Abstract : Glazes are colored translucent pictorial layers used since the XVth century by the Primitive Flemish painters. A glaze layer is constituted of pigments dispersed in a linseed oil medium. It gives to a painting a striking appearance because the color is built up inside the volume of the layer. In order to understand this peculiar visual effect and to describe it quantitatively, we study the angular and spectral repartition of the light scattered by glazes: By noninvasive and without any contact goniospectrophotometric measurements in backscattering and bidirectional configurations on samples made by a contemporary painter; By proposing a model of incoherent multiple scattering of light inside the matter; it leads to solve the radiative transfer equation and to determine the physical properties of glazes. The auxiliary function method is a new solving method allowing to express the scattered light fluxes in terms of incident and observation angles. Unlike the first and single modeling applied to works of art and dated 1931, we provide a bidirectional modeling, that has been validated by comparison with experimental results. In the particular case of varnishes, unscattering layers, an analytical expression is obtained for the reflectance factor. The measured and computed light fluxes allow to quantify the glaze visual appearance. It is then possible to quantify the color modulation in terms of the number of superposed layers or the gloss due to the plane surface of the fine glaze layers. Applications are conceivable for works of art preservation and restoration like, for example, the visual aspect prediction of a restoration.