On Monday, July 6, 2020 at 8:16:33 PM UTC-4, Elizabeth Boepple wrote:
I made a horrible error. I thought I was ordering (online) a fusible interfacing, and got Heat n Bond. I tried using it as the filter layer for masks, and they come out so stiff they look horrible! Does anyone know a way to soften this stuff so it doesn't feel like the patches our parents used to sew on dungarees? Thanks!

My dear it came out stiff because that is how fusibles work. The intent is for you to iron two pieces of fabric together and melt a plastic glue in between to make them stick to each other. To get a soft hand on such things you have to get one of the pricier ones, and even so I do not see how it could work as a filter layer. You cannot breathe through a layer of plastic, and no matter how airy a fusible looks it is coated on one side, impregnated with, or made entirely of, that meltable plastic.
The inner layer of vacuum cleaner bags works well as filter material, and is one of the things that got good ratings when my house looked it up.

NightMist