot scratches are clearly recognizable. Each scratch in glass is accompanied by horseshoe-shaped so called chatter marks (fig. 12). Different from the horseshoe chatter marks of a normal, let us say „cold“ scratch, the legs of the horseshoe chattermarks in the thin solidified glass surface of hot glass (above 500°C) are partly or wholly molten back, depending on the heat of the glass. As characteristic marks of a „hot“ scratch, only short bows or lines remain from the horseshoe chatter marks (fig. 13).

Just such small bows are shown by the typical scratches of the investigated ribbed bowl fragment (fig. 14). The scratches on ancient glasses obviously are a kind of a “production defect”, occurring quite regularly till late antiquity on non-blown glass vessels.   

The question remains: what caused the scratches? We find the rotary scratches on those surfaces where the hot glass had contact with its mold. The scratches must have been generated by a movement of the glass against its turning mold for instance. The ancient mold material basically must have been gypsum plaster which does not stick to the glass. Its evaporating water content generated for a limited time a separating steam layer between mold and glass – warranting basically a smooth glass surface of the molded glass [20]. However, the ancient mold material obviously was not as homogeneously fine grained as modern plaster. Therefore, after the water content of the plaster was exhausted and the separating steam layer disappeared, the ominous traces could be generated as scratches in a shiny surface. 


A new explanation for the manufacturing of the Amphora

If the rotary traces on the Amphora are not grinding marks but scratches, one has to think again about its manufacturing. The rotary traces on in- and outside confirm a mold contact on both sides. How does one get a comparably thin glass layer between the outer mold and the core? Hilmar Staude already rightfully precluded pouring the glass into a two-shelled mold since he expected air inclusions. Hot glass never is fluid enough to be poured into narrow cavities, and it does not solidify at a certain temperature, its viscosity raises successively instead. The contact with the comparably cool mold already causes the glass to become stiffer and to block the pouring channel. The often assumed melting of crushed glass in a two-shelled mold likewise needs a critical view. In case of the Amphora ist must be precluded since it would not deliver a bubblefree glass. Finally, the extreme variation of the wall thickness does not support a sagging process, that means the shaping of a flat glass cake over a core by gravity and heat. Only one of the oldest glass working methods is possible which so far for the ancient glass vessel production rarely is considered – perhaps since it is in low esteem today: pressing. The pressing of glass needs only comparatively low temperatures. Moreover through a steam-bolstered short contact with the mold a shiny surface is possible which does not need any troublesome finishing. While grinding in antiquity did not the play role which we assume today, stationary or rotary pressing was often used. Turning during pressing – for instance by fixing the mold onto a turntable or a potter’s wheel – is helpful to prevent the sticking of the glass to the mold, and in addition to get an even, fast and more thinwalled distribution of the glass mass. Through rotary pressing, the scratches could have been generated. Different compared to real grinding marks, such scratches appear separately in a shiny surface (comp. fig. 2). Molds which were made from gypsum plaster or a related material become brittle from the contact with the hot glass since the heat causes the water content (including the crystal water) to evaporate. The molds crumble easily and could not be used a second time. Ancient pressed glasses therefore are no mass production.

    Fig. 15: The bottom part of the Amphora, H. 26.4cm.

    Fig. 16: The pressing of a large footed bowl with foot and stem

    Fig. 17: Correction of an excentrically applied stem and foot.

    Fig. 18 a: The pressing of the top part of the Amphora with coat- and core mold

    b: Removing the top part of the coat mold

    c: Applying a new mold for the wide opening, fast turning to widen the top part of the neck, pressing on additional hot glass for the broad rim.

The manufacturing of the bottom part of the Amphora (fig. 15) is identical with the manufacturing of the big footed bowls with core and coat mold (fig. 16) [21], where stem and foot were pressed on - either together with one mold or successively with separate molds. For the Amphora, the stepped transition between foot and stem suggests a common mold for foot and stem – as illustrated. The pressing required a fast execution “hot in hot”, resulting sometimes in unintentional deviations from the rotational symmetry. This could be an uneven wall thickness or – as in case of the Amphora - a somewhat excentric application of the foot. The cavity of the foot - according to Staude  “excentrically and slantingly carved out“ – is far more convincingly explained by pressing with a hastily excentrically and slantingly introduced plunger than by a deviation from the rotational symmetry during the grinding of a blank which is clamped to a lathe. Since the thick solid stem retained the heat and herewith the plastic state longer than the rest of the vessel, it permitted a simple correction to enable the vessel to stand straight (fig. 17). Only a slightly deformed or oblique stem remained from this procedure.

According to the traces, the top part was made similar to the bottom part. The rim only made the process a little more complicated (fig. 18a, b, c). Over a fitting core the top part was pressed first with a thoroughly slim neck. Since the gypsum plaster became soft and brittle after the contact with the hot glass, the coat mold could easily be cut through on the hight of the planned sleeve, the top part of the used mold was removed and replaced by a new mold for the wide opening [22]. The centrifugal force of a fast rotation and perhaps some tooling loosened the top part of the slim neck from the core mold and made it adapt to the new mold for the opening. A thick ring of hot glass was pressed on with a fitting plunger or a suited tool to become the broad applied rim. The first impact of  the centrifugal force after mounting the new top mold could have pressed this mold on one side stronger against the neck and caused this way the one-sided flattening of the neck which is still visible above the sleeve. This sleeve was applied after the now brittle mold was broken off the still hot glass.  

     Fig. 19: The Glass Amphora and their metal parts of gilded copper

      a  bud shaped lid knob

      b  upper handle fixtures, embellished with a maple leaf

      c  lower handle fixtures, embellished with a mask

      d  belt with an embossed and chiselled foliage tendril

      c  satyr with wine skin as faucet

The Amphora consists of 13 separate parts (fig. 19). The fitting, but not snapping lid of a slightly differently colored glass material may be a later addition or a replacement. It was simply pressed, perhaps from a ring of hot glas, producing an opening which only had to be enlarged or smoothed by grinding to accept the gilded knob made from gilded copper. The bottom part of the Amphora received two drilled holes on opposite sides – assumedly made with the grindstone tip of a drill – to insert the gilded copper faucets (fig. 20). The application spots for the molded or pressed and hot formed handles were roughened and the handles with their copper-gilded fixings were cemented onto the vessel. Finally the two halves of the vessel with their perhaps edgy ground rims were cemented to each other and the joint covered with a gilded copper-belt.

    Fig. 20: The bottom part of the Amphora, the inside with rotary scratches, the deformed onset of the stem and two openings


A surprising discovery

It is worthwhile to finally investigate the sleeve of the Amphora a little more in detail (fig. 21). Its internal diameter is c. 5 cm, this is about the size of a girls bracelet. The hot applied bulging ring for the sleeve – which in our opinion covers the application site of the mold for the wide opening – was parted with a knife-like tool into a broad middle part and two narrow rings at the sides. This makes the sleeve identical to a celtic glass bracelet of the form Haevernick 6c [23]. The fragment of a celtic glass bracelet from Manching shows a related fluctuating structure of the indented grooves (fig. 22) [24]. The bracelets Haevernick 6c are dated just like the ring from Manching to the late Middle Latène – a time overlapping in its final phase with the assumed manufacturing time of the Amphora. Is this a coincidence? It is not known where the Amphora was made – it seems certain that it does not come fron a celtic oppidum. Olbia on the other side is located somewhat outside of the known area of distribution of the glass bracelets [25]. Perhaps there existed contacts by migrating craftsmen? In any case, the bracelets and the sleeve of the Amphora share the secret of their seamless manufacture. Even if the similarity between bracelet and sleeve were coincidental, for this technological reason it is not irrelevant.

     Fig. 21: The sleeve of the Amphora with a broad bulging middle ring and two bulging rings beside.   

    Fig. 22: Fragment of a celtic glass bracelet of the late middle Latène time from Manching. Clear glass with yellow foil

     Fig. 23: The making of the sleeve by spinning a thin glass thread

The manufacturing technique of the glass bracelets is still debated. Perhaps the sleeve of the Amphora can help to solve this puzzle. Since the Amphora obviously was made on a turning or potter’s wheel, the sleeve could have been made as a bulging ring by a very fast drawn and coiled (spun) thin glass thread – just like a glass bead – and it was subsequently profiled by indentations with a knife (Abb. 23). The spinning of glass threads is a technique used for the making of the body and long uninterrupted decor threads of late cored vessels [26], and for the reticella bowls – also time-related to the Amphora – which are vessels with smooth walls made by coiling glass spiral threads to look comparable to wicker baskets [27]. Perhaps this was the way also the seamless celtic glass bracelets were made[28]. It would be one more example for the logic relationship, for continuity in combination with creativity in the development of the ancient art of glass working.


[1] G. Platz-Horster, Die Berliner Glasamphora aus Olbia. Journal of Glass Studies 37 (1995) 35-49.

[2] G. Platz-Horster / J. Welzel, Antike Technik auf dem Prüfstand. Antike Welt 79/1 (2009) 65-68. Welzel presents a masterly copy of the Amphora, made by sagging, fusing, drilling and grinding. See note 8.

[3] H. Staude, Zur Herstellung der großen hellenistischen Glasamphora aus Olbia. Annales du 7e Congrès de l’Association Internationale pour l’Histoire du Verre, 1977 (Liège 1978) 445-451.

[4] E. M. Stern / B. Schlick-Nolte, Frühes Glas der alten Welt. Sammlung Ernesto Wolf (Stuttgart 1994).

[5] Platz-Horster (note 1).

[6] Platz-Horster (note 1) fig. 6, here drawn with the assumed shift of the axis.

[7] 1975 mentioned F. Fremersdorf indeed ‚gedrechseltes’ (lathe turned) glass, in: Fremersdorf, Antikes, Islamisches und mittelalterliches Glas, Cat. Museo Sacro Bibl. Apostolica Vaticana 5 (Vatikanstadt 1975), 11/12. A. von Saldern described 2004 the knob- like central protrusions of ancient glasses, enclosed by a protruding ring (comp. here figs. 8b and 9) as the point where the glass was fixed to the turning lathe. See v. Saldern, Antikes Glas, Handbuch der Archäologie (München 2004) 183/184. A. v. Saldern quotes D. F. Grose, Early Ancient Glass (New York und Toledo 1989) 256.

[8] Platz-Horster / Welzel (note2). Telling are among other features the different sleeves.

[9] R. Lierke et al., Antike Glastöpferei. Ein vergessenes Kapitel der Glasgeschichte (Mainz 1999) 13-15; idem, Geritzt, geschliffen und geschnitten – die Evolution der frühen Glasschneidekunst. Antike Welt 34/4 (2003) 345-356; idem, The “turning” of ancient glass vessels. In: M. Feugère, J.-C. Gérold (eds.), Le tournage des origins à l’an Mil. Actes du colloque de Niederbronn, octobre 2003, Monographies instrumentum 27 (Montagnac 2004) 169-178. Most recently: R. Lierke, Die nicht-geblasenen antiken Glasgefäße / The non-blown ancient glass vessels (Deutsche Glastechnische Gesellschaft Offenbach 2009) 30/31, 56/57 .

[10] An overview at Platz-Horster (note 1).

[11] Reticella glasses were made from glass threads or rods which were spun around with thin glass spirals. The tiny spiral threads in the surface show no injuries despite the existing ‚grinding marks’. Lierke et al. 1999 (note 9) figs. 88, 89. Example of a goldband glass from R. J. Charleston, Masterpieces of Glass. A world history from the Corning Museum of Glass (New York 1990) 37, fig. 5.

[12] For an illustration of the internal traces of the Portland Vase see W. Gudenrath in: D. Whitehouse et al, Reflecting Antiquity. Modern Glass Inspired by Ancient Rome (Corning 2007) 72. For a comment see Lierke 2009 (note 9) 65. More examples there p. 30/31.

[13] To the narrow necked bottle fragment: C. Weiß, U. Schüssler, Kameoglasfragmente im Martin von Wagner Museum der Universität Würzburg und im Allard Pierson Museum Amsterdam. Jahrbuch des Deutschen Archäologischen Instituts Bd. 115, 2000 (Berlin 2001) 199-251, fig. 6-8.

[14] An explanation of the manufacturing of a small cameo bottle and the formation of the scratches see Lierke 2009 (note 9) e. g. 65, 69.

[15]The former director of the Kunstmuseum Düsseldorf/Glasmuseum Hentrich, Dr. H. Ricke, has to be thanked especially for permitting repeated investigations of this fragment in Mainz.

[16] The investigation was performed by  Ch. Eckmann, M. Fecht and Dr. S. Künzl.

[17] Dr. S. Greiff, RGZM Werkblatt-No. 07/150, 27. 8. 2008; see Lierke 2009 (note 9) 58.

[18] Dr. I. Zetsche, Archäologisches Museum Frankfurt/Main, kindly permitted the investigation which was executed by Dr. M. R. Lindig and his team in the Schott AG, Mainz..

[19] M. R. Lindig, Untersuchung der umlaufenden Spuren auf antikem Glas. In: Lierke et al. 1999 (note 9) 15-16. Otto Lindig, Projekt 6001- Sichtfehlerbewertung, Jenaer Glaswerk Schott & Gen., Mainz, 20. 4. 1976.

[20] About ancient mold material: Lierke et al. 1999 (note 9) 79-80.

[21] Ibidem 49 or Lierke 2009 (note 9) 34.

[22] After the contact with the hot glass which caused the loss of the crystal water the now brittle gypsum may easily be cut. It would also be possible to employ a two part mold and exchange the top part. 

[23] Th. E. Haevernick, Die Glasarmringe und Ringperlen der Mittel- und Spätlatènezeit auf dem Europäischen Festland (Bonn 1960) T. 17.

[24] The illustrated example has 5 ribs, different from the sleeve of the Amphora with 3 ribs. R. Gebhard, Der Glasschmuck aus dem Oppidum von Manching (Stuttgart 1989) T. 25, Nr. 338.

[25] Haevernick (note 23) T. 35.

[26] Example: Stern / Schlick-Nolte (note 4) Nr. 51. Cored vessels made with wound glass threads were still manufactured at the beginning ot the first century.

[27] See note 11.

[28] I’m grateful to the glass artist Alois Wienand from Boxtal-Freudenberg for intense conversations on the topic. His excellent replicas are made with a related, but not identical method. We agree especially that the ancient glass is a so called long glass (long workable in a plastic state). See also Lierke 2009 (note 9) 10.