I have old Kodak 4" glass filters - actually a gel with glass on both sides. I mount them inside the camera, behind the lens, with a modified Cokin filter holder. If I mount these things in front, the glass sometimes causes flair. They are K1, K2, and K3 - I think that's the same as the modern numbers 6, 8 and 12. I. too, have a lot of trouble with orange filters, but Clyde Butcher told me he uses orange almost exclusively.