B72 Retouching Gels are retouching/inpainting media with remarkable properties. The rheology and handling properties of the medium are modified by the development of a liquid crystal phase in a solution of retouching resin.

The viscosity of a normal resin solution depends on the concentration of the resin. This dependency can be easily observed as it happens all the time on the palette when a solution is mixed with pigments and when the paint is applied onto the painting. The viscosity of the paint changes constantly as solvent evaporates and when more solvent is added to dilute the paint once it becomes too thick to be easily applied by brush.  This simple relation between the concentration, the viscosity of the medium and time it takes for the paint to be too thick for brush application, imposes certain limits on what can be achieved using a simple resin solution for painting.

It is difficult to paint wet-in-wet on a large area and for a long time. Low viscosity paint is too runny and tends to bleed uncontrollably. An increase in viscosity gives a paint which is easier to control and apply precisely but thickens too quickly for painting wet-in-wet. Glazing over large areas is also difficult for the same reasons.

It is impossible to paint impastos with a diluted solution of resin at low concentration. The increase in concentration makes paint too sticky be applied easily, and painting impastos wet-in-wet is also impossible.

Paint for painting impastos can be made by the use of a thickener such as fumed silica. Dispersed in a resin solution in adequate quantity it will produce gel of the right consistency for painting textures and impastos; or at least for painting a few impastos before the paint becomes too thick. During the evaporation of the solvent the concentration of resin increases, and also the proportion of the thickener in the paint increases, making it impossible to be applied by brush less time than paint made with the solution alone. Paint on the palette thickens rapidly, turning quickly into a hard lump.

The graph bellow illustrates a comparison in the behaviour of different retouching media.

B72 Retouching Gels were developed in order to overcome the limitations of conventional retouching media. Their behaviour is more complex than of that of Newtonian fluids and cannot be described simply in terms of their viscosity but rather in terms of their rheology, which is behaviour under applied mechanical stress.

The rheology of the retouching resin solution is modified by the development of a liquid crystal phase. The lyotropic liquid crystal phase is formed by amphiphilic molecules, a building material for micelles, which become more and more organised, so giving liquid crystal phases of increased order. The hardness/softness of the gel depends on the degree of order.  It is therefore possible to produce thick resin solutions with low resin concentration. The properties of a gel retouching medium can be modified by the concentration of the resin, development of the liquid crystal phase of lower or higher order and the choice of solvent. Solvents with larger molecules, which are building blocks of micelles, will produce harder gels.

The liquid crystal phase B72 Retouching Gel with the addition of fumed silica thickens faster. This is caused by the presence of silica. However, it dries slowly enough to allow the painting of impastos wet-in-wet.

Under mechanical stress the order of a liquid crystal phase is disturbed and diminished. When Gel is mixed with the pigments on the palette it becomes more fluid. After application on the painting, the paint thickens again as the micelles reorganise themselves into a more ordered structure. This allows painting and glazing wet-in-wet without the risk of paint bleeding and mixing uncontrollably, even with solutions of a low concentration of resin.

The drying behaviour and rheology of paint made with B72 Retouching Gel makes it possible to paint wet-in-wet, to paint impastos, to paint glazes wet-in-wet and to paint on a larger scale than with a simple solution of retouching resin.

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