Choosing an Acid Free paper allows for a greater lifespan of images and documents.

Acid Free paper is a paper which has a PH of 7 or slightly higher (making it PH Neutral or Alkaline). Acid in the paper comes from a chemical compound called Lignin, which occurs naturally and basically holds wood fibres together. Lignin itself is not acidic, but over time, as the lignin deteriorates it gives off an acid and as it does so, the paper will turn yellow and become brittle.

Pulp during the paper making processJust note that not all Acid Free papers are lignin free and the lignin in an 'Acid Free' paper will still deteriorate over time- although this period of time is generally many, many years and will still outlive most of us! Acid Free papers are manufactured to eliminate most (if not all of, in some cases) the lignin, as well as to neutralise the acid present during the pulping process. This is done by two processes, one being to separate off the lignin present, by the use of chemicals. The next process is where the acids already present in the wood pulp are neutralised with the use of an alkaline such as Calcium Carbonate. Sometimes from here a process called Buffering will be carried out, where the alkaline is added in excess to prevent future Acid attacks, either from any lignin present or from external influences that create acid formation (such as atmospheric Sulphur Dioxide).