Jakob Jakobsen, An Etymological Dictionary of the Norn Language in Shetland…
“ This development doubtless began in the 18th century, at the time that Norn was passing out of use, and later continued during the 19th century, at which time Norn is only found mixed with Lowland Scottish. This development hardly dates further back than to the 18th century, as there are only comparatively few words with the äi form only; while i and äi alternate as a rule, “ai” gradually becomes more and more predominant. In not a few words, however, the i-form alone accurs. ”
