Etching glass III

A surprise shipment of ammonium bifluoride meant that I spent another hour re-masking some frosted glass with Sellotape, adding a design in nail varnish, then sealing it into a plastic tub containing 10% sulphuric acid with 20% of the new bifluoride.
The equation for solution of silica in hydrofluoric acid is generally given as
SiO2(s) +6HF(aq) → H2SiF6(aq)+2H2O(l)
The result is fluorosilicic acid. After an hour it was obvious that ammonium bifluoride was, as expected, a much stronger etching agent than the sodium salt. However the demands of administration of courses for next term (starting 14th April) mean that I will have to shelve the project for a few days at least.

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