See "Etch Rates for Micromachining Processing-Part II," Journal of
Microelectromechanical Systems, vol. 12, no. 6, Dec. 2003, pp. 761-778,
online at
http://microlab.berkeley.edu/labmanual/chap1/JMEMSEtchRates2(2003).pdf  .
Note the fused silica is the same as fused quartz.
    --Kirt Williams