Mixed cultures of microorganisms -- a fungus, a cyanobacterium, and an alga -- were incubated on Corning high-alkali low-lime glass (similar in composition to selected types of unstable historical glass) for six months. Wet and dry controls were utilized for testing in conjuction with mixed cultures. Evidence for microbiologically caused deterioration of the glasses was gathered with scanning electron microscopy (SEM). Visual and SEM observation revealed that both glass types supported large populations of microorganisms. The surface of sodium-rich glass was affected by moisture alone, as noted in the wet control, while the potassium-rich glass showed spalling and fissures in the dry control not seen in the wet control. Potassium-rich glass was found to have deep circular pits associated with fungal hyphae. Sodium-rich glass did not have pitting, instead spalling of layers was evident, with etching of the glass surface by the mocroorganisms. Other surface irregularities were noted for both glass types in both controls and experimentals. This set of experiments points out three distinct possibilities associated with the growth of microorganisms on the surface of glass: 1) Direct attack of the surfact and/or subsurface by the microorganisms, which can be differentiated from other observable effects; 2) mechanical disruption by microbial adhesion phenomena; and 3) constant wetting of the surface by water held in place by the microbial biofilm.