Sandblasting
To cut glass or dress stone you normally need something harder than the workpiece, pressed against it and moved. That means a tool, and a tool has a shape, so it can only reach where its shape allows and it blunts as it works.
A grain of sand is not harder than steel, and it cuts steel anyway. Fire it fast enough and the energy arrives in a spot so small that the surface fractures locally. Every grain takes an invisible chip. There is no tool edge, so there is nothing to blunt and nothing to reach around — the abrasive goes wherever the stream goes, and follows any shape.
US Patent 108,408, "Improvement in Cutting and Engraving Stone, Metal, Glass", granted 18 October 1870 to Benjamin Chew Tilghman of Philadelphia. He claims working hard materials "by means of a stream of sand or grains of quartz", driven by a steam jet he preferred to run above 300 pounds per square inch.
Read the safety step before you buy anything. This blueprint deliberately does NOT use Tilghman's quartz sand.
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