You're lucky it was only dust Darcey, I bought a mirror recently from a facebook selling page, a well to do person as I know from our area, well, a dusting wouldn't have cleaned it, I've tried steaming it, I've done everything with it, I believe she must have had it on a table and let a child play with vaseline or similar on it but took a good pic of it to put online as it was only when I got it home and in the light that I seen the scum on it