also any glass thats not optical glass may suffer from light bends at different colours, hence why specticals and camera glass is coated. currently i'm looking into using a jar of marmite to shoot through, has a good stop and depending on how much you have eaten the grad might be ok too........................ i also ordered some nd gel sheets in a few different stops for a giggle, they cheaper than tinned farts and i have other uses for them any way.
No, no, no, no, no! Vegemite Caladina, vegemite. Just a thought, don't shoot through the jar. Apply a generous layer to the front of your lens, maybe using your lenscap as a spreader. Once you have finished shooting, you then have a tasty snack that you can lick off the front element. One's mind boggles! Gary