Terra verde: Overcoming the problem of transparency by crystal engineering

Abstract Terra verde, or Green Earth, a naturally-occurring mineral and green pigment consisting of celadonite or glauconite, is one of the most commonly-used pigments in works of art and their conservation. Its widespread use is owed primarily to its insensitivity to light, its neutral pH, and its high stability in air and moisture – attributes that collectively make it virtually unreactive with other components in an artefact. Notwithstanding this, the use of terra verde in the conservation of objects of cultural heritage is hindered due to its minimal tinting strength and hiding power, which makes the pigment effectively transparent when applied on the canvas with the most common binding media. This gives rise to problems in retouching during the restoration process. We report a robust and reliable method to overcome the problem of transparency through the application of crystal engineering techniques on the pigment Verona Green Earth, or Verona Terra Verde (VTV). The purchased pigment was co-crystallised with a selection of different co-formers and analysis was carried out on both starting compound and product via IR and XRF spectroscopy, hot-stage microscopy and powder X-ray diffraction. The results of the experiments conducted show an increase in the pigment's opacity without affecting its colour intensity.