Never have seen this before. This is the consistency of talcum powder.
I asked my brother who has been in thousands of attics in his HVAC career, and he has only seen it once.
We both think it is powdered gypsum, but I wanted some other opinions. House was built in 1920’s.
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
The stuff in my pic is a lot finer than that. I don’t think that’s it unless they can grind it to a powder.
Send me a sample, will analyze it for you at our lab.
Look like perlite to me. If not perhaps silica? Never seen silica used before though.
Are you looking at the first pic I posted, or the two perlite pics? The perlite is coarse, this stuff is ground down to a fine powder.
Also, according to the perlite website link, it has been used since the 1940’s. This house was built in 1928.
I think I’ll have it analyzed, just for curiosity’s sake.
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.