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Non-archival paper that has yellowed over time.
In the post November is Paper Month, Susan asks:
What makes paper archival … or how long will it last?
Paper in it’s normal state is naturally acidic, leading to eventual degradation. However, most paper made today is acid-free due to a couple of factors. The filler used in paper used to be China clay, whereas now it’s chalk, which naturally has a high pH, making the paper pH neutral. It’s also often treated with magnesium or calcium carbonate, giving the paper an alkaline surplus. This protects the paper from further acidification, such as from sulfur dioxide that occurs naturally in the air.
Acid-free paper has a projected lifespan of 500 to 1000 years, depending on quality. As grades of paper increase to “conservation grade”, the standards become stricter. Lignin levels are kept below 1% in archival paper (lignin causes paper to become yellow and brittle over time).
True archival grade paper is made from a different material altogether — cotton rag. Used in documents of exceptional legal or historic value, cotton is significantly more durable over time at a neutral pH when compared to paper made from wood pulp.
I hope this answers your question, and all are welcome to contribute to the comments with additional information.