Val et al., 2014: CHEMICAL PREPARATION OF DINOSAUR EGGSHELLS
used acids are organic, such as acetic acid CH3-
sodium hexametaphosphate (NaPO
3)6 (Figure
COOH (C
2H4O2) (Figure 3A) and formic acid H-
3D), a salt that transforms calcium carbonate
COOH
(CH2O2) (San Andres Moya and de la
into
sodium
carbonate.
This
agent
is
Viña Ferrer, 2004). In very hard matrices,
commercially available with different pH. For
inorganic acids have also been used. Some
the present study, we used the agent with a pH
examples are hydrochloric acid (HCl), (Figure
value of six, and thus slightly acidic. Therefore,
3B) and sulfuric acid (
H2SO4). We have also
we included it among the acids, even though it
tested oxalic acid H
2C2O4 (Figure 3C), an
is not considered an organic acid. The
organic acid 3000 times stronger than acetic
treatment with (NaPO
3)6 is widely used in other
acid, and commonly used for eliminating iron
fields of heritage conservation and preparation,
oxide concretions from archeological iron
and it yielded excellent results when used for
(Mourey, 1987). Nevertheless, there are other
eliminating carbonate concretions. In our case,
agents that can act on calcium carbonate in a
treatments
performed
using
sodium
less aggressive way, and that transform
hexametaphosphate (NaPO
3)6 have been highly
calcium carbonate into other carbonates that
effective and poorly aggressive (Val, 2007; Val
are more soluble in water, and thus easier to
et al. 2010). Table 1 shows the concentrations
eliminate without using acids. One example is
working best for each treatment.
Table 1: acidic agents.
CHEMICAL AGENT
%
OBSERVATIONS
Formic acid
H
-COOH
(CH
2O2)
10%
Surface highly damaged
Acetic acid
CH
3-COOH
(C
2H4O2)
10%
Surface highly damaged (Figure 3A)
Hydrochloric acid
(HCl)
10%
Surface highly damaged (Figure 3B)
Oxalic acid
H
2C2O4
10%
Surface slightly eroded (Figure 3C)
Sodium Hexametaphosphate
6 (pH 6)
(NaPO
3)6
15%
Optimal cleaning (Figure 3D)
Table 2: alkaline agents.
CHEMICAL AGENT
%
OBSERVATIONS
Potassium hydroxide
KOH
10% Optimal cleaning % (Figure 4A)
Sodium hydroxide
NaOH
4%
Surface slightly eroded (Figure 4B)
Sodium hexametaphosphate
(NaPO
3)6
15% Surface slightly eroded
Alkalies: for dissolving silicates present in
of eight, but contrary to the results obtained
matrices composed of 50% carbonate and
with the acidic version, the alkaline Sodium
50% silicate marls
hexametaphosphate did not yield positive
results. Table 2 shows the concentrations for
Alkaline agents dissolve silicates present in the
obtaining the best results for each treatment.
matrix without attacking the calcium carbonate
of the eggshells. As can be observed in figures
Other chemical agents
4A and 4B, the most effective treatments are
the ones using potassium hydroxide (KOH;
Mixtures to dissolve carbonate in hard
Figure 4A), as shown in the absence of
matrices with ferric iron
degradation in the surface and the good
The dissolution of matrices containing high
observation of al
microstructures of the
amounts of iron oxide is problematic, since
eggshell. However, they are difficult to apply
there are no established protocols or guidelines
and neutralize. In those cases, we highly
for doing so. Thus, we test ed new and old
recommend neutralization using an ultrasound
treatments
used
in
paleontological
bath twice with 80 ml of distilled water (double
conservation/preparation
(Rutzky et al.,
the volume) during 30 min (double the time of
1994),
and
also
in
other
fields
of
exposition). Moreover, we tested sodium
conservation/restoration of cultural property
hydroxide (NaOH; Figure 4B; Fernández et al.,
(Mourey, 1987). This type of matrix is very
2005), but in a lower percentage, due to its
hard and resistant to many treatments,
more aggressive character, and because we
including mechanical work. The most commonly
could note more abrasion on the surface and
used method working with this type of matrix is
degradation of the microstructure of the
the Waller Method (see Figure 5A; Waller,
eggshell.
Finally,
we
used
Sodium
1980; Blum et al., 1989; Rutzky et al., 1994).
hexametaphosphate (NaPO
3)6, with a pH value
The Waller method uses a solution of sodium
33
Journal of Paleontological Techniques