The Hungarian roof tile plants settle near a discovered clay mine in every case thus they work with different content clays. Examining characteristics of certain clay layers is needed to ensure stable product quality. Some clay layers with significantly different content occur among the presently used ones, which need an extensive examination for the production of good quality items. Results of high quartz content clay usability examinations are shown in this paper. The aim of examinations is to determine the usability of frequently occurring high quartz content clays. In the course of tests high quartz content clay mineral was mixed in increased amounts to clay mineral for roof-tile production provided by the industrial partner. Shrinkage, porosity and mechanical properties were examined as a function of mixing ratio.