Digital BW, The Print

was headed out the door with a portfolio of quadtone prints (all in 
plastic sleaves).  then I noticed that the edge at the open end of 
the sleave on all the prints had turned yellow!  also, a number of 
prints that had been stacked up for a few weeks all had gone yellow!  
This crap is certainly not archival.  What is the next step for 
archival quality and the same printing results?  I am getting worried 
about the prints that I have already sold (with my claims of archival 
quality etc.).

thanks,

mark