(Not all Fabrics are created equal!)
Different fabrics vary in their ability to block water droplets like the ones carrying the novel coronavirus when you cough, sneeze, or talk. This difference is apparent in the slow-motion videos shown here for water sprayed through three different pillowcases.
Shown here are images of each fabric including magnified photographs of their weaves. Notice how each fabric has a different weave. The weave in the left most image is fairly open with large gaps between the yarns. The middle and right weaves are tighter with less gaps. These tighter weaves and knits are recommended for DIY cloth masks.
Shown here is a simple DIY test to assess the suitability of a fabric for a cloth mask. While this test in no way replaces typical standards, it should help you eliminate fabrics that are completely unsuitable for mask construction. Note that the spray bottle should be set to the “spray” setting, not the “stream” setting.