But that reason is stuck in a descriptive clause modifying "lime-plaster" or "a coating of lime-plaster". You wouldn't write "Walls were sometimes whitewashed

But that reason is stuck in a descriptive clause modifying "lime-plaster" or "a coating of lime-plaster". You wouldn't write "Walls were sometimes whitewashed with a coating of lime-plaster which was white and because lime kills off bacteria." The longer sentence makes it less jarring until you try to parse it carefully, but still very poor style. I don't think I'm being some kind of prescripivist grammar Nazi here: the rest of the piece is written reasonably professionally and I'm sure the editors would fix that if they spotted it.