The identification of organic colorants and their color preservation in cultural objects has been a major challenge in heritage science. Research has shown that in artworks such as manuscripts from the Lorvão, Santa Cruz and Alcobaça monasteries, organic yellows were ubiquitously applied but are still to be identified [1,2]. Significantly prone to degradation, these colorants have earned the name 'lost yellows'. The key factors and degradation mechanisms of this color fading are yet to be disclosed. In particular, yellow dye plants were locally grown, providing important geographical information on an artworks’ production.

Pioneering studies by experts allowed an interdisciplinary approach for the characterization of yellow dyes and provide key knowledge on their plant sources. Dominique Cardon has extensively contributed for the study of natural dye sources for textile dyeing [3]. Jo Kirby and Mark Clarke have disclosed written sources for a better understanding of these artists ‘materials [4,5]. Richard Laursen and his group have unveiled the chromatographic profiles of yellow dye plants [6-8].

Identification of plant-based yellows in faded colors has been done through the detection of specific degradation products, which were proposed as markers using chromatographic techniques [9,10], although the assessment of their conservation condition and the causes of degradation is still in its early stages [3,4,9,11]. Furthermore, yellows found in textiles and as paints (lake pigments) are obtained by complexation with metal salts such as alum, forming organometallic structures that are yet to be identified. This hinders the development of better analytical and conservation strategies [12,13].

To understand the degradation mechanisms that are in play in such complex materials, it is necessary to have references prepared with as much historical accuracy as possible. These are used to assess the natural evolution of these colors and simulate the aging of these systems. This can only be accomplished by combining ethnographical and technical art history, preparing historically accurate reconstructions, and studying the chemical profile of the yellow-plant species [3]. Nabais et al. has developed an innovative methodology supported by the use of highly characterized references, which allowed the identification of original colors in medieval manuscripts and textiles [14-21].

These studies have proven that to advance knowledge on the characterization and identification of yellow colors, one must bring together humanities and natural sciences towards a better understanding and conservation of cultural heritage. YES project will carry out the first systematic study of yellow dye sources in Portugal, understanding their use, manufacture, and cultural importance, creating new knowledge on the production of yellow dyes, their degradation mechanisms, and their precise identification in artworks.