Gelled mediums were widely used during the 19 th century, added to paint on the palette to modify its rheological, optical and drying properties. Many variations of gelled mediums exist in the literature, all with the common basis of mastic resin or varnish, drying oil and a lead compound. With the aim of unveiling the chemistry of such systems, reconstructions were carried out not only following the historical recipes, but in addition, exploring variations and simplifications to get a better understanding of the key ingredients and interactions inducing gelation. Different parameters were tested to establish whether these have an impact on the gelation mechanisms: the influence of the nature and proportions of the ingredients and the preparation process. These reconstructions and their FTIR characterisation reveal the formation of metal-resin soaps, but also underline the specific role of lead in the gelation.