Well, this is interesting. I was at Ingelby's of Glemsford, yesterday. They're a specialist manufacturer of paints, limewash, varnish and lacquer. I asked them about clay paint, and they said that they had tested some a year or two back, and found that for the first week or so after application they remain vapour-open ("breathable"), but thereafter they were no more breathable than a modern acrylic paint. Whilst stressing that this was a pretty basic test not done in laboratory conditions, they were categoric that there was an issue, and that the un-regulated word "breathable" was being pushed to its absolute limits by the manufacturers of clay paints. If you want something that remains vapour-open, then it's limewash or distemper.........and not clay paint.
I'm just the messenger, and not a chemist. I can't couch for the veracity of their findings.
I'm just the messenger, and not a chemist. I can't couch for the veracity of their findings.