CONSERVATION OF A MONUMENT OUTDOOR BRONZE SCULPTURE: LINE'S STATUE IN TORN, POLAND.
INTRODUCTION:
The social Committee of Torn in Poland has been working few years for
founding a monument of the great scientist and author of polish dictionary
Bogumil Linde. A bronze sculpture was initiated by the artist Zygfryd
Godzilewski for this purpose. Ceremony of the monument's exposure and its
transfer to the society of metropolis Torun was connected with a scientific
session in 20 December 1916 year. The ceremony included biographical and
historical part about Linde, his dictionary and his scientific achievements .
DESCRIPTIVE AND ANALYTICAL STUDY
Over natural size (195 cm high, and small bronze plinth about 70×70 ×15 cm), the bronze statue
of Lind presents him as a mature man in staying position with a left straight
leg putted forward. Head is shown slightly forward, has a little curdled hair.
His face has a big nose and deeply based eyes, show concentration and
consideration. The left hand is sited on the hip and drew an elegant long coat.
Under the coat is visible vest, and under his neck is foulard. Right hand holds
a big book, as attribute characterized Lind's
work and scientific position.
The Pedestal is covered with a polished granite
plates, three from each side. The inscription on the middle plate at the street
side is „ BOGUMIL SAMUEL LINDE 1771 – 1847, AUTHOR OF THE POLISH DICTIONARY”.
Engraving is painted with a dark colour corresponded to the bronze statue. The
space between plates is filled with a cement mortal 0,5 - 2 cm. The monument is
placed in front
of Copernicus .
Universty'slibrary and refers to it thematically:
The Lind's statue and small plinth under it is castes from bronze
alloy. Where the under pedestal was made of cement and brick, outside covered
with the polished grey granite plates. Pointing with grey cement, uncolored,
filler sand about for 1,5 .mm granulation.
The sculpture is made in casting technique. It is empty inside that’s
easy to proof by knocking on the surface.
The main compounds of the statute are tin and copper. Very small two samples were taken, the first one from
the statue and the second one from the small plinth, to identify qualitatively
and quantitatively the original metal or the alloy of the statute by using
Atomic Absorption Analysis.
The resulted data of the
analysis represented in table [1] showing that the statute and its small plinth
made of bronze where:
The major metal is copper
The admixture metal : tin
The traces metals: zinc, phosphor, iron and sulfur.
Table 1, the elements that
composite the original composition of the statute
No. of samples
|
Copper Cu %
|
Tin Sn %
|
phosphor P %
|
Zinc Zn %
|
Iron Fe %
|
sulfur S %
|
Total %
|
1
|
90.
|
6.30
|
1.10
|
1.00
|
.50
|
.90
|
99.80
|
2
|
89.60
|
6.80
|
1.20
|
1.10
|
.50
|
.70
|
99.10
|
Adding of these trace
elements to the bronze alloy affected in its properties. The reason for
employing low tin bronze could be the wish of the artist to keep the red color
of copper and to improve the casting properties of the metal. Tin oxidizes,
aids the fluidity of the molten metal (by reducing the melting temperature),
and improves work ability .
Iron has a limited solubility
in bronze (0.5%) and above this limit it will exist as discrete particles
dispersed throughout the alloy. The presence of greater than 0.5-wt% iron will
have a deleterious effect on the corrosion resistance of bronzes .
This analysis was proofed by exact observation of bronze surface after
cleaning that emerged that the alloy is homogeneous, without points of
segregation of components. Good homogeneity can be result of addition compounds
like zinc and phosphor which increase the homogeneous of alloy.
CONDITION AND DESTRUCTIVE FACTORS:
The state of preservation was
satisfactory, the metal surface is in a good condition without occurrence of deep
corrosion centers, but whose visual
appearance was poor.
The statue covered with a uniform dull black layer covering green color
which disfigured its true golden brown color and its appearance. [Photos 1, 3
and 9]. This dark or black appearance is
commonly due to the association of sooty matter with the oxides and sulfides of
copper.
In contrast to the overall
wild black patina that was very coherent and not easy to remove, patches of
light green corrosion with many un-aesthetic spots on whole surface of the
statute can be seen on the head, shoulders, the frock coat, both
shoes, and the metal plinth [photos 5, 7 and 10].
The cement mortar was partially crumbled out from granite plates. The
plates were dirty and covered with black crusts. They emerge in as a result of
rinsing binder from cement [photo 11].
The inscription on the middle plate „ BOGUMIL SAMUEL LINDE 1771 – 1847,
AUTHOR OF THE POLISH DICTIONARY” was faded or paled and need to be re-colured
[photo 13].
The bronze sculpture's state
of preservation was good, but aesthetically, it had changed dramatically since
its dedication and it is in a need to be cleaned and conserved.
One sample from this blackish
green layer was taken to be analyzed by X-ray diffraction method [XRD]. The aim
of this analysis is identification the corrosion compounds in order to decide
whether it is authentic, stable, and suited to certain kinds of conservation
treatment.
The powder of the sample was studied by X-ray diffraction, using a
Philips X-ray Diffract meter with Cu Kα radiation
in Center of the antiquities conservation researches, Supreme Council of Antiquities Egypt.
From the X-ray diffraction
scan showed in Figure, and the obtained corrosion compounds are given in table
2.
Fig.1: X-ray diffraction scan
of the corrosion sample from the surface
of the statute.
Table 2: the corrosion compounds that identified on the statute's surface.
No.
|
Compounds
|
Formula
|
1
|
Brochette
|
CuSO4.3Cu[OH]2
|
2
|
Tenorite
|
CuO
|
3
|
Cup rite
|
Cu2O
|
4
|
Quartz
|
SiO2
|
The mainly reason to progress the corrosion of the surface is the
aggressive city atmosphere. And because
it has been standing in the center of the city, near to the commuter routes that connect Torun with
the other cities, dirt and pollutants had so heavily accumulated.
In addition to snow and rain
streaking in winter that disfigured the surface of the bronze sculpture, some
corrosion may have been effected by bird droppings, but there is no reason to
doubt that most was atmospherically induced, and sun rays in summer may speed up any
chemical reactions that attack the surface for long time.
The presence of Brochette in the corrosion products that identified on
the surface resulted from exposing the
statute to sulfur-bearing atmosphere that most likely formed from sulfur
compounds produced by burning of fuels.
Copper's oxides Cup rite and
Tenorite
were developed on the statute's surface as a result of exposing to air. The
first one forms as a sealing layer and has protective properties but the
increase both in temperature and in oxygen pressure breaks this layer.
So the
lattice defects are distributed unevenly throughout this layer and various
gases are able to penetrate. The abundant oxygen supply generally leads to
surpassing of the oxygen equilibrium pressure [Cu2O/CuO] and
subsequently to the formation of a thin black layer of the second oxide
Tenorite CuO on Cuprite Cu2O .
The identified Quartz may be residue of the die that used in casting the
statute.
Treatment And Conservation:
The treatment was designed to
improve the sculpture's appearance, and to protect and conserve the bronze
metal itself. The treatment consisted of the following steps:
Removal of surface accretions
by mechanical cleaning
After some experimentation in
an inconspicuous area [the bottom part of the bronze plinth], with scrub
brushes, nylon scrub pads, and a cleaning paste made from 10% of Rouchle solution [150g
Potassium Sodium Tartarate and 50g sodium hydroxide in 1L water] condensed with
wooden dust to remove the dull black layer on the surface of the statute, we found that the most successful method was cleaning the wild patina
with sandblasting method.
The process was made by use of clear sand with
fraction 0,18mm. at pressure from 3-6 bar. The process was controlled with a
suitable distance from the surface, it gave good control. After preliminary
positive results the whole sculpture was cleaned. Procedure was successively
made from top to the bottom with all difficult reached places of sculpture.
During the night object was covered with wrapping foil to protect from possible
influence of environment, rain, birds etc.
It was decided also to sandblasting the black crusts on the granite
plates with use of low pressure and with the help of scalpel. Old crumbled
cement joints were mechanically removed and replaced with new one.
Patination of the surface:
It is decided fabricate natural protective layer to obtain Cuprite layer
on the bronze surface with 15- 30 % solution of hydrogen peroxide H2O2.
It was applied on the metal surface by brush. Three times the whole sculpture
was covered to get a deep brown patina. For underlining the form of sculpture,
it was rubbed with delicate scratching material, and buffed with a diaper it has build dark and bright
appearance.
Conservation and protection against corrosion:
To preserve the sculpture surface it was covered with varnish- 10%
Paraloid B44 solution in toluene with addition of proper pigment (umber,
ochre). After this procedure, the metal has gotten a pretty warm tint and
additional colouristic unification.
For getting the impermeability of gases a protective layer of varnish
was covered. This layer was microcrystalline wax Cosmolloid 80H, 20-30 %
solution in white spirit. After evaporation of white spirit, the metal surface
was heated to let the wax to penetrate the pores, and then polished with soft
brushes.
The black crusts on the granite plates were cleaned by using fine
sandpapers, scalpel, and 5% solution of citric acid.
The broken cement joints were removed mechanically and a new layer was
applied with special tools.
The inscriptions paint on the pedestal was strengthen with a solution of
Paraloid B72 in toluene with addition of pigment.
The work was executed at advantageous weather conditions, temperature 28
°C degrees, without rain falls.
References:
1- Alojzy Tujakowski, Bogumil
Samuel Linde, Framer of First Polish Dictionary, Torun, 1977.
2- M. A. Giumlia-Mair, Investigation of Copper-based Hoard
from the Megalithic site of Al-Midamman, Yemen: An Interdisciplinary Approach,
[IN:] Journal of Archaeological Science, 2002, pp.195-209.
3- N. A. North, and I. D. Macleod,
Corrosion of Metals, [IN:] Conservation of Marine Archaeological Objects
, edited by C. Pearson, pp.68-98, Butterworth, London, 1987.
4- T. Stambolov, The Corrosion and Conservation of
Metallic Antiquities and Works of Art, A Preliminary Survey, Amsterdam, 1985.
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