Can anyone advise me what material is used for the white, soft, (flimsy) material used as a gasket which goes between the sight glass and a room sealed heater heat exchanger? The sight glass has got dirty, and I'd like to replace the gaskets if possible. (They're rectangular with a circular aperture and there's two - each side of the sight glass.) [It's a Potterton Levante heater if this is important.]
Many thanks.