It is easy to underestimate Wedgwood's "useful wares." However,  Robin Reilly in  Wedgwood The New Illustrated Dictionary (1996, p. 286) ranks the development of the hard vitreous stoneware body for mortars and pestles among Josiah I's major achievements and shows that their production was a very serious business to him.

The body is both non-absorbent and impervious to acids. Prior to the 1779 introduction of Wedgwood's innovation, mortars and pestles were made of stone or bronze -- both of which tended to react with the compounds, contaminate mixtures, or absorb oils. The experiment or the substance being prepared was thus spoiled--a serious matter in the preparation of medicines.

According to Reilly, Wedgwood produced mortars and pestles in a brown body as well, though Josiah described the white as "so much more beautiful."

Wedgwood considered himself a man of intellect and science as well as a manufacturer. Reilly further reveals that he supplied ceramic equipment for experimental work to scientists like Watt and Priestly free of charge. (Since the proprietors of Seekers do not consider themselves men of science, this rationale will be ineffective in negotiations for this or similar items.)

Unfortunately we are unable to find an explanation of the impressed mark "Clevden" on the mortar.