Hello GlisGlis,
You would not put the glass on the anvil. You would hold the edge of the glass against the side of the anvil. And, you would strike downwards on the opposite side of the glass.
So, the anvil and the glass would be in the same plane. And, the blow would run perpendicular to the plane, on the other side of the glass.
Here is a historical reference, I believe from around 1860. It is from California where a few native craftsmen were practicing lithic reduction right in to the early 20th century.
""The Shasta Indian seated himself on the floor, and placing the stone anvil, which was of compact talcose slate, upon his thigh, with one blow of his agate chisel he separated the obsidian pebble into two parts; then giving another blow to the fractured side, he split off a slab a fourth of an inch in thickness. Holding the piece against the anvil with the thumb and forefinger of his left hand, he commenced a series of continuous blows, every one of which chipped off fragments of the brittle substance. It gradually acquired shape. After finishing the base of the arrow-head, the whole being only a little over a inch in length, he began striking gentler blows, every one of which, I expected, would break into pieces. Yet such was his adroit application, his skill and dexterity, that in little over an hour he produced a perfect obsidian arrow-head. I then requested him to carve me one from the remains of a broken bottle, which, after two failures, he succeeded in doing. He gave me, as a reason for his ill success, he did not understand the grain of the glass. No sculptor ever handled a chisel with greater precision, or more carefully measured the weight and effect of every blow, than this ingenious Indian; for even among them arrow making is a distinct trade or profession, which many attempt, but in which few attain excellence. He understood the capacity of the mate-rial he wrought, and before striking the first blow, by surveying the pebble, he could judge of its availability as well as the sculptor judges of the perfectness of a block of Parian. In a moment, all that I had read upon this subject, Written by learned and speculative antiquarians, of the hardening of copper for the working of flint axes, spears, chisels, and arrow-heads, vanished before the simplest mechanical process. I felt that the world had been better served, had they driven the pen less and the plow more !" (American Ethnological Society,)"
