Titania ceramic discs, used in evacuated high-power microwave structures, can be made to withstand higher electric intensities by coating the ceramic surfaces with a thin layer of a lead-borate glaze. This improvement was noted in static and microwave fields, the value of the tangential electric intensity required to cause breakdown being similar in both cases. Factors which might influence breakdown, such as secondary emission and electrical resistivity, were found to be little different at glazed and unglazed surfaces. The glaze would appear to provide a protective coating to the ceramic, preventing tracking owing to a reduction of the material by local heating. Initial sparking at a glazed surface gives rise to a cleanup process, resulting in a higher breakdown strength.