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 \sim 9 - 17 for all calibrations .
Slight underlying systematics ( trends or flux discontinuities ) are on a level of \lesssim 0.2 mag 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 0.16 mag per century , and show that this trend can not be considered as significant .