trass
/tras
/(also tarras)
noun [mass noun]
A light-coloured variety of volcanic ash resembling pozzolana, used in making water-resistant cement.Although usually of lead, in Deptford the cooler was made of tarras, a cement which, like lead, did not dissolve in the acidic solution.
Origin
Late 18th century: from Dutch tras, German Trass, based on Latin terra 'earth'.