short communications
310 doi:10.1107/S0909049508043082 J. Synchrotron Rad. (2009). 16, 310–313
Journal of
Synchrotron
Radiation
ISSN 0909-0495
Received 13 October 2008
Accepted 17 December 2008
# 2009 International Union of Crystallography
Printed in Singapore – all rights reserved
A note on medieval microfabrication: the
visualization of a prayer nut by synchrotron-based
computer X-ray tomography
Peter Reischig,a,b Jorik Blaas,a Charl Botha,a Alberto Bravin,c Liisa Porra,c
Christian Nemoz,c Arie Wallertb and Joris Dika*
aDelft University of Technology, Delft, The Netherlands, bRijksmuseum, Amsterdam, The Netherlands, and
cEuropean Synchrotron Radiation Facility (ESRF), Grenoble, France. E-mail: j.dik@tudelft.nl
One of the most fascinating objects in the Rijksmuseum (Amsterdam, The
Netherlands) is an early 16th century prayer nut. This spherical wooden object
measures 4 cm in diameter and consists of two hemispheres connected with a
small hinge so that it can be opened. The interior of the nut holds wood carvings
with scenes from the life of Christ. These miniature reliefs show an incredible
degree of finish with carving details well beyond the millimetre scale. In the
present paper it is shown how synchrotron-based computer X-ray tomography
revealed the structure and fabrication method of the bead. The central part of
the relief was cut from a single piece of wood, rather than assembled from
multiple components, underlining the extraordinary manual dexterity of its
maker. In addition, a piece of fibrous material contained in the inner structure
of the bead is revealed. This may have served as a carrier for an odorous
compound, which would be in line with the religious function of the prayer nut.
Keywords: X-ray tomography; tomography reconstruction; cultural heritage;
prayer nut; wood.
1. Introduction
In medieval Europe religion was an omnipresent aspect in daily life.
The 15th century showed a strong tendency towards private worship.
The Christian worshipper tried to empathize with Christ by privately
meditating on His sufferings. All kinds of attributes, usually with
depictions of Christ’s passion, were used as support during prayer and
meditation. A common example is the traditional rosary, a string of
beads for counting when engaged in repetitive prayers (Winston-
Mien, 1997). Prayer nuts were sometimes attached to such rosaries.
These prayer nuts are a much more precious and luxurious type of
prayer item or devotionalia. Prayer nuts consist of two hemispheres
connected with a hinge and a catch, so that it can be opened during
prayer. The interior is decorated with refined wood carving of biblical
scenes. Their miniature format nicely reflects the personal and inti-
mate character of 15th century religious life (Falkenburg & Scholten,
1999).
The Rijksmuseum in Amsterdam owns a beautifully preserved
prayer nut from the early 16th century (Fig. 1). The nut is made of
boxwood and measures 4 cm in diameter. The outer part of the
hemispheres is decorated with a complex pattern with a Gothic motif,
while the interior shows the crucifixion and Christ carrying the cross.
Around the edge of both scenes one can read two Latin inscriptions,
commemorating the redemption through Christ. The scenes have
been carved with an exuberant virtuosity and an amazing attention to
detail. On a few cm2 the artist succeeded in recreating the crucifixion
of Christ with no less than 13 figures, three crosses, five horses and
two pikes. Despite this crowd, however, the illusion of depth in the
relief is very convincing. This is partly achieved by upscaling the
figures in the front and downscaling the figures in the back. In
addition, the scene is staged in three consecutive planes. In the front
we see various lamenting bystanders, behind them a line of horsemen,
partially viewed on their back, while the third plane shows again a
frontal view on the crucified figures. This sequence adds to the spatial
illusion, but also raises the question of how this miniaturized three-
Figure 1
Boxwood prayer nut with Christ carrying the cross and the crucifixion, c. 1515,
Rijksmuseum, Amsterdam, The Netherlands.

dimensional puzzle was made. Given the ingenious construction of
the illusionary space with its multiple planes and figures, would the
actual making of the object also depend on the assembly of different
components? Would such components be carved piece by piece, then
assembled and finally integrated in the nut? In order to answer these
questions we decided to perform an X-ray tomography experiment of
the prayer nut shown in Fig. 1. Our aim was to (i) obtain an overview
of the inner structure of the bead and (ii) hopefully shed light on the
construction of this delicate piece of late-medieval microfabrication.
2. Methods and techniques
Tomographic images of the bead were recorded using monochro-
matic X-rays available at the ID17 biomedical beamline of the
European Synchrotron Radiation Facility (Grenoble, France). The
set-up is described elsewhere in detail (Fiedler et al., 2004; Nemoz et
al., 2007). Briefly, the source of synchrotron radiation at ID17 is a
symmetrical multipole wiggler using a 70 mm gap. The fixed-exit two-
crystal Si(111) monochromator in Laue (transmission) geometry is
located at a distance of 141.5 m from the source (Suortti et al., 2000).
The beam energy E was 30 keV. The bandwidth of the monochro-
matic beam was
E/E’ 5
104 in this setting. The sample is placed
10 m downstream from the monochromator on an optical table, and it
can be rotated in the fan beam about a vertical axis for computed
tomography imaging. The distance between the sample and the
detector was 2.5 m. A scheme of the set-up is shown by Fiedler et al.
(2004). The high-resolution detector used in this study is called a
FReLoN CCD camera (Bravin et al., 2003). It has an active input
surface of 94
94 mm, where the incoming X-rays are converted
by a 100 mm-thick standard mammographic phosphor screen
(Gd2O2S:Tb, 5 g cm3 density) to visible light, which is then guided
by tapered fibre optics onto the CCD array of 2048
2048 pixels. By
this reduction an effective pixel size of 47
47 mm is achieved, and
the resolution is about 10 lines mm1 at the 5% level of the modu-
lation transfer function. The detective quantum efficiency is 0.3 at
33 keVand zero frequency (Coan et al., 2006). The height of the beam
is 0.7 mm so that on the detector only a few horizontal lines are
illuminated. For obtaining each tomographic slice, 1440 projections at
0.25 degree interval are recorded; after flat-field normalization,
images are reconstructed using a standard filtered back-projection
algorithm (Hamming filter).
The reconstruction of the top half of the object resulted in 667
slices, each of resolution 1133 (fixme) pixels squared. The unu utility,
part of the TEEM 1 software toolkit, was used to convert the recon-
structed slices to a single volume data set of 3.2 Gbytes. The recon-
structed slices form a volume describing the densities of the prayer
nut in a rectangular grid of discrete positions. To facilitate further
processing of the data volume, the values were quantized to an 8-bit
form, reducing the volume size to 816 Mbytes, which allowed it to fit
into the RAM of our workstation. We also used the unu utility to
create multi-planar reconstructions (MPRs), i.e. interpolated slices of
arbitrary orientation through the volume. The MPRs can be used to
inspect details of the interior structure. We made use of the Miter
direct volume rendering software, also part of the TEEM toolkit,
along with our own enhancements, to create interactive three-
dimensional renderings of the prayer nut. The volume renderer was
set up to show a single surface layer, representing the boundary
between wood and air. For this, a threshold density value was chosen
based on a histogram of all density values in the volume data set. The
volume renderings were used to obtain a more high-level overview of
the characteristics of the nut and to identify interesting features.
Based on this localization, the MPRs were used to zoom in on and to
study the identified positions. The volume renderings were also more
suitable for showing surface details. Finally, we produced cut-away
views to expose the internal structure of the object in context.
3. Results
Fig. 2 shows different two-dimensional sections along three axes
through the prayer bead. The corresponding planes are indicated in
translucence in the photograph, except for the section parallel to the
plane of view (e). Fig. 3 contains volume reconstructions projected at
different angles to enhance the perception of depth. The upper row
shows the inside of the bead with a vertical cut, so that some features
from Fig. 2 are included. The lower line shows the outer decoration of
the bead. Video animations of both reconstructions are included in
the digital depository of this journal.2
4. Discussion and conclusion
The sections in Fig. 2 reveal the shell structure of the bead. The object
consists of an outer hemisphere into which the inner wood carving
has been placed. Both parts are joined with two pins on the left and
right (not visualized). The back of the inner part is held in position by
a small pin [see section (d )]. Note that the relief has been cut from a
single piece. No interfaces could be detected that would indicate
some form of joining. Instead, the relief shows a continuous pattern
of year rings [(a) and (e)], which also applies to the outer shell. We
short communications
J. Synchrotron Rad. (2009). 16, 310–313 Peter Reischig et al.  Medieval microfabrication 311
Figure 2
Sections through all three axes of the prayer nut; the approximate position of the
sections is indicated in translucence. Section (e) runs parallel to the plane of view in
the photograph and is therefore not indicated.
1 http://teem.sourceforge.net/.
2 Supplementary data for this paper are available from the IUCr electronic
archives (Reference: GF5017). Services for accessing these data are described
at the back of the journal.

only noted two types of additions to the relief. First, the crosses and
pikes have been cut separately and were then added. A horizontal
section through the middle of the relief shows submillimetre drill
holes into which the crosses were placed (b). Furthermore, the upper
section of the relief, i.e. the arc above the crucifixion scene, has been
cut from another separate piece of wood. This was already noticed by
visual inspection (Fig. 1), but also becomes clear from cross sections
(c) and (d), where we notice the joints and a slightly increased
absorbance of this separate component. Thus, the relief has been
composed from four major parts: the outside shell with its Gothic
pattern, the inside relief of the crucifixion, smaller details like the
crosses and pikes, and finally the arc above the crucifixion scene. The
latter was taken away in order to have a large angle access to the
relief. This is necessary to carve figures with undercuts and to drill
holes from above.
Unfortunately, there are not many contemporary sources on the
production of prayer nuts. We do know that the making of rosaries
was a specialized craft in 15th century Europe with major production
centres in Southern Germany and Flanders (Winston-Mien, 1997).
These craftsmen were called paternosters, referring to the repetitive
prayer of ‘Our Father’, for which rosaries were used. Fig. 4 shows such
a paternoster at work. We see how a hollow semicircular drill is
placed on a wooden block in order to prepare multiple hemispheres.
The same block of wood would then be worked in an identical
manner from the back in order to create perfect orbs. A larger
globule would have been made for the prayer nut, which was then cut
in half and both sections were then hollowed out. The outer
decoration and the inner relief would have been made with an array
of small drills, chisels and knives. We suspect that the artist must have
used some form of optical magnification for the micro-carving,
probably a lens, which would have been available in the early 16th
century.
In between the inner and outer shell we notice a rather small
compartment in the tomography sections [Fig. 2, (a)–(d)]. Note how
the outer casing has an open structure with numerous holes. The top
of the outer hemisphere also shows a hole,
inside which a woven rope-like structure can
be seen with a thick knot in the bead’s
interior. The disentangled strings at the end
indicate the fibrous nature of the material. It
is suspected that the string, which has
broken off inside the drill hole, was origin-
ally used to attach the bead, e.g. to a rosary.
The knot remains hidden from the outside
and cannot be seen through the cavities of
the outer shell, as shown in the three-
dimensional volume reconstruction of the
sectioned bead.
Given the open structure of the casing, the
fibrous material of the rather large knot may
have also served another purpose. It has
been suggested that fragrances were some-
times enclosed in prayer nuts (Falkenburg &
Scholten, 1999). Also, in this case, the object
may have served as a so-called pomander
casing. Pomanders were generally made by
softening resinous substances and mixing
them together, often with dirt or clay, or
wax. These pomanders were often carried in
open boxwood cases, with piercings, or
carvings to let the scent out. Some pomander
cases even had sections for several different
scents of pomanders, as well as compartments for a sponge soaked in
aromatic vinegars. In a more modest form, simple pierced cases or
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312 Peter Reischig et al.  Medieval microfabrication J. Synchrotron Rad. (2009). 16, 310–313
Figure 4
Contemporary medieval depiction of the production of prayer beads, Hausbuch
(Amb. 317. 2, fol. 13r), Stadtbibliothek, Nuremberg, Germany,
Figure 3
Upper row: volume reconstructions with a vertical cut through the middle of the nut, revealing the shell structure
as well as the knot sandwiched between the outer and inner shells. Note the openings in the outer shell. Lower
row: volume reconstruction of the outer shell with Gothic motif.

just hollowed-out fruit were stuffed with herbs and spices (Clarkson,
1939). The presence of these odiferous substances would literally
increase the spiritual experience of the worshipper during prayer.
The fibrous material of the knot may have been the carrier for such
odorous compounds that would have been released through the open
structure of the outer shell. Pending further analysis, this suggestion
could not be corroborated.
However that may be, this examination has shown that synchro-
tron-based X-ray tomography can be applied successfully to study the
fabrication methods of medieval prayer nuts in a non-destructive
manner. It would therefore be worthwhile to examine more of such
objects and eventually determine developments in manufacturing
and compare prayer nuts from different workshops.
We would like to thank Robert van Langh and Frits Scholten from
the Rijksmuseum in Amsterdam for their support of this project.
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J. Synchrotron Rad. (2009). 16, 310–313 Peter Reischig et al.  Medieval microfabrication 313