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Environmental Earth Sciences ISSN 1866-6280 Environ Earth SciDOI 10.1007/s12665-012-2035-y

Differences in the quality of polishingbetween sound and weathered granites

Luís M. O. Sousa & BrunoM. M. Gonçalves

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SPECIAL ISSUE

Differences in the quality of polishing between soundand weathered granites

Luıs M. O. Sousa • Bruno M. M. Goncalves

Received: 1 June 2012 / Accepted: 29 September 2012

� Springer-Verlag Berlin Heidelberg 2012

Abstract Aesthetic characteristics are important when

rocks are used as construction materials. Among all the

surface finishes available, the polished finish is the one

which best enhances the rock attractiveness. Colour (C),

roughness (R) and gloss (G) are surface properties usually

used to assess the rock polishing. In this research, the CRG

properties were assessed in polished samples of three

ornamental granites with different textures and physical–

mechanical properties, one weathered and two sound.

C was evaluated in CIE-L*a*b* and CIE-L*C*ab hab systems,

R with Ra and Rz parameters and G using the 20�, 60� and

85� geometries. The weathered granite presents a notice-

able colour variation, especially in the b* parameter, while

the sound ones have a more homogeneous colour. Sound

and more textural homogeneous granites have a low sur-

face roughness and high gloss, which are uniformly dis-

tributed. The weathered granite shows a higher surface

roughness and lower gloss, with scattered distribution. The

mineral contacts are the main cause of the surface irregu-

larities. The results show the interdependence between the

CRG properties and stress the importance of the polishing

process for enhancing the aesthetic characteristics of the

granites. CRG properties could be used for quality control

in granite processing plants, thus avoiding noticeable dif-

ferences in the facades of buildings, particularly when

weathered granites are used.

Keywords Granite � Polishing � Colour � Roughness �Gloss

Introduction

Stone products can only be successful in the world market

if they can be competitive with other construction materi-

als. Several factors are considered for evaluating the

profitability of a particular project of rock exploitation.

Some of these factors are related to the characteristics of

the rock, such as physical–mechanical properties, fractur-

ing and texture; while others are external to the rock, such

as the market demand and environmental constraints dur-

ing the exploitation (Carvalho et al. 2008; Sousa 2010; Fort

et al. 2010; Demarco et al. 2011; Mosch et al. 2011).

Despite the several factors which control and, in some

cases, cause the use of a specific rock to be unfeasible, the

aesthetical properties are the basis for the selection of the

rocks because they define the architectural harmony and

enhance the visual perception of the building (Sanmartın

et al. 2011). Definitions of aesthetical properties are sub-

jective, since they depend on several factors, such as tex-

ture, colour, gloss, and surface finish (Sanmartın et al.

2011), which are weighted differently by users. When the

rock is polished, the rock texture, in terms of the shape,

size and arrangement of the minerals, as well as the colour

are enhanced. With this surface finish, the minerals are

perfectly distinguished and their aesthetic properties are

maximized, which is why this is the best surface finishing

for enhancing the value of the rock.

The objective of the polishing is to make the rock sur-

face as flat as possible, enhancing the colour by dimin-

ishing the roughness and enhancing the gloss (Benavente

et al. 2003; Silveira 2008). These three related parameters

L. M. O. Sousa (&)

Department of Geology, Universidade de Tras-os-Montes e

Alto Douro, Apartado 1013, 5001-801 Vila Real, Portugal

e-mail: lsousa@utad.pt

B. M. M. Goncalves

R&D Department, Transgranitos-Marmores

e Granitos do Alto Tamega, Lda., Apartado 26, Teloes,

5450-909 Vila Pouca de Aguiar, Portugal

123

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DOI 10.1007/s12665-012-2035-y

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(colour-C, roughness-R, and gloss-G) could be changed

during the rock processing, while the perception of the rock

texture can only be improved or worsen. Assessment of the

CRG properties is often used for evaluating the rock pol-

ishing process and also during the wear evaluation in

particular environmental conditions or applications

(Benavente et al. 2003; Lopez-Arce et al. 2010; Sanmartın

et al. 2011). However, the assessment of the CRG prop-

erties should always consider the polyphasic characteristic

of the rocks: randomly distributed minerals with different

properties.

The colour is a consequence of the different proportions

of the different minerals and it is also influenced by

the surface finishing (Prieto et al. 2010). The lightness

(L* CIELAB coordinate) is higher when the granites have a

lower proportion of dark minerals, i.e., biotite (Sousa and

Goncalves 2012). Colour assessment is used for several

purposes, such as rock characterization, evaluation of

monuments through time, assessment of resistance to the

weathering factors, quality control in processing plants and

cleaning interventions (Inigo et al. 1997; Garcıa-Talegon

et al. 1998; Fort et al. 2000, 2010; Inigo et al. 2004; Tiwari

et al. 2005; Bams and Dewaele 2007; Grossi et al. 2007a,

b; Saliu 2008; Alonso et al. 2008; Rivas et al. 2011; San-

martın et al. 2012).

However, this is a difficult parameter to evaluate. For

instance, the natural variation of the mineral colour and the

weathering degree are examples of factors affecting the

final result. In order to avoid the uncertainty in relation to

rock colour and texture evaluation, automatic classification

systems have been used in several research studies (Ramos

and Pina 2005; Gokay and Gundogdu 2008; Araujo et al.

2010; Alvarez et al. 2010). The CIE colour classification

system is one of the most widely used systems and allows

the colour to be classified without misinterpretations by the

producers and architects (Sanmartın et al. 2011). Moreover

rock heterogeneity must always be considered, by having a

representative number of shots in each evaluation (Prieto

et al. 2010; Rivas et al. 2011; Sousa and Goncalves 2012).

In granitic rocks, the number of shots depends on the grain

size, textural heterogeneity and sample size, but usually

several dozen is necessary. On the other hand, in more

homogenous rocks, like marble and limestone, three of four

shots could be enough (Fort et al. 2000; Tiwari et al. 2005;

Grossi et al. 2007a). According to Prieto et al. (2010), who

presented a procedure for determining the number of shots

in granites, 14 shots are sufficient in a sample of 36 cm2,

for 8 and 10 mm diameter measurement heads.

Polished and sawed rocks and cut surfaces have irreg-

ularities and imperfections according to several factors,

such as mineralogical composition, weathering degree and

grain size, energy applied, duration of the effort, size of the

abrasives and interactions with debris (Arbizu and Perez

2003; Xi and Zhou 2005; Filatov 2008; Wang et al. 2009;

Filatov et al. 2009a; Aydin et al. 2011). The frequency,

location and importance of the irregularities in polished

surfaces, determined by the roughness profile, help us to

assess some of the factors that influence rock polishing.

The roughness of stone materials is assessed for several

purposes, from archaeology to stone industry, but mainly

for the evaluation of the sawing and polishing processes,

the influence of weathering agents and the cleaning inter-

ventions (Ribeiro et al. 2007; Moropoulo et al. 2007;

Alonso et al. 2008; Yonekura and Suzuki 2009; Urosevic

et al. 2010; Lopez-Arce et al. 2010; Procopiou et al. 2011;

Ozcelik et al. 2011).

During the polishing of the sawed surface, the irregu-

larities become lower and the surface tends to be flat, at

least visually; therefore, the roughness or asperities

diminish rapidly (Benavente et al. 2003; Gorgulu and

Ceylanoglu 2008; Silveira 2008; Yavuz et al. 2011). Huang

and Xu (2004) observed SEM micrographs of granite

grinded in a special apparatus and found that the ductile

deformation is the predominant mechanism at the fine

grinding stages, while in the first stage (#150 grit) the

brittle deformation prevails. Moropoulo et al. (2007)

defined the friability index, considering the fracture density

and the surface roughness; therefore, the decrease in

roughness decreases friability and increases rock stability.

Traffic and weathering factors break down the mineral

assemblage and small mineral particles are removed and

roughness increases (Lopez-Arce et al. 2010).

The relationship between roughness and grain size is not

clear, since some results show a direct relationship (Shen

et al. 2006; Aydin et al. 2011) while others show an inverse

relationship (Xie et al. 2009). The roughness of the granites

is always higher than the roughness of their individual

grains, since the grain boundaries are the most important

cause of roughness (Shen et al. 2006; Xie et al. 2009).

When the grain is much larger than the profile length, the

probability of evaluating the roughness of one or two

minerals increases (Alonso et al. 2007). Xie et al. (2009)

found an inverse correlation between surface roughness

and granite crystal size, since the larger crystal size of

granite leads to a shorter boundary between adjacent

minerals and to a decrease in the action of crystal interface

on surface continuity.

These conflicting results concerning the relationship

between roughness and grain size reflect the complexity of

the problem, since different approaches and methodologies

are used that may or may not consider all the factors which

influence surface characteristics, such as the wear equip-

ment, depth of the crack, number of cracks, scale of

observation, crystal orientation and cleavages (Erdogan

2000; Xie et al. 2009). Ribeiro et al. (2011), by analysing

saw slabs, concluded that the micro-structural features are

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responsible for the differences in roughness and that the

rocks with higher roughness show higher abrasive wear

strength and uniaxial compressive strength. Shen and Xu

(2007) found that the hardness and the impurities in quartz

lead to a final coarse surface when compared with calcite

and fluorite minerals. The silicon dioxide content of the

stone is related to its strength characteristics and, therefore,

to the removal rate, power consumption and final surface

roughness (Sidorko et al. 2008).

As in colour assessment, roughness needs several mea-

surements to have a representative value, since most of the

measurement devices read a small portion of the surface,

sometimes in a same mineral. The number of the measures

used in the bibliographic references is highly variable, from

1 to 20 (Huang et al. 2002; Benavente et al. 2003; San-

martın et al. 2011; Aydin et al. 2011), depending on the

rock type and the sample area. All the authors refer to the

heterogenic character of the rocks and establish a mean

value. However, it is necessary to conduct research to

establish a universal methodology as Prieto et al. (2010)

have done for colour evaluation.

Gloss is the capability of a surface to reflect incident

light (ASTM 1995) and it is related to the polishing quality

and has an effect on the aesthetic attributes of the rock. The

measurement of specular gloss is achieved by comparing

the luminous reflectance of the samples to that of a cali-

brated gloss standard (Nadal et al. 2006). Specular gloss is

frequently used to assess the surface quality of the

dimension stone because it is related to surface roughness

(ASTM 1997; Huang et al. 2002; Gorgulu and Ceylanoglu

2008; Sanmartın et al. 2011). However, for high values of

glossiness ([70 %), there is no clear relationship with

roughness and the surface micro-porosity also plays an

important role (Sousa et al. 2007). Erdogan (2000) identi-

fies the factors that negatively affect the gloss: porosity,

distinct crystal boundaries, cleavages, fillings in the micro-

fractures, obliqueness between the crystal orientation and

the cutting plane and the mica content.

Similarly to roughness, gloss measurements in stone are

made essentially for evaluating the quality of the polished

surfaces. The number of shots in gloss measurements

varies according to the heterogeneities of the surface,

which in rocks could reach 20 measurements per sample

(Huang et al. 2002).

Studies conducted in several rock types show a clear

relationship between the above referred CRG properties

(Huang and Xu 2004; Yavuz et al. 2011). However, few of

them assess all the properties. Sanmartın et al. (2011)

conclude that no general conclusions could be drawn

regarding the influence of roughness on the colour of

ornamental granite. The relationship between gloss and

roughness is clear in many studies (Huang et al. 2002), but

it is more evident for low values of roughness (Sanmartın

et al. 2011). The gloss is linked with roughness, because

the flatter the surface, the more light is reflected. Therefore,

in the polishing process, with the progressive use of thinner

abrasives, the gloss and roughness change inversely

(Huang and Xu 2004; Gorgulu and Ceylanoglu 2008;

Yavuz et al. 2011). In the last stages of the polishing

process the rock acquired the best polish. Also, if the

polishing continues, the roughness does not diminish

because the continuous removal of small mineral particles

occurs (Filatov et al. 2009a).

Colour changes with the surface finish and, conse-

quently, with roughness and gloss, particularly with light-

ness (L*), since the darker rocks (low L*) show a higher

ligthness with the increase in roughness (Simonot and Elias

2003; Benavente et al. 2003; Sanmartın et al. 2011). All

these general relationships are subjective, taking into

consideration the poly-mineral characteristic of the rocks,

as well as the equipment and processing procedures, but

some of them can be clearly understood.

In a processing plant, small changes are expected in the

polishing, according to the variation in the polishing pro-

cess over time, with consequences in the quality of the

polishing of the slabs. Sousa et al. (2007) found significant

differences of glossiness in ceramic tiles polished under the

same condition, explained by the kinematics of the pol-

ishing. However, these changes are ignored if the quality

standards are accomplished and if we assume that the

differences between the polishing of different rocks are the

consequence of their own characteristics, rather than vari-

ation in the processing.

During the processing of rocks it is not common to

assess the characteristics of the polished surface by means

of standard procedures, therefore making it difficult to

improve or correct the quality of the final product. The

main purposes of this study are to evaluate the polished

surface of ornamental granites and to identify the differ-

ences and the accountable factors. The factors assessed

were colour, gloss and roughness of three ornamental

granites with different textural and weathering character-

istics, from samples collected in a processing plant.

Materials and methods

Three brightness varieties of Portuguese ornamental gran-

ites were selected for this study: Amarelo Real (AR)

granite, Pedras Salgadas (PS) granite and Branco Micaela

(BM) granite (Table 1). The first (AR) is yellowish brown

and the others (PS and BM) are greyish, and according to

the ISRM (1981) weathering degree, the granites can be

classified as W1 (PS, BM) and W2–W3 (AR). The differ-

ence in the weathering degree affects the physical–

mechanical properties (Table 2), and the more weathered

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variety of Amarelo Real granite is not suitable for pave-

ments and exterior use in wheat environments because its

high porosity diminishes its resistance to the action of ice

and salt (Sousa et al. 2005).

The samples, 100 9 100 9 20 mm sized and with pol-

ishing finish, were collected throughout a two-month per-

iod in an ornamental granite processing plant. The number

of samples of each type of granite used in the study varies

as follow: AR-129; PS-131; BM-124.

The colour was assessed with the X-Rite colorimeter,

0� viewing angle geometry and specular component included,

using the D65 illuminant, and were expressed using CIE-

L*a*b* and CIE-L*C*abhab (CIE 2004). In the CIE-L*a*b*

system, the colour is quantified according to three chro-

matic coordinates: L* parameter represents lightness or

luminosity (L* = 0 dark; L* = 100 white); a* parameter is

the red-green axis (a* [ 0 red; a* \ 0 green); b* parameter

is the yellow-blue axis (b* [ 0 yellow; b* \ 0 blue).

The attributes of the polar system CIE-L*C*abh ab systems

are the chroma (Cab* = (a*2 ? b*2)1/2) and the hue (hab =

tan-1 b*/a*). The 8 mm aperture was used because it is the

most common in similar studies (Grossi et al. 2007a);

however, larger apertures allow a faster determination

(Sousa and Goncalves 2012). Taking into account the area

of the samples and the number of representative measures

(Prieto et al. 2010; Sanmartın et al. 2011; Sousa and

Goncalves 2012), each colour assessment (one per sample)

results from the mean of 40 shots.

Granite gloss was quantified with the Novo-Gloss Trio

gloss meter (Rhopoint Instruments), using the 20�, 60� and

85� geometries, considering the mean of 15 measures in

each sample.

Although the 60� geometry is the most used in polished

rock surfaces (Silva et al. 2007; Yavuz et al. 2011) we used

the three geometries to perform a comparative study.

The roughness of polishing finish was assessed with

Mitutoyo SJ-201 roughness meter. The mechanical rugos-

imeters are still used extensively, despite the growing use

of laser profilometers (Avdelidis et al. 2004; Rousseau

et al. 2012). Six profiles of each sample were done, with a

length of 12.5 mm in each profile, and considering the

mean value. Results are expressed by Ra, the profile mean,

and Rz, the maximum amplitude, which is defined as the

greatest height between the valleys and the saliencies (ISO

1984; Ribeiro et al. 2011).

The surfaces of the samples were analysed under binoc-

ular stereoscope to identify the quality of the polishing and to

obtain a clear picture of the most frequent irregularities.

Spearman rank correlation coefficients were computed

with the IBM� SPSS� Statistics software program.

Results and discussion

Mean and standard deviation values obtained for the col-

our, gloss and roughness are presented in Table 3.

Colour

Brightness (L*) is very similar in the three granites. This

parameter is influenced by the percentage of biotite and

also by the hue of the other minerals (Sousa and Goncalves

2012). The values obtained by other researchers are very

different, since some rocks are not classified as granite.

When comparing the three granites, the higher value of

the b*-parameter (yellow/blue) and a*-parameter (red/

green) in the AR granite is remarkable. These parameters

are mainly controlled by the pigments present in the rock

Table 1 Petrographic characteristics of the studied granites (according Sousa and Goncalves 2012)

Commercial name Origin Type Colour Grain size (mm)

Amarelo Real (AR) Vila Pouca de Aguiar, N Portugal Two micas, medium size,

hypidiomorphic granular

Brow yellowish 1.8 ± 1.5

Pedras Salgadas (PS) Vila Pouca de Aguiar, N Portugal Biotitic, medium size, slight

porphyritic tendency, hypidiomorphic

granular

Greyish 1.4 ± 1.0

Branco Micaela (BM) Aguiar da Beira, N Portugal Two micas, fine to medium size,

hypidiomorfic granular

Greyish 1.3 ± 0.9

Table 2 Physical–mechanical properties of the studied granites

(LNEG 2012; granite abbreviations in Table 1)

Physical-mechanical property AR* PS BM

Compression breaking load (kg/cm2) 828 1,035 1,797

Compression after frost test (kg/cm2) 799 855 1,737

Bending strength (kg/cm2) 80 170 228

Volumetric weight (kg/m3) 2,577 2,618 2,630

PTN water absorption (%) 1.05 0.21 0.22

Open porosity porosity (%) 2.7 0.56 0.58

Linear dilation coef. (max. value) (10-6/�C) 7.1 9 7.5

Amsler wear (mm) 0.7 0.2 0.3

Resistance to impact (cm) 60 65 60–65

* Values for the more weathered and frequent variety

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forming minerals and/or by the weathering state (Hajpal

and Torok 2004; Wang et al., 2004; Torok and Hajpal,

2005; Urones-Garrote et al. 2011; Sousa and Goncalves

2012). In this specific case, the higher values of the

b*-parameter result from the weathering state that gives a

yellowish brown colour (Fig. 1) and also a higher chro-

matic variation. The sound granites (PS and BM) have a

more homogeneous coloration, and the small variations

among the samples are related to the textural characteristics

of the granites. As we can see in Fig. 2 the variation trend

of the a* and b* parameters in AR granite is uniform, thus

with the naked eye it is almost impossible to distinguish

similarities in hue among several slabs with similar colour

parameters. These results contradict those found in a recent

study, where two distinguishable groups were identified in

the same granite (AR) (Sousa and Goncalves 2012). The

weathered granites have a high demand but it is impossible

to preserve their colour because of the variations

throughout the outcrop, throughout the quarry and even

within the same block. For this reason two extreme vari-

ations are possible: the gradual variation, when the

weathering state is similar, probably in the case of the

blocks exploited in the same quarry; and the abrupt vari-

ation, when the blocks originate from different quarries or

from different zones in a same quarry.

In order to assess the magnitude of the variation

among the samples, the total colour difference was com-

puted (DEab* ) by applying the formula DEab

* =

(DL*2 ? Da*2 ? Db*2)1/2 (CIE 1976; Seve 1991) consid-

ering the extreme values of the colour parameters and also

the mean values shown in Table 3. The results (Table 4)

enable us to identify the L* parameter as being the most

Table 3 Mean values and standard deviation of the chromatic

(L*, a*, b*, Cab* and hab), gloss (G20, G60 and G85) and roughness

parameters (Ra and Rz) (granite abbreviations in Table 1)

Parameter Granite

AR PS BM

Colour

L* 68.7 ± 2.1 67.6 ± 1.4 67.1 ± 1.4

a* 2.2 ± 0.9 -0.4 ± 0.1 -0.3 ± 0.1

b* 10.7 ± 2.1 1.9 ± 0.4 1.8 ± 0.3

Cab* 11.0 ± 2.2 2.0 ± 0.3 1.9 ± 0.3

hab 79.5 ± 3.2 107.3 ± 5.9 104.8 ± 4.2

Gloss (gu)

20� 30.6 ± 7.1 37.7 ± 6.5 40.8 ± 6.0

60� 42.6 ± 7.2 54.1 ± 6.8 56.9 ± 5.5

85� 62.0 ± 6.0 82.4 ± 5.4 83.7 ± 2.3

Roughness (lm)

Ra 4.3 ± 1.5 1.0 ± 0.8 0.7 ± 0.2

Rz 82.0 ± 23.8 30.2 ± 14.2 21.7 ± 6.9

Fig. 1 Macroscopic characteristics of the granites used in this study.

AR Amarelo real granite, PS Pedras Salgadas granite, BM Branco

Micaela granite, the scale bar is 5 cm long

Fig. 2 Distribution of the a* and b* chromatic parameters (granite

abbreviations in Table 1)

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important for the colour variation and the AR granite as the

most heterogeneous. Some of the colour variations could

be addressed to the equipment/methodology, for instance

Sousa and Goncalves (2012) point out that colour variation

below 2.5 CIELAB units are considered to be a conse-

quence of the research procedures. The human eye only

can appreciate DEab* higher than 3.2 (Prieto et al. 2006). For

practical purposes a difference of 3 CIELAB units is

considered the upper limit of rigorous colour tolerance, i.e.

the limit of perceptibility of the colour changes (Berns

2000; Volz 2001; Benavente et al. 2003; Sanmartın et al.

2011). The results obtained (Table 4) are substantially

higher than 3 CIELAB units, especially in case of the AR

granite.

The results obtained are unequivocal about the hetero-

geneity of the colour, even if the mean values of the colour

parameters are considered. The tested samples are highly

variable and for all the granites it is possible to find sam-

ples distinguishable with naked eye, but which are more

significant in the AR granite. These results highlight the

importance of quality control of the colour variations in

altered granites, the most problematic type because of the

natural variation of the mineral coloration. This colour

variation must be considered in the selection of the lots sent

to each building to avoid chromatic variations that may

lead to eventual commercial conflicts. These weathered

rocks are subject to colour variation during the year as

a consequence of the absorption/evaporation of water

(Tiwari et al. 2005) which intensifies the natural variations.

This is one more reason for using similar tiles in the same

facade/panel.

Colour saturation (Cab* ) remains stable in PS and BM

granites; however in the AR granite, there is a general

tendency for decreasing when the samples became lighter

(Fig. 3). Similar results are reported by Grossi et al.

(2007a) in several limestones subjected to weathering tests,

without identifying a universal tendency, and also by

Benavente et al. (2003) in granite. However, Grossi et al.

(2007b) found an inverse relationship in Rosa Porino

granite samples, both in polished-uncoated and artificially-

coated surfaces. In the weathered granites it seems that the

colour is enhanced when the lightness decreases.

Roughness

Roughness values range from 0.7 to 4.3 lm, for the Ra

parameter, and from 21.7 to 82.0 lm, for the Rz parameter

(Table 3). The more sound and texturally more homoge-

neous granite (BM) shows the lower roughness, while the

other sound granite (PS) has slightly higher and more

heterogeneous values (Fig. 4). The presence of more cracks

in the bigger potassium feldspars can also justify this dif-

ference. The more weathered granite (AR) shows the

higher values and remarkable variability, comparable to

the roughness of sound saw granites (Amaral et al. 2008).

The grain size also has some influence in this result

because the coarse-grained granites usually have higher

roughness (Aydin et al. 2011). On the other hand, the

roughness could be influenced by the quality of the pol-

ishing and also by the rock heterogeneity that leads to

different values in same granite. For instance, in two

experiments with Rosavel granitoid, a porphyritic quartz

syenite with pink potassium feldspar megacrystals, very

different roughness values were found, 0.4 and 1.0 for Ra,

%4.0 and 35.4 for Rz (Alonso et al. 2007, 2008).

In the bibliography there is a big scattering of the

roughness values in polished ornamental rocks, with the

Table 4 Maximum colour differences (Max. DEab* ) considering the

extreme and mean values of the colour parameters (L*, a* and b*)

(granite abbreviations in Table 1)

Granite Max. DEab*

L* ranking a* ranking b* ranking Mean values

AR 11.1 10.9 10.9 6.7

PS 8.3 5.6 5.4 4.3

BM 6.9 4.6 4.2 3.6

Fig. 3 Relationship between the color saturation (Cab* ) and the

lightness (L*) (granite abbreviations in Table 1)

Fig. 4 Distribution of the roughness values (Ra) in test samples

(granite abbreviations in Table 1)

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Ra values ranging from 0.01 to 1.0, according to rock type

processing conditions (Filatov et al. 2009a; Gorgulu and

Ceylanoglu 2008; Xie et al. 2009; Yavuz et al. 2011), but

some values come from polishing devices that are not used

in the industry. The roughness values (Ra) of the BM and

PS granites are similar to those found in granitic rocks

collected in processing plants and/or with polished surfaces

(Alonso et al. 2008; Sanmartın et al. 2011). Aydin et al.

(2011) refer to Ra values of about 5.5 lm in surfaces cut

with an abrasive water-jet machine, slightly higher than the

value for the AR granite. Total roughness (Rz) values of the

sound granites (BM and PS) are similar to those found by

Alonso et al. (2008) in four Spanish granites.

The Ra and Rz values are linked for the three granites

studied; however, the increase in the mean roughness is not

concomitant with the total roughness (Fig. 5). The angle of

the linear fit diminishes with the increase of the roughness

because the mean roughness increases, but the maximum

roughness tends to become constant. This means that when

overall roughness increases, the depth of the cracks and

cavity valleys on the rock surface, which are the main

contributors for the Rz values, do not increase indefinitely.

The mechanism in the grinding of granite varies

according to the decrease in the size of the abrasive, and it

evolves from the brittle-mode removal to the ductile-mode

removal (Huang et al. 2002). According to these authors, a

decrease in roughness and a high gloss were formed in the

ductile-mode and increase when Ra is lower than 0.2 lm.

Detailed analyses under binocular stereoscope show that

the location of the irregularities (loss of material) occurs

preferentially in mineral contacts, as observed by other

authors in similar rocks (Erdogan 2000; Silveira 2008;

Filatov et al. 2009a). Other circumstances where the

roughness increases are the following: the interception

point between the fissures and the mineral contacts, and

when the cohesion between minerals is lower, such as

when numerous contacts and fissures are located in a small

area. This is according to the observations made by Lopez-

Arce et al. (2010), who found that the intra-granular

surface roughness is lower than the inter-granular rough-

ness. According to observations by Xie et al. (2009), the

micro-cracks on ground surface were mainly distributed

around mineral borders and were mostly derived from

fracture of crystal interfaces and disfigurement of crystal

interiors. Feldspar has low irregularities on the surface,

which corresponds to other studies (Alonso et al. 2008;

Lopez-Arce et al. 2010). The loosening of particles is more

frequent in quartz along the fissures and in the micas when

the cleavage has a lower angle to the polishing plane.

However, the relative magnitude of these aspects is dif-

ferent for the two types of granites studied. In the sound

granites (PS and BM) there is a higher frequency of irregu-

larities linked with biotite (the dominant mica), with loss of

material when the cleavage plane is oblique in relation to the

polishing plane, and there are visible grooves and a higher

wear than the other minerals. Lopez-Arce et al. (2010) show

that the roughness values in biotite depend on the orientation

of the mineral, being lower in the basal planes. In the more

weathered granite (AR) we can see the loss of material,

especially in the contact of the micas with others minerals, in

the mineral contacts linked with intra-granular fissures, in the

trans-granular fissures and also along the twinning planes in

feldspars. In this granite (AR), the loss of material is much

more frequent and only in this granite the loss of intra-

granular fragments from quartz and feldspars is observed.

These observations show that the brittle-mode removal,

as defined by Huang et al. (2002), is still observed, mainly

in weathered granites. In these granites the poor physical–

mechanical properties prevent the formation of a flatter

surface, since the release of mineral fragments occurs

continuously.

Gloss

The Amarelo Real granite shows the lowest values of gloss

in the three geometries used in the assessment (Table 3).

The gloss values found for polished rock surfaces are

variable, but the methodology for determining gloss, the

polishing process and the abrasive use are some of the

reasons for the differences (Erdogan 2000; Huang et al.

2002; Gorgulu and Ceylanoglu 2008; Yavuz et al. 2011). In

a recently published work, Sanmartın et al. (2011) descri-

bed gloss values in polished granites ranging from 67.4 to

89.3 gu (G60), slightly higher than that obtained in the

present study; while Huang et al. (2002, 2004), in experi-

ments with a special polishing machine, reach 90 gu.

The gloss values show some heterogeneity in the values

of the AR granite, where it is possible to identify two

families of samples with a distinctive gloss (Fig. 6). This

situation could indicate a different origin of the samples,

according to the weathering degree of the raw materials

and, therefore, a different capability to gain gloss. Another

Fig. 5 Relationship between Rz and Ra for the three granites (granite

abbreviations in Table 1)

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evidence is the possibility of dividing the studied granites

with the 85� geometry. In fact, the value of 70 gu defines

the boundary between the sound granites (BM and PS) and

the weathered ones (AR). From Table 3 it is possible to

verify that along with the increase of the angle

(20� ) 60� ) 85�) the difference between the sound

granites diminishes (BM and PS) and the difference for the

weathered ones increases (AR). Therefore, the 85 �geometry is the most suitable for distinguishing and com-

paring the gloss in the studied granites because it is more

sensitive to small differences. The grain size exerts some

influence on these results since for a weakly absorbing

medium like granites, a decreasing particle size increases

scattering, decreases light penetration and augments diffuse

reflectance (Lagorio 2004). In polishing experiments,

Huang et al. (2002) found a higher gloss in fine granite than

in coarse ones, with a difference of about 10 gu.

The results from the three geometries are correlated,

especially the 20� and 60� geometries (Fig. 7). In fact, the

gloss values from these two geometries match completely

and the coefficients of determination are high. The gloss

values of the 60� and 85� geometries are related in the AR

and PS granites but not in the BM granite. From this

research we can conclude that gloss can be obtained with

the 20� or 60� geometries. For the most homogeneous

granite (BM), the 85� geometry is not suitable for corre-

lating with gloss values from other geometries. In Fig. 7 it

is also clear that the samples from AR granite are separated

in two distinguish groups. In the BM granite, there are two

visible groups in the G60–G85 graph. These results

emphasize that the gloss measures can be used as a pro-

duction controlling instrument for polish quality.

Relationship between the surface characteristics

In the previous section the relationships between the colour

parameters (L*, a*, b*) were already pointed out, as well as

between the gloss values obtained with the different

geometries (20�, 60�, 85�) and between the mean (Ra) and

total (Rz) roughness. In this section the relationships

between the three surface characteristics will be analysed.

For each granite, the Spearman rank correlation coeffi-

cients are presented (Tables 5, 6, 7).

For the studied granites there is a tendency for gloss to

decrease as long as the overall roughness increases, as was

found in several types of rocks (Gorgulu and Ceylanoglu

2008; Sanmartın et al. 2011; Ozcelik et al. 2011). However,

in the present study this variation does not have the same

magnitude for the three granites (Fig. 8). The sound

granites (PS and BM) show a high decrease in gloss with a

slight increase in roughness, while the weathered granite

(AR) shows a slight decrease in gloss within a large

interval of roughness variation. In the AR granite the

roughness increases as a consequence of the deepening of

the valleys, so the impact in the gloss will be lower than

expected, since the reduction in the reflection area is not

proportional to the increase of the irregularity. In sound

granites with a smooth surface the increase of the rough-

ness has an important effect on light reflection. Usually in

other experimental studies the roughness/gloss values are

different because the methodology used is not similar;

however, the general pattern is the same. For instance, in

an experimental study conducted in granites, Huang and

Xu (2004), working only with roughness (Ra) lower than

0.2 lm, reached values in gloss similar to the ones

obtained in the present research with a 2 lm roughness.

Another interesting relationship occurs between lightness

and the a* and b* parameters in the AR granite. According to

Simonot and Elias (2003), the roughness modification of a

surface leads to a vertical shift of its reflectance spectrum and

the changes are more important in objects which are more

saturated initially, like the weathered granites. As noted, in

the sound granites there is a loss of small fragments in biotite

and muscovite, which has an effective contribution in the

dispersion of the falling light. The loss of mineral portions in

the weathered granites, particularly linked to biotite and

along the fissures, increases the roughness but does not sig-

nificantly affect gloss.

Fig. 6 Distribution of the gloss values obtained with the 60 � (a) and 85� (b) geometries (granite abbreviations in Table 1)

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A relationship can be drawn between gloss and the

L*-parameter (Fig. 9), which similarly was found by San-

martın et al. (2011) in polished granites. When the samples

become white, with fewer dark minerals, the capability for

reflecting the falling light is diminished. As pointed out

above, the irregularities in mica minerals are smooth, with

small deep irregularities, while in others minerals they are

deeper and therefore have a stronger influence in both

Fig. 7 Relationship between the gloss values of different geometries (granite abbreviations in Table 1)

Table 5 Spearman rank correlation coefficients between the surface characteristics in the Amarelo Real granite

L* a* b* Cab* hab G20 G60 G85 Ra Rz

L* 1.00

a* -0.61c 1.00

b* -0.51c 0.93c 1.00

Cab* -0.52c 0.96c 0.99c 1.00

hab 0.66c -0.98c -0.89c -0.90c 1.00

G20 -0.50c 0.56c 0.58c 0.58c -0.52c 1.00

G60 -0.58c 0.59c 0.58c 0.59c -0.56c 0.96c 1.00

G85 -0.58c 0.42c 0.39c 0.39c -0.42c 0.76c 0.85c 1.00

Ra 0.25c 0.02a 0.10a 0.10a 0.03a -0.08a -0.17a -0.36c 1.00

Rz 0.23b 0.10a 0.18b 0.17a -0.05a 0.05a -0.04a -0.22c 0.91c 1.00

a Not significant valueb Significant value at p \ 0.05c Significant value at p \ 0.01

Table 6 Spearman rank correlation coefficients between the surface characteristics in the Pedras Salgadas granite

L* a* b* Cab* hab G20 G60 G85 Ra Rz

L* 1.00

a* -0.30c 1.00

b* 0.19b 0.58c 1.00

Cab* 0.19b 0.55c 0.99c 1.00

hab 0.03a -0.74c -0.87c -0.84c 1.00

G20 -0.65c 0.13a -0.10a -0.10 -0.02a 1.00

G60 -0.70c 0.15a -0.07a -0.07c -0.05a 0.94c 1.00

G85 -0.59c 0.02a -0.14a -0.14a 0.07c 0.76c 0.86c 1.00

Ra 0.45c 0.15a 0.35c 0.33c -0.26c -0.55c -0.56c -0.58c 1.00

Rz 0.42c 0.20b 0.37c 0.36c -0.31c -0.49c -0.51c -0.56c 0.95c 1.00

a Not significant valueb Significant value at p \ 0.05c Significant value at p \ 0.01

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roughness and gloss. When the mica percentage decreases

(L* increases), the relative importance of the deeper

irregularities increases and then the gloss diminishes, as

found earlier by Erdogan (2000). Therefore, roughness

also plays an important role, since the flatter the surface,

the lower the lightness (Fig. 9). In Tables 5, 6 and 7, the

contradictory behavior between the L* parameter and the

gloss (inverse) and the roughness (direct) can be clearly

observed. When the surface becomes rougher, the gloss

diminishes but the lightness increases. So, for one specific

type of granite the best polishing will be achieved with

lower lightness. This clarifies even more the relationship

previously found between chroma and lightness (Fig. 3).

Conclusions

Concerning the colour parameters, the sound granites are

more homogenous and the weathered ones show high

variations. The results identify the L* parameter as the most

important for the colour variation in studied granites. In

weathered granite an AEab* of about 11 CIELAB was found,

while in sound granites the maximum colour variations are

slightly higher than the perceptability limit of colour dif-

ferences. In order to avoid problems in rock applications, a

quality control is needed for weathered granites. Whenever

necessary, the granite products should be separated in lots

with homogeneous colour, avoiding the side-by-side use of

rock pieces with different perceptible colour. A tendency

for an increase in the colour saturation with lightness was

observed in the weathered granite.

Roughness values range from 0.7 to 4.3 lm, for the Ra,

and from 21.7 to 82.0 lm, for the Rz, with the higher values

coming from the weathered granite. In sound granites the

surface irregularities are related with mica minerals, where

a high wear is frequently observed when the cleavage plane

is oblique to the polishing plane. In the more weathered

granite, the surface irregularities are a consequence of both

the mineral contacts and fissures. Only in the weathered

granite can the loss of intra-granular fragments of quartz

and feldspars be observed.

Gloss ranges from 42.6 to 56.9 gu (G60), with the lower

values found in the weathered granite. In this study, the 85�

Table 7 Spearman rank correlation coefficients between the surface characteristics in the Branco Micaela granite

L* a* b* Cab* hab G20 G60 G85 Ra Rz

L* 1.00

a* 0.08a 1.00

b* -0.28c -0.28c 1.00

Cab* -0.29c -0.35c 0.99c 1.00

hab 0.07a -0.62c -0.42c -0.32c 1.00

G20 -0.61a 0.00a -0.47c 0.46c -0.22b 1.00

G60 -0.58c -0.04a 0.43c 0.43c -0.17a 0.96c 1.00

G85 -0.12a -0.09a 0.09a 0.10a 0.10a 0.27c 0.37c 1.00

Ra 0.22b 0.02a -0.13a -0.14a 0.04a -0.19b -0.23c -0.17b 1.00

Rz 0.25c -0.00a -0.09a -0.11a 0.10a -0.25c -0.31c -0.16a 0.83c 1.00

a Not significant valueb Significant value at p \ 0.05c Significant value at p \ 0.01

Fig. 8 Relationship between the gloss (G60) and the mean roughness

(Ra) (granite abbreviations in Table 1)

Fig. 9 The gloss decreases with the increase of the lightness

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geometry proved to be more suitable for differentiating and

comparing the granite gloss because it is more sensitive to

small differences, but this issue needs to be further inves-

tigated in other granites.

Roughness and gloss have a clear relationship, which

varies according to the two granite types. In the sound

granites, there is a high decrease of gloss with a slight

increase in roughness; while in the weathered granite, the

increase in roughness causes a small decrease in gloss. In

weathered granites the increase in roughness is essentially

a consequence of the deepening of the valleys, rather than

the fissure widening and, therefore, the gloss does not

diminish significantly.

CRG properties are important for the aesthetic charac-

terization of the ornamental granites and should be con-

sidered in rock processing to define homogenous lots. The

importance of this evaluation is obvious for weathered

granites, where the physical–mechanical properties and

natural colour variation lead to higher heterogeneities. The

systematic evaluation of the CRG properties during rock

processing allows us to determine the natural variation of

the rocks and, therefore, to distinguish the influence of the

variations due to the processing.

Acknowledgments This research was supported by Centro de

Geociencias da Universidade de Coimbra, with funds from the FCT-

Fundacao para a Ciencia e a Tecnologia. Acknowledgments are due

to Dr. Karl-Jochen Stein for their constructive comments and

suggestions.

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