  We present a statistical analysis of the accuracy of the digitized magnitudes
of photometric plates on the time scale of decades. In our examination of
archival Johnson B photometry from the Harvard DASCH archive, we find a median
RMS scatter of lightcurves of order 0.15mag over the range B~9-17 for all
calibrations. Slight underlying systematics (trends or flux discontinuities)
are on a level of ~<=0.2mag per century (1889-1990) for the majority of
constant stars. These historic data can be unambiguously used for processes
that happen on scales of magnitudes, and need to be carefully examined in cases
approaching the noise floor. The characterization of these limits in
photometric stability may guide future studies in their use of plate archives.
We explain these limitations for the example case of KIC8462852, which has been
claimed to dim by 0.16mag per century, and show that this trend cannot be
considered as significant.
