phfitz said:
I just wanted to polish out some cleaning marks from a 100 year old B&L Tessar. I will use a 2 part acrylic/urethane seam sealer instead of pitch and see how that works out. I don't like the idea of cooking up roof tar, it would smell like death.
Based on my limited experience polishing an astronomical mirror, this is a very bad choice for polishing an optical surface. The process with pitch is to form the pitch to match the curvature of the glass surface by first hot pressing, and then cold pressing immediately before each polishing session. This ensures that you won't change the curve, or change it in (hopefully) predictable and controllable ways.
Polishing long enough to remove cleaning marks may or may not change the lens prescription enough to matter, but doing it with a lap that doesn't precisely match the glass is guaranteed to destroy the lens rather than improve it.
I agree with another poster -- rosin is a component of many optical pitch mixtures. Roofing tar is an expedient substitute and can work, but proper optical pitch is often genuinely pitch, as exuded by an evergreen tree, hardened by boiling, softened by adding turpentine, tempered by adding rosin -- and it's often a clear amber color rather than the black we're used to thinking of. Some optical pitches are made from asphalt, however, and road or roofing tar (actual petroleum based tars) can be tempered to make usable pitch.
No two-part setting adhesive or sealer is going to produce a usable lap, because it can't be pressed to conform after it initially sets. Pitch must *flow*, both in use and in pressing, to do its job.