Analysis of top layers of terra sigillata specimens of different origins having archaeological interest revealed illitic hydromuscovite composition. The elutriation process required to produce a clay suspension caused the uniform glaze slip. No evidence of any special additions was found. Variation of the x-ray diffraction intensity of potassium in comparison with that of aluminum and magnesium is given in relation to the irradiation time. Intensity profiles parallel to the surface are shown for the individual elements. Average compositions of glaze layers are tabulated for different origins. The phosphorus content of the glaze is 0.1-0.2% P₂O₅ throughout variations of raw clay. -- AATA