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[Information found in the previously
posted papers on Stradivari's varnish analysis and historic bass-bars
may be found in my new book, Stradivari, published by Cambridge
University Press in 2010. The paper posted below was given at the
International Workshop on Diagnostics and Preservation of Musical
Instruments, University of Bologna, Ravenna, May 20-21, 2010. An earlier
version of this paper was published in the Newsletter of the Michigan
Violinmakers Association, April, 2010.]
Dendrochronology: Tool of Truth or Deception?
By
Stewart Pollens
2010
Dendrochronology,
which may be defined as the dating of the year rings of wood, has only
recently been employed in the dating of violins. In 1953, Lottermoser
and Meyer attempted a relative dating of Italian stringed instruments by
comparing the year-ring patterns of three violins, though actual dating
was not achieved until the 1980s by Corona, Schweingruber, and Klein.[i]
In 1994, when I helped organize the Guarneri del Gesù exhibition at the
Metropolitan Museum of Art in New York, I invited one of the world’s
leading dendrochronologists, Dr. Peter Klein of the Ordinariat für
Holzbiologie of the University of Hamburg, to date the spruce tops of
the twenty-five violins assembled for that exhibition. Among the
conclusions gleaned from Klein’s findings are that Giuseppe Guarneri
apparently did not use well-aged wood in making his instruments (the
last datable ring is often just a few years earlier than the date on the
violin’s label), and that he occasionally used mismatched pieces for the
tops (whether this was by design, disinterest, or carelessness remains a
matter of conjecture).
I had met Dr. Klein many years before the Guarneri exhibition, for in
the 1980s and 1990s, he was often invited to the Metropolitan Museum of
Art to date panel paintings, and I would buttonhole him in the staff
cafeteria and ask him to date the wood of one or another instrument that
I was then studying or restoring, including violins, viols, lutes,
guitars, and harpsichords. I never had any reason to question his
findings, which were always in agreement with my assessments. A few
years after the Guarneri exhibition, I employed his services in dating
the wood used to make the “Messiah” violin. Though he initially
determined that the last datable year-ring of that instrument was 1738,
which postdated Stradivari’s death, members of the violin trade
maneuvered him into temporarily retracting his findings in exchange for
an opportunity to re-measure the “Messiah’s” year-rings at the Ashmolean
Museum; however, because of a dispute that developed during that
session, he was unable to leave the Ashmolean with his measurements.
Klein now declines to date violins, which is a great loss to those of us
who formerly relied upon his expertise and objectivity.
Then
John Topham, and subsequently, Dr. Henri Grissino-Mayer entered the
fray. Though lacking formal training in dendrochronology (in the case of
Mr. Topham), as well as experience in the dating of objects (Grissino-Mayer
is primarily involved in the use of dendrochronology in the study of
climate history) and, most importantly Klein’s proprietary master Alpine
spruce chronology, Topham and Grissino-Mayer claim to have
re-established the “Messiah’s” authenticity by employing what they call
a “floating” chronology derived from the “Archinto” and
“Kux-Castelbarco” Stradivari violas. Unfortunately, their chronology
only extended to 1685, and as dendrochronologists Angelo Mondino and
Matteo Avalle pointed out, this did not permit Topham and Grissino-Mayer
to discover a more compelling statistical match further along in time.[ii]
In 2005 Mondino and Avalle asserted that the last year-ring of the
“Messiah” grew in 1788, fifty-one years after the death of Antonio
Stradivari, though in a more recent study, they claim to have discovered
an even later date, 1844.[iii]
Disturbed by the multitude of conflicting dates, I decided to delve more
deeply and directly into this dating technique in the hope of learning
more about it and, perhaps, determine why dendrochronologists have
arrived at so many different dates for the “Messiah.” In 2007 I
purchased the Synchro Search computer program and the accompanying
Manual of Dendrochronology Applied to the Dating of Musical Instruments
written by Angelo Mondino and Matteo Avalle (Cremona Books, 2005).
Synchro Search is a graphing and statistical program for comparing
year-ring data with established master spruce chronologies. The program
disk includes a number of master chronologies that are in the public
domain, including the Ötztal-Obergurgler, Siebenlist-Kerner Ötztaler,
Obersaxon-Meierhof, Schweingruber, and Malcolm chronologies, which are
among the chronologies that have been used in dating violins. In using
the Synchro Search program, the graphed year-ring dimensions of the
violin in question can be advanced with the click of a computer key
against graphs of any of the master chronologies until a match is
found. As one proceeds, the program automatically computes statistical
data, including Gleichläufigkeit (similarity or G index),
Pearson’s r correlation coefficient (see below), Student’s t,
mean sensitivity, serial correlation coefficient, overlapping index, and
linear regression. The program can also be set so that the computer will
locate the best matches, which is very convenient when master
chronologies are hundreds of years in length.
In my
investigation of this dating procedure, I did not wish to muddy the
“Messiah” waters any further, so I decided to use Synchro Search
in an attempt to establish the age of a large (17 ½”; 444 mm) viola,
apparently Brescian, that the late Jacques Francais attributed to
Gasparo da Salò (1540-1609). Making year-ring measurements of an old,
heavily restored violin or viola top can be difficult. There may be
“patches coming through” made with transplanted or later wood, painted
year-rings, as well as dents, scratches, and heavily discolored or
retouched varnish that obscure the grain. One must be aware of seams,
wings, and grain direction. If even a single year-ring is missed, this
can prevent the discovery of a match with the master chronology, so one
must be very careful in measuring. At that time, Klein used a little 7X
loupe fitted with a metric scale that came in contact with the surface
of the instrument. The advantage of this device is that it can be
carried around in a pocket; the disadvantage is that it must be pushed
along the surface of the instrument as one moves from year-ring to
year-ring. When using a loupe, it is necessary to call out the
measurements to a recording secretary, and this is another
inconvenience. I devised a piece of equipment that is more accurate and
convenient to use—unfortunately, it is cumbersome. It consists of a
low-power stereo microscope (mine is an Amscope 7-45X on a boom stand)
with one eyepiece fitted with a cross-hair reticule. Below the
microscope, a violin/viola cradle (International Violin Co. #T-97) is
attached to a 250 mm rack-and-pinion linear translation stage (Edmund
#R56-796) that is linked via a brass angle bracket to a 12”-300 mm
Mitutoyo Digimatic linear scale with SPC output (see figure). To use
this device, I clamp the instrument (with its strings, bridge, and
tailpiece removed) in the cradle and start by aligning the earliest
year-ring (which is typically the ring at the edge of the lower bout of
the instrument—though one should carefully check the year-ring shading
to make certain of the orientation) with the cross hair in the
microscope’s eyepiece. Using the rack-and-pinion of the translation
stage, I then carefully align the next year-ring against the cross hair
and press the button on the linear scale’s output device (see figure).
This enters the measurement into a Microsoft Excel spreadsheet, which I
have programmed to subtract the previous measurement from the last one
entered, so that I wind up with a column of year-ring widths. Most
violins have tops made with book-matched slabs of spruce, so one must
stop at the center joint. These tabulated widths should then be
multiplied by a factor of 100 so that the graphed data can be more
readily compared to the master chronologies (dendrochronologists work in
units of 1/100 mm, so 1 mm is expressed as 100).[iv]
The column of figures must then be copied out of Excel as a text file
(.txt file extension) so that the data can be “processed,” or compared
against any of the master chronologies that are supplied with the
Synchro Search program.
Returning to the Brescian viola: I compared my 74 measurements of
year-rings from the treble side of the instrument to the following
master chronologies: PCAB Giertz Obergurgler 1276-1974, LADE Siebenlist
Obergurgler 1511-1974, PCAB Schweingruber Obersaxen 1537-1995, and Media
46° 1276-1994. I employed a straightforward matching of the year-ring
sequence with these master chronologies and did not use indexing,
segmentation, or any of the other statistical manipulations that
dendrochronologists often resort to. Tabulated below are the final
dates for the viola’s treble-side sequence where there was 60% or better
Gleichläufigkeit (G index or similarity) with each of the master
chronologies. This was chosen as a cutoff point because it represents a
degree of probability of over 95%, and because this percentage of
agreement between the year-rings of an object and a master chronology is
sometimes considered a “match” in dendrochronological reports and
literature.[v]
The reader may be surprised by the number of “matches” that were found:
Final Year Gleichläufigkeit
PCAB Giertz Obergurgler
1350 62.3%
1417 61.6%
1445 65.7%
1463 65.7%
1500 61.6%
1522 62.3%
1540 60.2%
1554 64.3%
1572 65.7%
1583 63.0%
1600 62.3%
1618 61.6%
1732 63.6%
1756 62.3%
1758 65.7%
1839 63.6%
1863 62.3%
1901 65.0%
LADE Siebenlist Obergurgler 1511 62.3%
1665 65.0%
1680 62.3%
1704 63.0%
1732 63.0%
1750 60.9%
1792 60.2%
1810 60.9%
1826 63.0%
1826 65.7%
1834 65.7%
1949 65.0%
PCAB Schweingruber Obersaxen 1690
61.6%
1756 65.0%
1758 62.3%
1784 64.3%
1810 65.0%
1895 63.0%
1925 60.9%
1953 67.8%
1959 60.9%
Media 46°
1522 62.3%
1551 63.6%
1572 63.0%
1611 60.2%
1625 60.2%
1657 70.5%
1665 65.0%
1696 60.9%
1710 60.9%
1712 62.3%
1732 60.9%
1754 62.3%
1756 63.6%
At 60% Gleichläufigkeit or higher (representing a probability of
95% or better), 52 matches were found, and at 65% Gleichläufigkeit
or higher (representing a degree of probability of around 99% or
better), 13 matches were found (with final dates of the series ranging
from 1445 to 1953). It is also disconcerting that the PCAB Giertz
Obergurgler, LADE Siebenlist Obergurgler, and PCAB Schweingruber
Obersaxen master chronologies produced very few dates in common (1732
and 1756); the Media 46° chronology exhibited several concordances
(1522, 1572, 1665, 1732, and 1756), but this would be expected as it is
a composite derived from over thirty chronologies, including the other
three that I used. The 1657 date (which has the highest
Gleichläufigkeit of 70.5% and a probability of 99.9%) is of interest
because violas of this large size were still being constructed at this
time, though if correct, it would rule out Gasparo da Salo as the maker,
as he died in 1609. However, I should point out that I subsequently
discovered a date of 1805 with a considerably higher Gleichläufigkeit
of 75.3% using the CEBR Schweingruber Ceader Valley (Cipro) chronology.
I must admit that I did not initially seek a match with this master
chronology because its synchronization dates extend only from 1675
through 1981, and my inclination was to search for dates using master
chronologies that had earlier starting points. This only demonstrates
that a supposedly “objective” person, such as myself, may overlook a
master chronology because its inclusive years appear to fall outside the
date that one hopes or expects to find.
In
general, my experiences with the Alpine master chronologies supplied
with the Synchro Search program (which include the PCAB
Siebenlist-Kerner Ötztaler and PCAB Gierz Obergurgler chronologies that
were employed by Grissino-Mayer, et. al., in fixing a date to their
floating chronology when attempting to date the “Messiah”) have not
yielded overwhelmingly convincing matches with a number of Italian
instruments that I have tried to date.[vi]
The reason for this may be due to the fact that these master
chronologies were constructed from wood samples taken from Alpine
regions or altitudes that do not have precisely the same climatic
conditions as the locale where the Cremonese and Brescian makers
obtained their wood, which I believe was likely from the Italian Alps
directly north of Brescia. Many of the master chronologies in the public
domain are derived from wood found in the Ötztal and Obergurgl mountain
ranges, which are close to Innsbruck, Austria. It is also possible that
the Synchro Search program itself is at fault—some dendrochronologists
have been critical of certain of its statistical operations, including
its method of calculating Pearson’s r correlation coefficient and
Student’s t.[vii]
However, determining Gleichläufigkeit is fairly straightforward,
and it is often the only measurement cited in dendrochronological
reports, including those of Peter Klein.
In conclusion, I would strongly advise violin historians,
authenticators, appraisers, and purchasers to be extremely wary of the
dates that are being published in auction catalogs, certificates, and
scholarly journals.
[i]
W. Lottermoser and J. Meyer, “Über die Möglichkeiten einer
Dendrochronologie von altitalienischer Geigen,”
Instrumentenbau-Zeitschrift 12 (1958), pp. 295-297; E.
Corona, Ricerche dendrocronologiche su due violini del XVIII
secolo,” L’Italia For. e Mont., XXXV, 3 (1980), pp.
112-115; F. H. Schweingruber, Tree Ring Basics and
Application of Dendrochronology (Dordrecht, 1983); P.
Klein, H. Mehringer, and J. Bauch, “Tree-Ring Chronology of
Spruce Wood and Its Application in the Dating of Stringed
Instruments,” Icom Committee for Conservation 7th
Triennial Meeting Copenhagen 10-14 September 1984 Preprints
(1984), pp. 84.1.69-84.1.72.
[ii]
Angelo Mondino and Matteo Avalle, Manual of
Dendrochronology Applied to the Dating of Musical Instruments
(Cremona, 2005), pp. 85-107.
[iii]
Mondino and Avalle, Manual of Dendrochonology,
pp. 85-87; Mondino and Avalle, New Dendrodating Procedure
Exercises (Cremona, 2009), p. 91-92.
[iv]
Below
are the raw measurements taken from the viola multiplied by 100:
236, 189, 175, 165, 119, 135, 140, 173, 169, 150,
116, 176, 190, 169, 202, 174, 132, 205, 193, 205, 212, 204, 200,
216, 215, 220, 188, 198, 156, 161, 205, 188, 167, 157, 172, 193,
151, 181, 189, 244, 196, 199, 173, 214, 177, 214, 183, 182, 192,
171, 166, 195, 181, 171, 205, 168, 185, 175, 127, 172, 151, 170,
162, 170, 225, 237, 260, 274, 257, 262, 245, 116, 128, 175.
[v]
Mauro Bernabei, Jarno Bontadi, and Gabriele Rossi
Rognoni, “A denrochronological investigation of stringed
instruments from the collection of the Cherubini Conservatory in
Florence, Italy,” Journal of Archaeological Science 37
(2010), p. 195.
[vi]
Mondino and Avalle, Manual of Dendrochronology, p. 68.
[vii] I
would like to thank Peter Ratcliff for pointing this out.
Personal communication, 2010.
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The author entering year-ring measurements
into the computer.

The author's violin cradle linked to a
rack-and-pinion linear translation stage and digital linear scale.
Below are some illustrations from the
previously posted article on Stradivari's varnish:

The author with epi-fluorescence
microscope. The microscope's uv light source and special excitation and
barrier filters help distinguish varnish layers. A digital camera feeds
images directly into computer.

Cross section of pegbox cheek of "Pawle"
Stradivari cello photographed with epi-uv fluorescence, showing varnish
layers. Note pigment particles in lower layer.

Cross section of rib of the "Paganini"
Stradivari cello (1736) photographed with epi-uv fluorescence, showing two
distinct varnish layers

Cross section of rib of Stradivari violin
(1734) photographed with epi-uv fluorescence, again showing two distinct
layers.


Varnish made by author from recipe derived
from the analyses of Stradivari's varnish. The varnish consists of linseed
oil, pine resin, and natural, mined red ochre pigment. Note similarity in
color and transparency to the varnish on the "Alard" Strad (1715), above. |