Crack tips in silica glass in moist atmosphere are filled with an equilibrium liquid condensation of a few hundred nanometers length. Not only does this local environment affect the chemistry of slow crack propagation by stress corrosion, but it also has an important mechanical effect due to its highly negative Laplace pressure. The present article presents an original technique for measuring the physical properties of the liquid condensation in terms of the Laplace pressure and critical condensation distance. This is achieved by combining in situ atomic force microscopy measurements of the condensate length and optical determination of the crack closure threshold in a double cleavage drilled compression specimen.