methods of polarising microscopy and sem to assess...

DRAFT

12th International Congress on the Deterioration and Conservation of Stone

Columbia University, New York, 2012

1

METHODS OF POLARISING MICROSCOPY AND SEM TO ASSESS THE

PERFORMANCE OF NANO-LIME CONSOLIDANTS IN POROUS SOLIDS

Elisabeth Ghaffari,1 Thomas Köberle2 and Johannes Weber1

1 University of Applied Arts Vienna, Institute of Arts and Technology, Section of

Conservation Sciences, Salzgries 14/1, A-1013 Wien, Austria

2 Geologie-Denkmalpflege-Bauforschung, Nordstrasse 39, D-01099 Dresden, Germany

Abstract

Attempts to evaluate the efficacy and harmlessness of a consolidation treatment for

porous mineral materials have to deal with the task to measure relevant properties at

sufficient in-depth resolution. Non- to low-invasive methods, such as drill resistance or

ultrasound velocity measurements, prove useful in this context, but need eventually to

be complemented by other means of analysis which provide more precise topographic

and micromorphologic information. In view of this, the present study, performed in

frame of the EU-project STONECORE, aimed to assess some of the relevant features

related to a stone consolidant based on nano-lime by methods of microscopy. Focus is

put on the in-depth distribution and the bonding properties of the consolidant after evaporation of the solvent, an issue of specific interest for the final result of a treatment.

The treatments tested were performed on a sieve fraction of crushed stones and

mortars from various sources in order to eliminate textural characteristics of the

different materials. The analyses included polarising microscopy (PL) on thin-sections

and scanning electron microscopy (SEM). By use of these methods, the consolidant

could be well traced in the pore system of all samples, and quantitative data by digital

image analysis on the rate of pore filling at varying depth from the surface of treatment

could be calculated. The results show that, despite full penetration of all samples, the

precipitation of the consolidant was partly governed by its backward migration towards

the surface. Reducing the rate of evaporation could significantly contribute to achieve a

more even distribution. For the given mode of treatment, substrates rich in quartz had

especially high gradients of pore filling from the surfaces inwards. No clear impact of the zeta potentials on this effect could be established.

In addition to the above, an approach is presented to identify the conversion of the

calcium hydroxide to carbonate. Both PL and SEM proved useful in that respect. They

can be employed to replace, in a more precise and significant way, the usual check of

pH by liquid indicators. It was thus shown that moisture plays a key role in the

formation of calcium carbonate from the hydroxide.

Keywords: consolidant distribution, nano-lime, SEM, microscopy, image analysis

1. Introduction

In the course of conservation of decayed stone and mortar, the impregnating consolidation primarily aims at bridging those contacts between single grains or grain

fragments, which had been weakened by the action of weathering. The size of those

gaps can widely vary from a few micrometers up to several millimetres. In a similar way,

DRAFT



2

the shape, location and hence the accessibility of the defects to be consolidated differ

significantly between objects and even within one object, depending on its petrographic

nature and the specific decay symptoms. This range of different conditions can only be

met with a variety of carefully selected consolidants, be it of inorganic or organic nature.

Another issue of relevance is the capacity of a product in its liquid state to penetrate

a given pore structure in a sufficient way, and to react or precipitate in the right place

with no major backwards migration of the consolidant upon evaporation of the liquid. In

that context, parameters like the molecule or particle size of the consolidant, the

properties of the solvent or the suspending agent in respect to the pore system of the substrate are of importance. It is generally believed that small molecules or finer

particles penetrate better than coarse ones. In addition, the electric potential of all phase

surfaces involved in the consolidation must be taken into consideration, though no

precise prediction of its impact on the above parameters can be given.

Amongst all possible consolidants, inorganic systems are frequently preferred

because of their presumed better compatibility with the mineral substrates. One of the

most traditional inorganic consolidants is lime in the form of Ca-hydroxide, applied

either as a solution – lime water - or as aqueous dispersions – diluted lime wash. Several

drawbacks of these systems are due to either a low concentration of the active

component (Ettl & Wendler, 2005), or to its tendency to agglomerate to particles too

large to penetrate the pore system of the substrate. The latter problem is sometimes met by using alcoholic dispersions of Ca-hydrate (Giorgi et al., 2000) or by using specific

dispersion devices to obtain smaller agglomerates in the range of a few micrometers

(Jägers, 2000). In order to obtain Ca-hydroxide in significantly smaller size stable in

suspensions, recent developments have focussed on nano-sized systems of lime in

organic solvents, mostly alcohols. They are synthesised in different ways and yield

particles in the size range of approx. 20 to 200 nm. (Daniele & Taglieri, 2010,

Ziegenbalg, 2011). The success of such treatments for the consolidation of stone and

mortars is varying, depending, amongst others, on the above mentioned substrate

parameters.

One group of nano-sized lime systems traded under the brand name of CaLoSiL was researched in the frame of the EU-funded “STONECORE” project between 2008

and 2011. This project has dealt with the development and test application of calcium

hydroxide nano-particles with sizes in the range between 50 and 250 nm, stably dispersed in different alcohols (ethanol, n-propanol, iso-propanol). Amongst the main

issues of interest were the penetration behaviour of the systems into the porous structure

of several natural and artificial mineral materials, as well as the evaluation of their

consolidating effect, their micromorphological aspects, and the carbonation of the

Ca(OH)2-consolidant (Ziegenbalg 2011).

The efficacy of consolidation treatments was assessed through partly well

established, partly novel methods of measuring the bulk mechanical properties of the

stone or mortar before and after treatment, such as e.g. the drill resistance in depth and

the ultrasound transmission velocity along with other low-destructive approaches

(Valach et al. 2011; Ziegenbalg 2011). These methods were complemented by

topography-related investigations by means of optical and electron microscopy

supplemented by X-ray microanalysis. The mechanical methods proved generally useful yielding quantitative results, but by their nature they failed to provide clear information

DRAFT



3

about the exact depth of penetration and precipitation of the consolidant, its precise

topographic location in the substrates’ pore system, and the micromorphological

features. It was the latter which used to compliment the information on the kinetics of

carbonation obtained by measurements of the pH and X-ray diffraction.

The paper presents the most relevant tests and measurements of microscopy aimed

at the evaluation of the nano-lime consolidation effect in laboratory tests. In-situ

applications were also evaluated, they are however not included in this contribution.

Main aim is to present and discuss the approach of combined methods of light and

electron microscopy as an interesting alternative to mechanical methods for case studies of conservation in the practice.

2. Topographic and morphologic studies of nano-lime consolidants in different

porous solids

Aim of the study was to assess the properties in terms of penetration and

precipitation of CaLoSiL in different substrates – stones and mortars. In that context, the in-depth distribution and the bonding of the consolidant to the grain surfaces after

evaporation of the solvent is of specific interest for the final result of a treatment. While

measurements of the physico-mechanical parameters before and after a given treatment

yield important information on the bulk effect, the above mentioned properties can be

best assessed by methods of microscopy. In this context, Pintér et al. (2008) presented

the usefulness of a combined approach for ethyl silicate consolidants, while its

suitability to study lime-based systems was so far not well established. The present study is focussed on detecting possible impacts of the mineralogical

and chemical composition of a variety of substrates on the performance of CaLoSiL treatments. To that end, it was decided to conduct the test on a single sieve fraction of

crushed fragments of those materials. Thus, the impact of petrographically and/or decay-

related differences in pore and grain sizes could be largely eliminated.

The distribution of the consolidant was analysed by scanning electron microscopy

(SEM) on polished sample sections, and by polarising microscopy (PL) of thin sections.

An attempt was made to interpret the results against the zeta potential of the

fractions.

2.1 Substrates In earlier experiments it was found that penetration behaviour and distribution of

CaLoSiL is neither significantly controlled by particle size and corresponding pore size, nor by the variety of available solvents, but rather by the mineralogical nature of the different materials.

Seven rocks and four mortars were used to achieve a broad variety of substrates.

They are listed in Table 1.

2.2 Sample preparation and laboratory treatment

The rocks and mortars were reduced to small pieces in a laboratory crusher. A

fraction of 0.09 to 0.160 mm was obtained by dry sieving. The sands were filled into

plastic cylinders placed on air-permeable disks (glass-frits), as illustrated in Figure 1. No

further compaction of the sands was accomplished before treatment. The amount of sand

was determined gravimetrically. The samples were then treated with CaLoSiL E25, a

DRAFT



4

calcium hydroxide suspension in ethanol with an average particle size of approx. 150

nm, at a concentration of 25 g/L. The treatment was performed by dropping the product

onto the top surface of each sample until full penetration to the bottom was observed.

Table 1. Substrate materials tested in this study

Samples were freely left to dry under laboratory

conditions - only in the case of one sample from the

“Greek limestone” was the surface covered to reduce

the rate of evaporation. The treatment was repeated

several times wet-in-wet, until the fractions appeared saturated. After 24 hrs of curing under laboratory

conditions, the treatment procedure was repeated. The

experiment was finished after another 72 hrs. Then the

sections were prepared for microscopic observation.

2.3 Section preparation and methods of analysis

The whole compounds including the frit supports

were vacuum-embedded in a blue dyed epoxy resin (Araldite 2020). Petrographic thin sections of about 25 µm thickness as well as polished sections were produced

perpendicular to the surface of treatment.

The thin-sections were observed by PL (Jenavert), photographs were taken with a

microscope camera (Leica DFC290). The polished cross sections were coated with

carbon and studied by SEM (Philips XL 30 ESEM, 20 KV, high vacuum, back-scattered electron detector-BSE), equipped with an energy-dispersive X-ray analyser (Link-ISIS).

The SEM-micrographs, taken at low magnification of 100 times for the full depth of a

sample, were assembled to composite images by use of a Photoshop software. Pores,

No

.

Material Short description Main constituents

1 Drachenfels

trachyte

volcanic rock with mainly sanidine in a dense micro-

crystalline groundmass

silicates

2 Römer tuff volcanic tuff, mainly of feldspar and lithic fragments in

an altered, formerly vitreous groundmass

silicates

3 Leuben mortar historic dolomitic lime mortar with quartz aggregates quartz + Mg-carbonates

4 Lab-made lime

mortar

weakly bound lime mortar with quartz aggregates quartz + Ca-carbonate

5 Lab-made dolomitic

lime mortar

weakly bound dolomitic lime mortar with quartz

aggregates

quartz + Mg-carbonates

6 Greek limestone compact limestone (grainstone) with fossil and lithic

fragments

Ca-carbonate

7 Maastricht

limestone

fine-grained, highly porous biocalcarenite consisting of

uniform lime particles

Ca-carbonate

8 Sterzing marble coarse-grained white crystalline marble Ca-carbonate

9 Kremersand Mori commercial non-natural mixture of aggregates from

yellow marble and fossil limestone

Ca-carbonate

10 Aachen marl marl, a clayey, mainly micritic carbonate rock Ca-carbonate + silicates

11 Dahlen stucco historic white gypsum plaster without aggregate gypsum

Figure 1. Illustration of sample treatment

DRAFT



5

aggregates and consolidant were edited in different pseudocolours to ease their visibility

and to allow for digital image calculation. The latter was performed using a Leica QWin

Plus software. Exemplary SEM micrographs are shown in Figures 2 and 3.

Figure 2. Consolidant from CaLoSiL

treatment, filling the pore systems Figure 3. Consolidant from CaLoSiL

treatment, forming bridges between grains

2.4 Topography and morphology: results and conclusions

By SEM/BSE as well as by PL techniques, the consolidant revealed well visible in

all substrate materials. Given its higher resolution, SEM/BSE additionally allows for a

detailed characterisation of crystal shape and orientation for each of the studied

substrates. In view of this, pseudocolour editing of the SEM-micrographs proved a

practicable, though time consuming task. In such way, the basis for a number of useful digital image calculations could be laid, enabling to assess the specific performance of

the consolidant for a given material in a quantitative way.

The decision to use loose aggregates rather than compact material for the test

treatments afforded a representative comparison between mineralogically different

specimens, and compensated the lack of real pore geometries.

It was shown that the liquid consolidant had penetrated through the full depth of all

samples. In contrary to the materials in their compact state which sometimes show

difficulties of impregnation with CaLoSiL, the penetration of the liquid into the loose substrates was no limiting factor at all.

The decisive factor was found to be the precipitation of lime on top and in

subsurface areas of both surfaces of the specimens. This caused an uneven distribution

of CaLoSiL through the depth of impregnation for virtually all samples. The resulting gradient, an important factor in context with a successful and harmless consolidation

treatment, is more pronounced for materials containing high amounts of quartz (samples No. 3 and 5) than for the rest of the specimens. Another effect is due to the presence of

smaller grains in some samples, an unexpected technical consequence of dry sieving.

The zeta potential of the substrates, measured for their granulations in ethanol and

in water, respectively, revealed of no clear significance for the success of a treatment,

neither in terms of total average pore filling nor in respect to the gradients of

precipitation. It must be mentioned in this context that even no clear dependence on the

mineral composition and their zeta potential could be detected.

For the case of the “Greek limestone”, the different conditions of curing – free

evaporation of the solvent (sample No. 6a) vs. prolonged curing by covering the sample

DRAFT



6

surface (sample No. 6b) – proved of high significance for the development of gradients:

the latter sample showed no increased concentration of the consolidant at or near the

treated surface. This is an indication that, at least in some cases, the frequently observed

formation of a white haze on treated surfaces, as e.g. reported by Dähne (2011), is

caused by migration and should be prevented by suitable measures of protection.

Figure 4

Figure 7

Figure 5a Figure 8a

Figure 5b Figure 8b

Figure 5c Figure 8c

Figure 6 Figure 9

Figure 4. SEM-BSE image of sample 3, full sample depth with frit support on the bottom

DRAFT



7

Figures 5. a-c Details of Figure 4. (5a) top of sample, (5b) central area, (5c) bottom area Figure 6. Distribution of consolidant in aggregate samples of historic lime mortar, sample 3. Bold line refers to this sample

Figure 7. SEM-BSE image of sample 6a, full sample depth with frit support on the bottom Figures 8. a-c Details of Figure 7. (8a) top of sample, (8b) central area, (8c) bottom area Figure 9. Distribution of consolidant in aggregate samples of “Greek limestone”, uncovered (sample 6a). Bold line refers to this sample Table 2. Performance of the nano-lime consolidant for the different substrates

No. Substrate

material

Zeta potential

of substrate

(mV)* t

ota

l p

oro

siy

(ar

ea-%

)

in

dep

th d

istr

ibu

tion

of

conso

lid

ant

Agglomeration of consolidant Degree of pore filling

by consolidant

in w

ater

in e

than

ol

on tr

eate

d s

urf

ace

(w

hite

haz

e)

in

su

bsu

rfac

e

are

a

in

bo

tto

m a

rea

on

bo

tto

m

su

rfac

e

av

g. ac

ross

fu

ll

dep

th (

area

-%)

in

su

bsu

rfac

e

are

a

fact

or of su

rfac

e ex

cess

co

nso

lidat

ion

1 Drachenfels

trachyte -23,7 -2,6 55,38 good +++ + + ++ 16 24 1,55

2 Römer tuff -22,24 -4 52,35 good + - - ++ 10 13 1,29

3 Leuben

mortar -17,35 3,8 48,43 poor ++ +++ ++ ++ 7 55 7,79

4 Lab-made

lime mortar -17,85 3,05 48,18 fair ++ + + ++ 12 21 1,77

5 Lab-made

dolomitic

lime mortar

-7,1 7,95 48,80 poor + +++ ++ ++ 7 41 5,44

6a Greek

limestone -18,2 -1,05

54,78 good + ++ - + 22 41 1,83

6b -- “ --

covered 49,65 fair - - +++ +++ 20 14 0,67

7 Maastricht

limestone -16,45 3,75 53,59 fair + - ++ ++ 10 15 1,49

8 Sterzing

marble -9,15 5,75 50,09 fair + + - + 13 28 2,14

9 Kremersand

Mori -14,25 2,05 54,23 good ++ +++ - + 32 72 2,27

10 Aachen

marl -19,2 -1 56,50 good +++ + - - 35 62 1,77

11 Dahlen

stucco -5,15 -3,9 53,44 good +++ +++ - + 24 56 2,34

* The values for the zeta potential of the substrate materials were supplied by IBZ Freiberg. CaLoSiL

E 25 has

a zeta potential of 38 mV (IBZ Freiberg)

3. Carbonation

Apart from the visualisation of the presence of the consolidant, a big advantage of

both SEM and PLM lies in the possibility to evaluate the state of carbonation of the

consolidant in a more precise way than measuring the pH which is usually carried out by

DRAFT



8

the phenolphtalein test: by means of SEM, the micro-structure of the consolidant gives a

clear indication of the presence of carbonate, while by PLM this is proved through its

optical properties.

The distribution of the consolidant in the pore structure of the treated material was

evaluated by the two most common microscopic methods in material research, i.e. the

high-resolution scanning electron microscope (SEM) and the petrographic polarising

microscope (PLM).

In order to find out the limits and advantages of the two different methods for the

detection of CaLoSiL, a special survey was carried out. Polished thin sections of differently treated materials were carefully investigated.

They were produced with a water-free lubricant to avoid sample disturbance by water

and carbon dioxide reactions. This type of preparation is very convenient for both

microscope procedures. By use of PL and SEM, a defined sample area was examined,

and the presence of the consolidant was documented by micrographs. These records

could then be compared with each other.

Both methods show reliable results, as shown in Figures 10, 11 and 12. The

consolidant is detectable in more than 95 % of the cases with both microscopic

approaches. For the remaining samples, sometimes SEM seams to yield more significant

results, sometimes PL is advantageous.

Apart from the visualisation of the presence of the consolidant, a big advantage of

both SEM and PLM lies in the possibility to evaluate the state of carbonation of the

consolidant in a more precise way than measuring the pH: by SEM, the micro-structure of the consolidant gives a clear indication of the presence of carbonate, while by PLM

this is proved through its optical properties.

CaLoSiL, which is deposited in the pores by evaporation of the alcohol, consists of

Ca(OH)2. The birefringence (Δ) of portlandite appears moderate, with a value of 0,029 (Tröger 1967), but due to the small particle size the interference colour show a first

order grey.

The particles than convert in CaCO3 by carbonation, that means CO2 uptake in

presence of small amounts of water. The precipitated calcite show a much higher

birefringence with Δ = 0,1719 (Kordes, 1960). Even in the smallest areas of the section,

submicron-sized particles can be clearly observed. The increase in interference colours

due to the formation of the carbonate makes these particles visually stand out.

In Figure 11, the non carbonated area of CaLoSiL can be seen (area b1, compare to area b in Figure 10), while the outermost layer towards the pore appears carbonated

(b2 in Figure 11).

DRAFT



9

Figure 10. PLM photograph of

CaLoSiL (area b) in a loose

sand of carbonate rock fragments (a). C is a pore filled

with blue dyed resin. Plane polar light

Figure 11. PLM photograph of

carbonated CaLoSiL (b2) and

not carbonated CaLoSiL (b1)

in crossed polar light

Figure 12. Same area as in Fig. 10 and 11, seen by SEM

4. Discussion and Conclusions

The approach to use the SEM to detect consolidants in the pore system of solids is

not novel, though it is not frequently used in conservation studies. A prerequisite to produce photographs suitable for further digital image analysis is a distinct grey value of

the consolidant in the SEM-BSE image. Whenever this applies, a number of

micrographs must usually be produced, in order to cover reasonable areas of a sample

section if one wants to study the full depth of penetration. The effort is considerable, but

indispensable to fully understand the in-depth distribution of the consolidant on test

samples, an important factor which has produced many unsatisfactory or even harmful

results in the practice of conservation.

An alternative approach can be seen in polarising microscopy on thin sections. This

method, well known by geologists and scientists from related fields, needs not only

optimal section preparation, but also relatively high skills in using the microscope in the

best possible way to recognise structures and phases, and to interpret observations. Its advantage lies in the low magnification, allowing to study and record larger areas of a

sample section when compared to SEM. Disadvantages of PL are the limits of resolution,

and the fact that micrographs suitable for digital image analysis can be produced just in

rare cases. Thus, PL can be viable to check for the presence of a consolidant, however

without quantifying its amount within certain areas of the section.

It may be advisable to use polished thin sections which can be studied by both

methods, PL and SEM, in order to achieve the best possible results.

When it comes to the assessment of the progress of carbonation of consolidants

based on calcium hydrate, however, both tools are equally useful.

The selected way of specimen preparation for the tests presented in this contribution,

i.e. by using a sieve fraction of crushed stone material, limits the significance of the

observations to some extent. Nevertheless, some conclusions useful for the application of consolidants in the practice can be drawn. Thus, it was shown that the most critical

step in the course of impregnation of an open porous solid may be the instance when the

liquid phase evaporates, which might cause a backwards migration of the precipitate.

Reducing the rate of evaporation can greatly help minimise this effect, thus preventing

the formation of over-consolidated subsurface areas or even surface layers.

DRAFT



10

It was shown that, for a given application procedure of a consolidant to a specfic

pore system, as experimented in this study, the mineral nature of the solid seems to be of

relevance to the above effect of migration – the backwards migration proofed most

pronounced for substrates rich in quartz. The assumption that the zeta potential of the

solid would yield a measure of significance, however, was not supported by the data.

One could rather assume that the surface morphology of the grains – naturally differing

for different types of minerals, especially when they underwent crushing – might be a

decisive factor for the degree of “trapping” the precipitate by chemo-physical forces.

More investigations are needed to support this hypothesis. Finally, it can be stated that the nano-sized lime used in the study can yield a good

consolidant for mineral materials, provided that care is taken for its proper mode of

application. The amount of shrinkage of the precipitate in the pore space is limited,

especially when compared e.g. to most ethyl silicate systems. The slow rate of

carbonation in the absence of moisture is probably due to the dense packing of the nano

particles. This needs not be considered a significant risk, however, since measurements

of mechanical strength have proofed the efficacy of the consolidant even in the

hydroxide state. Whenever moisture would enter the pores, it will result in the

immediate, isotopic crystallisation of calcium carbonate.

Acknowledgements The present study was financially supported by the European Commission through

its 7th FP research project EU-213651 STONECORE. Thanks are due to the project

partners who supplied the materials and provided fruitful discussions.

References Dähne, A. 2011. ‘Evaluation of lime nano-sols for conservation of wall paintings,

mortars and stucco’, EWCHP-2011, Proceedings of the European Workshop on

Cultural Heritage Preservation, Fraunhofer IRB Verlag.

Daniele, V., Taglieri, G. 2010. ‘Nanolime suspensions applied on natural lithotypes: The

influence of concentration and residual water content on carbonatation process and

on treatment effectiveness’, Journal of Cultural Heritage, 11:102-106. Ettl, H., Wendler, E. 2005. ’Strukturelle Putzfestigung mit Kalkwasser? Grenzen und

Alternativen’, Beiträge zur Erhaltung von Kunst- und Kulturgut, 1:129-133.

Giorgi, R., Dei, L., Baglioni, P. 2000. ‘A new method for consolidating wall paintings

based on dispersions of lime in alcohol’, Studies in Conservation, 45:154.

Jägers, E. 2000. Dispergiertes Weisskalkhydrat für die Restaurierung und

Denkmalpflege: Altes Bindemittel-Neue Möglichkeiten, Petersberg: Michael Imhof

Verlag.

Kordes, E. 1960. Optische Daten zur Bestimmung anorganischer Substanzen mit dem

Polarisationsmikroskop, Weinheim: Verlag Chemie GmbH.

Pintér, F., Weber, J., Bajnóczi, B. 2008. ‘Visualisation of Solid Consolidants in Pore

Space of Porous Limestone Using Microscopic Method’, Proceedings of 11th international congress on deterioration and conservation of stone, Torun, Poland, 15

-20 September 2008. (Ed.) Jadwiga W. Lukaszewicz, Piotr Niemcewicz, 473-480.

DRAFT



11

Tröger, W. E. 1967. Optische Bestimmung der gesteinsbildenden Minerale, Teil 2

Textband, Schweizerbart´sche Verlagsbuchhandlung, Stuttgart, 69.

Valach, J., Hasníková, H., Dobrzynska-Musiela, M. et al. 2011. ‘Influence of material

and consolidant type on aggregate strengthening’, EWCHP-2011, Proceedings of the

European Workshop on Cultural Heritage Preservation, Berlin 2011,

Fraunhofer:IRB Verlag.

Ziegenbalg, G. 2011. ‘Stone conservation for refurbishment of buildings (STONECORE)

- A project funded in the 7th framework programme of the European Union’,

EWCHP-2011, Proceedings of the European Workshop on Cultural Heritage Preservation, Berlin 2011, Fraunhofer: IRB Verlag.

methods of polarising microscopy and sem to assess...

Documents