  We derive the plage area evolution over the last 12 solar cycles employing
data from all Ca II K archives available publicly in digital form known to us,
including several as yet unexplored Ca II K archives. We analyse more than
290,000 full-disc Ca II K observations from 43 datasets spanning the period
1892-2019. All images were consistently processed with an automatic procedure
that performs the photometric calibration (if needed) and the limb-darkening
compensation. The processing also accounts for artefacts plaguing many of the
images, including some very specific artefacts such as bright arcs found in
Kyoto and Yerkes data. We have produced a plage area time-series from each
analysed dataset. We found that the differences between the plage areas derived
from individual archives are mainly due to the differences in the central
wavelength and the bandpass used to acquire the data at the various sites. We
have empirically cross-calibrated and combined the results obtained from each
dataset to produce a composite series of plage areas. "Backbone" series are
used to bridge all the series together. We have also shown that the selection
of the backbone series has little effect on the final plage area composite. We
have quantified the uncertainty of determining the plage areas with our
processing due to shifts in the central wavelength and found it to be less than
0.01 in fraction of the solar disc for the average conditions found on
historical data. We also found the variable seeing conditions during the
observations to slightly increase the plage areas during activity maxima. We
provide the so far most complete time series of plage areas based on corrected
and calibrated historical and modern Ca II K images. Consistent plage areas are
now available on 88% of all days from 1892 onwards and on 98% from 1907
onwards.
