The smoothing stone on our 1915 Henry Lang beveling machine was mined by the Newcastle Stone Company in England. This was the only mine known that had the consistency of sandstone, free of any imperfection that would scratch the glass being smoothed.
The company is no longer producing these stones, they are being replaced by modern, man-made stones. So far, I have worn one out, but have 3 in reserve that I found over the years.
A finely adjusted stream of water “lubricates” the surface, from there, the quality of the bevel is up to the skills of the craftsman.
Starting with firm pressure and finishing with a light touch gives the finished piece a smooth surface but leaves the slight facets of a hand-made bevel. This is what gives the finished window the desired prismatic effect.
What I have found most interesting is how the process requires constant focus and attention, leaving little room for the mind to wander.