Slip is a semifluid clayey type of material used in the ceramic industry for coating earthware products, such as bricks and tiles, thus providing distinct surface colors or patterns. In the present work a characterization was conducted to identify the mineralogical as well the chemical and physical properties of geological materials existing in the slip used by industries in the north region of the state of Rio de Janeiro, Brazil. The basic natural materials composing a slip, namely clay and kaolin, are responsible for changes not only the slip plasticity but also in its viscosity and adhesion to the surface of the ceramic piece. The slip characterization was performed in terms of particle size analysis, Atterberg limits and actual grain density. The chemical composition was obtained by X-ray fluorescence and the thermal properties by differential thermal analysis. The results indicated that marked differences exist between slip from distinct origins. A few slip samples failed to present the required properties.