A. Bailão, S. Šustic, Retouching with mica pigments, e-conservation Journal 1, 2013, pp. 45-60
Available online 22 November 2013
doi: 10.18236/econs1.201308
Retouching with mica pigments
Ana Bailão and Sandra Šustic
Available online 22 November 2013
doi: 10.18236/econs1.201308
Retouching with mica pigments
Ana Bailão and Sandra Šustic
Abstract
Mica pigments are defined as a type of interference pigments with high reflective and refractive properties. In the second half of the 20th century, these pigments have become an additional material to the retouching palettes of conservator-restorers, in particular for reconstructing the losses of metallic surfaces. However, their feature to manipulate incident light can produce large colour shifts depending of the texture of the surface, the viewing angle and illumination settings. Given the lack of literature on retouching with mica pigments, this study aims to investigate their specific features and handling properties in this particular application. The first part of this paper explores the composition and optical attributes of mica pigments, while the second deals with their application in retouching damaged metallic surfaces on three-dimensional wooden objects. It is shown that the selection of pigments of the same geometrical structure and a constant source of illumination are some of the main factors that need to be taken into consideration while retouching with mica pigments.
Mica pigments are defined as a type of interference pigments with high reflective and refractive properties. In the second half of the 20th century, these pigments have become an additional material to the retouching palettes of conservator-restorers, in particular for reconstructing the losses of metallic surfaces. However, their feature to manipulate incident light can produce large colour shifts depending of the texture of the surface, the viewing angle and illumination settings. Given the lack of literature on retouching with mica pigments, this study aims to investigate their specific features and handling properties in this particular application. The first part of this paper explores the composition and optical attributes of mica pigments, while the second deals with their application in retouching damaged metallic surfaces on three-dimensional wooden objects. It is shown that the selection of pigments of the same geometrical structure and a constant source of illumination are some of the main factors that need to be taken into consideration while retouching with mica pigments.
1. Introduction
Depending on how pigments interact with light, they can be divided into three basic types: absorption pigments, metal effect pigments and interference pigments. The main principle of absorption pigments, which include the traditional organic and inorganic pigments, is absorption and/or diffuse scattering of light. Metal effect pigments are contemporary pigments that are made of aluminium, zinc, copper or brass [1]. Finally, interference pigments, which are the focus of this paper, consist of various layers of a metal oxide deposited onto a natural mineral, mica [2].
Mica-based interference pigments were introduced in the 1960's by DuPont with products based on titanium dioxide coatings on a muscovite substrate [2]. Further developments were driven mainly by Merck and Mearl. The synthesis of mica pigments with a transparent iron (III) oxide layer started in the late 1970s [2]. Currently, they make about 80% of the total interference pigments sold [2, 3]. The special effects of these pigments, such as angle-dependent colour or decorative texture, enabled them a growing economic significance and exploitation in many industries among which the automotive, printing, plastics, cosmetics, coating and ceramic industries [4-9]. In addition, these pigments have found their purpose in conservation and restoration practice as well. In Portugal, for example, mica pigments are considered as an optional retouching material when matching a colour of a damaged metallic surface, most often found on three-dimensional wooden objects, such as sculptures and/or decorative frames, and in paintings, especially panel paintings.
In general, damaged gilded areas can be restored with traditional or non-traditional gilding materials and techniques [10, 11]. The exploration of non-traditional, yet reversible, materials in gilding conservation is an ongoing process. There are several options on the market: gold powder [11], metal pigments (like bronze powders) or mica pigments mixed with a natural binder (Arabic gum) or synthetic resins (Paraloid B72, Plextol B500/ Plextol D360, among others). The metal powders have been used extensively as original surface treatments for more than a century [12]. Due to several important advantages in comparison with metal pigments, such as non-toxicity and resistance to tarnish, mica pigments are considered as an adequate alternative to bronze powders. Furthermore, in retouching losses of specific objects where the use of gold or silver leaves is interpreted as a false historical, mica pigments are regarded as a substitute material. Needless to say, they are also less expensive than metal leafs, making them more accessible to conservator- restorers.
However, in some cases, their optical abilities can also be a drawback. Depending on the illumination settings and the viewing angle, mica pigments can yield a large colour shifts making the retouching process more laborious and unpredictable. In addition, retouching areas can also be very difficult to depict photographically for documentation purposes.
This paper aims to study more in-depth these materials in order to attain greater control over their optical attributes in retouching two-dimensional and three-dimensional metallic surfaces. Thus, the first two sections deal with the morphological and optical properties of mica pigments. The objective of the last section is to make evident various associated problems that may occur during the process of retouching and also to offer some practical guidelines to improve the results of colour matching.
2. Composition of Mica pigments
The mica term has been in use by the modern paint industry to be a generic name for the whole group of micas [7]. In mineralogy, micas are a group of hydrous potassium aluminium silicates. Numerous sheet silicate minerals are recognized, but the name often used and with the greatest commercial value and use is muscovite, which is grounded into different pigment grades [2,8,13,14].
Muscovite can be found in nature in many distinct forms as: thin flexible cleavable hexagonal plates (Figure 1), small anhedral disseminated grains, compact scaly masses or synthetically crystallized to the required dimensions. When pure, it is colourless or white but due to the presence of impurities it may be brown, blue, green, pink or red [8]. These impurities include metals such as Titanium dioxide (TiO), Chromium (Cr), Iron (Fe), Aluminium (Al), Cobalt (Co), Nickel (Ni), Zinc (Zn) and Copper (Cu) [14]. These metals form an oxide-hydroxide surface layer that covers the mica platelets making them colourful [14].
Mica pigments are available in shades of gold (Figure 2), copper and silver with red, blue, or violet highlights which can be mixed between them. For the purpose of retouching gilded or silver surfaces, the two most important coatings are titanium dioxide, since it produces colours that appear as a silvery iridescence, and iron oxide, which produce colours that resemble bronze and copper. Gold and brown hues can be obtained by the application of iron oxide layer over a coating of titanium oxide [2, 3]. Because the coating is already oxidized, they can be expected not to oxidize further and not discolour [12].
However, interference pigments can also be classified by their structure and manufacture method. For example, high refractive index substances like titanium dioxide or iron oxide may be deposited on a transparent substrate such as mica or silicon dioxide or even in aluminium oxide. According to Cramer [15], such pigments are manufactured using wet-chemical processes, while other interference pigments, made with an aluminium layer as internal reflector, are manufactured in high vacuum.
Left to right:
Figure 1. Macrophotography of muscovite. Photo by Frederico Henriques.
Figure 2. Examples of mica pigments in two shades of gold: Pearl Luster IRIODIN Colibri Pale Gold (right) and Pearl Luster IRIODIN Colibri Royal Gold (left).
Figure 3. Interference effect of two different shades of mica pigments with 10-60 µm from Merck (top) and Kremer Pigments (below).
Left to right:
Figure 1. Macrophotography of muscovite. Photo by Frederico Henriques.
Figure 2. Examples of mica pigments in two shades of gold: Pearl Luster IRIODIN Colibri Pale Gold (right) and Pearl Luster IRIODIN Colibri Royal Gold (left).
Figure 3. Interference effect of two different shades of mica pigments with 10-60 µm from Merck (top) and Kremer Pigments (below).
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3. Optical Properties of Mica pigments
As previously stated, mica pigments obtain their optical properties through interference. These are not the colours viewed in reflected light, where colour is a result of light absorption (subtraction) of certain wavelengths and reflection of other wavelengths of the pigment. Instead, the colour of these pigments is due to their specific structure which induces interference, a physical phenomenon where wavelengths are overlapped on each other occurring light reflection and light scattering (Figure 3), hence their name [16]. The final colour effect is similar to that of soap bubbles or oil slicks on water.
While retouching metallic surfaces, the treated objects are often exhibited at different locations with various lighting sources and, as a result, colour mismatches can be quite common. Normally, the justification for the optical colour differences of the mica pigments can be explained by metamerism. This phenomenon is often related to the fact that an observer can see the accurate colour match under an illuminant, but not under other. However, according to Johnston-Feller [17], there are three types of metamerism which, in the case of retouching with mica pigments, can occur at the same time.
The first one, mentioned above, is illuminant metamerism and it occurs while exposing the treated object to different lighting sources. This type creates the so-called metameric pair when two colours appear to be a match under one light source but mismatch under other.
The second type is observer metamerism. It happens when the examined colour match is acceptable for one observer and unacceptable to other. In this case, the metamerism is less apparent in relation to the changes in lighting conditions. Therefore, this phenomenon depends not only of the individual mechanism of the observer’s visual system but also upon the type of illumination in the room where the object is stored.
Finally, the geometric metamerism, associated with the surface different characteristics and with the material chemical properties, is an unfortunate common occurrence when retouching with mica. It consists in the visualisation of colour change while moving the angle of illumination and observation [17].
Other factors for metameric pairs and geometrical metamerism may be the amount of different pigment types used with mica to obtain a given colour in retouching, the variations in gloss, surface texture, and ratio of pigment to binder [18]. For this reason, it is important to retain that the simpler the mixture, the lesser the chance is for pronounced metamerism of the resultant colour match.
To summarize, mica pigments can change their colour depending of the light source, angle of observation and observatory itself. But, are these the only factors that induce colour diversity of mica? For professional use, it is crucial to know the material limitations to overcome them. Thus, the conservator-restorer requires more in-depth analyses of these pigments so that he can bring them under very tight matching tolerances.
4. Colour Diversity Phenomenon
As mentioned earlier, mica pigments are complex materials that have a similar behaviour to metallic pigments and are, as Mayer [18] noted, one of the colouring ingredients that can cause metameric pairs and geometric metamerism. In a paint or coating they act as mirror-like reflectors, which mean they have their effect at the gloss-angle. This is the angle at which we see a reflection of the light source on the surface of a paint or coating. Budde defined that the gloss angle is “determined as the ratio of the specular reflectance of a given sample to that of a black glass reference standard for angles of incidence of 20, 60 or 85 degrees” [19].
For a correct visualization and measuring of the interference pigments, it is necessary to use a multi-angle spectrophotometer. As Cramer mentioned, the ideal angles of illumination are 75º (steep), 45 degrees (classical) and 25º (flat) with constant aspecular angle of 15º [20].
One of the reasons for the colour change is related with the thickness of the metal oxide layer that covers the mica platelets. For example, layers of titanium dioxide with around 50 nm thick produce a silvery iridescence; when the coating thickness increase, the iridescence changes through yellow, red, blue and green (Figure 4) [2, 3]. The iridescence brightness decreases if the coating is thicker than 160 nm. When the coating is of iron oxide, it refracts the light as much as titanium dioxide but it adds a characteristic reddish colour [2, pp. 44, 49; 3]. Other reasons are the refractive index of the titanium dioxide layer and the angle of illumination [20].
In addition, the lustre and hiding power of the mica coating can be affected by the particle size. Namely, the sizes accessible on the market vary from 5 µm to 600 µm. Due to their smooth surfaces and uniform reflection of incident light, bigger particles will produce high lustre of the coating. Smaller particles, on the other hand, have flat surfaces with more edges and corners that will increase light scattering and refraction. This will result in better hiding power [2]. Moreover, small particles can minimize the geometrical metamerism, especially at gloss angle. These characteristics give them precedence over larger particles in retouching practice.
5. Retouching with mica pigments: products and their application
Mica pigments are often used as an alternative to metallic leafs and bronze powders when reconstructing the damage of gilded areas on two and three-dimensional wooden objects. As previously stated, mica powders are non-toxic and do not tarnish as metal does. When mixing mica with a binder that does not yellow in time, it is believed that the conservator-restorer can create a surface coating that will retain its original appearance for many years. Moreover, unlike metals, mica is inert, not reacting chemically with the binder during or after the application [21]. However, the usual mica pigments that conservator-restorers use are covered with a wet-chemical method of metal oxide layers. Hence, in spite of being materials with good permanence to light, they are less stable than natural mica.
The conservator-restorer can find these coated mica flakes already mixed with absorption pigments, usually the transparent quinacridones or a transparent iron oxide.
Metallic paints are also available in the market. For example, Liquitex is composed of mica flakes coated with titanium and acrylic emulsion. It is readily reversible and does not change colour [12, 21]. These interference pigments are produced by some well-known manufacturers.
Among the mica pigments available in the market, one of the most used in Portugal is the so-called Iriodin (Merck). These pigments are constituted by a mica core coated with one or several layers of titanium dioxide. With a refractive index of 1.6, it is not electrically conductive, and withstands temperatures of 800° C, being stable to UV light, temperature and humidity. It has a wide colour range of silver-white, red and bronze-coloured earth tones, and interference to gold lustre. It is supplied in dry powder form and may be bound in aqueous media or in various varnishes, and applied by brush or spray. Also, because of its wetting characteristics, it is recommended to use polar solvents with similar characteristics to get good dispersion of the pigments and to avoid modifying their properties, specially the brightness [22].
Yet, the validity of retouching with mica pigments is still questionable. When the conservator-restorer tries to make a colour matching, there will be an apparent difference between the mica pigment and the original metal leaves. Specifically, when light hits the metal leaf, it reflects off of a single plane, but when it hits a pigmented surface (as with a mica pigments coating), it reflects off of multiple micro surfaces. This makes the retouched area appear grainy compared to the metal leaf. The grainy appearance may be a concern but it is not always visible from all angles or a distance, especially if the original metal leaf was applied on mordant. Probably the conservator-restorer will have fewer problems when retouching the shadowed concave areas of carved or moulded ornaments on decorative picture frames, in comparison to convex burnished surface of a leaf originally applied on a silky clay base (bole) (Figure 5). The dimension of the loss area has also influence on the colour change. Colour variations are less significant in small losses (Figure 6).
Left to right:
Figure 4. Colour change due to the thickness of the titanium dioxide (TiO2) that covers the mica platelets.
Figure 5. Example of losses retouched with mica pigments mixed with yellow ochre (PY 43) gouache on a bole gilded surface. PY 43 was added to obtain the yellowish effect of the gilded surface and also to increase the coating hiding power. Note the mismatch between the luminance of the original burnished areas on the left and dull retouched surface on the right.
Figure 6. Small loss before (left) and after (right) being retouched with mica pigments.
Left to right:
Figure 4. Colour change due to the thickness of the titanium dioxide (TiO2) that covers the mica platelets.
Figure 5. Example of losses retouched with mica pigments mixed with yellow ochre (PY 43) gouache on a bole gilded surface. PY 43 was added to obtain the yellowish effect of the gilded surface and also to increase the coating hiding power. Note the mismatch between the luminance of the original burnished areas on the left and dull retouched surface on the right.
Figure 6. Small loss before (left) and after (right) being retouched with mica pigments.
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Integration of larger areas can be very difficult because the colour variations appear more evident to the observer’s eyes. The laws of optics dictate that the maximum reflectance of interference pigments will be shifted towards shorter wavelengths as the angle of illumination is varied from steep, near-normal incident to flat, near-grazing incidence. Accordingly, when the loss is large and without many of the original metallic surfaces around it, it will probably be less significant to the observer’s eyes. However, when the opposite occurs, it will certainly be a very difficult task to resolve (Figure 5). In complex situations, it is advisable to limit the retouching with mica pigment only on small damages, which are lesser than 1.5 cm, so the metameric phenomenon will be reduced to the observer.
Mixing two different interference pigments may create even bigger colour shifts while the viewing and illumination angles change, because each interference pigment has its own characteristic and geometrical structure [16] as well as its own angles of reflection. Hence, mixtures with the same geometrical structure are recommended to achieve more precision in colour matching.
Figure 7 (left). Combined effect of mica pigments mixed with absorption pigments.
Figure 8 (right). Scheme of the light scattering showing the gloss angle.
Figure 7 (left). Combined effect of mica pigments mixed with absorption pigments.
Figure 8 (right). Scheme of the light scattering showing the gloss angle.
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Mixing mica pigments with traditional absorption pigments, in order to distress the surface or to create a particular patina effect, can lead to further problems. In Portugal, it seems common to make a homemade mix of mica pigments and watercolours or gouache paints. When coloured pigment particles are mixed with mica pigments, the created colours are a mixture of the base colour plus the effect of the mica. In such mixtures, the interference colour of the mica pigments dominates at the gloss angle, whereas the colour of the absorption pigment can be seen from all other angles [2]. This can also create certain difficulties when making a photograph of the retouched area. For example, if red pigment is mixed with mica, the paint will appear red because red pigments at most angles will absorb green and blue light and scatter the red light in all directions, but the mica particle is producing reinforced blue light only (Figure 7), creating a purple colour at the gloss angle (Figure 8). Consequently, the paint may appear purple at the flash angle (red and blue mix together); changing the angle of the illumination may reinforce the red colour (Figure 9). In these cases, it may be better to tone the dried layer of pure mica pigment with the additional glaze layer of selected absorption pigment in an adequate medium.
Moreover, the colour base under the mica layer can influence the final appearance of the coating. Thus, when using mica pigments to make metallic paints, the final appearance of loss can be altered by varying the colour base. For example, a gold coloured mica powder will take on a different hue if applied over a yellow, red, white, or black surface (Figure 10). It is recommended to make the base layer on the loss surface that has a semi-gloss or glossy finish. Usually, the use of the similar colour base as the original adhesive layer under the metallic surface (bole or mordant) and the use of a binder with the refractive index closely to the original adhesive can lead to more accurate colour matching results.
Mica pigments can also be brushed onto a surface that has been coated with a binder, rather than mixing the binder and pigment before the application. If the pigments are carefully applied directly onto the prepared loss surface, it is possible to achieve more opaque and evenly brilliant surface.
The type of light source can greatly affect the retouching area. Hence, when possible, the conservator-restorer should explore the lighting conditions of the place where the object will be exhibit after the restoration process in order to adjust the retouching place and set up new solutions to minimize metameric failures.
Left to right:
Figure 9 (left). Testing the colour change: the same pattern of mica paint with gold reflection under different viewing angles. Note the reinforce-ment of the red colour as the angle of the illumination changes. The third image illustrates the colour appearance at the front side observation.
Figure 10 (right). Testing the effect of the different colour base: gold coloured mica powder mixed with Gum Arabic applied over the layer in gouache paints (left), yellow ochre (PY 43) (center) and burnt sienna (PBr 7) (right). The same pattern under selected viewing angles.
Left to right:
Figure 9 (left). Testing the colour change: the same pattern of mica paint with gold reflection under different viewing angles. Note the reinforce-ment of the red colour as the angle of the illumination changes. The third image illustrates the colour appearance at the front side observation.
Figure 10 (right). Testing the effect of the different colour base: gold coloured mica powder mixed with Gum Arabic applied over the layer in gouache paints (left), yellow ochre (PY 43) (center) and burnt sienna (PBr 7) (right). The same pattern under selected viewing angles.
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6. Conclusions
Mica pigments have become an additional material for retouching losses of metallic surfaces on the flat surfaces of paintings and in three-dimensional wooden objects due to their morphological structure and capacity to generate a remarkable array of colours. Yet, their feature to manipulate incident light can produce large colour shifts depending of the viewing angle and illumination settings that can be a problem in the retouching process.
In fact, this alternative technique to metal leaf can never fully recreate the reflective surface of the original. The uneven light diffusion on a surface can make the retouching area appear grainy and matte in comparison with the smooth and glossy surface of a metal leaf. Therefore, it is advisable to limit the retouching with mica pigment only on small damages, which will minimize the metameric phenomenon.
Yet, this research provided several practical guidelines that can help optimizing the colour match. The first step in this process is to mix the interference pigments with the same geometrical structure provided by the manufacturer and to avoid the mixtures of absorption and interference pigments. It is convenient to use small size particle to minimize the geometrical metamerism, especially at gloss angle viewing. Other important factors are the selection of appropriate colour base and the use of a binder with a refractive index close to the original adhesive layer under the metal leaf. Moreover, the application of the pigment powder directly onto the loss surface previously coated with a selected binder can also be practical solution in certain cases.
Finally, it is important to ensure a constant lighting source and ideal angles of illumination where the object will be retouched, documented and exhibited afterwards. The use of instruments to measure the optical properties of mica pigments and illumination sources are advisable, whenever possible.
7. Acknowledgements
This work has been supported by Fundação para a Ciência e a Tecnologia (FCT) scholarship SFRH/BD/ 69783/2010 and QREN - POPH, co-funded by the Portuguese Government and European Union by MCTES. A special thanks goes to the Croatian Conservation Institute and also to paintings conservator Frederico Henriques for providing Figure 1 and technical information.
8. References
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Ana Bailão
Conservator-restorer
School of the Arts, Portuguese Catholic University (Porto, Portugal)
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Ana has a diploma in Conservation and Restoration by the Polytechnic Institute of Tomar (2005) and a MA in Painting Conservation by the Portuguese Catholic University (2010). She is currently a PhD candidate studying methodologies and techniques of retouching at the same university, in collaboration with Centro de Investigação em Ciência e Tecnologia das Artes (CITAR) and Instituto del Patrimonio Cultural de España (IPCE). Since 2004 she practices conservation and restoration in her own studio in Lisbon.
Sandra Šustic
Conservator-restorer
Croatian Conservation Institute (Zagreb, Crotia)
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Sandra Sustic has a diploma in conservation-restoration of easel paintings and polychrome wood. She is currently pursuing a PhD in Art History at the Faculty of Philosophy in Zagreb, Croatia. Her doctoral research deals with the history of conservation-restoration practice in Croatia. Her doctoral research is focused on the history of restoration practice in Croatia. Her areas of interest are theoretical and practical aspects of retouching paintings, technical art history, historically accurate reconstructions of paintings and education. Currently, she works at the Croatian Conservation Institute.
