Common glass consists of silica $\left(\mathrm{SiO}_{2}ight)$. When HF reacts with silica it will form $\mathrm{H}_{2} \mathrm{SiF}_{6}$ or SiF $_{4}$ Both can soluble in water which means that a fresh surface of $\mathrm{SiO}_{2}$ will be exposed to attack by HF.