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        Natural or Artificial? Multi-Analytical Study of a Scagliola from Estoi Palace Simulating Imperial Red Porphyry
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        Natural or Artificial? Multi-Analytical Study of a Scagliola from Estoi Palace Simulating Imperial Red Porphyry
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        Natural or Artificial? Multi-Analytical Study of a Scagliola from Estoi Palace Simulating Imperial Red Porphyry
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In this paper the characterization of a gypsum plaster sample from the end of the 19th century simulating imperial red porphyry using a multi-analytical approach is presented and discussed. The results of X-ray diffraction (XRD), thermogravimetric and differential thermal analysis (TGA-DTA), physical and mechanical properties are summarized. In order to have further insight into the microstructure, polarized light microscopy (PLM), scanning electron microscopy coupled with energy dispersive X-ray spectrometer (SEM-EDS), and micro Raman spectroscopy analyzes were also made. They helped to clarify the main issues raised by the other complementary analytical techniques and allowed the establishment of interrelations between the different properties, providing important information about the materials, the skills, and the technological development involved in the art of imitating noble stones with gypsum pastes. This study also contributes to our knowledge concerning the preservation of these types of elements that are important in the context of European decorative arts and rarely reported in the literature.


The knowledge of how to manufacture and use gypsum plasters is thought to have been introduced in Europe by the Greeks and was influenced by the Egyptians and the Minoans. They passed it to the Romans (Stark & Wicht, 1999), and it remained popular until the end of the Classical Antiquity.

In Medieval times, its use almost disappeared in Europe, emerging again with the presence of the Arabs between the 8th and the 15th centuries (Gárate-Rojas, 1999). Nevertheless, it was only in the 17th century that gypsum plasters definitely assumed great relevance within the European decorative arts, a role held throughout the whole Baroque and Rococo periods.

The possibility of imitating noble materials, like stones and wood, often richer in color and veining than the originals, contributed to the maintenance and even expansion of its use until the end of the 19th century (Gárate-Rojas, 1999; Stark & Wicht, 1999).

Contrary to what is thought, gypsum application was not limited to interior relieved decorations of walls, architectural elements, or as a base for decorative frescoes, but also included masonry mortars (Ghorab et al., 1986; Ragai et al., 1987; Ragai, 1988 a , 1988 b , 1989; Kawiak, 1991; Turriano, 1996; Vogel et al., 1999; Fischer & Vtorov, 2002; Middendorf, 2002; Lucas, 2003 a , 2003 b ; Igea et al., 2010, 2012; Philokyprou, 2012), floor screeds (Gourdin & Kingery, 1975; Kawiak, 1991; Fischer & Vtorov, 2002; Philokyprou, 2012) and exterior wall coatings (S.N.I.P., 1982; Fischer & Vtorov, 2002; Lucas, 2003 a , 2003 b ; Sanz, 2009; Bakr et al., 2012). In these studies, the samples usually exhibited a higher mechanical and weathering resistance than ordinary gypsum plasters (resulting from the hydration of β-calcium sulfate hemi-hydrate (Wirsching, 2005)). In some cases, they were also more durable than other materials used for the same purposes, namely natural stones (Pecchioni et al., 2005).

Whenever microscopy observations of these gypsum-based coatings were made, the main microstructural features pointed out were the presence of squat, stocky, sometimes almost spherical crystals that formed a compact microstructure (Kawiak, 1991; Fischer & Vtorov, 2002; Middendorf, 2002; Lucas 2003 a , 2003 b ; Sanz, 2009; Jakubek et al., 2010; Schlütter et al., 2010; Philokyprou, 2012), instead of prismatic needle-like crystals that formed a highly porous (~50%) structure, typical of the most common gypsum plasters (Wirsching, 2005).

The art and technology of imitating different types of marble, also known as stucco-marmo (or scagliola, in Italian), appeared in Italy in the 17th century. This material was composed of gypsum plaster, animal glue, pigments, and water and was profusely used throughout Europe during the 18th and 19th centuries (Arcolao, 1998; Gárate-Rojas, 1999; Turco, 2008). According to Turco, most of them are still in very good condition.

Portugal was not an exception. Although the inventory of this heritage has not yet been made, the few studies about it highlight its importance and show many examples of high-quality works (Vieira, 2002, 2008). To know the techniques and the materials used in their execution is paramount to prevent the loss of many specimens and to establish a methodology of conservation and restoration based on the main principles that promote preservation of the world’s cultural heritage, namely the Krakow Charter of 2000 (DGPC, n.d.).

In this context, a gypsum plaster sample from the end of the 19th century simulating imperial red porphyry was characterized. The mineralogical and chemical composition and microstructural properties were determined using a multi-analytical approach. X-ray diffraction (XRD), thermo-gravimetry and differential thermal analysis [thermogravimetric and differential thermal analysis (TGA-DTA)], polarized light microscopy (PLM), scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS), and micro Raman spectroscopy analyzes were made.

Physical and mechanical properties were also determined, including water absorption by capillarity, hygroscopic behavior, water vapor permeability, dynamic modulus of elasticity, and compressive strength using test procedures adapted to irregular shaped samples (Veiga et al., 2004).

In this paper, the contribution of microscopy on the clarification of most issues raised by other techniques allowed the establishment of interrelations between the different properties and provided important information about the materials, the skills, and the technological development involved in the art of imitating noble stones, so important in the context of the European decorative arts.

Materials and Methods

Case Study

The Estoi Palace is situated in the village of Estoi, district of Faro, in the south of Portugal. Its construction started in the middle of the 19th century, but it was only finished between 1893 and 1909, with the decoration of the house and gardens. The decorative program of the Palace was conceived by Domingos Meira, considered to be the most important Portuguese plasterer of the second half of the 19th century (Mendonça, 2012). It results from a mix of architectonic styles, where Neoclassic, Neorocaille, and Art Nouveau are predominant. The Estoi Palace is said to have “the finest plaster ceilings in the Algarve.” In 2008, an extensive works campaign promoted the restoration and adaptation of the building to a luxury hotel.

The sample studied, detached due to the occurrence of anomalies, belonged to a decorated door-frame of the Noble room (Fig. 1).

Figure 1 Noble room of the Estoi Palace after restoration: (a) localization of the sample site; (b) detail of the door-frame simulating imperial red porphyry.

The sample was ~110 cm in height, 13 cm in width, and the thickness varied from 4 to 8 cm. The decorative layer represented only a few millimeters of its structure. The core consisted of a very thick, less dense white plaster reinforced with sisal fibers and an iron wire (Fig. 2).

Figure 2 Sample details: (a) as it arrived at the lab; (b) side view, where two layers are visible.

Experimental Data

The analytical characterization started with a photographic recording followed by a detailed visual observation, sometimes assisted by an Olympus SZH stereo-zoom microscope equipped with a video camera Olympus DP20.

Afterwards, the sample was dried at 40°C for ~12 h to enable the detection of potential hygroscopic compounds or soluble salts whose crystalline structure could be damaged if the sample were dried at higher temperature. The sample was then split into several fractions to be used in the various techniques; and each sample layer was also analyzed separately (Fig. 2b).

In order to study the textural properties of the different layers (stratigraphy) and to identify the mineralogy and morphology of the aggregates and possible pigments used in the binder fragments of the sample were impregnated under vacuum with an epoxy resin, followed by cutting and polishing procedures. The polished surfaces and polished thin sections obtained were observed using an Olympus SZH stereo-zoom microscope and an Olympus BX60 petrography microscope in transmission using crossed polarized light (XPL) and plane polarizers in the observations made at National Laboratory for Civil Engineering (LNEC). At Bremen Institute for Materials Testing (MPA Bremen), a Zeiss Axiophot Fluorescence microscope was used.

XRD analyses were performed separately in both layers to determine the mineralogical composition of the binder and other possible constituents such as aggregates. A Philips X’Pert diffractometer with Fe filtered cobalt Kα radiation (λ=1.7903 Å), operating at 35 kV and 45 mA was used. Powder diffraction data were collected in the range 3°–74° (2θ) in steps of 0.05°/s. XRD diffractograms were done on powder particles (<106 μm) and the data were analyzed using cards from the International Centre for Diffraction Data Powder Diffraction Files (ICDD PDF).

The TGA-DTA provided additional data on the quantitative composition of the samples, namely the relationship between the gypsum and carbonate contents. The samples, prepared as mentioned for XRD, were tested in a Setaram TGA92 TGA-DTA simultaneous analyzer, from room temperature to 1,000°C using an argon atmosphere with a uniform heating rate of 10°C/min.

Micro Fourier Transformed Infrared (FT-IR) spectroscopic analyses were performed in a Nicolet Nexus apparatus coupled with a Continuum microscope (15x objective) and a MCT-A detector cooled with liquid nitrogen. The spectra were acquired in transmission mode with a resolution area of 100 μm, spectral resolution of 4 cm−1 and 256 scans, using a Thermo diamond compression cell.

SEM-EDS observations were made allowing further insight into the microstructure and crystal morphology of the sample matrix and provided semi-quantitative chemical analyzes of individual particles and phases. The microscope used at LNEC was a JEOL JSM-6400 coupled with an OXFORD energy dispersive X-ray detector (EDS), both on polished [with backscattered electrons (BSE) images] and freshly fractured surfaces [using secondary electrons (SE) images] that were previously sputtered with gold-palladium film in a BALTEC sputter coater.

In the observations at MPA Bremen a field emission scanning electron microscope Hitachi S 4000 coupled with an EDAX EDS detector was used. The samples were coated with carbon.

The micro Raman spectra were obtained in a Renishaw InVia Raman microscope with a diode laser operating at 785 nm, 50× objective, spatial resolution down to ca. 1 µm, and the acquisitions were made with one scan during 10 s.

Water absorption tests were performed using capillary absorption by a contact technique developed for irregular and/or friable samples (Veiga et al., 2004).

The water vapor permeability was determined using the method described in EN 1015–19:1998 (1998), adapted to the size of the sample. This method is based on measurement of the water vapor flux through the sample due to the pressure differential between the internal (relative humidity ~100%) and the external surface conditions (relative humidity=50±5%) at 23±2°C. The vapor flux was quantified by the weight variations of the test devices.

The procedure used to evaluate hygroscopicity (Magalhães & Veiga, 2007) consisted of measuring the weight variations of samples previously dried at 40°C until constant mass and then placed in a climatic chamber at a temperature of 23±2°C and increasing moisture conditions: 30, 50, 70, and 90% (adsorption conditions). The samples were kept at each moisture level until constant mass was achieved. They were then subjected to the same relative humidity levels in reverse order (desorption conditions) using the same weighing procedure.

The pore size distribution curves of each layer were determined using mercury intrusion porosimetry based on ASTM D4404–84:2004 and carried out using the Filling Apparatus and Autoscan 60 of Quantachrome. Each sample consisted of two specimens previously ground to granular form and dried to constant mass at 40°C. After that, they were placed in individual bags and maintained in a desiccator until testing. At the beginning of the test procedure they were subjected to degasification for at least 30 min, at room temperature, using a vacuum of 50 or 60 mmHg. For the contact angle a standardized value of 140° was used in all determinations.

The bulk density, needed for calculation of the dynamic moduli of elasticity and the compressive strength values, was evaluated using the water displacement method based on measurement of the water volume variation inside a graduated cylinder right after the insertion of previously weighed fragments of the sample (as a whole) and of each layer.

The dynamic moduli of elasticity were determined using the method based on measurement of the velocity of high frequency sound waves (ultrasonic pulse velocity) through the materials under study, allowing the calculation of elastic parameters (EN 12504–4:2007, 2007). Special exponential probes with pointed ends were used to provide good and precisely located contact with the sample surfaces.

The compressive strength test was performed on an electromechanical testing device complying with the requirements of EN 1015–11, from the Spanish company Hoytom S.L., model HM-S, with a load cell of 200 kN. The load rate of 100 N/s was adjusted according to EN 1015–11 so that failure occurred within a period of 30–90 s.

Results and Discussion

Visual Observation

In general terms, the door-frame simulating imperial red porphyry was composed of two layers, an inner one with white color (layer “1”) and an external one, thinner and of reddish color (layer “2”). In a front view, it seems made of stone. The brightness and stiffness of the decorative layer, together with the perfect porphyry effect given by colored particles, are remarkable and highlight the high quality of execution. These aspects were confirmed by observation of the sample with a stereo-zoom microscope (Fig. 3).

Figure 3 Stratigraphic observation: (a) polished surface, where three different layers are visible; (b) interface between layers “1A” and “1B”; (c) interface between layers “1B” and “2,” and detail of layer “2” where light purple and orange particles are visible.

The stratigraphic observation allowed acknowledging the presence of a double layer (referred as “1A” and “1B”) in the inner part (Fig. 3a). Looking at Figure 3b, a small fracture at the top is visible with a slight difference in color and texture that starts diagonally from there, indicating that this very thick element was produced in two steps, one immediately after the other, as the interface between them is almost imperceptible. When observed with higher magnifications this area is less perceptible, but looking directly into a fractured sample a preferential fracture pattern through a smoother surface is clearly visible as the joint surface (Fig. 4):

Figure 4 Fracture pattern attesting the existence of a joint interface in the white layer: “1A” and “1B” layers.

The perfection of the union between layers “1” and “2” is also remarkable (Fig. 3c) and will be further illustrated in the “SEM-EDS and PLM observations” section.

In layer “2,” the required decorative effect was achieved through the use of two types of particles (arrows of Fig. 3c): those with a light brown effect, thought to be quartz on the first observations at the stereo-zoom microscope due to its apparent transparency, and the orange ones, thought to be pieces of burned clay.

The information obtained by visual observation of the sample is summarized in Table 1.

Table 1 Summary of Visual Characteristics Observed.

Mineralogical Characterization

The qualitative mineralogical composition of the different layers of the sample was determined by XRD and the results obtained showed that gypsum is the main constituent. Calcite is only present in layers “1A” and “1B.” Anhydrite and hematite were detected in layer “2” (Table 2), but the use of aggregates of different origin (like quartz or fired clay) was not confirmed: the traces of quartz and feldspars found are due to their common occurrence in the Earth’s crust; dolomite and celestine are impurities associated with the gypsum deposits.

Table 2 X-Ray Diffraction (XRD) and thermogravimetric and differential thermal analysis (TGA-DTA) Results.

Notation used in XRD evaluation: ++++very high proportion (predominant compound); +++high proportion; ++medium proportion; +weak proportion; –not detected.

Trc, traces.

Hematite, considering the colors of layer “2,” was clearly used as pigment.

Finally, anhydrite can be either a sub-product of the calcination process or an impurity of the raw material. However, in the case of layer “2,” its use could only be clarified by SEM-EDS and will be further discussed in the corresponding section.

Quantification of the gypsum and calcite contents was assessed using TGA-DTA analysis and this confirmed the results of XRD (hematite and anhydrite do not undergo transformations with weight losses associated in the temperature range used, i.e. from room temperature to 1,000°C).

Similar TGA curve profiles of layers “1A” and “1B” (Freire et al., 2009), corresponding to similar weight losses, indicate that they were made of the same gypsum plaster mixture, which is also in agreement with the perfect interface between them (Fig. 3b). The difference in color is probably caused by the incorporation of sisal fibers in “1A” and the lower porosity of “1B” may possibly be due to the fact that layer “2” applied over it was pressed hard during execution.

Another important feature of these sample layers is the low and very similar weight losses obtained between 250 and 600°C (around 0.6%, Freire et al., 2009). These results are not surprising for the two internal layers as the sisal fibers of “1A” were retained in the sieve during preparation of the sample for analysis, but are somehow unexpected for the external one (layer “2”). In fact, the smoothness and brightness of its surface seem to indicate that some kind of organic addition was used, like wax or oil. However, in preparation of the samples the criterion of collecting only their core was always respected, in order to avoid contamination between consecutive layers. That is probably the reason why the hypothetical additions were not detected in layer “2” by this method.

Table 2 summarizes the information obtained by XRD and TGA-DTA analysis.

Micro FT-IR Analysis

Micro FT-IR spectroscopy was used to investigate the presence of organic compounds in layer “2.” Extraction of about 1 g with ether during 24 h was performed. The resulting solution was filtered and the filtrate was dried and analyzed by micro FT-IR.

The presence of beeswax was detected, a compound commonly used to polish the external surface and give it the brightness of a real stone (Gárate-Rojas, 1999).

The presence of other iron pigments, like goethite, was also investigated but further analyses, namely using micro Raman, did not confirm this hypothesis.

Micro Raman Analysis

The micro Raman spectra of the sample’s different constituents (Fig. 5) were obtained in order to complement the mineralogical composition. The data used for their interpretation is presented in Table 3.

Figure 5 Micro Raman spectra of the different constituents of the sample (performed by Isabel Pombo Cardoso).

Table 3 Data Used for the Interpretation of the Micro Raman Spectra (Bell et al. 1997; Burgio & Clark 2001).

The bands wavenumbers are presented in intervals according to the values given by the experimental conditions of the bibliographic references used.

a Synthetic pigment that replaced lapis lazuli since 1828.

The “1” spectrum, with bands at 414, 492, 1,008 and 1,135 cm−1, confirmed that gypsum is its main constituent.

In layer “2,” the presence of anhydrite (bands at around 609, 671, and 1,134 cm−1 and a shoulder at 1,017 cm−1) and hematite (224 and 290 cm−1) can also be detected in all the different colored sub-samples, with the paste having a significant concentration of this pigment, followed by the light purple and orange aggregates, in decreasing order. The purple paste and the light purple aggregate spectra also showed the presence of carbon black (broad bands at 1,330 and 1,590 cm−1) and ultramarine blue (545–547 cm−1). The main function of the first was to darken the pastes; the second, when used in very small quantities, usually had the purpose of giving a purple hue.

The bands at 492–494 cm−1 in the three spectra of layer “2” result from the presence of gypsum (493 cm−1, weak intensity), anhydrite (498 cm−1, medium intensity) and hematite (495 cm−1, medium intensity).

Finally, the presence of other iron pigments was not confirmed.

Physical and Mechanical Properties

The results of the physical and mechanical tests performed are summarized in Table 4.

Table 4 Physical and Mechanical Properties (Freire et al. 2011).

Ccc, capillary coefficient by contact; Sd (d=10 mm); thickness of the air coat of equivalent diffusion (Sd) through a 10 mm thickness test specimen; UPS, ultrasonic pulse velocity; DME, dynamic modulus of elasticity; Cs, compressive strength.

The values found indicate that layer “2” has about twice the density of layer “1,” which is in agreement with all the other properties determined, both separately (capillary water absorption, ultrasonic pulse velocity and dynamic modulus of elasticity) or as a whole (water vapor permeability and compressive strength) where the influence of layer “2” has also been noted. In fact, the water vapor permeability is much lower than that of the most common gypsum plaster materials (Sd<0.10 m) and the compressive strength is higher (Freire et al., 2011).

SEM-EDS and PLM Observations

In order to have further insight into the microstructure, polished surfaces, fractured surfaces, and thin section preparations were observed by SEM-EDS, whereas the thin sections were also observed with PLM.

The perfection of the interface between layers “1” and “2,” already illustrated in Figure 3c, is now assessed by SEM (Fig. 6).

Figure 6 Interface between layers “1” and “2”: (a) SEM-BSE (polished surface); (b) SEM-SE (fractured surface), where the presence of sisal fibers in layer “1” is perceptible (white arrows). SEM, scanning electron microscopy; BSE, backscattered electrons; SE, secondary electrons.

Using SEM, significant differences in density/porosity of the two main layers are clearly detected (Fig. 7).

Figure 7 SEM-SE images of fractured surfaces showing the difference of density/porosity between both layers: (a) layer “1”; (b) layer “2.” SEM, scanning electron microscopy; SE, secondary electrons.

At higher magnifications, it appears that the morphology of both parts is also very distinct: layer “1” has mainly prismatic, needle-like crystals (Figs. 8a, 8c) and layer “2” has squat, stocky crystals (Figs. 8b, 8d).

Figure 8 Images at higher magnifications showing the different crystal morphologies of the binder: layers “1” (a) and “2” (b), SEM-BSE mode; layers “1” (c) and “2” (d), SEM-SE mode. On the right side of (c) and (d) images of fractured surfaces are the respective EDS spectra, that are very identical and both referring to calcium sulfate dihydrate. SEM, scanning electron microscopy; BSE, backscattered electrons; SE, secondary electrons.

So, they seem to correspond to completely different compounds or, if to the same compound, at least formulated with different additives. However, as stated before, XRD, TGA–DTA analyses, and now the EDS spectra show that they are mainly made of gypsum (CaSO4.2H2O). Besides, in the TGA-DTA results the temperature range where organic compounds usually decompose (250–600°C) presented a negligible loss of weight. It is very unlikely that any of the layers had organic additives incorporated in the paste.

The only differences in composition seem to be the presence of hematite (Table 2 and Fig. 8b) and other pigments (Table 3) and a higher content of anhydrite (Tables 2 and 3) in layer “2.” Could one, or both, of these factors be responsible for such a huge alteration on the crystal growth and interlock and on the density/porosity? And besides that, are there any other differences between these two layers?

A plausible explanation for the distinct crystal morphologies of layers “1” and “2” is that they were prepared with different gypsum phases: mainly hemihydrate in the former and anhydrite in the latter.

But how were these two phases obtained separately? Two possibilities can be pointed out:

  1. (a) The calcined gypsum used for both was the same, and the plasterers separated the phases according to the final product they wanted to obtain.

    In fact, in traditional kilns it is impossible to have total control of the calcination process. Parameters like temperature, type of raw material, time of the process, and size and relative position of the stones have an important influence and the resulting products are always multiphase gypsum plasters (in the larger stones, e.g., the borders are usually anhydrite but the nucleus is often raw gypsum). All these facts were widely known by the plasterers so that, whenever needed, the different phases (dihydrate, hemihydrate, and anhydrite) could be separated from the start (Cardoso, 2006; Sanz, 2007; Villanueva, 2004):

  2. (b) They were really products prepared differently: the plaster for layer “1” was calcined at lower temperatures than the plaster used in layer “2.”

Nevertheless, none of these procedures guarantees a 100% efficient separation of phases, as there are always remains of the so-called “firing products.”

Seeking answers to these questions, additional SEM observations of thin sections and polished and fractured surfaces of both layers allowed characterizing them in a more complete way.

Layer “1”

Further images of the typical micro structure of layer “1,” more precisely in the area of “1B,” are shown in Figure 9.

Figure 9 Images of thin sections showing the microstructure of layer “1B”: (a,c) PLM, crossed nicols; (b,d) SEM-BSE mode. Notation: G, gypsum grains, some partly dissolved (dashed lines) or totally dissolved (thicker arrows); L, limestone grains; Q, quartz; F, Na, K feldspars; M, Marl; dashed arrows, round pores, indicative of the use of a relatively fluid paste. PLM, polarized light microscopy; SEM, scanning electron microscopy; BSE, backscattered electrons.

Besides confirming that it is a very porous micro structure, the thin section observations gave a more complete overview of the composition of the matrix. The images of Figure 9 show that layer “1” has a relatively high amount of fine grains (grain size <0.5 mm), namely limestone, quartz, feldspars, and marl.

The last three compounds are considered to be impurities of the raw material. For limestone, this explanation did not seem enough to justify its more frequent occurrence. The results obtained by TGA–DTA analysis indicated the presence of 10% calcite, which could also have the contribution of the addition of calcitic aggregates or hydrated lime. However, neither of these two hypotheses was confirmed: the limestone grains did not show the shape/appearance of having been processed (milled); the calcite crystals resulting from the carbonation of lime would have a different morphology than those observed in layer “1” (Fig. 10).

Figure 10 Layer “1B”: (a) detail from Figure 9b (dashed frame) showing a limestone grain (L) and a partly dissolved gypsum grain (G); (b,c) fractured surface images of compact agglomerates of precipitated micro grains filling some voids of the matrix, SEM-SE mode; (d) EDS spectrum of the agglomerate of micro grains in (c), showing they are composed of calcium carbonate. SEM, scanning electron microscopy; SE, secondary electrons; EDS, energy dispersive X-ray spectrometer.

In fact, some calcite formations observed, both in fractured surface (Figs. 10b, 10c) and in thin sections (Fig. 11d), have a different aspect, seeming to be constituted by compact agglomerates of precipitated micro grains that fill some voids of the matrix. The respective EDS spectra show they are composed of calcium carbonate (Fig. 10d), which can justify the total carbonates content of layer “1.”

Figure 11 Layer “1B”: (a) detail from Figure 9d, taken in the largely dissolved gypsum grain (dashed circle at the bottom right side): dihydrate showing lamellar structure (thick white arrow); primary anhydrite showing the typical holes (black arrows); granular anhydrite (dashed arrows) surrounded by dihydrate; (b) EDS spectrum of the primary anhydrite; (c) thin section showing a grain of fibrous pseudo-morph dihydrate, PLM, crossed nicols (performed by José Mirão); (d) thin section showing another type of limestone particle (left) and a primary anhydrite grain (upper right), PLM, crossed nicols; (e) detail of the same primary anhydrite grain of (d) showing holes, SEM-BSE mode. PLM, polarized light microscopy; SEM, scanning electron microscopy; BSE, backscattered electrons.

The micro calcite grains probably originated by the presence of water in the background (a lime-based mortar). The dissolution of some calcite and the transport of its constituents between the background and the sample analyzed is very plausible, resulting in the precipitation of calcite crystals in the voids of the latter (as said before, the sample was already detached due to the occurrence of anomalies).

The matrix also contains gypsum grains that are remains of the calcination process and should not be considered as aggregates. Many of these gypsum grains show dissolving structures, which is a phenomenon difficult to explain satisfactorily. All stages of dissolution can be observed (Figs. 9b, 9d). In the final stage (total dissolution), the result is the formation of irregular shaped and sized voids like the ones shown in Figures 9b and 9d (thicker arrows). However, round pores are also present (Fig. 9d, dashed arrows), indicating that the gypsum paste of layer “1” has been used in a relatively fluid consistency.

Going back to the dissolving grains, at higher SEM-BSE magnifications, it is possible to see that in some the remaining particles are a mix of dihydrate and anhydrite (Fig. 11). The gypsum-anhydrite differentiation by SEM-BSE is very effective as the resulting images represent “atomic number contrast” which, in turn, depend on the density and atomic number of the elements present in the test specimen (Jordan et al., 1991). As the average atomic number of anhydrite is 11 and that of gypsum is 7, the two phases can be clearly distinguished by grey level differences, with anhydrite appearing brighter than gypsum.

The majority of the anhydrite crystals resulting from the calcination process are fibrous or strip-shaped (called thermo-anhydrite) and crystalize as dihydrate. Their original shape is usually kept pseudo-morph, i.e. the dihydrate grains grow in the same direction as the beam of fibers of the anhydrite grains (Fig. 11c), with a quite similar orientation, though with a more squat morphology (Figs. 11a, 11c).

Granular anhydrite is characterized by round edges and melting marks (Fig. 11a). It results from the calcination of raw material at higher temperatures (usually above 350°C) and it is very difficult to hydrate, being called “insoluble anhydrite.” Its proportion rises with the increase in firing temperatures.

The morphological features observed in Figure 11a are consistent with the mechanism of hydration described by Sievert et al. (2005): a partial dissolution of the anhydrite grains at the surface is followed by adsorption of the resulting hydrated Ca2+ and SO4 2− ions. When the thickness of the adsorbed layer increases, it starts to crack and water molecules come in contact with the fresh surface of anhydrite, resulting in the formation of gypsum nuclei. The crystallization of gypsum starts. The larger the amount of gypsum formed, the more difficult the hydration of the remaining anhydrite. If the temperatures of calcination are higher, the solubility of the anhydrite obtained is lower, leading to a higher content of grains that do not hydrate, as observed in layer “2.”

In this case, the previously referred circulation of some water inside the matrix (very frequent in old buildings) has probably allowed the slow transformation of anhydrite to dihydrate to proceed. However, this circulation of water can also be responsible for some dissolution of the dihydrate formed, which could explain the different stages of dissolution of these type of grains.

The primary anhydrite is natural anhydrite that exists in the raw material. When its grains present holes they are said to have been “thermally damaged” (Figs. 11a, 11e). This type of anhydrite remains unchanged after hydration of the paste, even when treated with additional water (Schlütter et al., 2010).

Fractured surface observations allowed a different perspective of the quartz grains and of a round pore (Fig. 12a) as well as the sisal fibers embedded in the gypsum matrix that were already visible with the naked eye (Figs. 12b, 12c).

Figure 12 SEM-SE images of layer “1” showing: (a) quartz grains (white arrows) and a round pore (upper right corner); (b) sisal fibers embedded in the gypsum matrix; (c) the same as (b), but at a higher magnification. SEM, scanning electron microscopy; SE, secondary electrons.

Another important observation in the micro structure of layer “1” is the formation of secondary phases that result from the “thermal activation” of some of the impurities present in the gypsum stone, namely marls often consisting of mixtures of calcite, dolomite, and clay minerals (Fig. 13). When subjected to high enough temperatures, the partial decomposition of the dolomite grains result in calcium and magnesium richer phases (equations 1 and 2) that, in contact with sulfur trioxide (from the partial dissociation of gypsum) (equations 3 and 4) are able to produce the respective calcium and magnesium sulfates (CaSO4 and MgSO4) (equation 5) (Fig. 14).

(1) $$\eqalign{ {\rm CaMg}\left( {{\rm CO}_{3} } \right)_{2} \,_{{{\rm (s)}}} \,\buildrel {450{\minus}700\,^{\circ}{\rm C}} \over \longrightarrow \!\mathop{{{\rm CaCO}_{3} _{{{\rm (s)}}} {\plus}{\bf{MgO}} \,_{{{\bf (s)}}} {\plus}{\rm CO}_{2} _{{{\rm (g)}}} }}\limits_{{\downarrow 700\,^{\circ}{\rm C{\minus}900}^{\circ}{\rm C}}} \quad(1)$$
(2) $${\rm CaO}\,_{{{\rm (s)}}} {\plus}{\rm CO}_{2} \,_{{{\rm (g)}}} $$
(3) $$\eqalignno{ {\rm CaSO}_{4} .2{\rm H}_{2} {\rm O}_{{{\rm (s)}}} \,\buildrel {700{\minus}1180\,^{\circ}{\rm C}} \over \longrightarrow \,\cr\mathop {{{\rm CaSO}_{4} \,_{{\left( {\rm s} \right)\,{\rm (}}} _{{{\rm partially}{\rm .}\,{\rm dissociated)}}} {\rm {\plus}2H}_{{\rm 2}} {\rm O}_{{{\rm (g)}}} }}\limits_{{\downarrow\,\gt\,1180^{\circ}{\rm C}}} $$
(4) $${\rm CaO}\,_{{{\rm (s)}}} {\plus}{\bf{SO}} _{{{\bf{3}} \,{\bf{(g)}} }} $$
(5) $${\rm CaO}\,_{{{\rm (s)}}} {\plus}{\rm MgO}\,_{{{\rm (s)}}} {\plus}{\bf 2}{\bf{SO}} _{{\bf 3}} _{{{\bf{(g)}} }} \,\to\,{\rm CaSO}_{4} \,_{{{\rm (s)}}} {\plus}{\bf{MgSO}} _{{\bf 4}} \,_{{{\bf{(s)}} }} $$

Figure 13 SEM-BSE images of thin sections of layer “1B”: (a) detail from Figure 9b (full line frame) showing a limestone grain (L) and a thermally damaged marl grain (M); (b) detail of (a) (white frame): the bright grains are calcium richer phases resulting from partial decomposition of the calcite present in the marl [EDS spectrum (c)] and the dark “matrix” are thermally changed marl impurities [EDS spectrum (d)]. SEM, scanning electron microscopy; BSE, backscattered electrons; EDS, energy dispersive X-ray spectrometer.

Figure 14 SEM-BSE image of the matrix in layer “1,” showing a round pore on the right side and irregular voids resulting from the dissolution of larger anhydrite/gypsum grains, some partially filled with CaSO4.2H2O crystals (dashed arrows) and others totally filled with MgSO4 (full arrows). The EDS spectrum refers to the last voids content. SEM, scanning electron microscopy; BSE, backscattered electrons; EDS, energy dispersive X-ray spectrometer.

Similar observations were made in German high-fired gypsum mortars produced for restoration purposes (Schlütter et al., 2010). However, as stated before, the crystal morphology of layer “1” indicates that the material used in its production was mainly hemihydrate (Fig. 8), which does not need very high temperatures to be obtained (~150–200°C) (Wirsching, 2005).

So, the presence of some phenomena that only occur at much higher temperatures (like the presence of holes in primary anhydrite grains or the thermal activation of marls), can be due to either the presence of very small particles of raw material in the kiln (almost powdered) that are completely burned during the calcination process, or the hemihydrate used resulted from manual separation of the different phases of gypsum obtained, which is not a 100% efficient process and always has small quantities of other phases mixed in (unburned and over burned).

Layer “2”

Layer “2” has a much denser microstructure than layer “1” (Fig. 15). As expected, the hematite pigments are more visible in the binder matrix (paste) than in the colored “aggregate particles.” In PLM images they appear like black holes as they cannot be transmitted by light due to their particle size.

Figure 15 Microstructure of layer “2” with hematite grains pointed to by arrows: (a) dark purple matrix with the gypsum colored aggregate particles perceptible, PLM, crossed nicols; (b) detail of (a) (white frame); (c) the same area as (b), but in SEM-BSE mode and at lower magnification: the dashed frame corresponds to Figure 8b; (d) binder matrix with a largely dissolved former anhydrite grain (dashed line), SEM-BSE mode. PLM, polarized light microscopy; SEM, scanning electron microscopy; BSE, backscattered electrons.

Similarly to layer “1,” using the SEM-BSE mode, it is possible to see that in some of the larger dissolving grains the remaining particles are a mix of dihydrate and anhydrite (Fig. 16). However, layer “2” shows a much higher content of granular anhydrite than layer “1,” indicating that the calcination temperatures of the plaster used to prepare it were higher.

Figure 16 Microstructure of the matrix in layer “2” at SEM-BSE showing granular anhydrite grains surrounded by dihydrate: (a) detail of Figure 15d (white frame); (b) detail of (a) (white frame); (c) binder matrix with a partly dissolved anhydrite grain corresponding to a colored aggregate; (d) detail of the matrix of the aggregate grain in (c). SEM, scanning electron microscopy; BSE, backscattered electrons.

The calcium sulfate dihydrate crystals resulting from rehydration of high temperature calcined gypsum are non-prismatic, squat, with interpenetration textures, leading to high density and low porosity structures (Lucas, 2003 b ; Tesch & Middendorf, 2005; Schlütter et al., 2010); all these features also occur in layer “2” (Fig. 8d).

Multiphase gypsum plasters, particularly those with a high content of anhydrite II, are usually prepared with higher plaster/water ratios than hemihydrate plasters, which directly influence their porosity/density. They also have a quicker initial setting time (mainly due to the presence of anhydrite III) and a longer final setting time (due to the presence of anhydrite II) (Wirsching, 2005; Schlütter et al., 2010). This means that in a multiphase gypsum plaster mechanical strength and workability both can be improved (Wirsching, 2005; Tesch & Middendorf, 2005), very useful issues when thinking about the intended stone-like effect of the numerous decorated gypsum plaster pieces similar to this door-frame sample from the Noble room of Estoi Palace.

Different morphologies of the paste exhibiting crystals of higher dimensions and occurring in areas of the matrix where they have more room to grow were also observed in layer “2” (Fig. 17). The respective EDS analyzes made in all the (e) and (f) areas of Figure 17 [images (a) to (d)] reveal that they are all gypsum. The exception is the crystal of Figure 17g that, in spite of having a similar spectrum, has a morphology typical of anhydrite.

Figure 17 ad: SEM-SE images of the paste in layer “2” showing different morphologies of gypsum; (e) typical EDS spectrum of the matrix, obtained in the (e) areas; (f) typical EDS spectrum of the different crystal shapes of gypsum, obtained in the (f) areas; (g) SEM-SE image of a fibrous grain (typical shape of thermo-anhydrite) and respective EDS spectrum (h). SEM, scanning electron microscopy; SE, secondary electrons; EDS, energy dispersive X-ray spectrometer.

The gypsum fake “aggregates” in the paste were also investigated by SEM. They were not visible in the images of fractured surfaces but in polished surface and thin sections, using the BSE mode, its presence was noticed (Fig. 18). These observations reinforce the fact that the colored “aggregates” are the result of previously prepared pigmented pastes made of the same gypsum as the matrix, that were ground after hardening and drying. Only after that were they mixed in the “main paste” while it was still fresh. Such procedure is in agreement with a recipe to imitate granites and porphyries referred by Arcolao (1998).

Figure 18 Images of the purple matrix (PM) of layer “2” with orange (O) and light purple (LP) aggregates: (a) polished surface with stereo-zoom microscope (white frame of Fig. 3c); (b) the same as (a) but in SEM-BSE mode; (c) thin section, PLM with plane nicols, authored by José Mirão. PLM, polarized light microscopy; SEM, scanning electron microscopy; BSE, backscattered electrons.

The three different colored parts of layer “2” were analyzed using SEM-EDS and the respective spectra are shown in Figure 19. The main difference observed is the higher quantity of iron (Fe) in the purple matrix, meaning that hematite is more concentrated there than in any of the aggregate particles. These results are in total agreement with those of micro-Raman presented in the previous section.

Figure 19 Comparative EDS spectra of the three different colored parts of layer “2.” EDS, energy dispersive X-ray spectrometer.

The thin section observations allowed clarifying how hematite was used to obtain the different colored effects present in the decorated layer: in the orange aggregates it was used as powder; in the light purple aggregates in small (but not powdered) particles; in the purple matrix both kinds of hematite are present (Fig. 20).

Figure 20 Thin section observations of layer “2” (PLM, crossed nicols) showing the different grain sizes of hematite pigments (white arrows): (a) light purple aggregate; (b) orange aggregate and purple matrix (images authored by José Mirão). PLM, polarized light microscopy.

The particle size of hematite determines the respective color effect: the bigger the particles, the darker the effect is; powdered hematite produces an orange color. Thus, it seems illogical that the purple matrix, which has both grain sizes of hematite, is the darkest part of the sample. This issue is explained by the presence of other pigments, namely carbon black, detected in the micro-Raman analyses.

Another important observation is that powdered hematite is amorphous, though not detectable by XRD, which means that layer “2” must have higher hematite content than the one given by this analytical technique.

Hematite’s presence was also observed in fractured surface. Figure 21 is a SEM-SE image of a crystal of fibrous anhydrite surrounded by hematite grains that are visibly well embedded in the gypsum matrix.

Figure 21 SEM-SE image of the matrix in layer “2” showing a crystal of fibrous anhydrite (A) and grains of hematite (H) and respective EDS spectra. SEM, scanning electron microscopy; SE, secondary electrons; EDS, energy dispersive X-ray spectrometer.

An additional detail that strengthens the argument that the layer “2” structure results from the hydration of a multiphase gypsum plaster predominantly made of anhydrite II is that the colored “aggregates” are the result of previously prepared pigmented pastes also made of gypsum. The accelerating effect that hardened gypsum particles have in a mixture of water and a hemihydrate or a multiphase gypsum plaster is well known (Wirsching, 2005). As the presence of additives is unlikely in this sample’s core, it seems that it was probably necessary to prepare the gypsum paste of layer “2” using a gypsum plaster deliberately calcined at higher temperatures (according to some morphological aspects observed by Frank Schlütter in thin sections, they were around 300–400°C) and/or for a longer time, in order to be mainly made of anhydrite with low setting time (anhydrite II).

Further features, namely minor constituents that are impurities of the raw material(s) used in the preparation of layer “2” (matrix and “aggregates”) were also detected through the thin sections and fractured surface observations using SEM and PLM. In Figure 22, only celestine and calcium phosphate are impurities of the raw material; hematite is an addition, as already discussed.

Figure 22 SEM-BSE image of layer “2” showing grains of three compounds: hematite (H), calcium phosphate (Ca-P) and celestine (Ct) and respective EDS spectra. SEM, scanning electron microscopy; BSE, backscattered electrons; EDS, energy dispersive X-ray spectrometer.

Another example of an impurity is given in Figure 23. The EDS spectra of the grain and of the small bright area observed in it showed that it is dolomite with an inclusion of feldspar.

Figure 23 SEM-BSE image of the matrix in layer “2” showing a grain of dolomite (D) with a feldspar inclusion (bright area, F) and respective EDS spectra. SEM, scanning electron microscopy; BSE, backscattered electrons; EDS, energy dispersive X-ray spectrometer.

In fractured surface observations some grains with an unusual shape were detected and also identified by EDS as feldspars (Fig. 24).

Figure 24 SEM-SE image of the matrix in layer “2” showing two grains of feldspars and respective EDS spectrum. SEM, scanning electron microscopy; SE, secondary electrons; EDS, energy dispersive X-ray spectrometer.

Similarly to layer “1,” the formation of secondary phases, like magnesium sulfate, has also been identified in layer “2” (Fig. 25).

Figure 25 Microstructural observations of layer “2” showing some voids filled with MgSO4 (white arrows): (a) PLM, crossed nicols; (b) SEM-BSE image; (c) EDS spectrum of the voids’ content. PLM, polarized light microscopy; SEM, scanning electron microscopy; BSE, backscattered electrons; EDS, energy dispersive X-ray spectrometer.

The main microstructural characteristics of layers “1” and “2” are now summarized in Table 5.

Table 5 Summary of the Microstructural Observations of the Layers “1” and “2.”


The use of a multi-analytical approach to study an ancient gypsum plaster sample designed to imitate imperial red porphyry proved to be very fruitful, resulting in a compilation of quite complete information about the materials and the technological features used in its manufacture. Such knowledge is extremely valuable for the establishment of restoration methodologies of these outstanding decorative elements whose presence in buildings of higher heritage value in the Portuguese architecture, mainly belonging to the 19th century, is significant.

In this paper, some properties of the sample analyzed are summarized. Measurements of density, capillary water absorption, dynamic modulus of elasticity and compressive strength were made and they confirm the existence of significant differences between both constituent layers.

A deeper insight into the microstructure was made using microscopy techniques. The main results are presented and discussed and the following conclusions can now be drawn:

  1. (a) The presence of different minor constituents (impurities) in the two layers means that the raw material used was not the same for both;

  2. (b) The distinct crystal morphologies found for the same compound (calcium sulfate dihydrate) indicate that the gypsum phases used as binder were also different:

  • in layer “1” the plaster was mainly composed of hemihydrate; and

  • in layer “2” the starting product was predominantly anhydrite II.

So, the firing temperatures were clearly higher in the material used in layer “2” than in layer “1.” However, its reactivity (XRD results showed that the anhydrite content is low) seems to indicate that the temperature of calcination was predictably around 300–400°C, much lower than that indicated for the production of medieval high fired gypsum mortars used in exterior surfaces (around 900°C) (Lucas, 2003 a , 2003 b ; Sanz, 2009; Schlütter et al., 2010).

This fact is very important because it indicates that some control of the calcination could already be achieved. In the medieval times, it was easier to obtain a temperature of 900°C in the kiln than a much lower one (Lucas, 2003 a ) as is the case of 300–400°C.

A possible explanation for this technological improvement is that Estoi Palace decoration works took place at the end of the 19th to the beginning of 20th centuries, under the direction of Domingos Meira. As stated before, the work of Meira was well recognized around Portugal and also abroad: he was awarded in the international exhibitions of Paris (1900), Chicago (1904), and Rio de Janeiro (1908), and his workshop was responsible for the most important plasterworks in Portugal from the mid-1800s (Mendonça, 2012). A high production demanded a high volume of related materials, namely gypsum, so in the last decade of the 19th century Meira was the owner and manager of a gypsum steam plant, in Lisbon, equipped with two kilns, in order to control the quality and production of the material used in his works (Mendonça, 2012).

  1. (c) Even though the plaster used to prepare layer “1” was mainly hemihydrate, some substances typical of high firing temperatures were observed in its microstructure, like different forms of anhydrite and thermally changed marls. This can be due to the presence of small grains of raw material in the kiln that dehydrate earlier than the average gypsum stones and easily reach much higher temperatures during the time that the calcination process takes (over burned products).

  2. (d) XRD and SEM-EDS results indicated that the colored aggregate particles used were made of the same base material as the matrix where they were embedded.

  3. (e) PLM observations of thin sections allowed concluding that those colored aggregates visible in the decorative layer correspond to previously prepared pastes with different additions of pigments. After hardening and drying they were ground and the resulting “aggregates” were then mixed with a new paste, a procedure in agreement with a compilation of old plaster recipes.

  4. (f) Besides hematite, the use of micro-Raman allowed detection of the presence of two additional pigments in the purple matrix of layer “2” and in the light purple aggregates: carbon black and ultramarine blue.

  5. (g) Micro FT-IR analyzes revealed the presence of beeswax at the surface of the sample, probably used to polish it and give it a “stone effect.”


M.T.F.’s PhD research study was supported by the scholarship SFRH/BD/40128/2007 from Fundação para a Ciência e Tecnologia (FCT).

The authors would like to thank the technicians Paula Menezes from the Materials Department of LNEC, and Ana Duarte, Dora Santos, and Bento Sabala from the Buildings Department, for their support on the execution of the experimental work; LNEC for the support within the project 0204/11/17692 Materials with Historical Interest—Durability and Characterization, and the research center CERIS from Instituto Superior Técnico, University of Lisbon.


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