I have not looked at the Format glass grads, I looked a lot at Format glass ND's most of the first half of 2012 and never found one screw in or sheet that did not have a magenta cast. The screw in 105 and 95mm were the worst.
The Lee grads seem to work fine for me, still resin, but very clean and no issues I can find.
Lee also makes solid ND's 0.3, 0.6 and 0.9 out of glass and they are excellent. However they are only 4 x 4, not 4 x 6.