In this work, the concentration of rare-earth ions in doped silica whispering gallery lasers (WGLs) is controlled by evaporation. The fabrication of WGLs is used to experimentally evaluate the evaporation rate (mol/μm) and ratio (mol/mol) of erbium and silica lost from a doped fiber during heating. Fixed lengths of doped silica fiber are spliced to different lengths of undoped fiber and then evaporated by feeding into the focus of a CO2 laser. During evaporation, erbium ions are precipitated in the doped silica fiber to control the erbium concentration in the remaining SiO2, which is melted into a microsphere. By increasing the length of the undoped section, a critical point is reached where effectively no ions remain in the glass microsphere. The critical point is found using the spectra of the whispering gallery modes in microspheres with equal sizes. From the critical point, it is estimated that, for a given CO2 laser power, 6.36 × 10-21 mol of Er3+ is lost during the evaporation process for every cubic micron of silica fiber. This is equivalent to 1.74 × 10-7 mol of Er3+ lost per mol of SiO2 evaporated. This result facilitates the control of the doping concentration in WGLs and provides insight into the kinetics of laser-induced evaporation of doped silica.