Archival paper and inks: what makes an art print last

Archival paper is paper made to change as little as possible over many years. It keeps its colour and stays strong. This page explains what archival means in real numbers, why ordinary paper yellows, what the labels on art paper mean, and which inks last longest. It belongs to the guide to art prints and paper, which covers the whole path from a digital file to a print on the wall.

Why paper ages at all

All paper is made of cellulose, the fibre found in plants. The Canadian Conservation Institute, part of the Government of Canada, explains that paper breaks down in two main ways. The first is acid: acid in the paper cuts the long cellulose chains into shorter and shorter pieces, and the paper becomes weak. Higher humidity speeds this up. The second is oxidation, a reaction with oxygen that metal traces, air pollution, bleach residue and light all speed up.

The Institute gives numbers for how much the storage climate matters. At room temperature (20°C) and 50% relative humidity, an acidic paper can suffer serious damage in a few decades. At 20°C and 30% humidity, a typical wood pulp paper lasts twice as long as it would at 50%. Its general rule is that every drop of 5°C in temperature halves the rate of chemical decay. To show the scale, it takes an acidic paper that lasts 50 years at 25°C: the same paper would last 100 years at 20°C and 500 years at 10°C.

So archival paper has two jobs. It starts with as little acid and as few harmful substances as possible, and it carries a reserve that keeps neutralising acid that forms later.

A short history of paper quality

The University of Illinois Library runs a preservation guide for collections, and its page on paper gives the history in order.

The guide says the Library of Congress suggests treating any wood pulp paper made before the 1980s as possibly acidic from alum-rosin sizing. It also says that ground wood pulp, used for newspapers since the mid-1860s, keeps its lignin and should still be treated as unstable today.

The Canadian Conservation Institute adds an important point for modern art paper: wood pulp can make lasting paper when it is processed well. Modern papers made from highly processed chemical wood pulp, with alkaline additives such as calcium carbonate, can last as long as rag paper.

What the archival labels mean

Paper packets and print descriptions use a handful of terms. Here is what each one means.

Acid-free

The University of Illinois guide says acid-free is the paper industry's term for paper with a neutral pH of 7 or an alkaline pH above 7, and no acidic pulp, lignin or sulphur. Acid-free paper can be made of cotton or of wood pulp. The guide adds a warning: acid-free status is measured when the paper is made, and it is no guarantee that the paper will stay stable. Acid can form later from the paper itself, from the air or from things it touches.

Lignin-free

Lignin is the natural substance that holds wood fibres together. The Canadian Conservation Institute says lignin in wood pulp makes paper discolour and leaves it sensitive to sunlight, so it yellows and darkens. Paper mills can remove lignin with chemicals during pulping. Cotton has no lignin to begin with.

Buffered, or alkaline reserve

A buffered paper contains extra alkaline material, usually calcium carbonate, that neutralises acid as it forms. The Image Permanence Institute says a reserve of at least 2% calcium carbonate captures acids from the air, from the paper itself or from a print it touches, and that it is used up as it reacts.

Permanent paper

Permanent paper has a real standard behind it. The Library of Congress explains that the National Information Standards Organization defines it in ANSI/NISO Z39.48-1992 (reaffirmed in 2009). The standard asks for:

• a pH between 7.5 and 10.0,
• an alkaline reserve of at least 2%,
• less than 1% lignin,
• good tear resistance.

The University of Illinois guide says papers that meet this standard are designed to last several hundred years under normal use. It names a similar international standard, ISO 9706, published in 1994. Paper that meets either standard may carry an infinity symbol inside a circle, or a printed statement of compliance.

The United States also has a law on the subject. The US Government Publishing Office's paper standards quote Public Law 101-423, passed in October 1990, which states that federal records, books and publications "of enduring value be produced on acid-free permanent papers."

Cotton rag

Cotton rag paper is made from cotton fibres instead of wood. The University of Illinois guide says rag papers are often found in good condition after centuries, because cotton and linen fibres have a neutral pH. The same guide says fine art rag paper today is made of long cotton rag fibres and cotton linters, the short fibres left over when cotton is processed.

Optical brighteners

Some papers contain optical brighteners, which the US Government Publishing Office also calls fluorescent brighteners. They are added to make the paper look brighter. The University of Illinois guide says brighteners and tinting agents may not be lightfast, and can cause fading, yellowing or brittleness over time. The US Government Publishing Office bans optical brighteners in its paper grades that contain 50% or 100% cotton or linen.

The coating on inkjet art paper

An art print needs more than a stable sheet. Inkjet ink spreads into plain paper, so inkjet papers carry a thin top coating that holds the ink near the surface. The Image Permanence Institute, a research laboratory at Rochester Institute of Technology, calls this coating an image receiver layer. It says the first ones were added to art papers such as watercolour paper, using mineral particles in a starch or plastic binder.

The coating ages too. The Institute says some inkjet papers can yellow, and their coatings can break down, even at room temperature. For this reason it recommends cool storage for inkjet prints kept for the long term. The page on storing unframed prints turns that advice into steps for a home.

Ink: the other half of an archival print

A stable sheet can still carry an unstable image, so the ink matters as much as the paper. The giclée printing page explains the two kinds of inkjet ink, dye and pigment, and how each one sits on the paper. For a long life, three further points matter.

Mixed ink sets

The Image Permanence Institute's guide to digital prints says printers use all dyes, all pigments, or a mix of the two, where the colours are dyes and the black is pigment. Pigment has tended to resist light, moisture and air pollution better than dye. So in a print from a mixed printer, the coloured parts of a photo are the weaker part. In the Institute's tests with ozone, a gas in indoor air, printed text lasted better than photographic images. The researchers suggested this may be because text is more often printed with a pigment black. Ask which ink prints the colours before you choose a printer or a print shop.

The pairing of ink and paper

The Institute says the same kinds of print can age at different rates from one product to the next. In its guide to digital prints it gives an example: two images printed on different makers' inkjet printers, on the same type of paper, may age at different speeds because of small differences in the ink and paper recipes. For this reason the Institute recommends treating each print as an individual and checking its condition from time to time.

Open coatings and air

Paper with a porous coating takes in ink through tiny spaces between mineral particles. The Institute says those spaces stay open after the print dries, so the colours are exposed to the air and fade more easily from air pollution. Papers with a swelling plastic coating take the ink fully inside the coating, which makes those prints more resistant to scuffs and to fading from air pollution. The page on lightfastness and fading explains how light and air fade prints, and how fading is measured.

A checklist for choosing archival paper

When you print a digital portrait or read the description of an art print, look for these points:

1. Fibre: cotton, or a purified chemical wood pulp.
2. Acid-free and lignin-free: both stated.
3. Buffered: an alkaline reserve, usually calcium carbonate.
4. A named standard: ANSI/NISO Z39.48 or ISO 9706, or the infinity symbol.
5. Brighteners: whether optical brighteners were added.
6. Ink: pigment ink for the longest life, dye ink only if you accept more care.
7. Coating: an inkjet coating made for the ink you will use.

What this means for a royal pet portrait

Purrealism art prints come in four sizes, 8 × 10, 12 × 16, 18 × 24 and 24 × 36 inches, on matte or glossy paper. You choose the size and the finish in the portrait studio. If you choose the digital download and print it yourself, this checklist is how you compare papers at a print shop or in a store.

Paper quality is only half of a long life. The Canadian Conservation Institute's figures show that heat and humidity speed up decay in any paper, so keep an unframed print in a cool, dry room away from heaters, and follow the storage steps linked above until it is framed. To pick the portrait itself, the gallery shows finished examples in several styles.