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LuxeSci Show Notes: S2E9 - Art Restoration

Welcome to LuxeSci, a podcast to re-ignite your wonder by exploring the science behind luxury items.  Well, we’ve talked about almost all the colors, how we see color, how to make paint and even interviewed an artist about his process.  So what’s next…well…once these beautiful works of art are created?  How can they last?  How do we preserve cultural and artistic history?  This week we’re talking about art restoration and preservation.

Art restoration

  • Encyclopedia Britannica - any attempt to conserve and repair architecture, paintings, drawings, prints, sculptures, and objects of the decorative arts

  • That have been adversely affected by negligence, willful damage, or more usually, the inevitable decay caused by the effects of time and human use

  • Conservation - maintenance and preservation of works of art and their protection from future damage 

  • For paintings - this means the correct choice of display and storage to minimize aging

  • Ex - parts of the paint and ground will come up from the surface (cleaving, flaking, blistering or scaling)

  • Traditional method - reinforce the back of the canvas by attaching a new canvas (lining or relining)

  • Iron a new canvas on using adhesive made of warm animal glue and farinaceous paste and sometimes a plasticizer

  • Still used in Italy and France

  • 19th century method - uses thermoplastic wax-resin mixture

  • Original done with heated irons but in 1950s the vacuum hot table was introduced

  • Two canvases are heated with molten adhesive and joined together on an electrically heated metal plate

  • Covered with a membrane

  • Air between the canvases is evacuated with pump through holes in the corner of the table

  • Adhesion occurs on cooling

  • However - this can cause darken canvases and thinning paint layers

  • 1960s - “heat-seal” adhesives - synthetic resin formulations (ethylene-vinyl acetate copolymer) 

  • More recently - cold-setting polymer dispersions in water using a low-pressure suction table with a powerful downdraft of air

  • This and the smaller suction plate are used widely now

  • Can be equipped with heating elements and humidification systems 

  • Control humidity, heat and gentle pressure to perform a variety of treatments

  • Changing of original pigment by excessive light exposure

  • Ex - copper resinate (green used from 15th to 18th century) becomes a deep chocolate brown after prolonged light exposure

  • Restoration - repair or renovation of artworks that have already sustained injury or decay

  • Early attempts at restoration were not usually successful

  • One technique was to cover the painting with wood-ash and then wipe it off with water and that formed an alkaline substance that was definitely not good for the painting

  • By the late 19th and early 20th century there was a growing sense that art and science would have to merge to preserve art 

  • 1920s - a group of people assembled at the Fogg Art Museum with Edward Forbes being a pivotal figure.  He advocated technical investigations which included x radiography and imaging to view the internal composition of the object or work of art

  • Two others - Rutherford John Gettens and George L Stout wrote - Painting Materials: A Short Encyclopedia - essential resource for artists 

  • Steps for restoration

  • Initial analysis

  • Assessing loss of paint - done by infrared imaging

  • Use cameras with fixed wavelengths - helps distinguish different pigments because materials reflect and absorb various wavelengths differently

  • Removing discolored varnish 

  • Repairing the painting

  • Immediate coat of varnish to separate the new paint from the old

  • Conservator will inpaint damaged areas using a dry pigment mixed with synthetic non-yellowing solvents

  • Science

  • Paper from Michele Baglioni et al, University of Florence in 2021.  Published in Molecules

  • Looks at advances in material science and technology and how it can be used in conservation of cultural heritage and preservation of artifacts

  • Cleaning

  • Nanostructured fluids - colloidal systems based on surfactants.

  • Above a certain concentration, surfactants self-assemble into micelles small aggregates with hydrophobic core

  • And one or more organic solvents to these solutions

  • Ex - oil and water emulsions

  • First use was in 1986 when a NSF consisting of an aqueous cleaning fluid with n-dodecan, n-pentanol and SCS (surfactant) was used to solubilize and remove wax stains from the surface of Renaissance frescoes in Florence, Italy.

  • Solubilization of apolar molecules inside the core of the micelles is responsible for the detergent properties of the NSFs - drives the removal of low molecular weight chemicals that are commonly found in soil and grime. 

  • Biocleaning

  • Using microorganisms and their metabolic processes for conservation

  • In particular - removal nitrates and sulfates from stone and elimination of black crusts and removing the aged organic compounds from frescos

  • Consolidation

  • In 1996 - there was a major flood of the Arno river in Florence, Italy

  • Consequently, a number of artworks were contaminated with salts and organic compounds

  • This messes with the cohesion of the layers of paint on murals and frescoes

  • One physical chemist from the university of Florence set out to help fix that

  • He re-created lime (the original binder) by using ammonium carbonate and barium hydroxide solutions

  • This set into calcium carbonate upon reacting with atmospheric CO2

  • This was turned into the formulation of Ca(OH)2 nanoparticle dispersions in short chain alcohols

  • Crystalline platelets of Portlandite are transformed into calcium carbonate on reaction with CO2

  • Hydroxide-bearing mineral and is the naturally occurring form of calcium hydroxide

  • Major bonding agent in cement and concrete

  • Alcohols were used to increase the amount of active material delivered and avoids alkaline-sensitive pigments and binders

Dimos Notes

Discoloration of a painting surface can be caused by various factors. A yellow appearance may be due to traditional varnish deteriorating – old varnish is made from botanical ingredients and therefore decays over time causing a yellow or darkened appearance.

Fireplaces and candles are often the cause behind a darkened appearance – toxic particles from soot or smoke can become embedded and lead to a strong visual disturbance.

As many historic paintings have been on display over hundreds of years, it is likely that at some point in their life they have faced smoke contamination from fireplaces or nicotine.

Yellow hues can also be caused by nicotine contamination upon and within the varnish, building up over centuries or decades.

Paintings may appear clouded or with a light veil within the varnish, this can be caused by moisture becoming trapped or mold spores, this is often seen following a flood or escape of water which has affected the artwork. 

Varnish Removal is the most common type of restoration and easiest way to change the appearance of a painting.



When carrying out pigment analysis, a combination of different techniques and instrumentation are used to give our interpretations more certainty. These techniques include:

  • Konica Minolta photospectrometer CM-2600d/2500d 

  • Polarised Light Microscopy (PLM)

  • Optical Coherence Tomography (OCT)

  • Infra-red reflectometry

  • X-ray Fluorescence (XRF)

  • Scanning-Electron Microscopy Energy Dispersive X-Ray Spectroscopy (SEM-EDS/X) 

Any of these analytical methods can be combined with comprehensive technical photographic examination as well as cross-section analysis to best contextualize and interpret results.

In 2004-5, infrared reflectography (IRR) revealed the preliminary sketches for a completely different painting hidden beneath the surface of Leonardo's Virgin of the Rocks. Why did Leonardo abandon his original design and replace it with the one we see today recorded in paint?

Optical coherence tomography (OCT) is a different infrared tool applied as an additional visualization and evaluation technique on a selection of the reconstructed paints. This non-invasive imaging method typically uses low coherency near-infrared light source, which allows penetration of translucent paints and capture of high-resolution depth profiles of layered structures [19,20,21,22]. In our study, OCT was particularly useful for showing differences in translucency and texture when the paint reconstructions containing smalt were compared to those without smalt such as in works by Rembrandt.

X-ray Fluorescence (XRF)

Unlike PLM, XRF is a non-invasive analytical method that does not require any samples to be taken from the artwork. This makes it an extremely ethical form of analysis and means it can be used with any painting. XRF works by firing x-rays at the object and, depending on its material composition, the x-rays will either be absorbed, transmitted (travel through) or scattered. The record of the spectra can be cross referenced from metallurgy and geology comparative databases available to obtain the best analysis.

XRF does have some limitations. Like PLM, it is best for detecting inorganic pigments, so organic pigments such as lake pigments etc are hard to identify. However, in combination with other analysis methods, this does become easier. 

In 2019, XRF mapping not only confirmed this earlier discovery of the figure of the Virgin, but uncovered tantalizing details showing an earlier depiction of the angel and baby Jesus. Revealed in X-ray maps of the element zinc,


 the position of the figures and their relationship with each other in this abandoned composition is very different to that in the final painting.

While we may never know why Leonardo changed his mind before committing to the painting we see now, combining XRF, IRR and Hyperspectral Imaging (HSI) shows just what might have been!

Macroscopic X-ray fluorescence imaging (MA-XRF) assisted by computational analysis, in combination with SEM-EDX analysis of paint cross-sections, provides new information about the distribution and composition of the smalt paints in the painting. 

Use of smalt in Rembrandt’s late works

Rembrandt’s late works, which date from the 1650s and 1660s, are characterized by their loose, sketchy appearance, lively brushwork and unusual surface roughness. Some of the technical aspects—described in the seventeenth century as ‘the rough manner’—are also to be found even in his earliest works [12]. Clearly, Rembrandt did not comply with the change in taste that took place in the second half of the seventeenth century, when fine handling of the paint and smooth surfaces were more highly appreciated than this rough manner. As a result Rembrandt’s late works differ radically from the polished works of most Dutch painters from this period. One of the unique features of Rembrandt’s late works is the use of large amounts of smalt: a potash-silica glass colored blue with cobalt [3,4,5]. Smalt was widely used in the seventeenth century as an inexpensive substitute for the costly ultramarine. The pigment was available in different grades, from pale gray to deep blue, with the color being dependent on the particle size and cobalt content [6].

XRF Figures:


The lead L-line map (Fig. 2d) reveals Rembrandt’s expressive working manner in the light areas of Homer’s face and slightly raised right hand. Here broad brushstrokes of thickly applied lead white paint, finished with pink and yellow tints, were applied over a brown underlayer that is partly left exposed. The lively gray brushstrokes of his hair and beard also show up in the Pb map (orange cluster in Fig. 3c). Most striking are the highlights of the golden shawl. Visible in both the Pb and Sn maps (Fig. 2e), a characteristic pattern of rectangular ridges in the lead–tin yellow containing paint testifies to the use of a palette knife (dark red cluster in Fig. 3c). Rembrandt only started to use a palette knife for the creation of bold surface relief in the 1650s [33]. The Pb map also reveals broad diagonal brushstrokes in a lead white-containing paint at the top of Homer’s right sleeve that could be interpreted as highlights of folds in the cloak (dark yellow cluster in Fig. 3b). These are hardly noticeable in the visible light image, but suggest that the cloak was originally more detailed and voluminous than its present appearance suggests. The copper K-line map (Fig. 2f) shows its presence in the area of Homer’s belt indicating the use of a copper-containing pigment, in addition to smalt (confirmed in cross-section, see Fig. 5d). The iron and manganese K-line maps (Figs. 2g, h) show the presence of iron and manganese in all the smalt paints, pointing to the admixture of earth pigments. The presence of calcium (calcium K-line map, Fig. 2i) can be associated with either chalk, bone black or the substrate of a yellow lake. The many losses in the painting, especially in the lower portion of the painting, were restored in 2005–2006 with chalk fillings, and retouched with earth pigments, also containing Fe, Mn and Ca. Normally these would be difficult to distinguish from original paint in the MA-XRF maps; however, they can be distinguished using t-SNE analysis (green cluster in Fig. 3d).

Scanning-Electron Microscopy Energy Dispersive X-ray Spectroscopy (SEM-EDS/X)

SEM-EDS/X is perhaps the most conclusive form of analysis for looking at pigments as it gives the breakdown for a wide range of elements and can be visualised alongside samples to easily show where the particular elements are in the paint layer. It allows for very minute control over the analysis which can target individual paint layers for very precise results.

Owing to its level of precision, the main application of SEM-EDS/X is for authentication purposes and forgery detection. Unlike XRF, it can be used to detect different types of organic pigments as well as unconventional materials and additives. This also makes it extremely useful in the examination of modern and contemporary works where the characteristics (and potential ageing problems) of certain materials are less well-documented.

Historical research and identifying artist materials/techniques

Often a museum or owner will want to understand more about how their artwork is created. This could include an artistic style, geography and era. Combined with other forms of examination such as technical photography and cross-section analysis, pigment analysis can provide that key final step in understanding the way an artist constructed their painting. Understanding pigments used can help document the artist’s methodology in order to place the work within the sphere of art history as well as giving us insight into the history of pigment production and use. In cases where the artist is known, pigment analysis can help build a bank of knowledge about their technique and material choices.

Above: a detail from a 19th century beach scene following restoration 

Identification of non-original materials and out-of-period alterations

This is particularly useful when an artwork has been recently purchased from an auction or it is believed that past restorations have been carried out without regard to the history of the artwork. Pigment analysis (especially XRF) can help identify large areas of non-original intervention before treatment begins to help both conservator and client bring the painting back to its intended state. This is particularly valuable when intervention is historic as it is much harder to definitively detect via other methods. 

Above: a skull which had been hidden beneath a new layer on an oil painting, discovered and unveiled by our team

Authentication and forgery detection

For instance, a pigment like cadmium yellow was not synthesized or used in art until the 19th century, so it could not possibly have been used in a painting claiming to be by a 16th-century painter. Pigment analysis can help identify inconsistencies to aid in forgery detection and authentication.

On the other hand, where an artist is known and is documented to have used certain materials (or indeed avoided others) pigment analysis can aid in the attribution of their works when compared with other known technical studies. These applications are very useful for clients who have purchased works at auction or have recently inherited works.

Above: a lost signature which was found following investigation and restoration by our conservators


By directing x-rays through a painting and collecting the resulting image on radiographic film, x-rays can be used to highlight what is underneath the paint surface that we see today.  Some of the information that can be collected from the x-radiographs include pentimenti (compositional changes), areas of previous fills or older repairs, as well the location of tears and holes in the canvas or paint layers.

X-rays are shorter in wavelength than ultraviolet radiation, and the energy of that radiation allows the x-rays to penetrate through the paint surface.  The depth of penetration depends on the thickness and the density of the material that the x-rays are moving through.  For example, lower-density materials, such as the canvas or lighter weight elements such as carbon black, do not absorb as much x-ray radiation, and appear darker in the resulting x-radiograph.  Materials with higher densities, like heavy metal pigments such as cadmium red and lead white, absorb x-rays quite easily and show up as white or brighter grey in the x-radiograph.

Creating a system for an X-ray of a painting:

An alpha-numeric grid system is devised to aid in the positioning of the x-radiographs, ensuring an overlap of 1¾” between each exposure.

Schematic of x-rays penetrating throughthe painting, and being captured on the x-rayfilm. The processed image is seen to the far right.

A method for securing the x-ray film to help maintain the grid system during data acquisition. This involves two steps; first, the location of each exposure was marked on the verso of the stretcher bars and second, a specialized film holder was designed to facilitate the process over a two-day time span.  A 22¾”x24½” x-ray film holder using sheets of polycarbonate with rare-earth (neodymium) magnets mounted in each corner.  The magnets are padded with a soft synthetic rubber to protect the surface of the painting 

For the x-radiographic examination, an x-ray tube in conjunction with high-resolution digital-based phosphor x-ray film are matched. Images are captured at 30kV of radiation, which offer better quality in the resulting image.  To avoid any radiation exposure, teams stayed outside of the cage during capture times.

 A digital scanner is used to scan the films as soon as they were exposed.


  • Art conservation - maintenance and preservation and protection from future damage

  • Art preservation - repair and renovation of damaged art

  • Portlandite - natural form from calcium hydroxide and major bonding agent in cement and concrete

Cocktail party facts

  • What’s the difference between art conservation and preservation?

  • Where did modern art conservation science originate 

  • How did scientists create touchable art

Thank you for listening to this episode of LuxeSci.  Please tell at least two people about this podcast.  This is the best way to help us get noticed and find new listeners.  A special thanks as always to my audio engineer Dimos.  Our theme music is Harlequin Mood by Burdy


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