a hermetic electro - optic display cell has an enclosure for a liquid electrolyte comprising a wall of a plastics material , a lower inorganic base and an upper window both of which are impervious to the electrolyte . impervious gasket means seals at least the lower edges of the wall to the base when clamped together . to render the wall hermetic an inorganic coating is provided around the exterior faces and along the edges at least as far as the gasket means . metal is the preferred coating except for transparent faces where silica may be employed .