The oldest known glasshouse in Amsterdam was recorded in 1597, founded by a Venetian glassmaker, Antonio Obizzo, but it was short lived.9 A new glasshouse was founded in 1601 by the merchant Jan Janz Carel and run by Jan Hendriksz Soop, his son-in-law; it operated until around 1630. De Twee Rozen glasshouse was founded in 1621 and operated in Amsterdam until 1679.10 It was undoubtedly one of the most important glasshouses in seventeenth-century Amsterdam.11 It was also the glasshouse where the alchemist and early chemist Johann Rudolf Glauber (1604–1670) experimented with his innovations in glass technology; and it appears to have been internationally known at the time and is thought likely to be the glasshouse illustrated in the 1669 Latin edition of Christopher Merret’s translation of Neri’s L’arte vetraria12 as well as in Johann Kunckel’s edition of Neri’s book, published in 1679.13

Originally, De Twee Rozen was situated at Keizersgracht (referred to hereinafter as Phase 1), but it was moved to a new location further from the city center at Rozengracht in 1657 (Phase 2).14 In 1662 a “touristic” city guide, the Beschrijvinge der wijdt-vermaarde koopstad Amstelredam (Description of the widely renowned merchant city Amsterdam) by Melchior Fokkens, was published, with an entire chapter on the glasshouse as one of the wonders of the city. It describes a whole range of drinking vessels manufactured in the glasshouse, including, besides the normal drinking glasses, complete opaque white vessels (lattimo) and vessels with blue or white rim edges, goblets with lids, flutes, and various other tablewares.15 Among the FdV vessels, there are examples made of filigree glass with multicolored canes, ice glass, millefiori glass, and many others types of glass and decorative techniques. Beads and plates of mirror glass also constituted an important part of the production. Amsterdam in the seventeenth century was an important trading center and exported glass, among various other commodities, to almost all of Europe, and De Twee Rozen products would have constituted an important part of this trade.

Abundant glass production material was recovered from the Monumenten en Archeologie, Gemeente Amsterdam (MenA) excavations at Keizersgracht in 1981, found during dredging of the canal as part of sewage works (MenA site KG10). These were originally attributed by Baart16 to the Soop glasshouse and dated to 1601–1610. However, in 2013 the site was reassessed as the waste from the first site of De Twee Rozen (i.e., 1621–1657).17 Unfortunately the previous misidentification has been widely quoted in the literature, including in archaeometric studies where glass compositions have been attributed to the Soop glasshouse and/or dated to the period 1601–1610.18 The erroneous attribution has important implications: it has been used as a chronological anchor for beads from archaeological sites in North America19 and as a reference point for the history of glassmaking in Amsterdam.20

Excavations at Rozengracht in 200621 revealed material from the second location of De Twee Rozen, the period from 1657–1679 (MenA site R021)—Phase 2. Figure 1 shows the typical examples of colored glass shards excavated there. Unfortunately, no entire objects were unearthed in the excavations.22 It is from this site that most of the new material analyzed here was derived. Forty-five colored glass fragments from Rozengracht in the collection of the MenA were sampled, including 3 polychrome fragments, to give a total of 50 glasses (24 whites, 26 blues). In addition, we had previously analyzed 24 colorless glasses from the site, and as these have been discussed in only a preliminary way,23 they are also included here to provide a full understanding of the glass produced at the later site.

For comparative material from Phase 1 (Keizersgracht, site KG10), the initial location of De Twee Rozen, we are largely dependent upon previously published material. We present only a single new analysis, which is of a translucent green glass. Published full major-element analyses of colored glasses from Keizersgracht are limited to 14 samples presented by McCray and Warren,24 but a neutron activation investigation of bead- and bead production–related material from the site by Karklins and others25 provides a substantial body of information on the colorants and opacifiers used, although the analytical technique could not produce full major-element analyses. In addition, analyses of 38 fragments of weakly colored or colorless glass from the site are provided in De Raedt’s thesis26 and later also discussed by De Raedt and others.27 Hulst and Kunicki-Goldfinger28 also analyzed 13 samples of colorless glass, 10 of which had previously been analyzed by De Raedt; and McCray and Warren also analyzed several samples of colorless glass.29 Overall, these publications provide a useful dataset for the first site of the glasshouse.

Both locations of De Twee Rozen glasshouse at Keizersgracht (Phase 1, marked as KG10) and at Rozengracht, (Phase 2, marked as RO21) and the actual location of the Soop glasshouse (KLO10) are shown on a map of Amsterdam (Fig. 2).

Forty-six glass fragments from the collection of the MenA were sampled using a diamond tool. The samples were embedded in an artificial resin and polished with diamond pastes down to 0.25 μm grade.

A Nikon SMZ1000 zoom stereomicroscope with a CoolPix 4500 digital camera attachment was used to investigate the structures and morphology of the cross sections, following which they were coated with a thin layer of carbon and analyzed using a CamScan Maxim 2040 scanning electron microscope equipped with Oxford Instruments ISIS or INCA energy dispersive X-ray spectrometers (SEM-EDS) at the School of History and Archaeology, Cardiff University, Cardiff, United Kingdom. Back-scattered electron (BSE) imaging was used to identify compositional changes on the cross sections of complex samples. For quantitative elemental analysis, the electron beam was rastered at a magnification of 500x over an area of fresh glass for 100 s, at 20 kV accelerating voltage. For complex samples, for the analysis of small areas of interest, the electron beam was rastered at high magnification or used in spot mode. Count-rate on metallic cobalt was around 4000 cps. Standards were pure oxides and minerals, and quantification was carried out using the ZAF method. Oxide weight percents were calculated stoichiometrically. Analytical totals were typically between 98% and 102% and have been normalized to 100% for comparative purposes. Detection limits were about 0.1% for most elements, but higher for SnO₂ and Sb₂O₅, at around 0.3%. Corning Reference Glass B30 was used as a secondary standard. Good agreement between recommended and analyzed results was obtained in the case of most components (Table 1).

Details of samples and their compositions are presented in Table 2. The table includes both the new analyses (items 1 and 15–64) as well as the analyses of colorless glasses from De Twee Rozen glasshouse (items 2–14 and 65–88) that were published previously.31 The table is arranged by the site (Phase 1, KG10: items 1–14; Phase 2, RO21: items 15–88) and color of the sample analyzed. Three complex, multicolored objects are also presented separately in Table 3.

The present analysis allows us to add a chronological dimension to the introduction of “Venetian” glassmaking technologies at De Twee Rozen and compare them with the introduction of the manufacture of glass vessels in Venetian forms. It appears that the earlier production in Phase 1 was based upon a simple mixture of sand and unpurified barilla ash. Although this glass shows some similarities in composition to Venetian vitrum blanchum, it should be emphasized that the silica source for vitrum blanchum was much purer, with low alumina and iron oxide and based upon crushed pebbles from the River Ticino rather than sand.59 The use of the term vitrum blanchum to describe this type of northern European product is problematic, as it differs not only in the detail of its chemical composition but also in technological approach: it used sand and western Mediterranean barilla ash rather than pebbles and Levantine rochetta. This type of formulation was widely used, for example, in northern France from at least the end of the sixteenth century60 as well as in Spain and in Italy from earlier times.61 Therefore, while the production of façon de Venise vessels at De Twee Rozen may be associated with Venetian glassworking techniques, this does not appear to have been accompanied by a specifically Venetian glassmaking technology, as the sand–soda ash glass formulation used was widely established across much of northern and southern Europe at this time. It is clear that, at least in this case, the spread of FdV production and Venetian cristallo technology are not closely linked.

The data indicate that a Venetian-type cristallo technology was well established sometime after the move to Rozengracht in 1657 (Phase 2). This was almost 40 years after the establishment of De Twee Rozen and the production of glass in the Venetian style in Amsterdam. We cannot, however, be sure of the precise date at which cristallo technology was introduced at De Twee Rozen, as the nature of the deposited glass-production materials means that they may each represent short periods of production in their respective locations.

It should not be surprising that the introduction of glassworking techniques and styles preceded the introduction of cristallo-type technology to purify the plant ash and reduce the tint of “colorless” glass, as the types of know-how that had to be acquired for these technologies were very different. Blowing a glass object is an embodied skill, learned by experience and in practice exercised with only limited reflective decision-making—the skilled artisan’s responses to changes in workshop conditions, glass temperature, viscosity, vessel form, and so on are almost intuitive.62 A glassblower who had experience making other forms could, with practice, have learned to produce FdV forms through observation and application of their pre-existing skills. Manufacture of a standard soda ash–based glass, more or less equivalent to Venetian vitrum blanchum but using sand rather than crushed pebbles, would also have been a procedure fairly familiar to those who had previously melted inland wood ashes with sand to make a potash-rich glass. However, in Venice, the preparation and mixing of raw materials and their melting and coloration was the province of a specialist craftsman, a conciatore.63 Cristallo was based upon practices which were not yet widely known or understood—it required the purifying of the ash by a procedure of dissolution and evaporation, then the replacement of some of the lost lime and magnesia (probably in the form of an addition of vitrum blanchum to the batch),64 and direct personal experience of the process would have been extremely beneficial, if not essential. Artisans who blew the vessels would not necessarily have possessed the experience and know-how to replicate the arcane practices involved in cristallo technology. It is therefore understandable that these two types of knowledge would have spread at different speeds and in different ways.

A second change that occurred around the time of the 1657 change in location was the introduction of opacification with antimony. Antimony was the typical opacifier in glass from the Late Bronze Age up until the fourth century CE, when the supply of antimony appears to have failed.65 Antimony continues to be found in decorative glasses in later objects, such as enamels and glass tesserae in mosaics, but is generally considered to represent recycled older material.66 From the fourth century, tin oxide (cassiterite) became a primary opacifying agent and was dominant in medieval and Renaissance Venetian glass as well as in European glasshouses manufacturing FdV glass. The oldest known recipes for opaque white glass date from the sixteenth century and are included in the anonymous recipe book.67 Recipes and archaeometric analysis consistently indicate that tin was always accompanied by lead, typically in ratios from 1:2 to 3:1,68 which is similar to the ratios in opaque whites from De Twee Rozen Phase 1, in the range from 1:1 to 3:1.69

The first direct written reference to the use of antimony in making lattimo comes from 1640, when such a recipe appeared in the Darduin manuscript.70 It was less expensive than tin and the opacification seems to have been no less effective.71 It therefore appears that De Twee Rozen glasshouse was moved to its second location at Rozengracht only a dozen or so years after the first known written mention of the use of antimony to produce lattimo. Corroboration of the possibility that De Twee Rozen was an early adopter of antimony opacification in white glass depends upon the availability of analytical data. Verità and Biron72 note the use of lead antimonate yellow in Venetian enameled glass from the fifteenth century, but at this time white was invariably made using tin oxide opacifier. However, Verità and Zecchin73 identified antimony opaque white enamel on a Venetian polychrome goblet from the second half of the sixteenth century which contained 3.50% Sb₂O₃ (i.e., 3.9% Sb₂O₅), although unlike the Rozengracht white glass, it contained only 0.12% PbO. Several authors have identified apparently isolated examples of the use of antimony opacification in seventeenth-century glass (in vessels excavated in London74 and Portugal,75 and in beads excavated in North America76). While the use of antimony to produce opaque white glass in Venice may therefore have begun earlier than its mention by Darduin, at present its earlier occurrence elsewhere appears to have been limited. Experiments with antimony and glass have been reported based on a crucible recovered from an alchemical laboratory at Oxford;77 and in his 1662 commentary on Neri’s L’arte vetraria, Merret provides a recipe for the use of antimony to color enamel glass white, which he describes as “…whilst a secret of great value, but now commonly enough known to the furnaces,”78 suggesting that this understanding had been achieved only recently. The exceptional characteristics of the opaque white glasses—the use of a relatively pure plant ash base, the addition of lead, the omission of manganese decolorizer—all point to the sophistication of this technology and that, wherever it was developed, it must have been preceded by substantial developmental work and experimentation. If this particular approach was not developed at De Twee Rozen, the knowledge was presumably imported as a package.

Another novel approach observed in the Phase 2 assemblage is the coloration of translucent blue glass, which was typically prepared in Venice based on cristallo formulation, according to written sources.79 This is also confirmed by chemical analyses by various researchers,80 and our results are consistent with these findings. These translucent blue fragments (often called turquoise, sky color, or so on, as already mentioned) of vessels or production wastes that can be linked with the production of vessels were colored with brass (CuO 1.3–1.8% and ZnO 0.3% on average) with the possible addition of manganese (0.5% on average). Though there are many recipes from the discussed period (e.g., L’arte vitraria by Neri) describing coloration of glass with the use of brass and manganese (or without manganese), there is very scarce comparative material among the archaeological and/or historical artifacts that have been analyzed. The enamels or other forms of decoration described in the literature as turquoise are typically opacified and are almost manganese free; some of them also contain a small amount of cobalt.81 A seventeenth-century blue goblet (?) with millefiori rods from the Monastery of Santa Clara-a-Velha in Portugal constitutes an exception. It is decorated with transparent turquoise blue, among other decoration containing 4.03% CuO and 0.47% MnO; however, it represents vitrum blanchum–type technology rather than cristallo.82 We did not find in the literature any example of a translucent blue (turquoise?) Cu-rich glass with similar composition to our Cu-rich glasses from Rozengracht, which was the body glass of a vessel as opposed to a decorative element. From this point of view, Amsterdam’s Cu-rich blues might represent a less common technology of vessel-glass coloring in the seventeenth century.

Overall, glassmaking techniques appear to have been far more sophisticated in Phase 2 than those used in Phase 1. Just how these purification and coloration technologies came to be introduced at De Twee Rozen is open to speculation, but it may be pertinent that the director of production from 1667 was Nicalao Stua, a Venetian master.83 Stua is likely to have had direct knowledge of Venetian glassmaking methods and might have introduced them. Second, it is tempting to see a role for the chemist and alchemist Johann Rudolf Glauber, who from time to time rented a furnace at the Rozengracht glasshouse for his experiments, including the coloration of glass. Glauber had also experimented with the purification of wood ash to produce potassium salts of higher quality.84 He can be expected to have been interested in the techniques used to purify the ashes and color the glass. Unfortunately, the extent of their interaction at Rozengracht is unknown, and there is a comment that Glauber had ceased using the glass furnaces by 1663, apparently before the arrival of Stua,85 although he is known to have worked in Amsterdam until 1670.

It has been suggested that the development of lead crystal glass in late seventeenth-century England was in part dependent upon the importation of the practice of adding lead, from the Netherlands.86 We have not identified any colorless high-lead glass at Rozengracht, but lead oxide concentrations of up to 4% occur in the antimony-opacified white glasses, while up to 6.3% occurs in cobalt blue. This is considerably less lead than present in successful English crystal, but Brain expresses the view that the earliest English crystal would have contained significantly less than 16% PbO. A further point of interest is his suggestion that the earliest English crystal utilized a method for the addition of lead to glass proposed by Glauber in 1651,87 before the move of De Twee Rozen to Rozengracht.

The recognition that the Keizersgracht site represents production material from Phase 1 (1621–1657) of the De Twee Rozen glasshouse has allowed the rare opportunity to compare production from two periods of a glasshouse in northern Europe known to have been manufacturing glass à la façon de Venise. New analytical data, mainly from De Twee Rozen’s Phase 2 at Rozengracht, have been compared with earlier analyses and have allowed the identification of a number of technological changes which were introduced during the 50 years of operation of the glasshouse.

In particular, it has been possible to identify the introduction after the glasshouse’s change in location of the purification of the barilla plant-ash flux, following the approach to cristallo used in Venice, as well as the introduction of antimony rather than tin as an opacifier in white glass, a technology which was relatively new in Venice itself and which appears to have required a distinctive base glass with high-alkaline earth oxides and the addition of lead. Other distinctive approaches of the Rozengracht period include the production of translucent copper blue in a cristallo-type glass matrix. It appears that the glass from the Phase 1 period was overwhelmingly based upon a simple sand–plant ash mixture, whereas in Phase 2 a more sophisticated technology used cristallo-type glass, except for specific colors.

We have drawn particular attention to the lag between the introduction of FdV glass and the introduction of cristallo-type technology. Although not often explicitly acknowledged in the literature, this difference in the rate of spread of the fashion for FdV glass vessels and the technology needed to produce glass of the highest quality (and on which the reputation of the Venetian glass industry was based) is to be expected. The types of knowledge required were the provinces of different craftsmen in Venice itself. The relatively slow spread of the know-how needed to make cristallo may even have been a contributing factor in the development of other crystal glass types in England and Bohemia. It emphasizes the need for more chemical analysis of well-dated glass assemblages of the period so that the changes that occurred may be better understood.

De Twee Rozen serves as a good example showing that products of one glasshouse do not have to be characterized by the same chemical composition and that a glasshouse operating over several dozen years may produce glasses which differ significantly in technology and composition. Had the material from the two periods been present in a single dump, the sequence of changes would have been almost impossible to confirm or even identify.

The role, if any, that the presence from time to time of Glauber at De Twee Rozen had in the development of the new technologies used there is unclear, but in view of his interests in glass coloration, in the purification of ashes, and the addition of lead to glass, it is very tempting to speculate that he had some influence.

The material analyzed here is still not sufficiently comprehensive to produce robust conclusions in all areas, especially with regard to Phase 1 of the glasshouse operation at Keizersgracht (1621–1657). On the other hand, considering all of the work drawn upon here, it is arguable that the production of no other glasshouse in northern Europe manufacturing FdV glass has been so thoroughly studied. The present results therefore constitute material of importance for further comparative studies of FdV glass in Europe, with a view to tracing the origins of individual items, as to date it has not been possible to connect the majority of FdV objects to any specific glasshouse. Furthermore, this paper provides the first such detailed approach to De Twee Rozen products, in which the compositions are connected to specific objects, information which is missing in a number of previous publications.

The comparative material for the discussed glasses from De Twee Rozen is very small. We hope that further studies on Venetian glass and glass of the vessels made in the Venetian style, especially from the seventeenth century, will allow us to clarify, advance, and possibly correct our present findings. There is an absolute need for trace-element data on the vessels as well as more research on the composition of glass used to decorate vessel glass. There is also a need to advance research on glass beads from both locations of the glasshouse De Twee Rozen. These bead and vessel glass technologies did not have to be the same. But the beads might be quite close to the technology of polychrome canes and other decorative colored elements applied during the manufacturing of the vessels.