A short note on Egyptian blue

Abstract
A <strong>short</strong> <strong>note</strong> on Egyptian blue
Gian Antonio Mazzocchin *, Danilo Rudello, Carlo Bragato, Francesca Agnoli
Dipartimento di Chimica Fisica, Università Ca’Foscari di Venezia, Calle Larga S. Marta 2137, 30123 Venice, Italy
Received 15 April 2003; accepted 6 June 2003
We studied first the feasibility of the reaction synthesis of EB in molten alkaline carbonates; then we changed to “solid phase” syntheses,
comparing the yields according to the proportion of the various components. In molten carbonates azurite was produced, which turned to
malachite at room temperature. Three “dry ways” were performed: the stoichiometric one, with Na 2CO 3 as a flux, the Schippa and Torraca
recipe, and a “Bolognese” method, which uses NaHCO 3 as a flux. Good results in colour and yields were obtained with the Schippa and
Torraca recipe, with the stoichiometric molar ratio and with the “Bolognese method” at 860 °C.
Research aims. – In the frame of the research carried out on the Roman age wall paintings, the authors put their interest in the checking of
the different recipes leading to the synthesis of Egyptian blue. In order to deepen the study of the mechanism of Egyptian blue formation, the
synthesis in molten carbonates was tried, but it did not lead to Egyptian blue but to azurite and in the end malachite.
© 2003 Elsevier SAS. All rights reserved.
Keywords: Egyptian blue; Synthesis and characterisation; FT-IR; XRD; SEM; EDS
1. Introduction
EB appeared in Egypt during the third millennium BC and
was used at least for 3000 years as a precious pigment,
spreading in the whole Roman Empire [1–4].
Starting from the end of the XIX century to beginning of
the XX century, many researchers began to analyse and
synthesise this compound, investigating the effects of cooking
temperature and flux nature and concentration.
The use of X-rays allowed the identification of EB as
calcium and copper tetra silicate (CaCuSi 4O 10), and showed
that EB and the mineral cuprorivaite, discovered by Minguzzi
in the Vesuvio lava in 1938 [5], are the same thing.
The synthesis of this pigment “caeruleum”, described by
Vitruvius in De Architectura in the first century BC, was
performed by heating a mixture constituted by siliceous
sand, lime (or calcareous sand) and copper compounds (minerals
or bronze fragments) and a flux (soda, natron or plant
ash). Vitruvius did not expressly mention calcium carbonate
among EB components, maybe because the sand used in
Pozzuoli, had the right limestone content; medieval copyists
did not indicate it among the necessary compounds and
consequently, the synthesis of EB was not possible anymore.
* Corresponding author.
E-mail address: g.a.mazzocchin@unive.it (G.A. Mazzocchin).
© 2003 Elsevier SAS. All rights reserved.
doi:10.1016/j.culher.2003.06.004
Journal of Cultural Heritage 5 (2004) 129–133
This is likely to be the cause of the quick decline of use of EB
in medieval and renaissance painting, but the main cause was
the decline of Roman Empire with the consequent loss of
knowledge and technologies.
There is a general agreement on the working temperature,
that must be kept between 850 and 950 °C.Above 1100 °C, in
fact, green frit forms from and EB decomposes [6,7].
In this work, we attempted first the synthesis of EB in
molten alkaline carbonates; then we changed to “dry phase”
syntheses, comparing the yields according to the proportion
of the various components.
2. Materials and methods
2.1. Muffle oven and crucibles
www.elsevier.com/locate/culher
The used muffle was a ZB1 (Carlo Erba). It was calibrated
by means of the known melting points of salts: NaNO 3
(507 °C), NaClO 4 (482 °C), NaCl (807 °C) and Na 2CO 3
(851 °C). The results showed a precision of ±5 °C with
respect to the set out temperature.
The used crucible were:
• stainless steel, which was discarded because of iron
contamination problems;

130 G.A. Mazzocchin et al. / Journal of Cultural Heritage 5 (2004) 129–133
• porcelain, which was discarded because of aluminium
contamination problems, besides the difficulties in removing
the compound from the walls of the crucible;
• nickel, which presented the same problems of the stainless
steel, even if to a small extent;
• zirconium, which was acceptable but not resisting
enough: in fact, after some use it weakened and got
punctured;
• terracotta, undoubtedly the best and lowest-cost one.
2.2. Reagents
Chemicals were generally of analytical grade. Libya and
Egypt sands were used in some cases, but the best results
were obtained with finely ground rose quartz.
2.3. Optical microscopy
The samples were observed by means of an optical microscope
Wild-Leitz M8 with a 19.2× to 256× zoom. The
samples were illuminated using a movable fibre glass system.
2.4. Scanning electron microscopy (SEM)
SEM images were taken using a Jeol (Tokyo, Japan) JSM
5600 LV equipped with an Oxford Instruments 6587 EDS
microanalysis detector. The images were taken under low
vacuum conditions where samples did not show charging
effects; in this way, it was possible to avoid the coating of the
samples with a high conductance thin film (gold or graphite
films).
EDS microanalysis was made to obtain information on the
elemental composition of the sample.
2.5. X-ray diffraction (XRD)
X-ray powder diffraction was used to identify the different
crystalline phases present in the pigments. A Philips X’Pert
vertical goniometer with Bragg–Brentano geometry, connected
to a highly stabilised generator, was used for XRD
analysis. Cu-K a Ni-filtered radiation, a graphite monochromator
on the diffracted beam and a proportional counter with
pulse height discriminator were used. Measurements in a
5–60° range were taken with a step size of 0.05° and 2°s per
point.
2.6. Infrared spectroscopy (FT-IR)
Absorption spectra in the IR region were collected using a
Nicolet Magna 75 FT-IR spectrometer. The spectra were
given as an average of 32 measurements. Samples were
diluted in KBr pellet (IR grade, Merck).
3. Results and discussion
The first synthesis attempts were carried out in molten
carbonates at 860 °C.
Twenty grams of Na2CO3 were mixed with 20 g K2CO3 and the mixture transferred in a large zirconium or porcelain
crucible and put in muffle oven at 860 °C. The molten
mixture was colourless; 2.5 g of CaCO3,2gofCuOand6g of SiO2 were then added to the molten mixture. The molar
ratios of SiO2:CuO:CaO were 4:1:1, as required by the stoichiometry
of the product to be synthesised.
After the addition of CaCO3 the solution remained
colourless-pale yellow, while after the addition of CuO it
became yellow-green. After SiO2 addition, the liquid began
to bubble because of the formation of CO2 according to the
2– 2–
reaction: CO3 + SiO2 → SiO3 +CO2↑
After 2hinovenmuffle,themolten mixture was poured
on a stainless steel plate where we observed beautiful blue
drops (Fig. 1a). Leaving the product at room temperature, it
absorbed humidity and its colour became green (Fig. 1b)
[8,9].
FT-IR spectra of the blue product showed the characteristic
bands of carbonates, while the bands typical of EB,
around 1000–1050 cm –1 Fig. 1. (a) Drop of blue obtained by a synthesis carried out in liquid phase of
molten carbonates; (b) the colour of the blue product left at room temperature
changed to green.
, shifted to the ones of malachite.
We cannot completely exclude also the formation of
amounts of copper silicate and calcium silicate.
Therefore, we began to study the “dry syntheses” mixing
the required amounts of compounds and grinding them to-

Fig. 2. (a) Content of the crucibles at the end of the reaction carried out
following the recipe by Schippa and Torraca; (b) content of the crucibles at
the end of the reaction carried out following the “Bolognese mixture”.
gether in an agate mortar and, then, transferring the mixture
in muffle oven on small terracotta plates. When the amount of
Na 2CO 3 flux exceeds the molar ratio 0.5–1, one is allowed to
observe the formation of a semi-solid grey mass, which was
difficult to be removed from the terracotta surface. This did
not allow to take, grind and put the product again in muffle.
Anyway, the terracotta crucibles or plates revealed to be
suitable for the assays carried out at these temperatures.
We repeated some experiences already reported by Bianchetti
et al. [6] which deals with mixtures described by
Hansen and Jensen [11] but the used molar ratios did not
allowed us to get good EB products.
On the contrary, by using stoichiometric ratios (4:1:1), as
suggested by Bruni et al. [12], and adding Na 2CO 3 quantities
to have a 0.5–1 molar ratio range, we obtained better results
in the colour of the product, kept in muffle oven only for 3 h.
(These results were obtained using the following amounts of
reagents: 6 g of SiO 2, 2.5 g of CaCO 3,2.0gofCuOand2gof
Na 2CO 3 at 860 °C.)
Attempts to use borax or Na 2SO 4 as a flux did not lead to
better results.
The role played by different flux as Li 2CO 3 or K 2CO 3 was
tested and the results were similar to the ones obtained using
G.A. Mazzocchin et al. / Journal of Cultural Heritage 5 (2004) 129–133
Fig. 3. FT-IR spectra of the “Bolognese mixture” sample (a), of the stoichiometric
sample (b) and of a Roman EB sample found in Vicenza (c).
Na 2CO 3, being careful that the fusion of the reaction mass
was easier and leading to a dark grey product, which firmly
sticks to the terracotta plate [2–4].
Some assays were carried out to check the effect of the
physical status of SiO 2 using colloidal silica gel following
the recipe by Schippa and Torraca [10], which used molar
ratios of 4.0:1.1:1.0:0.2 (with copper amounts slightly higher
than the stoichiometric ones).
In these experiments, we got good quality products taking
care in the repeated grinding of the product and leaving it in
muffle oven for 12 h (Fig. 2a).
In our lab, we also tested a “Bolognese mixture” [13],
characterised by the following molar ratios: 10:1.4:1.4 for
Si:Ca:Cu, respectively, with NaHCO 3 as flux in a molar ratio
of1(Fig. 2b) (typical quantities were: 6.02 g of SiO 2, 1.45 g
of CaCO 3, 1.73 g of malachite, 0.80 g of NaHCO 3).
We used a simple method to evaluate the yield of EB in the
obtained product, from the amount of calcium carbonate
residue.
131

132 G.A. Mazzocchin et al. / Journal of Cultural Heritage 5 (2004) 129–133
Fig. 4. EDS spectra of the “Bolognese mixture” sample (a), of the stoichiometric
sample (b) and of a Roman EB sample found in Vicenza (c).
Thus we were able to verify that the EB obtained using the
recipe by Schippa gave a yield around 90% similar to the
stoichiometric method, while the “Bolognese” gave yield up
to 97%.
The processes of re-treatment, performed in muffle oven,
had a positive effect also for the stoichiometric mixture with
Na2CO3 as a flux, as reported also by Bruni et al. [12].
In Fig. 3, the FT-IR spectra of a Bolognese sample (a), of
a stoichiometric one (b) and of a roman EB sample (c) found
in Vicenza [14] are shown.
As one is allowed to see, the typical bands of EB (medium
intensity bands at 1005 and 1050 cm –1 and low intensity
bands at 1170 and 1240 cm –1 ) are present, but in our products
the carbonate amount is lower (band at 1450 cm –1 ) [15].
In Fig. 4, the EDS spectra of the same three products are
shown: again, calcium amount is higher in the roman sample,
in which a small amount of magnesium is also present. In
Fig. 5, the corresponding XRD spectra are shown. The similarity
of the spectra confirms that the synthesis product is
cuprorivaite in all cases.
In Fig. 6, the SEM image of a stoichiometric product
heated up to 1100 °C is shown. The resulting product was
green, and showed a rounded molten surface shape due to the
decomposition of EB at high temperatures [6,7].
4. Conclusions
Our tests confirm most of the data already reported by
many authors [2–4]. In fact good yields in EB can be obtained
with stoichiometric calcium, copper and silica ratios,
using Na2CO3 as a flux. The reiterated grinding of the products
had a positive effect, just like as a 12 h permanence in
muffle oven at 850–900 °C. We also confirm the EB decomposition
at temperatures higher than 1100 °C, also leading to
the formation of green frit.
As far as the “Bolognese mixture” is concerned, our tests
confirm that the recipe used by the students of the course
Material Chemistry and ceramics technologies (Faenza) in
the Inorganic Chemistry laboratory gave good yields, even if
the component molar ratios are not the stoichiometric ones
[2–4]. NaHCO3 becomes Na2CO3 at temperature higher than
250 °C, while malachite begins to decompose at 300 °C and
all the carbonate decomposes above 650 °C. All these data
seem to suggest that the reaction leading to EB is favoured by
the “fresh preparation” of the oxides reacting on the surface
of finely grounded SiO2 [4].
It is also known that silicon dioxide content in EB varies
from 58.5% to 88.7% [4,16], thus it is not surprising that we
got good yields with SiO2 percentages ranging from 57% to
77%.
FT-IR, EDS and XRD data indicate that the same product
can be obtained either with the stoichiometric and the “Bolognese
method”. On the contrary, the synthesis in molten
carbonates, through the dissolution of silica and the formation
of silicates, did not give any result as EB synthesis is
concerned.

Fig. 5. XRD spectra of the “Bolognese mixture” sample (a), of the stoichiometric
sample (b) and of a Roman EB sample found in Vicenza (c).
G.A. Mazzocchin et al. / Journal of Cultural Heritage 5 (2004) 129–133
Fig. 6. SEM image of a stoichiometric product heated up to 1100 °C.
References
[1] D. Ullrich, Egyptian blue and green frit: characterization, history and
occurence, synthesis, PACT 17 (II.3.1) (1987) 323–332.
[2] F. Delamare, De la composition du bleu égyptien utilisé en peinture
murale gallo-romaine, Scienze e materiali del patrimonio culturale 4
(1998) 177–195.
[3] F. Delamare, Le bleu éegyptien, essai de bibliographie critique, Scienze
e materiali del patrimonio culturale 4 (1998) 143–163.
[4] J. Riederer, Egyptian blue, in: E.W. FitzHugh (Ed.), Artists’Pigments:
a Handbook of their History and Characteristics, vol. 3, Washington,
National Gallery of Art, 1997, pp. 23–45.
[5] C. Minguzzi, Cuprorivaite: un nuovo minerale, Periodico di Mineralogia
9 (1938) 333.
[6] P. Bianchetti, F. Talarico, M.G. Vigliano, M.F. Ali, Production and
characterization of Egyptian blue and Egyptian green frit, Journal of
Cultural Heritage 1 (2000) 179–188.
[7] P. Baraldi, F. Bondioli, C. Fagnano, A.M. Ferrari, A. Tinti, M. Vinella,
Study of the vibrational spectrum of cuprorivaite, Annali di Chimica
91 (2001) 679–692.
[8] J.L. Pérez-Rodrìguez, C. Maqueda, M.C. Jiménez de Haro,
P. Rodrìguez-Rubio, Effect of pollution on polychromed ceramic
statues, Atmospheric Environment 32 (6) (1998) 993–998.
[9] R.J. Gettens, E. West FitzHugh, Azurite and blue verditer, in:
E.W. FitzHugh (Ed.), Artists’Pigments: a Handbook of their History and
Characteristics, vol. 2, Washington, National Gallery of Art, 1997,
pp. 23–35.
[10] G. Schippa, G. Torraca, Contributo alla conoscenza del Blu egiziano,
Bollettino dell’Istituto Centrale di Restauro 31–32 (1957) 97–107.
[11] F. Hansen, O.I. Jensen, Farkevemi, Unorganiske pigmenter, G.E.C
GAD, Copenhagen, 1991.
[12] S. Bruni, F. Cariati, F. Casadio, L. Toniolo, Spectrochemical characterisation
by micro-FTIR spectroscopy of blue pigments in different
polychrome works of art, Vibrational Spectroscopy 20 (1999) 15–25.
[13] M.C. Iappalucci, private communication.
[14] G.A. Mazzocchin, F. Agnoli, I. Colpo, Investigation of Roman age
pigments found on pottery fragments, Analytica Chimica Acta 478 (1)
(2003) 147–161.
[15] P. Mirti, L. Appollonia, A. Casoli, R.P. Ferrari, E. Laurenti, A. Amicano
Canesi, G. Chiari, Spectrochemical and structural studies on a
Roman sample of Egyptian blue, Spectrochimica Acta 51 (3) (1995)
437–446.
[16] M.S. Tite, M. Binson, M.R. Cowell, Technological examination of
Egyptian blue, in: J.B. Lambert (Ed.), Archaeological Chemistry, vol.
3, Washington American Chemical Society, 1984, pp. 215.
133