What is ...Acid Free Paper?

What is ...Number 1
This is the first of my "What is ....? posts. Published in the middle of the month, they will cover various aspects of paper, printing and finishing in greater depth. Many of these subjects are complex, so these posts are intended to be a brief description and summary.

What is ...Acid Free Paper?
It is an unfortunate fact that that no paper and printed literature will last forever. Paper is a 100% natural product which is affected by light and air and can yellow and become brittle over time.

Papers described as "Acid Free" are more resistant to the effects of ageing and will are more suitable for producing literature for archival purposes.

Acid free symbol

Acid free papers can be produced from all kinds of cellulose fibres. The papers made from wood fibre which are best able to resist ageing are manufactured from fully bleached chemical pulp and do not contain any residual lignin. Lignin is the stuff that makes the walls of plant cells strong and it's tough to break down and is acidic by it's nature (it’s like tree sap but is not actually sap). To produce acid free papers it is essential to remove the lignin to trace amounts.

During the papermaking process, 'sizing' is added both internally (in the pulp) and on the surface of the sheet. The purpose of 'sizing' (which is added to most paper types) is to resist the penetration of liquid in the finished product and in the case of printing papers, to assist with printability. The chemicals used in sizing for acid free papers are alkaline in nature and chalk (calcium carbonate) is generally used to size the surface (which is also alkaline). The paper will also be 'buffered' with a solution which prevent changes in the pH. To be regarded as truly "acid free" the end product needs to have a pH value of between 7.5-10.0

Today, the vast majority of  papers produced for the print market are acid-free and this is mainly because paper mills now use chalk (calcium carbonate) rather than china clay as the filler in the manufacturing process. Chalk reacts with acids and therefore the pulp needs to be chemically neutral or alkaline.

Although acid free papers have a longer life and are regarded as 'archival', they are not necessarily regarded as true 'archival papers' ...that maybe a future subject for another  "What is ...."

 

Posted by Justin Hobson 21.01.2014