Negative thermal poling introduces second-order nonlinear effects into silica glass. The effects are studied using the charge separation model allowing for the third-order nonlinear effect to be anisotropic. The second-order nonlinear coefficient χ(2) is found to be consistent with the results reported previously by Arentoft et al. (1999), Ren et al. (2002), and Marckmann et al. (2001) and the third-order nonlinear coefficient χ(3) is found to be anisotropic but constant during poling.