10.6092/issn.1973-9494/9230
Lo studio riguarda modi innovativi e sostenibili per aumentare l’idrofobicità delle malte a base di argilla utilizzate nel restauro, applicando oli naturali sulla superficie di campioni selezionati. Il fine è trovare un nuovo modo per proteggere la superficie di questi materiali, ma anche di promuovere l’idea di sostenibilità utilizzando oli da cucina naturali e riciclati. Pertanto, gli oli applicati per la protezione superficiale delle malte erano olio di canapa, olio di borragine (naturale) e olio di cottura esausto riciclato. I test condotti sui campioni trattati sono stati: test di assorbimento capillare, test di essic­cazione, porosità, test del tubo di Karsten, osservazione stereoscopica, alterazione del colore con cartografia Munsell, test del vapore acqueo e assorbimento del test delle gocce d’acqua. I risultati indicano il ruolo protettivo di tutti gli oli poiché riducono la po­rosità e l’assorbimento di acqua. La borragine e l’olio di canapa tendono ad alterare il colore dei campioni ma offrono una maggiore idrofobicità. L’olio di canapa ha permes­so l’essiccazione più rapida dei campioni. Il test del vapore acqueo indica nuovamente il basso livello di perdita d’acqua dalla superficie dei campioni trattati, mentre per il test specifico è stato creato un campione con una miscela di olio di cottura riciclato all’interno della massa argillosa.
application/pdf
1973-9494
9230
en
Maria Stefanidou
Conservation Science in Cultural Heritage
surface protection
Use of Oils for the Protection of Clay Mortars
Text.Serial.Journal
https://conservation-science.unibo.it/
18
Articles
2018-12-31
2019-03-18
2018-12-31
2019-11-05
121-133
CSCH
Maria Stefanidou
2018/12/31
10.6092/issn.1973-9494/9230
121
Aristotle University of Thessaloniki, Thessaloniki
1973-9494
surface protection
en
133
Use of Oils for the Protection of Clay Mortars
https://conservation-science.unibo.it/article/download/9230/9065
18
U     SE OF OILS FOR THE PROTECTION OF CLAY MOR-
      TARS

    Aspasia Karozou*6
    Laboratory of Building Materials, School of Civil Engineering
    Aristotle University of Thessaloniki, Thessaloniki, Greece

    Maria Stefanidou
    Laboratory of Building Materials, School of Civil Engineering
    Aristotle University of Thessaloniki, Thessaloniki, Greece



    Keywords: clay mortars, oils, surface protection


    1. Introduction

   Nowadays, promoting sustainable construction materials such as earthen mortars
is a significant matter in terms of resource scarcity, energy efficiency, pollution and
economy. Since the building sector accounts for around 40% of the world’s energy
use, according to the World Business Council for Sustainable Development (WBCSD),
the benefits offered by the use of earth in construction are significant. A material of low
carbon footprint, earth is recyclable, inexpensive and easy to use; it has, moreover,
been used for generations and has led to remarkable results. Today, it is once more in
the spotlight. However, despite the beneficial aspects, earthen materials present sev-




                                                                                                    CONSERVATION SCIENCE IN CULTURAL HERITAGE
eral weaknesses that need to be eliminated in order to become commercial. The lack
of specific regulations in most parts of the world, the low response to seismic reaction
and its great vulnerability to water intake are the main reasons that earthen construc-
tion is still one step behind the more popular industrial materials such as cement [1].
Hence the need to find new ways of protecting earthen structures from damaging
factors, so as to promote their use in modern construction. Furthermore, as far as cul-
tural heritage is concerned, the majority of monumental structures worldwide consist
of earthen materials that need to be protected and restored. Innovative techniques that
are in agreement with the principles of restoration, while providing low environmental
impact are considered a necessity.
   Earth construction is the oldest technique known to man. More than 9,000 years
ago adobes were used to create magnificent structures around the Mediterranean ba-
sin, Asia and the ancient known world. Various examples of building structures based
on earthen materials can be found even in Central America. The city of Chanchán in
Peru (around 850 AD) and the village of Taos in New Mexico (1000–1500 AD) are two
examples that indicate the knowledge of earthen construction techniques. The most
common techniques known for earthen construction are wattle and daub, cob, rammed
earth and adobes. In the wattle and daub technique a woven lattice of wooden strips
called wattle is daubed with a viscous material usually made of some combination of


*   Corresponding author: akarozou@gmail.com

                                                                                              121
                                                                               wet soil, clay, sand, animal dung and straw. It was used for almost 6000 years [2].
                                                                               Rammed earth is a method of creating a compacted wall from the compression of a
                                                                               soil mixture, while in the cob technique masses of clay mixed with sand and straw are
                                                                               formed manually and are placed one above the other, thus forming monolithic struc-
                                                                               tures with high stability. Lastly, adobes refer to the well-known sun-dried types of brick
                                                                               [3].
                                                                                   Numerous methods exist for the enhancement and protection of clay-based ma-
                                                                               terials. Mankind has for centuries taken into consideration the protection of earthen
                                                                               structures by adopting different strategies according to their surroundings. Such strat-
                                                                               egies involved orientation of the building in accordance with rain direction and the
                                                                               use of protective coatings and additives [4]. The use of additives can modify certain
                                                                               aspects so as to manage shrinkage, mechanical characteristics and to obtain water-
                                                                               proofing capabilities. There are various categories of additives that can be used, such
                                                                               as mineral additives (sand, pozzolan and ashes), synthetic additives (Portland cement,
                                                                               hydraulic lime, hydrated lime, gypsum, soap bitumen), vegetable additives (fibers, oils,
                                                                               fats, tannins, latexes and molasses) and animal additives (fibers, excrements, urine,
                                                                               blood, casein, animal glues and oil-fats) [5]. The use of coatings for surface protec-
                                                                               tion is also common, in order to offer permeability, durability and water resistance to
                                                                               sensitive earthen materials. Water resistance is achieved through the use of polyvinyl
A. Karozou, M. Stefanidou - Use of oils for the protection of clay mortars




                                                                               alcohol, ethyl silicates, epoxy and acrylic resins [6]. Although alkoxysilanes and alkoxy-
                                                                               polysiloxanes were commonly used as treatment agents [7-10], recent studies have
                                                                               tried to exploit natural resins such as linseed oil in order to be more compatible with
                                                                               earthen materials [11-13]. Moreover, recent studies refer to increasing hydrophobicity
                                                                               through the use of vegetable oils and spent cooking oils inside the mass of concrete
                                                                               and lime respectively [14,15]. Nevertheless, most of these projects deal with cement
                                                                               or lime-based mortars and not clay-based ones.
                                                                                   The aim of this study is to find new environmental friendly techniques in order to
                                                                               preserve and protect earthen materials from deterioration and thus enhance water
                                                                               resistance. For this purpose, various oils, both natural and recycled, were applied on
                                                                               the surface of sample mortars, so as to increase the hydrophobicity of the specific clay
                                                                               produced in the laboratory that was used. The oils applied for the surface protection
                                                                               of the mortars were hemp oil, borage oil (natural) and recycled spent cooking oil, elin
                                                                               (provided by the company Elin and used as a code name for the oil). Besides water
                                                                               repellent capabilities, breathability is also an important aspect that enables the water
                                                                               vapor to be transported from the inside to the outside of the material’s mass.


                                                                                 2. Materials and techniques

                                                                                  For the purposes of this study a specific clay mortar was manufactured in the labo-
                                                                               ratory and the various oils were applied as agents for surface treatment. The speci-
                                                                               mens created were of a rectangular shape measuring 4 × 4 × 5 cm and tiles with a
                                                                               dimension of 20 × 20 × 3 cm. The clay used comes from the area of Crete and is rich in
                                                                               lime and in addition to aluminum compounds contains quartz, calcite (CaCO3) and gyp-
                                                                               sum (CaSO4) in small percentages. The clay was sieved to obtain a grain diameter of
                                                                               less than 0.5 mm. The sand used for the mortars was river sand of silica composition,
                                                                               similar in color to that of the clay and with a grain size between 0-4mm. The mixture
                                                                               proportions of the mortar by mass were 1 (clay):2.5 (sand ratio) while the water: binder


                                                                         122
ratio was equal to 0.6 in order to achieve the desired workability of 15±1cm (tested by
flow table). Surface treatment was conducted by brush and the main goal was to form
a waterproof surface that allowed the materials to breathe. The procedure for applica-
tion of the oil for all specimens was as follows:
    •     First application by brush, once in each direction (right to left and left to right).
    •     After two hours, the remaining proportion of oil that was not absorbed was
          wiped off and left in laboratory conditions of 25±1°C
    •     After 24 hours a second layer of oil was applied twice in each direction
    •     The specimens were left to dry in laboratory conditions for 15 days after the
          application of the oils.
    All tests were conducted after the period of 15 days. The oils used were the follow-
ing:
    Natural oils:
    •     Borage oil (indicated as B)
    •     Hemp oil (indicated as H)
    Recycled cooking oil:
    •     Elin oil - Fatty Acid Methyl Esters (indicated as E)
    Hemp oil was chosen as a treatment agent since it is widely used for the protection
of wooden structures, while offering waterproof capabilities and weathering resistance;
it contains, moreover very low levels of toxic VOCs (volatile organic compounds), mak-
ing it an excellent alternative to synthetic and petroleum-based polymer coatings [16].
Borage oil, on the other hand, is widely used in medicine due to the many favorable
properties it possesses, mostly to treat skin disorders and rheumatoid arthritis [17,18].
Since it is native to the Mediterranean region, it was decided that an experimental
study on its properties in construction as a coating would be of interest. Furthermore, in
order to exploit industrial waste oils, recycled cooking oil (fatty acid methyl esters) was
also chosen as a treatment agent and was provided by a Greek company (Elin). The
main purpose of this study was to determine ways of protecting against water penetra-




                                                                                                        CONSERVATION SCIENCE IN CULTURAL HERITAGE
tion. For this reason, the following tests were performed:
    •     Capillary absorption according to EN1015-18 & drying test
    •     Porosity according to RILEM CPC11.3
    •     Absorption of water drop test (heuristic test)
    •     Karsten tube penetration test (CSTLI7500-TQC)
    •     Surface roughness measurements using Mitutoyo profilometer and recording
          the roughness parameter (Ra) values - which are used widely as a one-di-
          mensional roughness parameter and concerns the arithmetic average of the
          absolute values
    •     Water vapor permeability test (EN 1015-19:1999)
    •     Optical observation of color change using Munsell charts
    •     Stereoscopic observation using microscope LEICA WILD M10
    •     Microscopic examination by SEM (JEOL840A JSM)
    For the purpose of comparison reference composition samples were left untreated
(recorded as A).


   3. Experimental results

   In order to study the behavior of the samples for water penetration, various tests


                                                                                                  123
                                                                               were performed, such as a capillary absorption test, a porosity test and the heuristic
                                                                               test of water droplet absorption test. The heuristic test measures the time it takes for
                                                                               a single droplet to be absorbed by the surface of the specimens. These experiments
                                                                               record the behavior of the samples when they come into contact with water. Moreover,
                                                                               roughness measurements were recorded for all the surfaces of each sample so as to
                                                                               calculate the average Ra value. Ra is a roughness parameter deriving from the arith-
                                                                               metic average value of the filtered roughness profile determined from deviations about
                                                                               the center line within the evaluation length. Table 1 reports the results of the physical
                                                                               properties of the samples and those of the tests mentioned previously.


                                                                               Table 1. Results of the physical properties and tests.

                                                                                                                     Capillary                             Water
                                                                                     Water
                                                                                                      Porosity       Coefficient        T                  penetration
                                                                                     absorption                                                 Ra (mm)
                                                                                                      (%)            (g/cm2             (min)              value
                                                                                     (%)
                                                                                                                     *min1/2)                              (ml/min)

                                                                               A     7.04             13.71          0.43               0.58    0.111599   0.241
A. Karozou, M. Stefanidou - Use of oils for the protection of clay mortars




                                                                               B     4.34             8.87           0.28               19.0    0.233832   0.128

                                                                               E     4.21             8.46           0.29               2.50    0.227467   0.100

                                                                               H     5.20             10.55          0.32               2.00    0.108695   0.155

                                                                                  It is clear that the values of the treated samples B, E and H, in the case of water
                                                                               absorption, porosity and capillary coefficient differ in comparison with the untreated
                                                                               samples. The protective role of the oils applied is obvious since water absorption and
                                                                               porosity are lower in each case. It can be noted that the samples treated with hemp oil
                                                                               present values close to the reference samples A. Moreover, roughness values are in
                                                                               agreement with the time measured for the absorption of one water droplet, indicating
                                                                               the hydrophobic nature of the samples treated with borage oil. However, the results
                                                                               obtained for time absorption indicate the low hydrophobic nature of both the E and H
                                                                               specimens. The capillary absorption test results are recorded in Figure1.
                                                                                  The surface of reference sample A shortly after contact with the water was de-
                                                                               stroyed. The capillary coefficient calculated after conducting this test, indicates the low
                                                                               water uptake for all three treatments, while for the reference sample A, the capillary
                                                                               coefficient was calculated using the last measurement taken. Besides damage of the
                                                                               surface of sample A, the drying procedure was conducted as a reverse capillary test
                                                                               for all samples, naturally without the presence of water (Figure 2). Duration of the test
                                                                               was until stabilization of the samples’ mass. Both A and H samples possess the longest
                                                                               drying period, with A sample presenting a massive weight loss. This fact is explained
                                                                               by the large water uptake and the unaffected surface that allowed the water to move
                                                                               more easily from the inside out. The samples treated with borage and Elin oil present
                                                                               a slower rate of water evaporation, butt indicate good drying behavior by allowing the
                                                                               sample to breathe. The samples treated with hemp oil show the highest water intake,
                                                                               yet the drying procedure indicates a slower rate of water loss compared to the other
                                                                               treated samples.


                                                                         124
                     Figure 1. Capillary absorption of the specimens.




                                                                                                CONSERVATION SCIENCE IN CULTURAL HERITAGE




                         Figure 2. Drying test of the specimens.

   Another way to test water absorption on a horizontal surface is through the Karsten
tube penetration test. Karsten tubes were applied horizontally onto the surface of the
samples using plasticine and the tubes were filled with 2 ml water. The test lasted ten
(10) minutes in total for each sample, while every 15 seconds the water absorbed was

                                                                                          125
                                                                               recorded. The results are presented in Figure 3, while the water penetration value for
                                                                               each sample is indicated in Table 1. Again, the results show the hydrophilic nature of
                                                                               the untreated samples and the protective nature of the treated ones. The hemp sam-
                                                                               ples once more present a similar behavior to the untreated specimens compared to
                                                                               the other samples B and E. However, B and E samples show a slower rate of uptake
                                                                               of 0.128 and 0.1 ml/min respectively.
                                                                                  The breathability of the samples is of the utmost importance since moisture entrap-
                                                                               ment is a catastrophic factor for porous materials. If appropriately treated water can be
                                                                               prevented from reaching the upper surface and evaporating [19]. For this reason, the
                                                                               water vapor test was conducted on both treated and untreated tiles. Each tile was ap-
                                                                               plied on top of a container half filled with water using silicon and was then preserved in
                                                                               a chamber with steady conditions (20 oC, 60%RH). The weight loss of the tile-container
                                                                               system indicates the passage of water vapor through the tile and into the atmosphere.
                                                                               The system was measured for four months and the results are presented in Figure
                                                                               4. As expected, the weight loss of sample A was the highest compared to the treated
                                                                               samples at a value of 4.64%. Despite the previous results that indicate the similar be-
                                                                               havior of hemp treated samples to the reference ones, it is noted that the H specimens
                                                                               show the lowest water loss of the tile-container system with a value of 2.10%. The
                                                                               values of both samples treated with borage and Elin are very close with 2.65% and
A. Karozou, M. Stefanidou - Use of oils for the protection of clay mortars




                                                                               2.68% weight loss respectively.




                                                                                                           Figure 3. Karsten tube test results.




                                                                         126
                       Figure 4. Weight loss in the tile-container system.

    The water vapor permeance Λ was calculated from the data collected so far. Again,
it can be concluded that the untreated samples present the highest water vapor flux in
comparison to the treated samples, according to the high value of the Λ factor. From
Table 2 it can also be noted that H samples prevent the passage of water vapor com-
pared to samples B and E that again present similar behavior.

Table 2. Results of water vapor permeability test.

Water vapor permeance Λ (kg/m2 *s*Pa)
Α                          B                         Ε                       Η
1.00118E-11                5.1293E-12                5.89365E-12             4.49E-12




                                                                                                CONSERVATION SCIENCE IN CULTURAL HERITAGE
   All samples were examined to detect alterations on their surfaces, in terms of color
change, cracking and roughness. In order to determine alterations in color after treat-
ment, Munsell charts were used. The results are listed in Table 3 below. Both B and H
samples differ from the color of the untreated samples A, in terms of value and chroma,
and developed a darker surface. On the contrary, the E samples showed small altera-
tions compared to the reference samples, with a slight differentiation in hue.
   Alterations in the color of the samples were also detected through stereoscopic
observation (Figure 5). Moreover, shrinkage cracks were evident on the surface of all
specimens, with H samples showing a smoother surface. This element is in line with
the results of the roughness measurements, since H samples presented the lowest
Ra value of 0.108 mm. Samples treated with elin oil presented many shrinkage cracks
and a rougher surface. The rougher surface spotted through microscopic observation
comes in agreement with the Ra value of 0.227 mm, indicating the roughness of sam-
ples treated with elin oil. Despite the highest roughness value in the samples treated
with borage oil (Ra=0.234), the microscopic observation reveals a relatively smooth
surface in accordance with E and A samples. Macroscopic observation showed that
in all cases the oils were able to penetrate up to a depth of 2.0 mm from the surface,
with a maximum penetration depth of 5.32 mm in the samples treated with borage oil
(Figure 6).



                                                                                          127
A. Karozou, M. Stefanidou - Use of oils for the protection of clay mortars




                                                                               Figure 5. Stereoscopic images of the various specimens: A) Untreated sample; B) sample treated
                                                                               with borage oil; C) sample treated with Elin oil; D) sample treated with hemp oil (scale-1000μm).




                                                                                                Figure 6. Area of penetration of sample treated with hemp oil.




                                                                         128
Table 3. Optical observation of color changes using Munsell charts.
              Hue           Value             Chroma           Name
A             2.5Y          8                 2                pale yellow
B             2.5Y          6                 3                dark yellowish brown
E             5Y            8                 2                pale yellow
H             2.5Y          7                 3                pale yellow




                                                                                                         CONSERVATION SCIENCE IN CULTURAL HERITAGE


Figure 7. SEM images of the various specimens A) Untreated sample, B) sample treated with
borage oil, C) sample treated with Elin oil, D) sample treated with hemp oil (scale-300μm for A,
scale-200μm for B, scale-100μm for C and D).



   Similar to the stereoscopic observation are the results conducted using a scanning
electron microscope (SEM). The surface of the untreated sample A is smoother than
the treated sample’s surface, while the borage treated specimens present a much
more stable and consistent surface (Figure 7). Micro roughness is observed on B, H
and E samples which can justify the hydrophobic properties in accordance with the
relevant literature [9].




                                                                                                   129
                                                                                     4. Conclusions

                                                                                   Using oils for the protection of clay-based materials has given some promising re-
                                                                               sults. The clay structure becomes more resistant to water penetration in each test
                                                                               conducted using the various oils. The decreased tendency of the treated specimens
                                                                               to absorb water and the good drying behavior of all the samples make the oil coatings
                                                                               used suitable for surface protection. After concluding the tests, as expected, the un-
                                                                               treated samples presented the highest values in water absorption and porosity, while
                                                                               the time of absorption for the water droplet was high. The high values of capillary coef-
                                                                               ficient and water penetration compared to the treated samples indicate the protective
                                                                               role of the coatings used.
                                                                                   It was noted that the B and E samples presented similar behavior in almost every
                                                                               test conducted, while the H samples were closer to the behavior of the untreated sam-
                                                                               ples. The high values of Ra and time of absorption of the water droplet indicate the
                                                                               hydrophobic nature of borage oil, while the alteration in color is considered to be a
                                                                               negative aspect. Despite the high level of water absorption and porosity of the hemp
                                                                               treated samples, they had the lowest weight loss during the water vapor permeability
                                                                               test, a fact that indicates prevention of the water vapor flux. The smooth compact
                                                                               surface of the hemp samples spotted through microscopic observation, are in line with
A. Karozou, M. Stefanidou - Use of oils for the protection of clay mortars




                                                                               the low Ra value measured. In comparison to the B and E samples, the H specimens
                                                                               show a good drying behavior, while a negative aspect is the significant color alteration
                                                                               of the surface.
                                                                                   As mentioned above, treated E samples shared similar characteristics with B sam-
                                                                               ples and the results indicate the hydrophobic nature of the coating applied. However,
                                                                               despite having the lowest water penetration value and relatively low values of poros-
                                                                               ity, water absorption and capillary coefficient, the fast absorption of the water droplet
                                                                               and the cracks developed on the surface diminish its protective role. Nevertheless,
                                                                               the positive aspects of the Elin oil application were that it was fast drying, had good
                                                                               water vapor permeability behavior and produced an insignificant color alteration of the
                                                                               samples. Also, the low water penetration value presents resistance against horizontal
                                                                               water penetration.
                                                                                   In conclusion, there are many positive aspects in each of the coatings applied. How-
                                                                               ever, further investigation and perhaps modification of the oils used is proposed. The
                                                                               durability of the treated samples proved greater compared to the untreated ones, mak-
                                                                               ing the protective role of the agents obvious. The use of such materials in construction
                                                                               is promising in terms of sustainability and their use is thus to be encouraged.


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                                                                                  Biographical notes

                                                                                  Stefanidou Maria, Associate Professor is the director of the Laboratory of Building
                                                                               Materials in the Civil Engineering department of the Aristotle University of Thessaloniki.
                                                                               Her interests are the study of traditional mortars and the composition of compatible
                                                                               ones as well as the improvement of their properties by sustainable methods and by
                                                                               using nanotechnology. Her research interests include producing and testing artificial
                                                                               stones, bricks, mortars and grouts for restoration purposes as well as experimenting
                                                                               with innovative techniques for building materials such as the use of phase change
                                                                               materials, self-healing renders and nano-modified coatings.

                                                                                  Karozou Aspasia, M.Sc. is currently working as a PhD student at the laboratory
                                                                               of building materials in the civil engineering department of the Aristotle University of
                                                                               Thessaloniki. She holds a Diploma in civil engineering and has a Master’s degree in
                                                                               conservation and restoration of monuments. Her main research interest focuses on
                                                                               finding innovative ways to protect and enhance clay for use in restoration projects.

                                                                                  Summary
                                                                                  In this study an effort has been made to find innovative and sustainable ways of
A. Karozou, M. Stefanidou - Use of oils for the protection of clay mortars




                                                                               increasing the hydrophobicity of clay-based mortars used in restoration by applying
                                                                               natural oils on the surface of selected samples. The concept was to find a new way of
                                                                               protecting the surface of these materials, but also to promote the idea of sustainability
                                                                               by using natural and recycled cooking oils. Thus, the oils applied for the surface protec-
                                                                               tion of the mortars were hemp oil, borage oil (natural) and recycled spent cooking oil.
                                                                               The tests conducted on the treated specimens were, a capillary absorption test, drying
                                                                               test, porosity, Karsten tube test, stereoscopic observation, color alteration using Mun-
                                                                               sell charts, water vapor test and absorption of water droplet test. The results indicate
                                                                               the protective role of all the oils, since they reduce porosity and water uptake. Borage
                                                                               and hemp oil tend to alter the color of the specimens, but offer increased hydropho-
                                                                               bicity, while the latter allowed the faster drying of the samples. The water vapor test
                                                                               indicates again the low level of water loss from the surface of the treated specimens,
                                                                               while for the specific test a sample with an admixture of recycled cooking oil inside the
                                                                               clay mass was created.

                                                                                  Riassunto
                                                                                  Lo studio riguarda modi innovativi e sostenibili per aumentare l’idrofobicità delle
                                                                               malte a base di argilla utilizzate nel restauro, applicando oli naturali sulla superficie di
                                                                               campioni selezionati. Il fine è trovare un nuovo modo per proteggere la superficie di
                                                                               questi materiali, ma anche di promuovere l’idea di sostenibilità utilizzando oli da cucina
                                                                               naturali e riciclati. Pertanto, gli oli applicati per la protezione superficiale delle malte
                                                                               erano olio di canapa, olio di borragine (naturale) e olio di cottura esausto riciclato. I
                                                                               test condotti sui campioni trattati sono stati: test di assorbimento capillare, test di essic-
                                                                               cazione, porosità, test del tubo di Karsten, osservazione stereoscopica, alterazione del
                                                                               colore con cartografia Munsell, test del vapore acqueo e assorbimento del test delle
                                                                               gocce d’acqua. I risultati indicano il ruolo protettivo di tutti gli oli poiché riducono la po-
                                                                               rosità e l’assorbimento di acqua. La borragine e l’olio di canapa tendono ad alterare il
                                                                               colore dei campioni ma offrono una maggiore idrofobicità. L’olio di canapa ha permes-
                                                                               so l’essiccazione più rapida dei campioni. Il test del vapore acqueo indica nuovamente


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il basso livello di perdita d’acqua dalla superficie dei campioni trattati, mentre per il
test specifico è stato creato un campione con una miscela di olio di cottura riciclato
all’interno della massa argillosa.




                                                                                                  CONSERVATION SCIENCE IN CULTURAL HERITAGE




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