Which preposition to use with whenas

in Occurrences 2%

Whenas in silks.

at Occurrences 1%

To anyone coming from the outside, this angle of the forge would always seem weird and even mysterious even when the furnace was blazing and the sparks flying from the anvil, beneath the smith's powerful blows, or whenas at presentthe fires were extinguished and this part of the shed, innocent of windows, was in absolute darkness.

Which preposition to use with  whenas