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Paper and pH

Selecting the Right Paper is Easier when you Learn the Differences Between pH neutral and Acid-free Papers

When discussing artists' papers, the primary concern is with permanence, because paper is more than just a work surface-- it's an integral part of the whole creation. For permanence, the paper fibers used must be of a pure cellulose nature (plant tissue and fiber). Cotton is 100% cellulose, and high alpha woodpulp is 93% cellulose-- both considered to be of excellent permanence. The presence of acids will contribute to the deterioration of papers, and the manner in which the paper is used and cared for will greatly determine its life.

Currently, more and more manufactureres and archivists are proposing new definitions and standards offering correct terminology. The discussion continues, and poses the following questions and arguments.

A commonly used term to describe artist paper is 'neutral pH.' Many customers and retailers assume the neutral pH is the same as acid-free. While an acid paper with buffers added could be subject to deterioration or yellowing if the acid remaining in the sheet or formed during aging exceeds the buffering capacity.

The term 'pH of paper' is essentially undefinable because the fibers, additives, and absorbed water do not conform to the basic definition of pH established for aqueous solutions.

The acidity or alkalinity of a paper is expressed as a pH value. pH is a measurement of the concentration of hydrogen ions in auqueous solution. A neutral pH environment is the equal balance between the hydrogen ions. (HO is actually H+ and OH-: the positive ion is balanced against the negative) If more hydrogen ions are introduced, the balance goes toward an acidic solution. If more clacium carbonate is added, the solution becomes more alkaline. The pH scale :

pH Chart

Of the methods used to determine the pH of a paper, the most widely used. and the ones which tend to produce the more consistent values, are the 'wet end' extract and the 'cold water' extract pH procedures. The wet end extract is done in the beginning of the process, when the pulp is about 99% water and 1% fiber. The cold water extract is performed on air-dried paper. Paper (1.0g) is placed on a beaker with distilled water, broken down with a stirring rod, and covered. After one hour, the mixture is stirred again and (without filtration) the pH is measured with a glass electrode pH meter.

Buffers, such as calcium carbonate, may be added to change the acidic value of the pulp mixture guaranteeing the alkaline reserve. During the papermaking process, many manufacturers add a minimum of 2% calcium carbonate allowing the paper to be manufactured with a neutral or alkaline reserve. Acid free papers are considered to have no free acids or a pH measure of 6.5 pH or higher.