Applying the art of war to dating
We conclude that the luminescence detected here by Raman spectroscopy is a marker characteristic of coprecipitated pigments made from an ilmenite ore with high levels of Nd, such as are found in the Mc Intyre mine deposit.
Concentrations of REE in different samples of TITANOX pigments compared to those of the ilmenite from the Mc Intyre mine (New York) and average concentrations of the four paintings analyzed in Fig. Error bars on the TITANOX and ilmenite samples indicate a 5% instrumental error, while those on the painting average indicate SD of the four different paint samples. Concentrations of REE in different samples of TITANOX pigments compared to those of the ilmenite from the Mc Intyre mine (New York) and average concentrations of the four paintings analyzed in Fig. Error bars on the TITANOX and ilmenite samples indicate a 5% instrumental error, while those on the painting average indicate SD of the four different paint samples. To determine whether the luminescence marker for coprecipitated pigments could be used to infer creation dates of works of art, we performed a survey of 109 oil paintings of known provenance created between 19 by artists working in the United States, which should contain paints dating from the period in which luminescent titanium white pigments were manufactured.
Facile Raman-based detection of this luminescence, along with characteristic peaks of rutile, anatase, calcium sulfate, or barium sulfate, can identify the type of titanium white pigment and narrow its manufacture date range.) was introduced as a pigment in the early 20th century and by mid-century was as popular as lead or zinc whites.
Although titanium dioxide does occur in mineral form and therefore has been found in archaeological and historic contexts (), the detection of titanium white is most often a telltale sign of modern overpainting (or outright forgery) of older artworks.
The coprecipitated pigments display the luminescence peaks, whereas the pure pigments do not.
Calcium sulfate–coprecipitated titanium white paints from four paintings dating from 1950 to 1963 contain much higher levels of REEs than Grumbacher Titanium White paint made from non-coprecipitated anatase (Fig. La, Ce, and Nd are especially abundant in the coprecipitated pigments (from 3 to 6 μg/g), whereas in non-coprecipitated anatase, Ce is present at less than 1 μg/g and La or Nd are present at less than 0.01 μg/g.
Of the three abundant REEs, only Nd could plausibly give rise to the observed luminescence.
Lanthanum in its stable 3 oxidation state has no ), which correspond closely to the first three peaks of the coprecipitated pigments at 869, 875, and 882 nm.
We conclude that the titanium white pigment luminescence arises from neodymium incorporated into the calcium sulfate structure.), the primary titanium source for 20th century pigment manufacture, and become incorporated into the sulfate structure during the coprecipitation process.The ICP-MS analysis of samples of two coprecipitated pigments (TITANOX RCHT and TITANOX RC) and one pure pigment (TITANOX RA) shows that the coprecipitated pigments contain much higher levels of REE (Fig. TITANOX RC contains 18 times as much Nd as TITANOX RA, and TITANOX RCHT contains almost 400 times as much, indicating that the different methods of pigment preparation differentially affect trace levels of REE.