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The international symposium “Around Picasso”. A case about Cleaning Treatments on Sensitive Painted Surfaces

The international symposium Around Picasso: An insight into the relationship between material choices and failure mechanisms, held at the Museu Picasso de Barcelona in 2018, was a multidisciplinary forum coordinated by the museum’s Department of Preventive Conservation and Restoration. The symposium was enriched by the generous complicity of specialists from various public and private museum and university institutions.

As a monographic museum dedicated to Pablo Picasso, is it our responsibility to create avenues of dialogue between rigorous research and the promotion of the artist’s work. Through our platforms, we now offer a publication by Pierre-Antoine Héritier, one of the speakers at the symposium, with this interesting case study: Comparative Study of the Effects of Traditional and Innovative Cleaning Treatments on Sensitive Painted Surfaces: Cotton Swab Rolling vs. Combined Micro-Aspiration and Liquid-Dispensing System.

The article studies the real impact of cleaning sensitive painting surfaces, like those from the latter period of Picasso’s career, with an example of oleoresinous paint on board from 1968.

Reyes Jiménez

Comparative Study of the Effects of Traditional and Innovative Cleaning Treatments on Sensitive Painted Surfaces: Cotton Swab Rolling vs. Combined Micro-Aspiration and Liquid-Dispensing System

Pierre-Antoine HERITIER

Chief of Atelier HERITIER SàRL – conservator

Contact E-mail: pamfheritier@bluewin.ch

ABSTRACT

How preserve surfaces that are sensitive to leaching and polishing when subjected to cleaning treatments with aqueous solutions? How to remove stains preserving the original painted surface? This study assesses the effects of the traditional cotton swab rolling method and those of using Micro-Aspiration with simultaneous Liquid Dispensing (MALD) system. In this study the consequences of excessive rubbing on the perception of the painted surface as well as the efficiency of the MALD technology are shown through different examination techniques including colour and gloss measurements, UV-Vis, IR and 3D imaging among others.

1. INTRODUCTION TO SOME ISSUES AFFECTING THE APPEARANCE OF PAINTED SURFACES

In the last years significant research has been carried out in painting conservation to investigate cleaning techniques and materials in painted surfaces that are both vulnerable to polishing and sensitive to leaching. Research has recently attributed such superficial features to the use of commercial paints such as Ripolin or to a combined mixture of oleo-resinous paint and oil paint [1]. These effects can be observed in Figure 1. Figure 1 shows a detail of Picasso’s Buste de Mousquetaire (1968) made by superimposing different paint layers on a re-used wooden board coated with a matt, brown paint film which might have contributed to create this particular velvety and vulnerable painted surface.

Pablo Picasso, Buste de mousquetaire, 1968. a) Vista general

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Fig.1(a,b) Pablo Picasso, Buste de mousquetaire, 1968. A) General; b) Detail. Courtesy: Qoqa ©, Succession Picasso / 2018, ProLitteris, Zurich

 

1.1. Uneven painted surfaces: syneresis

The different techniques used by Picasso and their resulting optical appearance (e.g. dullness, bloom or gloss alternations, etc.) are inherent to each painting, being all of them extreme sensitivity features to the smoothing effect of the cleaning action with cotton swabs.

Another phenomenon observed on Picasso’s paintings is syneresis. (Fig.2) It consists of the shrinkage experienced by the polymer network during drying and the fact that solvents and some constituents of the paint are expelled onto the surface [2,3] This exudation phenomenon plays an important role on the flatness and final appearance of the painted surface since it modifies its optical qualities and dullness. When this syneresis phenomena takes place, it unfolds relatively quickly and becomes an inherent aspect of the painting.

The syneresis phenomena is typically noticeable on glycerophtalic paints as Ripolin or, for example, on polymerized drying oils such as stand oils. It is also often the case when wax has been added [4-7] . However, this effect is not consistent and varies according to the paint formulation -which can be changed by the manufacturer at any time-, and due to how the paint is mixed and applied by the artist.

Fig. 2 (a,b). Wet syneresis observed in two cans of Ripolin paint.

One good example of a changing formulation is the paint sample cards produced by Ripolin for Gris Perle Foncé n°10 (Fig. 3). Colour charts are rarely dated [8]. In this case it can be observed that the samples vary considerably, exhibiting different levels of oxidation, embrittlement and cracking. The colour changes suggest different paint compositions.

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Fig. 3 Hyperspectral curves for Ripolin’s colour charts paint samples from different batches of Gris Perle Foncé n°10

 

1.2. The notion of pruinosity

In the last years the examination of several Picasso’s paintings revealed that those that had never been cleaned still preserved this particular velvet aspect intact. This feature provides an added historical value to the artwork since it is the best evidence of not having been ever touched. Unfortunately, this is not the case in Picasso’s works from the 1950s and 1970s where this velvety and bloom appearance has usually been abraded or even removed due to past surface cleaning treatments.

In this paper, the biological term pruinosity is used to describe the optical effect observed on some painted surfaces that present a frosted or dusty looking coating on top of a surface[1] (Fig.4) and that recalls the bloom sometimes observed on the skin of prunes (Fig. 5). The term pruinosity blushing had actually been formerly suggested by Jan W. Gooch about lacquer paint layers and defined in his encyclopaedia of polymers as a milky opalescence that sometimes develops as a film of lacquer dries and is due to the deposition of moisture from the air and/or precipitation of one or more of the solid constituents of the lacquer; usually confined to lacquers that dry solely by evaporation of solvent [9].

Efectes vellutats i lluents sobre una superfície pintada.Fig. 4. Velvety and bloom effects on a painted surface.

Fig. 5. Colour difference in different spot measurements taken on the skin of a prune due to the bloom.

 

1.3. The effects of humidity on painted surfaces

Far beyond polishing problems and other irreversible changes in the superficial morphology of the paint film due to mechanical action, some chemical reactions can also contribute to modify the appearance of the paint film. For example, severe hydrolysis might result into the formation of tide lines in the paint film as a result of the migration of pigment particles by capillarity due to the vertical pervasion of water inside layers or to the horizontal spread [10]. In addition, oxidative phenomena can induce relevant changes in polarity and enhance the mechanical fragility of the paint film [11]. Depending on the materials used the potential risk of damage will change over time. This sensibility can also be enhanced by the specifics of the paint film, which plays an important role in the response of the painted surfaces to the action of the cleaning solution. According to this, the smaller the pigment size is, the larger its specific surface will be, being its chemical reactivity proportional to its specific surface [12,13] (Roche 2003; Perego 2015)[2].Finally, the use of moisture in aqueous cleaning treatments can induce severe stresses in the fabric and contribute to in-plane (crack formation) and out-of-the-plane deformation (tenting) [14].

1.4. The effects of surfactants on painted surfaces

The use of surfactants is still common practice in cleaning treatments. Research has shown that surfactant can be trapped on surface and could then modify the refractive index of paint films as well their physical characteristics thus altering the colour perception of painted surface in a similar way the refraction in blue iridescent butterfly wing changes due to the presence of solvent (Fig. 5).

The simple modification of the refractive index of air (n=1), to n=1.36 (with Aceton) changes the visual aspect. The interferential blue turns into green. When adding a liquid with the refractive index n=1.48 (Toluol), (close to the refractive index of chitin (n=1.57)), colours created by refraction are modified and chitin appears [19]. Figure 5 shows this effect on a butterfly wing.

Fig. 5. Change of refraction index on a butterfly wing.

 

1.5. Hydrophobic surfaces

Some modern and contemporary painted surfaces are matt, sometimes due to the scarce binding medium. In some other cases they are extremely sensitive to moisture as explained above, but some other times they can be extremely hydrophobic. Again recalling nature in an attempt to have a very precise sense of the superficial texture and appearance of such painted surfaces, hydrophobic painted surfaces recall the texture of lotuses. Lotuses present micrometric picks covered by nanometric waxy hairs which creates what is known as the fakir effect structure. Such surfaces represent a challenge for superficial cleaning treatments. (Fig.6 a,b).

Fig.6 (a-b) a) A drop of water on a butterfly wing. b) Lotus hydrophobic surface.

 

In the case of hydrophobic surfaces, the excessive rubbing during the cleaning action can produce irreversible damage in the form of compaction or micro-losses of the paint film, and also a mechanical change of the refractive index. It can also lead to the mechanical polishing of the painted surface. Polishing is the consequence of micro-abrasion that generates fragmentation and distortion of the material due to the ruggedness flattening. This crushing results into linear and parallel facets on the surface where a bright area where specular reflection is dominant [15].

2. Experimental. Methods and materials

2.1. Preparation of samples

In this study two types of samples were used. On the one hand, optical measurements were carried out on a painting on canvas presenting an oleoresinous paint mixed with oil and bought at a second-hand dealer that presented the pruinosity effect previously described (see Fig.13), simulating the appearance of Picasso’s Buste de mousquetaire. On the other hand, different tests were carried out on several representative custom-made paint samples consisting of a commercial MDF panels coated with a non-absorbent matt paint (average thickness between 35 to 40 µm) made with pigments bounded with a polyurethane resin to simulate a sensitive surface on a thin alkyd ground (5 µm thick) (Table 1). Then the samples were slightly coated with artificial dirt made of Cassel extract and applied evenly by spray.

Table 1. Series of custom-made representative samples.

REFERENCE DESCRIPTION
PB27+PW6 Prussian blue + Titan white
PG18 Viridian green
PR108+PW6 Cadmium red dark + Titan white

2.2. Cleaning procedure

In order to assess the impact of surface cleaning on the morphology of paint films, a comparative study using rolling cotton swab and MALD system[3] was carried out. The components of the MALD systems are shown in Figure 7 [16-18]. The comparison between the two cleaning methods was aimed to assess the consequences induced by the joint effect of humidity and mechanical action on a painted surface in terms of the colorimetric changes experienced as a consequence of the alteration of the gloss and dullness of the paint film.

Components del sistema MALDFig. 7. (a,b) Components of the MALD system.

The MALD system is an interesting device to perform the aqueous cleaning of painted surfaces, particularly hydrophilic surfaces -that usually present an intermittent polar/no polar character-, and which tend to be extremely reactive to moisture since. MALD allows the simultaneous action of a micro-suction system and a controlled application of the cleaning solution by brush-tools with hairs or fibers chosen depending on their softness and their low rubbing coefficient. The risks of surface modifications are therefore reduced. The MALD system also allows to break the drops of the cleaning solution by shearing them in a more efficiently way than when applied with a cotton swab but without the risks associated to it. Acting physically on the superficial tension forces eases moistening. In addition, the action of brush dispensing-tip allows reducing the angle of kinetic contact more efficiently than a cotton swab. Therefore, the value of the moistening rate increases. Furthermore, this type of tools with its various alternatives allows adjusting the nature of fibers, their softness and their positioning during the interventions. This aptitude allows the cleaning action to be more efficient. As a consequence, the contact between the liquid and the surface can be shortened. The duration of the operation is further reduced by the fact that cleaning solution and dirt are almost simultaneously sucked away. Of course, before proceeding, one must test the safety of the cleaning solutions, (pH and conductivity) before using them on painted surfaces (Fig. 8).

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Fig. 8 (a,b) Comparative action of cotton swabs and different brush dispensing-tips.

 

2.3. Description of equipment

In this research, several examination techniques have been carried out:

  • Examination under various wave lengths of monochromatic lights (forensic light sources): UV at 365 nm, also at 400nm, 455 nm, 505 nm, 530 nm, 590 nm, 625 nm: to detect the polished areas.
  • Infra-red photography at 1000nm, with a tangential illumination: to detect changes of material on the surface.
  • Glossmeter control and micro-photography (at 20°, 60°, 85° angle)[4] (devices HG268 and UBS endoscop MS100): before and after cleaning, on 13 different areas for each sample lighted with a retro-diffused lighting.
  • Luminance[5] tests (Luminancemeter device TES137). Quantification in Candela per m2 of luminance variation before and after cleaning of samples illuminated at a fixed angle (at 40°)[6] with a strong collimated light.
  • Colour measurements (colorimeters devices TES135A and Colorcatch Nano) before and after cleaning on 13 different areas for each sample. Coefficient ΔE00 (CIE) specifies the differences between the two colorimetric states. We have used this tool in order to measure cleaning efficiency of the two methods.
  • Hyper-spectral vision with Specim IQ camera: to obtain a datacube with very tight spectrums images of 2.7 nm large, in a wavelengths range between 400 and 1000nm. A raking exposure of samples has allowed to quantify hyperspectral values of cleaned areas and their potential modification after cleaning.
  • Microphotographs were taken with Dino-lite AM7013MZT.
  • 3D imaging was carried out with a 3D camera with high resolution acquisition (3.5billion pixels/m2) that allowed the inspection of the roughness of the painted surface and its potential changes.

3. Results and discussion. Assessment of the effects of cleaning

In this research, the quantitative and qualitative testing of the polishing effects of the two cleaning methods was performed on 38 custom-made representative samples (Fig.9) in order to assess the cleaning efficiency as well as the extent of interaction with the painted surface.

Fig.9. General view of all the studied panels.

Figure 10 shows the cleaning matrix carried out in selected custom-made representative samples and show the effects of swab rolling vs. the use of MALD system.

Fig. 10. Cleaning matrix on custom-made representative samples (PB27+PW6, PG108 and PR108+PW6) showing the effects of swab rolling vs. MALD system.

For this purpose, each sample was documented under 16 different examination conditions including colour and gloss measurements as well as hyperspectral and 3D imaging in order to document changes in the superficial texture of the different painted surfaces tested. The measurements were always taken before samples had been soiled and after having been cleaning with both the cotton swab and MALD system. Considering the magnitude of this study, in this paper only the results corresponding to the representative samples ref. PB27+PW6, PG18 and PR108+PW6 are shown.

When diffraction changes, colour perception is seriously affected, inducing reflection and luminance modifications and leading to an important chromatic change. In order to measure the impact of an excessive cleaning action on the appearance of the painted surface, measurements of the luminance, gloss and colour were taken before and after cleaning with a cotton swab rolling for both the test painting and the representative samples. The results corresponding to the luminance measurements showed an increase of brightness (specularity) in all the samples cleaned with cotton swab, regardless their composition. Colour measurements provided an indication of the cleaning efficiency. Whereas high ΔE00 values can be associated to a more significant increase of specularity (brightness) due to polishing, a low ΔE00 value is an indication of the cleaning efficiency since the difference between the measure taken before and after soiling is minimal, being close to 0. Finally, an increase of gloss was noticeable in the samples subjected to cotton swab rolling, being an evidence of its polishing effects on these samples. Figures 11, 12 and 13 are showing the Luminance, colour and gloss measurements for the selected representative sample after cleaning with swab rolling and MALD system.

Mesuraments de luminància, color i brillantor per a una mostra representativa PR27+PW6 després de neteges per fregament amb cotó i amb el sistema MALDFig. 11. Luminance, colour and gloss measurements for PR27+PW6 representative sample after cleaning with swab rolling and MALD system.
Mesuraments de luminància, color i brillantor per a una mostra representativa PG18 després de neteges per fregament amb cotó i amb el sistema MALDFig. 12. Luminance, colour and gloss measurements for PG18 representative sample after cleaning with swab rolling and MALD system.
Mesuraments de luminància, color i brillantor per a una mostra representativa PR108+PW6 després de neteges per fregament amb cotó i amb el sistema MALDFig.13. Luminance, colour and gloss measurements for PR108+PW6 representative sample after cleaning with swab rolling and MALD system.

Figures 11, 12 and 13 show the measurements taken on each of the three different colour samples and on 3 to the 13 checking areas and highlight an increase of luminance, and even more clearly, an increase of the shine caused by the polishing effect. Being particularly matt, it was almost impossible to avoid this effect in the samples and this is clearly noticeable in both types of cleaning. This difference is even more visible, and particularly serious, when the cotton swab technic is used, as figure 13 shows.

Color measurements also evidenced the different cleaning reached by the two methods, which allowed to compare their efficiency. It was noticed, first, that ΔE00 value is often below 5 (a value below 5 indicates that the visual differences between the surfaces, before and after, are almost not visible by naked eye). This evidenced that very little artificial dirt (or even no residues at all) was left on the cleaned surface. It also suggests that an efficient, but moderate, cleaning will result in values close to 5, whereas a cleaning with tendency to leaching or extraction will be close to 0. Indeed, because the measurements were taken before soiling and after cleaning, a value close to 5 means that the difference before/after is due to remains of the soiling, whereas a value close to 0 means an almost invisible difference that could be explained by an excessive cleaning. Nevertheless, despite MALD system seems to have a negligible effect on the painted surface (being therefore safer than cotton swab rolling), it must be used with care as shown by some of the examples presented in the tables.

Figure 14 shows the comparative action of cotton swabs vs. MALD system in sample PR108+PW6 where both the physical abrasion and the gloss change due to rubbing with the cotton swab can be observed.

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Fig. 14. Comparative action of cotton swabs vs. MALD system in a custom-made representative sample: a) Physical abrasion; b) Gloss change due to rubbing.

Luminance, colour and gloss measurements were also performed in the test painting. Figure 15 evidences the effects of removing the velvety superficial features which modifies dramatically the perception of the painted surface.

Fig. 15 (a-e) Estudi òptic del quadre de prova: a) Test de luminància; b) Estudi de la reflectància global; c) Test de brillantor (realitzat a 85˚, 60˚ i 20˚); d) Test de brillantor a 85˚ abans del fregament amb hisop de cotó; e) Test de brillantor a 85˚ després del fregament amb hisop de cotó.Fig.15 (a-e) Optical study of the test painting: a) Luminance testing; b) Study of global reflectance; c) Glossiness Testing (shooting at 85º, 60º and 20º); d) Glossiness Testing at 85º before the swab rolling action; e) Glossiness Testing at 85º after the swab rolling action.

Figure 16 shows the visual comparative study of the cleaning action through 16 different examination techniques of the selected samples.

) Estudi comparatiu visual de l’acció de neteja mitjançant 16 tècniques d’anàlisi diferents de mostres seleccionades.Fig. 16 (1-16) Visual comparative study of the cleaning action through 16 different examination techniques of selected samples.
1. Raking light; 2. Tangent light reflection; 3. Infra-red tangent light at 1000nm; 4. Hyperspectral imaging with tangent illumination (204 spectrums); 5. Incident Monochromatic light at 625nm ; 6. Incident Monochromatic light at 590nm ; 7. Incident Monochromatic light at 530nm ; 8. Incident Monochromatic light at 455nm ; 9. Incident Monochromatic light at 400nm ; 10. UV light at 365nm; 11. Visible light x50 and x250 12. Gloss observation at 20°, 60° and 85° before and after swab rolling cleaning; 13. Micro-photographies taken under retro-diffused lighting at 20°, 60°, 85° before and after cleaning with MALD system; 14. 3D scan; 15. Colour and gloss measurement; 16. Luminance measurement.

Finally, figure 17 shows the 3D view of sample PG18 (Viridian green) with a soiled part and some cleaned areas. This Artmyn innovative tool has been used since it offers a complementary and significant visual check thanks to its innovative features: modulated lighting, turned on according to different points of view, colorimetry measurements, specularity and the fact that the scanning allows to compare the sample in terms of surface structure before and after cleaning.

Visió en 3D d’una mostra PG18 (verd veronès) en què es veuen clarament les parts embrutades i netejades.
Fig.17. 3D view of PG18 sample (Viridian green) where the a soiled and cleaned areas are evident.

CONCLUSION 

In this paper, a qualitative and quantitative comparative study between the use of the traditional cleaning technique with cotton swab and the MALD system is presented.

In this study, the consequences of the polishing action when rolling a cotton swab with a cleaning solution were quickly visible on the different samples tested, and even some disruption of the paint film was noticed. Even if it was not possible to avoid these effects completely in the matt paint samples studied, MALD system has proved to be an innovative and efficient technique to avoid many of the risks inherent to the cleaning of vulnerable surfaces.

In the light of our professional experience, and considering the numerous tests carried out in the different custom-made representative paint samples made on purpose for this study, it can be stated that the advantage offered by the MALD system is twofold: a significant reduction of both the contact time needed between the cleaning solution and the paint surface to have an effective cleaning, but also a reduction of the pressure exerted on the painting. Both aspects contribute to limit significantly the interaction between the cleaning solution the paint film.

Further research is needed to compare the potential of the MALD system with that of gels and tissues commonly used in cleaning treatments.

 

The other published papers from the symposium can be consulted here:

https://www.springer.com/journal/42452/updates/17273076

Acknowledgements
Anita Durand, Artmyn, Caroline Dick, Diane Louise Lassonde, Diego Hernandez, Maria-Fernanda Héritier, Pablo Héritier, Paolo Cremonesi, Qoqa, Santiago Héritier

References

1. VVAA, Picasso Express (2011). Musée Picasso, Antibes

2. Delcroix G., Havel M. (1988) Phénomènes physiques et peinture artistique, Paris, EREC

3. Guérin C. (2011), Recensement des dégradations constatées au niveau de la couche picturale sur les œuvres de Pablo Picasso exécutées avec de la peinture d’aspect industriel entre 1915 et 1972, www.fabarte.org/dbfiles/mfile/400/495/FABA_2015_Etude_Claire_Gu_rin.pdf. (Accessed octobre 2019).

4. Fratrel J (1770) La cire alliée à l’huile ou la peinture à l’huile cire.

5. Dauzère C (1907) Recherches sur la solidification.

6. Petit J, Roire J, Vallot H (2001) Encyclopédie de la peinture, formuler, fabriquer, appliquer, EREC.

7. Béguin A (2001) Dictionnaire technique de la peinture, pour les arts, le bâtiment et l’industrie.

8. Gautier G (2011) Ripolin product lines in Picasso express, 22-5.

9. Gooch Jan W. (2010) Encyclopedic Dictionary of Polymers, Volume 1 Edit. Springer

10. Investigations of Tide- Lines on Edvard Munch’s Painting The Source Project Based Masters Dissertation Conservation Studies Spring 2014. Katrine S. Scharffenberg. Departement of Archaeology, Conservation and History. University of Oslo.

11. Roche A (2016) La conservation des peintures modernes et contemporaines, CNRS éditions.

12. Roche A. (2003) Comportement mécanique des peintures sur toile: Mécanismes de dégradation Edit. CNRS

13. Perego F. (2015) Dictionnaire des matériaux du peintre, éd. Belin. Paris

14. Fuster López L., Mecklenburg MF, Castell M, Guerola V (2008) Filling materials for easel paintings: when the ground reintegration becomes a structural concern in: Preparation for Painting: The Artist’s Choice and its Consequences, edited by Townsend, J. H., Doherty, T., Heydenreich, G., and Ridge, J., 180–186. London: Archetype Books.

15. Martineau R., « Brunissage, polissage et degrés de séchage », Les nouvelles de l’archéologie, 119 | 2010, 13-19.

16. Cremonesi P., Héritier P.-A. (2017) Un approccio innovativo alla pulitura di superfici dipinte sensibili: la combinazione simultanea di erogazione controllata di liquido e micro-aspirazione. Il Prato, Padova

17. Casoli A., Cremonesi P., Héritier P.-A., Volpin S. (2019) Analytical study to monitor the effectiveness of a combined liquid-dispensing and micro-aspiration system for the cleaning of modern oil paintings. In Conservation of Modern Oil Paintings K.J. van den Berg, I. Bonaduce, A. Burnstock, B.Ormsby, M. Scharff, L. Carlyle, G. Heydenreich, K. Keune (Eds.), pp. 523-534.

18. Cremonesi P. (2018) Combination of a liquid-dispensing and micro-aspiration device for the cleaning of sensitive painted surfaces. Studies in Conservation 63 (6): 315-325.

19. Berthier S. (2016) Comment fait le gecko pour marcher au plafond ? Ed. Belin

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  1. « Pruinescence, n. »  Oxford English Dictionary
  2. For example, the surface of a cube of 1cm, divided in cubes of 1μm, increases from a surface of 6 cm2 to a surface of 60000 cm2.
  3. Device that takes advantage of the joint use of micro-aspiration and controlled liquid-dispensing developed in close collaboration with researcher Paolo Cremonesi for the last two years as an alternative to swab rolling cleaning technique. With the aim of reducing the duration of humidification, a more advanced system is being currently tested, developed and manufactured through a 3D printer. This system mixes synchronized aspiration and drying in a dual-chamber. The first chamber (A) allows gentle aspiration through paired luer-lock brush dispensing tips (B), made of soft nylon hair (C). A thin space in the center of each brush eases aspiration of the liquids. The second chamber (D), inserted into the same unit, conveys a stream of purified air that can be regulated, and speeds up the drying of residual humid areas of the painted surface (E).
  4. According to the nature of the surface the measurements are taken under an angle of 20°, 60° or 85°. In order to determinate which angle is the most suitable to obtain relevant measurements, surface is first illuminated at 60°: the surface is considered as shiny when x>70GU (GlossUnity), semi-matt when x>10 GU and matt when x<10GU. In our case, even if all our measurements have been recorded simultaneously at 20°, 60°, and 85°, all measures taken under 60° are lower than 10GU, that is why we have compared samples before and after cleaning, under an angle of 85°.
  5. Luminance depends on light flow but also on aptitude of a surface to reflect light to a kind of artificial retina (Delcroix 1988).
  6. 40° is a standard angle, which avoids reflectance parasites.

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