I could be perlite, very unlikely it would be talc or gypsum. Perlite is easy to identify, grind it between two hard surfaces and it will sound like glass scratching. Both are super easy to identify using Polarized Light Microscope
Send us a sample, about the size of a dime in a small ziploc bag, will take a couple seconds to do the analysis with Polarized Light Microscopy coupled with dispersion staining-same instrument used for Forensic trace analysis. I did my PhD work in Forensic MicroAnalysis
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After years of exposure perlite will become very fine but you are correct, it was not in common usage in the 1920’s (unless it was added at a later date). Perlite is commonly used because it holds water, common usage included ceiling tiles and gardening
I don’t know about your area but in mine, many builders during that time period did not insulate the attics. Consequently, I often see the original roof structure but newer insulation in attics of those old homes.