White pigments were abundantly used in 20th century paintings, and relate to several degradation risks such as titanium white mediated photocatalytic binder degradation or zinc soap formation. Knowledge about the white pigments that were used is essential for risk assessments of 20th century collections. In this study, a representative set of 179 paintings in Dutch modern art collections were analyzed by portable X-ray fluorescence spectrometry. Subsequent explorative data analysis was performed, supported by qualitative findings from 140 home-made reference samples. Micro-samples were further investigated by X-ray diffraction and scanning transmission electron microscopy to gain information about crystal structure and surface coating. This study reveals that Dutch 20th century artists were hesitant to use titanium white. In the Netherlands, the use of this pigment is observed convincingly from the 1970s onward and initially mostly in non-oil binders. Additionally, lead white was used until late in the 20th century for paints and grounds but rarely mixed with titanium white in paints. The study also indicated that many CoBrA artists used zinc white, underlining the risk of degradation due to soap formation. Furthermore, this study highlights the different production processes of titanium white. Through the association of titanium with niobium, an impurity only present for sulfate processed pigments, the production process can be identified. The absence of niobium indicates the presence of rutile and is thus a sign of paint stability, as well as a ‘post 1959’ dating. This explorative study illustrates the value of a combination of data analysis approaches, which includes assessment of spectra as well as descriptive, bivariate and multivariate analysis, for medium-sized datasets gathered at similar conditions.@en

This study reports on the effect of artists’ paint formulation on degradation rates of TiO2-based oil paints. Titanium white oil paint exists in a multitude of different recipes, and the effect of the formulation on photocatalytic binder
degradation kinetics is unknown. These formulations contain, among others, one or both titanium dioxide polymorphs, zinc oxide, the extenders barium sulfate or calcium carbonate and various additives. Most research performed
on the photocatalytic degradation process focusses on pure titanium white-binder mixtures and thus does not take into account the complete paint system. Since photocatalytic oil degradation is a process initiated by the absorption of UV light, any ingredient or combination of ingredients influencing the light scattering and absorption properties of the paint films may affect the degradation rate. In this study three sets of experiments are conducted,
designed using the design of experiments (DoE) approach, to screen for the most important formulation factors influencing the degradation rate. The benefits of using DoE, compared to a more traditional ‘one factor at a time approach’ are robustness, sample efficiency, the ability of evaluate mixtures of multiple components as well as the ability to evaluate factor interactions. The three sets of experiments investigate (1) the influence of the TiO2 type, (2) the impact of different mixtures of two types of TiO2, ZnO and the additive aluminum stearate and (3) the influence of common extenders in combination with photocatalytic TiO2, on the photocatalytic degradation of the oil binder. The impact of the formulation on the degradation rate became apparent, indicating the shortcoming of oversimplified studies. The protective effect of photostable TiO2 pigments, even in a mixture with photocatalytic TiO2 pigments, as well as the negative effect of extenders was demonstrated. Furthermore, the ambiguous role of ZnO (photocatalytic or not) and aluminum stearate is highlighted. Neither can be ignored in a study of degradation behavior of modern oil paints and require further investigation.@en

This dissertation presents a study into the ultraviolet irradiation-initiated degradation phenomena occurring in titanium white containing oil paints, commonly referred to as photocatalytic degradation. The topic of this thesis can be summarized as: the (photocatalytic) properties of titanium white pigments in oil paints and their consequences for collections containing modern art. The thesis consists of three parts: 1) characterization of the use and properties of titanium white pigments, 2) understanding and monitoring the degradation of titanium white containing oil paints and 3) predicting degradation caused by titanium white pigments. Combining the results of these three parts leads to a risk management strategy for modern art collections presented as a conclusion of this thesis. Titanium white pigments were introduced in the 20th century as an alternative for lead white and zinc white. The pigments underwent a gradual development resulting in a large variety of pigments available throughout history. This variety ranges from very photocatalytic or ‘bad’ pigments, which severely speed up degradation, to photostable or ‘good’ pigments which can protect their environment from UV irradiation. Both ‘good’ and ‘bad’ pigments found their way into artist materials and thus into paintings. Hence the question ‘Titanium white, Friend or Foe?’. The pigment’s photocatalytic activity is highly dependent on the pigment’s crystal structure (rutile or anatase) and (inorganic) surface treatment. When a pigment is photocatalytic, radicals can form upon UV irradiation, which attack the oil binding medium and break it down to volatile components, leading to an effect called chalking: the pigment is unbound on the paint surface. Interestingly, while we know that ‘bad’ TiO2 pigments were used in modern oil paints, photocatalytic degradation problems have not been widely reported thus far. Several hypotheses for the lack of problems in collections are presented and investigated in this thesis. Additionally, characterization tools, predictive tools, and monitoring tools are developed, in order to provide solutions before it is too late. Simultaneously, analytical methods and research approaches, that are uncommon in the field of conservation science are explored to evaluate their applicability in answering cultural heritage research questions. @en

Fluorinated-titanium dioxide (TiO₂-F) nanoparticles in a pure anatase polymorph was precipitated from solution by hydrolysis of titanium oxychloride, using urea and ammonia as precipitation agents and potassium fluoride as a source of fluorine anion. A further wet attrition milling in presence of glycine completed by a heat treatment allowed an additional nitrogen doping of TiO₂ (TiO₂-F&N-HT). The morphology and crystalline structure of the as-synthesized powder was determined by transmission electron microscopy (TEM) and X-ray diffraction (XRD) and showed that TiO₂ powder was composed of nanoparticles with narrow size distribution which crystallized in the anatase phase. X-ray photoelectron spectroscopy (XPS) revealed that fluorine and nitrogen are present in TiO₂ as surface fluorination and interstitial doping, respectively. UV–vis diffuse reflectance spectroscopy (DRS) showed an increased optical absorption in the visible for TiO₂-F&N-HT sample. Under visible light irradiation, TiO₂-F nanoparticles showed a high photocatalytic performance, showing the high potential of an improved surface fluorination for Escherichia coli (E. coli) disinfection in suspension. These results show the importance of anatase-TiO₂ nanoparticles synthesis and modification by using a wet chemical approach leading to low aggregation and high specific surface area for effective bacterial inactivation. The co-doped TiO₂-F&N-HT powder showed slightly improved performance compared to the fluorinated sample. The significant degree of aggregation after the heat treatment is postulated as being a limiting factor in its photocatalytic activity.

@en

This work reports the analysis of the time-resolved photoluminescence behaviour on the nanosecond and microsecond time scale of fourteen historical and contemporary titanium white pigments. The pigments were produced with different production methods and post-production treatments, giving rise to a remarkable variability of titanium dioxide powders and, in some cases, to the formation of a complex surface of the crystal agglomerates. The pigments have been further characterized by Raman spectroscopy, scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy and inductively coupled plasma atomic emission spectrometry. Our study provides a clear view of the main features of the photoluminescence (PL) emission of anatase- and rutile-based pigments. For both the polymorphs of titanium dioxide the room-temperature photoluminescence emission is complex and involves different relaxation paths, related to shallow levels close to the conduction bands and mid-gap trap states. The PL behaviour appears to be little affected by post-production treatments such as organic and inorganic coatings. Instead, the presence of niobium impurities in the TiO₂ crystal lattice, as residues of the sulphate synthesis process, induce a remarkable quenching of the visible emission of anatase-based pigments. We confirm that rutile-based and anatase-based pigments are significantly different in terms of photoluminescence behaviour. This clear distinction is a valuable point for non-invasive pigment identification by in-situ photoluminescence spectroscopy. In particular, while many organic binding media emit in the visible region, the near-infrared emission of rutile is specific and can likely be used to identify the pigment in more complex materials as paints. This research paves the way to future studies of the photo-physical properties of titanium white pigments, which is imperative to understand the risk of degradation induced by the well-known photocatalytic activity of this widely used 20^th century pigment.

@en

Twentieth century paints often contain titanium dioxide and zinc oxide based white pigments that can range from photostable to highly photocatalytic. Photocatalytic pigments can cause the degradation of paint upon UV exposure, whereas photostable pigments may be benign or can protect paintings from degradation. Hence, knowing whether or not a pigment is photocatalytic is of high importance for risk assessment and the subsequent decision making process concerning storage and exposure conditions of objects. Here we present a proof of principle, focused on titanium white paints, for an easy-to-use and low-tech application of a commercial photocatalytic activity indicator ink (PAII) on embedded paint samples or cross sections. This test determines, qualitatively, if a photocatalytic pigment is present in a white paint sample. The PAII paint sample staining application shows an obvious color change, within five minutes of UV irradiation, for paint samples containing photocatalytic pigments. A microscope with a camera and a UV source are the only necessary equipment for the application of this method. A quantitative image processing protocol is also proposed as an extension of the staining method by applying open source software analysis to measure the color change using photographs. The test was evaluated on reference paints with well-characterized pigments and applied on samples from modern paintings by Piet Mondriaan, Robert Ryman, and Lucebert, indicating the presence of harmful photocatalytic pigments in these cases. The novel application of a commercial ink on paint samples offers a simple test, not just for assessment of photocatalytic activity of titanium white pigments, but which may in future be applied for the detection of photoactive forms of zinc white and other potentially harmful semiconductor pigments in art objects.

@en

As linseed oil has a longstanding and continuing history of use as a binder in artistic paints, developing an understanding of its degradation mechanism is critical to conservation efforts. At present, little can be done to detect the early stages of oil paint deterioration due to the complex chemical composition of degrading paints. In this work, we use advanced infrared analysis techniques to investigate the UV-induced deterioration of model linseed oil paints in detail. Subdiffraction limit infrared analysis (AFM-IR) is applied to identify and map accelerated degradation in the presence of two different grades of titanium white pigment particles (rutile or anatase TiO₂). Differentiation between the degradation of these two formulations demonstrates the sensitivity of this approach. The identification of characteristic peaks and transient species residing at the paint surface allows infrared absorbance peaks related to degradation deeper in the film to be extricated from conventional ATR-FTIR spectra, potentially opening up a new approach to degradation monitoring.

@en

Titanium white (TiO₂) has been widely used as a pigment in the 20th century. However, its most photocatalytic form (anatase) can cause severe degradation of the oil paint in which it is contained. UV light initiates TiO₂-photocatalyzed processes in the paint film, degrading the oil binder into volatile components resulting in chalking of the paint. This will eventually lead to severe changes in the appearance of a painting. To date, limited examples of degraded works of art containing titanium white are known due to the relatively short existence of the paintings in question and the slow progress of the degradation process. However, UV light will inevitably cause degradation of paint in works of art containing photocatalytic titanium white.In this work, a method to detect early warning signs of photocatalytic degradation of unvarnished oil paint is proposed, using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Consequently, a four-stage degradation model was developed through in-depth study of TiO₂-containing paint films in various stages of degradation. The XPS surface analysis proved very valuable for detecting early warning signs of paint degradation, whereas the AFM results provide additional confirmation and are in good agreement with bulk gloss reduction.

@en

Jackson Pollock’s Number 1A, 1948 painting was investigated using in situ scanning macro-x-ray fluorescence mapping (MA-XRF) to help characterize the artist’s materials and his creative process. A multivariate curve resolution-alternating least squares (MCR-ALS) approach was used to examine the hyperspectral data and obtain distribution maps and signature spectra for the paints he used. The composition of the paints was elucidated based on the chemical elements identified in the signature spectra and a tentative list of pigments, fillers and other additives is proposed for eleven different paints and for the canvas. The paint distribution maps were used to virtually reconstruct the artist process and document the sequence and manner in which Pollock applied the different paints, using deliberate and specific gestures@en

Titanium dioxide is the most abundantly used white pigment of the 20th century. The pigment is still in use, both in the production of contemporary art and for the conservation of older artwork as a retouching pigment. Unfortunately, next to its positive characteristics, the pigment has one major potential drawback: its photocatalytic activity that can cause degradation of artworks in which it is used. In this paper, we report on a new method to test the photocatalytic activity of different quality grades of titanium dioxide white pigments. This can be done quantitatively in a chemical lab or qualitatively in a quick and easy way, in a museum or artists' studio, with limited use of lab equipment. The photocatalytic degradation of an organic dye, acid blue 9, in an aqueous solution containing titanium dioxide, is followed over time by means of UV–Vis spectrophotometry. Dye solutions containing pigments with high photocatalytic activity lose their color within several hours of UVA exposure. On the other hand, dye solutions containing UV-stable titanium dioxide do not degrade within 24 h of UVA exposure. Insight in the photocatalytic activity of titanium white pigments, which can be obtained with this novel test, is of great importance for preventive conservation of modern art.@en