The purpose of this lab is to demonstrate that the failure strength of brittle materials is controlled by the size of its flaws. For example, a perfect piece of glass has a strength of a few gigapascals, but a piece of kitchen glassware might be 10,000 times weaker. The difference results from tiny scratches on the surface of the kitchenware. To demonstrate this effect, we use a pyramidal diamond point (a Vickers indenter) to introduce flaws into plates of glass. A computer-controlled indenter, designed and built by Joe Vocaturo, applies a prescribed load on the diamond; the students apply a range of loads to obtain a corresponding range of flaw sizes. A gallery of flaws is shown below; the colored ones are photographed under polarized light, which reveals the stresses around the flaw. We measure the flaw size with an optical microscope, then bend the plates until they break. The strength decreases as the flaw size increases, according to an equation developed from the theory of linear elastic fracture mechanics.