@joyce-drakeford Thanks for you question and for attending the webinar! So, there are different grades/levels of the polyethylene foam. If you are not using the archival one that might be why it is yellowing (the archival kind should not yellow under the proper conditions). So if it is the archival type, the next, probably most common cause, would be exposure to UV light. That is mostly okay for your fossils but it could break down the glues over time. It could also be caused by things that would harm your fossils over time like heat/humidity or oxidation. I also found this PDF with some info: http://fxi.com/assets/pdf/up_06_quality/Discoloration_Info_Sheet_-_111010.pdf