Chemical composition and deterioration of glass excavated in the ...

Abstract
Chemical composition and deterioration of glass excavated in the
15th–16th century fishermen town of Raversijde (Belgium) B
O. Schalm a, *, D. Caluwé b , H. Wouters c , K. Janssens a , F. Verhaeghe b , M. Pieters c
a Department of Chemistry, University of Antwerp, Universiteitsplein 1, B-2610 Antwerp, Belgium
b Department of Archaeology, Free University of Brussels, Pleinlaan 1, B-1040 Brussels, Belgium
c Institute for the Archaeological Heritage of the Flemish Community, Doornveld Industrie Asse 3, nr. 11, bus 30, B-1731 Zellik, Belgium
Received 24 November 2003; accepted 10 May 2004
Available online 2 September 2004
The chemical composition, as determined by electron probe X-ray microanalysis of a series of ca. 100 archaeological glass fragments,
excavated at the Raversijde site (Belgium) is discussed. In the 15th–16th century, Raversijde was a flourishing fishermen town located on the
shore of the North Sea, close to the city of Ostend. As a consequence of several battles that were fought in its vicinity, the site was abandoned
in the 16th century and was not occupied since then. It is one of the rare archaeological sites in Europe that contains a significant amount of
information on the daily life inside a small but affluent medieval community.
A comparison of the chemical composition of fragments of vessels and window glass encountered in Raversijde to those found in urban
centres in Belgium and to literature date on German and French archaeological finds shows that glass made with wood ash dominates.
Usually, it concerns artifacts with a predominantly utilitarian use. A few objects made with sodic (i.e., Na-rich) glass were also encountered,
likely to have been imported from Venice during the 15th century or in later periods from an urban centre such as Antwerp, where Façon-de-
Venice glass manufacturing activities were established near the start of the 16th century.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Historical glass; EPMA; Quantitative analysis; Glass composition
1. Introduction
Spectrochimica Acta Part B 59 (2004) 1647–1656
In the 15th–16th century, Raversijde was a flourishing
fishermen town located on the shore of the North Sea in
what is presently Belgium, close to the city of Ostend. The
town had a population of at least several hundreds and was
considerably larger than other villages in the area. On the
other hand, Raversijde was appreciably smaller than the
neighboring cities Ostend and Nieuwpoort. As a consequence
of several battles that were fought in its vicinity, the
site was abandoned in the 16th century and was not
B This paper was presented at the International Congress on X-Ray
Optics and Microanalysis (ICXOM XVII), held in Chamonix, Mont Blanc,
France, 22–26 September 2003, and is published in the special issue of
Spectrochimica Acta Part B, dedicated to that conference.
* Corresponding author. Tel.: +32 3 820 23 73; fax: +32 3 820 23 76.
E-mail address: koen.janssens@ua.ac.be (O. Schalm).
0584-8547/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.sab.2004.07.012
www.elsevier.com/locate/sab
occupied since then: in the period 1482–1492 its neighboring
Flemish cities Brugge and Ghent were at war with
Maximilian of Austria while in 1567 the Eighty Years’ war
between Spain and the Netherlands erupted. The town
finally disappeared after the siege of Ostend (1601–1604),
during which the Spanish cavalry used it as an encampment.
The remains of Raversijde are exceptionally well-preserved
and provide a unique opportunity to reconstruct its socioeconomic
history by combining information present in
historical documents with that which can be gathered from
the artifacts excavated at the site. Thus, it is one of the rare
archaeological sites in Europe that contains a significant
amount of information on the daily life inside a small but
affluent medieval community.
In what follows, the analysis results of a set of ca. 100
glass fragments, excavated at the Raversijde site, are
described and the relation between on the one hand the
shape and use of the glass objects and on the other hand

1648
their composition is discussed. Also, various glass compositions
that are encountered in Raversijde are compared to
those previously found in urban centres of the low countries
of the same period.
2. Background information
2.1. Evolution of glass compositions in northwestern Europe
up to the (post) medieval period
The process for glass production in northwestern Europe
in the period 800–1700 AD was based on melting a
siliceous constituent (e.g., quartz sand, crushed quartz
pebbles) with a fluxing agent (e.g., potassium- or sodiumrich
vegetable ashes) while sometimes an additional
calcium-rich material (such as limestone, seashells [1] or
marble) was used. In a number of cases, also a lead source
and small amounts of colorant, opacifier or decolorant
materials were added to the melt. Fig. 1 provides an
overview of the most important periods in the history of
glassmaking in northwestern Europe.
In the Antique to the post-Roman period, glass was made
with Natron as fluxing agent, a mixture of NaHCO3,
Na 2CO 3 and other Na-salts, found in Wadi Natrun (Egypt).
This resulted in a durable Na-rich glass which could be
worked into a variety of shapes. In view of its relatively low
Fe content, it could easily be rendered colorless by addition
of oxidizing products such as MnO2 or Sb2O5 [2,3]. After
the breakdown of the Roman empire and roughly until 1000
AD, knowledge on glass making was no longer available in
northwestern Europe while also some of the raw materials
were no longer available.
During the dwood and plant ashT period (800–1800 AD),
the dominant fusing agent for glassmaking in northwestern
Europe were ashes from wood and terrestrial plants. With
this agent, a calco-potassic glass (having SiO2,K2O and CaO
as major constituents) was made. According to Wedepohl
[4], in Germany, within the dwood and plant ash T period (see
Fig. 1), a subperiod between 1000 and 1400 AD can be
distinguished where glass was made from sand and wood
ash, and a second period between 1400 and 1800 where glass
was made with sand, wood ash and lime. The first glass type
is known as potassic glass, the second as calcic glass. Both
glass types have a typical greenish-yellow color and are
collectively known as forest glass or dWaldglasT. Wood ash is
O. Schalm et al. / Spectrochimica Acta Part B 59 (2004) 1647–1656
calcium- and magnesium-rich, but potassium-poor. In the
northern half of France, wood ash was also used for the
production of potassic and calcic glass, but additionally fern
ash was used for the production of potassic glass. Glass made
with fern ash is known as dVerre de fougèreT. Ferns contain
significant quantities of potassium and much less calcium
and magnesium. In the publications of Barrera and Velde
[5,6] on glass in the north of France during the same time
span, a distinction is made between three major periods:
period II from the 12th to the mid-15th century, period III
from the mid-15th century to the end of the 16th century and
period IV from the 17th to the late 18th century.
The production of sodic glass (having Na2O, SiO2 and
CaO as major constituents, similar to modern soda-lime
glass) in northwestern Europe started with the arrival of
Italian glassmakers during the 16th century in the urban
centres. They produced glass in the Venetian style (Façon-de-
Venise) with locally available raw materials [7]. This glass
has a slightly different composition than the genuine Venetian
glass, which was available in two varieties: (1) Vitrum
Blanchum that was in use from the 14th century onward, and
(2) Cristallo that was produced since around 1450 [8]. The
most prominent feature of both these types is that they are
colorless, transparent glass with a specific composition [8];
however, the more common Vitrum Blanchum was made with
less pure raw materials that the top quality Cristallo.
2.2. Excavated glass fragments at the Raverszijde site
Although only a small part of the town of Raverszijde
has been excavated by the Institute for the Archaeological
Heritage of the Flemish Community up to now, already a
substantial collection of artifacts was recovered (e.g.,
tableware, coins). Among these are ca. 700 glass fragments,
in direct opposition to the accepted notion that in the 15th
century, glass was so expensive that only the richest
communities and citizens could afford it. Moreover, the
abundance of glass fragments found at the Raversijde site
certainly is not an exception; also in the archaeological
depots of neighboring coastal towns and cities, the material
evidence is present that glass was in frequent use here since
the 15th century.
A visual inspection of the light green vessel glass that was
excavated in Raversijde and the neighboring centres revealed
that glass objects of the same shape of various quality and
surface finish were present. However, the level of decoration
Fig. 1. Most important glassmaking periods and subperiods in northwestern Europe.

among artifacts of the same shape varies considerably. Thus,
the question can be raised whether glass objects of similar
shape and function were available in these communities in
different price categories. The co-existence of glass of
different quality might (partially) explain the fact that next
to heavily or even completely deteriorated objects, also glass
artifacts were excavated on the same location that are in a
nearly perfect condition. Additionally, one can also wonder
whether or not objects with a predominantly utilitarian value
(such as window glass or bottles) were made in the same
kind of glass than objects having a high(er) esthetic value,
such as vessel glass (e.g., wine goblets).
2.3. Typological classification and pre-selection of the glass
fragments
Before a chemical analysis was performed on a subset of
the ca. 7400 excavated fragments, the latter were classified
according to their shape and color. Table 1 lists the results.
Among the vessel glass fragments, 319 fragments could
be ascribed to at least 117 individual specimens. The objects
were divided into different types: 15th century beakers (101
vessels), 16–17th century beakers (12 vessels) and bottles (4
vessels). In Table 2, an overview is given of the vessel
shapes encountered.
All the medieval vessel glass was utilitarian in nature,
i.e., simple blown tableware, made from a pale glass of
greenish color. Only 12 beakers could be associated with the
period of late 16th century inhabitation or that of occupation
by the Spanish cavalry. These vessels were more luxurious
in nature and could be further divided into a group of eight
colorless and one of four greenish beakers. Among the
bottles were two green 18th century wine bottles, one greencolored
16th century bottle and one colorless flask, probably
of the 15th century.
Window glass in profane buildings appeared from the
second half of the 15th century onwards and non-figurative
glass-windows with a geometrical network of lead was often
used. The window glass fragments associated with the
Raverszijde chapel (15th century) consisted of (a) greenish
glass panes decorated with glass paints, (b) glass panes
covered with a thin red flashed layer; and (c) (mainly) bluecolored
pot-metal glass. Only a few of fragments associated
with profane buildings were decorated with glass paints.
The beads that were excavated were used in jewels and
as decoration on clothing. For example, to one small piece
Table 1
Overview of the 706 glass fragments excavated in Raversijde classified
according to function and color of the artifacts they correspond to
Vessel glass Window glass Beads
Greenish 280 154 –
Colorless 38 26 2
Colored 1 10 9
Rest 72 148 –
Total 391 338 11
O. Schalm et al. / Spectrochimica Acta Part B 59 (2004) 1647–1656 1649
Table 2
The different typologies of the 101 vessels of which the fragments were
excavated in Raverszijde
Vessel type Number of objects
Maigelein 14
Maigelbecher 48
Octagonal Maigelbecher 20
Beaker with oblique ridges 12
Beaker with sharply bent ridges 3
Beaker with ridges in relief 1
Prunted beaker
See Ref. [17] for a description of the types.
3
of cloth five small beads were attached, made in a yellowcolored
glass. Also, one individually blown bead with a
soda-lime composition originating of the 15th century has
been found.
3. Experimental
The set of analyzed glass samples included glass of
different archaeological contexts, of different colors and of
different vessel shapes. By necessity, the analyzed set of
samples is not completely representative for the collection
of excavated fragments: completely deteriorated glass is
underrepresented in the analyzed set while the relative
abundance of the typologies of the excavated artifacts has
not been (completely) respected in the subset of glass
fragments that were analyzed.
Since excavated glass fragments can show considerable
surface weathering, it is in practice not possible to employ a
non-destructive method of analysis such as (microscopic) Xray
fluorescence analysis for obtaining reliable quantitative
data on the original glass composition. By employing
electron probe microanalysis (EPMA), reliable estimates
of the major composition, including the low-Z constituents
Na2O and MgO, can be obtained from relatively small
amounts of material.
Thus, small samples (typically 1 mm 2 to 1 cm 2 in side)
were removed from the selected glass fragments. Several
samples were embedded together into one block of resin and
polished to expose the original glass of the samples. The
resulting surface was carbon-coated and the major composition
of the glass samples was determined by means of
EPMA. These analyses were performed with a JEOL 6300
scanning electron microscope (SEM) equipped with an
energy-dispersive spectrometry (EDS) Si(Li) detection
system. The spectra were acquired for 100 s with a beam
current of 1 nA and at a voltage of 20 kV. The net elemental
X-ray intensities were calculated with the program AXIL [9]
(analysis of X-rays by iterative least squares) and a
standardless ZAF program was used to calculate the
composition [10].
Various National Institute of Standards and Technology
(NIST) and Corning Museum of Glass (CMG) standards
were used to validate the quantification procedure. Details

1650
Table 3
Criteria used to distinguish among different glass types
Glass type Criterion
Calco-potassic glass K2O+CaON22% w/w
Sodic glass Na2ON6% w/w
Lead glass PbON15% w/w
of the quantitative accuracy of the method for analysis of
glass can be found in more detail elsewhere [2]. At
concentration levels below 2% w/w, the relative accuracy
is between 5% and 10%. The reproducibility of the analysis
is largely determined by counting statistics and is generally
below the 5% level. The total experimental error on the TiO 2
determination therefore is smaller than 20% of the amount
present; the latter varies between 0.05% and 0.80% w/w [2].
In most cases, removing samples from irregular glass
fragments did not present a problem; in the case of more
precious objects, only small fragments were removed; since
the sample preparation procedure for EPMA analysis is
destructive, rare objects such as glass beads could not be
sampled.
From the set of 706 fragments, a subset of 119 samples
was selected for analysis. Among this subset were fragments
of 26 beakers of the 15th century, of 86 15th century
window glass panes, of 2 15th century beads and of 5
beakers of the 16th–early 17th century.
4. Results and discussion
4.1. Glass compositions encountered
By using the criteria given in Table 3, the set of analyzed
fragments could be divided into three groups: sodic glass (6
samples), calco-potassic glass (113 samples) and lead glass
(1 sample). For K 2O the concentrations varied continuously
between 2.5% w/w and 16% w/w, while the Na 2O concentration
was below 4.5% w/w (for the calco-potassic glass) or
well above 8.5% w/w (for the sodic glass). Thus, in the
Na2O-concentration distribution, a gap between 4.5% and
8.5% is observed. Barrera and Velde [5] reported a similar
gap between 4% and 6% for their set of compositions.
According to these authors, sodic glass often features a Na2O
concentration higher than 10% w/w. The calco-potassic glass
fragments contained SiO 2 between 50% and 63% w/w, with
an average of 57%; the sodic glass showed a slightly higher
SiO2 abundance: 63–69% with an average of 65% w/w.
Among the analyzed fragments, only two samples
containing a small amount of PbO (between 2% and 3%)
O. Schalm et al. / Spectrochimica Acta Part B 59 (2004) 1647–1656
were present. During the analysis of archaeological vessel
glass finds excavated in Antwerp (a city in the low countries
some 150 km distant from Raverszijde), small admixtures of
PbO (between 2% and 7%) were sometimes found as well
[7]. The occurrence of such small amounts of PbO therefore
does not appear to be exceptional; however, the relatively
low PbO concentration is not sufficient to classify these
fragments as lead glass.
The 15th century beakers and the great majority of the
window glass panes were made in a calco-potassic glass,
except for one sodic window glass fragment. The group of
analyzed window glass fragments contained three dark blue,
one light blue, one purple and one flashed (red) glass
fragment; however, these intentionally colored samples do
not have a composition which distinguishes them from more
clear, not intentionally colored, glass fragments.
Two beads were also analyzed and their composition is
listed in Table 4. A yellow bead originating from the piece
of cloth was made of lead glass (with 31% SiO2 and 68%
PbO as major constituents), a composition that rarely
occurred in the dwood and plant ashT period. Lead glass
not only has a high density and a low melting point, but also
a high refractive index, giving rise to brilliant colors [11].
The other bead analyzed was colorless and made of Vitrum
Blanchum glass. This artifact is the only object of sodic
glass composition that can be associated with the medieval
town of Raverszijde.
The 16th or early 17th century vessel glass fragments
that were analyzed were all made with a colorless sodic
glass or a colorless mixed-alkali glass, except for one object
that was made with a colorless calcic glass. This object also
featured a higher SiO2 concentration than the 15th century
calco-potassic glass fragments.
In many cases, the concentrations of the oxides present
above 1% w/w, introduced via the fusing agents, are the
most useful for grouping sets of glass fragments according
to their chemical composition. When the Na2O, K2O, MgO
and CaO concentrations were used to classify the potassocalcic
and sodic glass groups by means of hierarchical
clustering, the different compositional classes listed in
Table 5 were obtained.
4.1.1. Calco-potassic glass
The group of calco-potassic glass could be divided in a
large group of glass rich in CaO (Group 1, 104 analyzed
samples, having an average CaO content of 21.8F0.2% w/
w) and a second group rich in K2O (Groups 2a–2c, 8
samples). The compositional variation within Group 1
appeared to be too small to allow for a meaningful
Table 4
Major composition of the two beads in % w/w, as determined by EPMA
Color Na2O MgO Al2O3 SiO2 P2O5 SO3 Cl K2O CaO MnO Fe2O3 BaO PbO
Bead 1 Yellow 0 0 0.06 31.4 0 0 0 0 0 0 0.22 0 68.4
Bead 2 Colorless 14.6 2.3 3.8 65.4 0.43 0.23 0.77 3.8 7.2 0.70 0.71 0.21 0

Table 5
Average composition in % w/w of the different classes within the calco-potassic and sodic glass
Calco-potassic glass (113) Sodic glass (6)
Group 1 (104) Group 2a (4) Group 2b (3) Group 2c (1) Group 3 (1) Group 4 (1) Group 5a (2) Group 5b (2)
Na2O 1.83F0.08 2.9F0.7 1.0F0.1 0.6 8.9 11.2 12.7F0.2 15.0F0.4
MgO 3.83F0.07 7.2F0.3 5.9F0.2 3.5 1.90 4.3 2.9F0.2 3.0F0.8
Al2O3 3.1F0.1 1.7F0.1 1.2F0.1 1.0 1.04 2.14 1.6F0.2 3.0F1.4
SiO2 57.3F0.2 51.4F0.6 61.0F0.8 59.7 68.8 62.5 63F0.1 65.1F0.2
P2O5 2.21F0.05 3.5F0.3 2.21F0.03 1.02 0.09 0.26 0.13F0.02 0.3F0.1
SO3 0.23F0.02 0.16F0.08 0.11F0.08 0.44 0.31 0 0.29F0.02 0.30F0.08
Cl 0.34F0.01 0.24F0.01 0.41F0.03 0 0.47 0.58 0.48F0.08 0.7F0.1
K2O 6.6F0.2 13.5F0.3 15.1F0.8 14.7 11.6 5.3 8.8F0.6 4.0F0.3
CaO 21.8F0.2 14.5F0.5 11.8F0.3 16.9 6.24 9.8 8.7F1 7.9F0.7
MnO 1.34F0.07 2.0F0.9 0.57F0.05 1.25 0.32 0.52 0.42F0.03 0.5F0.2
Fe2O3 0.80F0.02 1.6F0.6 0.49F0.08 0.40 0.32 0.92 0.73F0.24 0.5F0.2
BaO 0.57F0.02 0.38F0.03 0.20F0.1 0.61 0.20 0.16 0.24F0.07 0.15F0.06
PbO n.d. n.d. n.d. n.d. n.d. 2.36 n.d. n.d.
The number of member fragments is indicated between brackets.
n.d.: not detected.
subdivision of this set into smaller clusters. The glass from
Group 2 contains somewhat more Na 2O and MgO than the
glass from Group 1.
The average composition of Group 1 corresponds to the
Calcic glass A composition described by Wedepohl [1], a
glass assumed to be made with beechwood ash and lime as
fluxing agents. For instance, the K2O, CaO and MgO
contents of Group 1 strongly resemble to that of glass
found at glass workshops in Thuringia [12] of the 15–16th
century, although the average sodium content of the
German glasses is higher (around 3% w/w). The glass of
Group 2 corresponds to Wedepohls Calcic glass B, made
with beechwood ash only or with a mixture of beechwood
and fern ash. In the analyzed set, no samples made with
O. Schalm et al. / Spectrochimica Acta Part B 59 (2004) 1647–1656 1651
Potassic glass C, made with fern ash only, were
encountered. In terms of its K 2O, MgO and CaO content,
it also resembles the high Mg/high K glass type that is was
found in Period III in France by Barerra and Velde and
attributed to early (14th century) glass production in the
Parisian and Western areas of France, such as Normandy
and Orleans [5,13].
In Fig. 2, where the concentration ratio (Na2O+MgO)/
CaO is plotted against CaO/(K 2O+CaO), the distinction
between Groups 1 and 2 is highlighted. The two ellipses
indicate the boundaries of Wedepohls Calcic Glass types A
and B. This plot shows that there is no relation between
glass type, shape and chemical composition among the
Raversijde finds.
Fig. 2. Plot of CaO/(K 2O+CaO) versus the (Na 2O+MgO)/CaO concentration ratios for the different types of calco-potassic vessel glass and window glass
excavated in Raverszijde.

1652
Fig. 3. Scatter plot of the Na 2O versus Cl concentrations. The line in the graph is only a visual aid to indicate the correlation.
A negative linear correlation between K2O and CaO is
observed, since these are the principal oxides in the ashes
employed for glassmaking. Such correlation was not found
for any other combination of detected constituents, except
for Na 2O and Cl. Their relation is shown in Fig. 3,
indicating that sodium has been introduced into the calcopotassic
glass by minute amounts of NaCl. According to
Gerth et al. [14] in studies of German glasses, the salt was
added in order to modify the viscosity of the glass. The
two encircled vessels which belong to Group 2 (see Fig. 3)
do not follow this tendency. To make the glass of these
two objects (a Maichelbecher and an octagonal Maichelbecher),
apparently another Na 2O source, poorer in Cl,
was used; other than that, there are no visual indications
that the Maigelbecher differs from the others of vessels of
the same type. The octagonal Maigelbecher, on the other
O. Schalm et al. / Spectrochimica Acta Part B 59 (2004) 1647–1656
hand, has a much finer surface finishing, suggesting that
this object may have belonged to a more expensive class
of glass artifacts.
4.1.2. Sodic glass
In view of the limited number of sodic glass compositions,
the classification shown in Table 5 in this case only
serves to highlight the fact that several Na-rich glass
compositions are represented at the Raversijde site, albeit
through one or two of the analyzed fragments only. The
composition labelled bGroup 3Q in Table 5 corresponds to a
mixed-alkali glass, i.e., a glass composition where the
concentrations of Na 2O, K 2O and CaO do not differ very
much from each other and are close to 10% w/w; the object
in Group 4 is separate from the other sodic glass artifacts
since it contains ca 2.4% w/w of PbO. Finally, Group 5
Table 6
Average composition of 15th century window glass and 15th century vessel glass, divided in calcic glass type A, calcic glass type B and sodic glass
Window glass Vessel glass
Type A (80) Type B (5) Sodic (1) Type A (23) Type B (3)
Na2O 1.9F0.1 1.2F0.2 11.2 1.52F0.08 3.3F0.8
MgO 3.88F0.07 6.4F0.3 4.3 3.5F0.1 7.4F0.4
Al2O3 2.9F0.1 1.3F0.1 2.1 3.7F0.3 1.7F0.2
SiO2 57.7F0.3 59F1.9 62.5 55.7F0.5 51.4F0.9
P2O5 2.08F0.06 2.3F0.3 0.3 2.65F0.07 3.7F0.4
SO3 0.24F0.02 0.11F0.05 – 0.21F0.08 0.20F0.1
Cl 0.35F0.02 0.35F0.04 0.6 0.30F0.02 0.24F0.01
K2O 6.5F0.2 14F1.2 5.3 7.2F0.2 13.6F0.4
CaO 21.8F0.2 14F1.1 9.8 21.7F0.3 14.2F0.5
MnO 1.12F0.06 1.5F0.8 0.5 2.1F0.2 1.7F0.2
Fe2O3 0.81F0.03 0.56F0.06 0.9 0.77F0.04 1.9F0.7
BaO 0.53F0.03 0.30F0.09 0.2 0.63F0.06 0.35F0.02
PbO – – 2.4 – 0.9F0.9

consists of objects made in Venetian Vitrum Blanchum or in
Façon-de-Venise glass, as found, e.g., in Antwerp in the
16th century [7].
4.2. Relation between glass composition and artifact type
In Table 6, the average composition of a number of
homogeneous shape/composition groups are listed. Both for
the vessel glass objects and for the window glass panes, a
distinction was made between calco-potassic glass of types
A and B.
It is clear that the vessel glass of Calcic type B is
somewhat richer in Na2O than the equivalent window glass,
although both groups feature a similar concentration of
MgO. Probably, in these vessels, sodium was introduced by
the use of a small amount of sodium-rich ash, hence the lack
of correlation between the Na 2O and Cl concentrations in
two of the three samples in this group (see Fig. 3).
The vessel glass of Calcic type A has a somewhat
higher MnO content than the equivalent window glass. It
is not unlikely that during the production of vessel glass of
Calcic type A, an additional amount of MnO2 (pyrolusite,
also known as glassmakers soap) was introduced into the
melt in order to decolorize the glass, something which
probably was not commonly done in the case of the
window glass.
4.3. Comparison with glass compositions excavated in
Antwerp
4.3.1. Vessel glass
In previous studies [15,16], analyses were performed on
a large series of 15–17th century glass remains excavated in
O. Schalm et al. / Spectrochimica Acta Part B 59 (2004) 1647–1656 1653
Table 7
Relation between typology and the type of glass
Vessel Type Catalogue
number
Glass composition
Bossed beaker 99 Vitrum Blanchum,
Group 5b
Bossed beaker 100 Façon-de-Venise,
Group 5a
Vetro-a-fili glass 102 Façon-de-Venise,
Group 5a
Colorless beaker
107 Calcic glass,
on foot
Group 1
Colorless beaker
on foot
All beakers were made in colorless glass.
the historical centre of the city of Antwerp, located at a
distance of ca. 150 km from the Raversijde site. The
composition of the calco-potassic vessel glass objects
excavated in Raversijde was compared with those recovered
in Antwerp, as shown in Fig. 4.
The vessel glass of Antwerp can clearly be divided into
three different compositional groups, coinciding with
Wedepohls Groups A–C. Among the Antwerp vessel glass,
Ca-rich Group A consists of 15th century Maigeleins and
Maigelbechers and of 17th century Roemers; Group B
contains the 16th century prunted beakers, Berkenmeiers
and Roemers while Antwerp Group C comprises the late
16th and 17th century colorless beaker types and 17th
century goblets [17].
It is striking to observe that almost all the 15th century
beakers of Raversijde have compositions that coincide with
that of Antwerp Group A; only three beakers belong to
Antwerp type B. This means that the Maigeleins and
Maigelbechers of Raversijde and Antwerp have a similar
Fig. 4. Scatter plot of K2O versus CaO for vessel glass artifacts excavated in Antwerp and Raversijde.
108 Mixed-alkali glass,
Group 3

1654
composition. However, the Raversijde compositions in both
Groups A and B contain somewhat higher amounts of K2O.
The group of analyzed late 16th–early 17th century
beakers were all made of colorless glass. Table 7 provides
an overview of the glass types and compositions encountered.
Remarkable is that the Vitrum Blanchum, Façon-de-
Venise and the mixed-alkali glass all contain somewhat
more K 2O than their equivalents excavated in Antwerp. The
window glass fragment of Group 4 has a composition
O. Schalm et al. / Spectrochimica Acta Part B 59 (2004) 1647–1656
Fig. 5. Scatter plot of the K 2O versus the CaO concentrations for 15th–17th calco-potassic window glass excavated in different locations in Belgium.
resembling that of Façon-de-Venise glass but with a small
admixture of PbO. One bead was also identified as having a
Vitrum Blanchum composition (Group 5b).
4.3.2. Window glass
In Fig. 5, the relation between the CaO and K2O content
of the Raversijde window glass samples is compared to
that from similar artifacts uncovered in several other
(urban) archaeological sites in Belgium. In total, 396
Fig. 6. The frequency distribution of K 2O for the 396 calco-potassic window glass compositions shown in Fig. 5 can be described by two Gaussian components
centered at 6.0% and 11.0% w/w, respectively.

window glass compositions, dated to 15th–17th century
contexts, are compared here. No clear difference between
the 15th century window glass of Raversijde of Calcic
glass type A and the 16th–17th century window glass from
the other cities is visible; as is the case in Raversijde, also
in the other investigated sites, the majority of the window
glass belongs to Calcic glass type A, with only a few
fragments featuring a composition that resembles to that of
Calcic glass type B.
Since the major difference between Wedepohl’s Calcic
glass types A and B is the K2O concentration, the frequency
distribution of this oxide was calculated for the abovementioned
set of 396 window glass compositions (see
Fig. 6). The distribution is dominated by a component
having a K2O content of 6.0F1.5% w/w, corresponding to
Calcic glass type A, and a smaller component having an
average K 2O content of 11F2% w/w, corresponding to
Calcic glass type B. The abundance ratio between these two
glass types is approximately 11:1 overall. It is remarkable
that the window glass of Raversijde contributes in a
comparatively strong manner to the frequency distribution
of Calcic glass B.
The results summarized in Fig. 6 show that with respect
to the production of commodity glass in the 15th–17th
century, the geographical region of Belgium can be
considered as one technological area, although it is possible
that during the 15th century Calcic glass type B is somewhat
more abundant than in the 16th–17th century.
5. Conclusions
In this work, the chemical composition of 119 samples
was determined by means of quantitative EPMA. Except for
the differences between calco-potassic glass, sodic glass and
lead glass, and the distinction between calcic glass with a
low amount of K2O (i.e., type A) and calcic glass with a
higher amount of K2O (i.e., type B), no groups could be
identified within the set by means of hierarchical clustering.
However, several subtle fluctuations were observed. The
samples were divided in several groups according to the
former function, age and the chemical composition of the
artifacts.
The set of 15th century beakers (26) could be divided in
23 samples of calcic glass type A (K2O: 7%, CaO: 22%)
and 3 samples of calcic glass type B (K2O: 14%, CaO:
14%). The calcic glass type B contains somewhat higher
amounts of Na2O and MgO than type A. For two samples
of type B, the amount of sodium did not follow the relation
between Na 2O and Cl that was noticed for the calcopotassic
glass. This is an indication that a small amount of
sodium-rich plant ash or recuperated sodic glass was
introduced to the batch instead of sea salt. Remarkable is
that the only analyzed vessel glass with a very fine
finishing was identified as calcic glass B with a sodium
source different from sea salt. This suggests that next to
O. Schalm et al. / Spectrochimica Acta Part B 59 (2004) 1647–1656 1655
ordinary quality glass objects, also more luxurious objects,
made with glass of slightly different composition, were
available.
With the set of 15th century window glass panels (86),
one sample was identified as being made in sodic glass with
a small amount of PbO. The rest of the window glass was of
Calcic glass type A (80) and Calcic glass type B (5).
Window glass and vessel glass were similar in composition,
except for type B. Although the MgO content for vessel
glass and window glass type B was similar, there was a clear
difference in the concentration of Na2O. Vessel glass type B
contained somewhat more Na2O (3.3% instead of 1.5%),
indicating that an additional sodium-rich ingredient was
used in their production. From the comparison of 15th
century window glass of Raversijde with that of other cities
in Belgium, it could be concluded that the period between
the 15th and the 17th century can be considered as one
technological period. Calcic glass of type A strongly
dominates glass of type B, with an approximate abundance
ratio of 11:1. The measurements also suggest that Calcic
glass type B was more common in the 15th century than in
the 16th and 17th century.
The chemical composition of the 15th century beads (2)
was completely different from the rest of the 15th century
glass. One yellow bead attached to a piece of cloth was
made of lead glass. The other bead was a colorless blown
bead. It was made of Vitrum Blanchum and is likely to have
originated from Venice.
The 16th–17th century vessel glass (5) contained a much
higher diversity in chemical composition than the 15th
century vessel glass. The analyzed set consisted of 2 Façonde-Venise
glass objects, 1 Vitrum Blanchum, 1 Calcic glass
type A and 1 mixed-alkali object.
It can therefore be concluded that both in the 15th
century, where the only glass that was available was
utilitarian in nature, had a greenish dWaldglasT color and a
calcic composition, as well as in the 16th century, where
also more luxurious vessels of colorless soda-glass were
available, citizens of the relatively small town of Raversijde
could afford to purchase these products and use them in
their households. Apart from the artifacts made in sodic
glass, of which the provenance can be established because
of the existence of specific chemical compositions, the
origin of majority of the glass fragments cannot be
established, in view of the high degree of composition
similarity among the calcic glass fragments found in
Raversijde and other centres in Belgium and the surrounding
regions.
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