Voltage pulsed modification of surfaces in air with a scanning tunneling microscope has been studied with a view to understanding the physical processes involved. Incremented negative pulses have been applied to a tungsten tip to determine the threshold for feature writing on gold. The primary event observed with virgin tips is pit formation, which is interpreted as due to the transfer of gold cations from sample to tip. Subsequent pulsing establishes a threshold for a secondary process in which hillocks form on the gold surface and which are thought to be the result of gold anion retransfer from tip to sample. The thresholds are 4.16 and 3.92 V, respectively, for the two processes. For one particular tip, which gave rise to a Y-shaped pit, a three-dimensional profile was subsequently obtained by crashing it at a remote site on the surface and imaging the crater formed. The geometrical parameters of the tip, so elaborated, allowed a model of the electrostatic potential between tip and sample during pulsing to be numerically evaluated by solution of Laplace's equation and the field pattern over the surface to be determined. For the blunt pyramid involved, there is substantial anisotropy though modest field reduction at positions some nm from the projection of the tip on the surface. It is then a straightforward matter to understand the Y shape of the pit formed on the surface by the initial pulse. Field evaporation is thereby confirmed as the operative process. Reference experiments using gold tips showed no threshold difference between primary and secondary modification, a result consistent with the field evaporation mechanism.