We extend to flat surfaces the fracture toughness method presented in Acts Materialia vol. 86 (2015) p.385 and measure in this manner the fracture toughness of fused quartz. Tests give 0.67 +/- 0.01 MPa m(1/2) which agrees with earlier microscopic and macroscopic test data for the fast fracture toughness of this material. Data show no signs of sub-critical crack growth; this observation is at variance with what one would expect from literature data on the phenomenon. (C) 2015 The Authors. Published by Elsevier Ltd.