Archeomatica_3_2018

Fig. 8 - Shows XRD scan for the corrosion products of the trumpet.
In general, copper and brass alloys are highly resistant to
atmospheric corrosion due to the formation of protective
layers of corrosion products, which reduce the rate of attack.
Thickness and composition of the corrosion product
layer formed are governed by the relative humidity and pollution
of the environment. Inside the instruments that are
regularly played, a very thin film of water at the surface can
be present for quite a long time and damage of the artifacts
(brass instruments) might occur on long-term (Elsener B.
et al. 2016).
The interior corrosion in brass wind instruments dues to the
effects of moisture and saliva inside the instruments, which
consequently increases the risk of metal corrosion inside
the instruments.
Measurements have shown that after 5 min playing the relative
humidity (RH) inside the instruments exceeds 90% and
it takes several days to reach ambient RH again (Scott A.
D., 2002, p.6).
Thus conditions for atmospheric corrosion, the formation of
a very thin film of liquid water at the surface [Craddock P.T.,
1995, p. 295; Hodges H., 1964, p.69; Selwyn L. 2004, p.53.)
are in principle present for quite a long time when the instruments
are regularly played and damage of the artifacts
(brass instruments) could be possible on long-term (Elsener
B. et al., 2016).
The solder joints of the trumpet are a soft soldering assembled
totally with tin-lead solder containing 90.39%% Pb
and 8.61% Sn, 0.47% Cu and 0.35% Zn (Table no.1). Soft solders
have been used to join pieces of metal together at low
temperatures (below 300°C), and produce joints in copper,
bronze, brass, and silver objects (Maryon H., 1941; Ashkenazi
D. et al., 2014 ).
It is generally composed of lead and tin mixed in proportions
that vary from as little as 30% lead (and 70% tin), to as
much as 98% lead (and only 2% tin) ((Maryon H., 1971; Goffer
Z., 2007). The flux is zinc chloride, resin, tallow, or some
other oily substance was used to prevent film formation and
then burn off (Cronyn J. M., 1990, p. 162).
This technique of soldering was used to attach a very small
piece of metal to an under layer of the same metal (Ashkenazi
D. et al., 2014, p. 51). In the trumpet, all connections
between tubes were made by means of a lap joint, in which
usually each tube fits into the next tube, moving toward the
bell (Van der Heide G. J. , 1991, pp. 122-150). But in the
trumpet the soft soldering process may be performed in a
later time to fix the disjointed tubes.
As a consequence of the joining of different metals, the behavior
and the rate of corrosion are remarkably influenced
by the intimate contact between metals with different
electrochemical potential. This contact induces the more
reactive and less noble metal to become anodic in a couple
strongly conductive to corrosion.
X-Ray diffraction analysis of the corrosion products of the
trumpet revealed the presence of different minerals includ-
ing Covellite Cu S, Paratacamite Cu2(OH)3Cl, Massicate PbO
and traces from Cuprite Cu2O, Calcite CaCO3 ( Fig. no.
8&Table no.2 ).
The formation of sulfates and chlorides (Covellite Cu S,
Paratacamite Cu2(OH)3Cl), resulted from the interaction
between surrounding environment and the trumpet. The
impact of sulfides on the corrosion of copper alloys has received
considerable attention, including published reports
documenting localized corrosion of copper alloys by Sulfate-reducing
bacteria (SRB), a diverse group of anaerobic
bacteria isolated from a variety of sulfur-containing. A porous
layer of cuprous sulfide with the general stoichiometry
forms in the presence of sulfide ions. Copper ions migrate
through the layer, react with more sulfides, and produce a
thick, black scale, which can be altered by oxygen from the
environment to a complex sulfide-oxide scale. The sulfide
scale does not confer much protection against further attack,
but the sulfide-oxide scale provides even less. Corrosion
products on copper alloys were more adherent and in
some cases difficult to scrape from the surface. In all cases,
bacteria were closely associated with sulfur-rich deposits.
There is one class of conditions under which biofilms appear
not to produce sulfide minerals.
The presence of basic copper chloride (paratacamite) is
related to the saline nature of the surrounding medium,
whereas the trumpet was preserved. Paratacamite always
found as a powdery, light green secondary corrosion layer on
the patina surface, while the compound of Atacamite occurs
as a sugary-looking coating of dark green glistering crystals
(Scott D.A., 2002, p.124). Often this dark green crystalline
Atacamite is altered to a paler green powdery product of
paratacamite (Gettens R.J., 1963; Frondel C. 1950).
The existence of lead in the soldering alloy as globules or
independent islands causes a non-homogenous structure. In
this case a galvanic corrosion tendency when the alloy is
exposed to moist air or soil. As a result of this reaction lead
corrosion products such as Massicate (PbO) was deposited
on the object surface.
The presence of Cuprite (Cu2O) as a trace in the corrosion
products due to the selective corrosion of the main alloying
element, which is re-deposited after dissolution onto the
surface of the objects, thus forming a copper enriched layer,
Cuprite is the most widely occurring alteration mineral
Fig. 9 (a, b) shows the both sides of the trumpet after treatment
and Conservation.
40 ArcheomaticA N°3 settembre 2018