Volume 21, Number 4
Nov 1997

How Stable are Photos on RC Papers?

The question has once again been raised regarding the stability of RC papers. RC, or resin-coated, papers differ from conventional photographic papers because the paper support has been laminated on both sides with a layer of polyethylene before being coated with the emulsion layer. The whiteness of the conventional or fiber base paper is created by an intermediate layer of gelatin mixed with barium sulfate, called baryta. In the RC paper, sometimes called PE papers in Europe (short for polyethylene), the whiteness is created by a titanium dioxide pigment in the upper polyethylene layer.

Virtually nothing has been written on the topic recently for many reasons. One problem is that papers have evolved significantly even in the last five to ten years. In fact, an enormous amount of technology has gone into the evolutionary process of these papers. RC papers have had a number of problems in the past, starting with edge seepage of chemicals and delamination during processing. Permanence was also an issue and RC papers have had problems with polyethylene/emulsion cracking, silver image deterioration, and staining from oxidized, migrated, incorporated developers.

The industry has tried to address these problems as best as it could. With the incorporated developers, for example, many companies have added anti-oxidants into the paper which is sandwiched between the polyethylene layers. More recently companies are doing away with the incorporated developers altogether, except in those papers that absolutely must have them, that is, rapid access machine processable papers.

To further complicate things, there are two schools of thought regarding the stability of materials. One side says that if the material meets the requirements of the national or international standards for manufacturing and processing and you store it in accordance with the standards, then it will last a long time. The other side says that many people and institutions can't store things in accordance with the standards and that this problem reflects real life. One must understand that in the latter situation, no company would make any kind of guarantee about the stability of their product under uncontrolled conditions.

ANSI, for example, makes the statement that "This standard applies to photographic ... records intended as storage copies, which should not be in frequent use. This standard does not apply to 'work' or 'use' copies." However, the reflection print standard allows a maximum storage temperature of 18°C (64.5°F) and a relative humidity of 30% to 50%. While these conditions are relatively cool and dry, they are not outrageously difficult to achieve for most collections. Under these storage conditions, both fiber and RC papers are expected to do very well.

Note that storage conditions include absence of light and reasonable protection against chemical contamination from industrial pollutants and poor quality enclosures. If a print is intended for archival use, these are the conditions under which it must be kept with very limited access.

The most common condition of uncontrolled keeping tends to occur because of long-term display. In this situation light is one of the most obvious problem. However, in any area with free public access, it is much more difficult to control temperature, relative humidity, and the ingress of air pollutants from the outside world. In Kodak's Conservation of Photographs, they state that, "In the case of black-and-white prints that are designated for long-term display, a negative should be made and carefully filed before the prints are put on display, if negatives do not already exist." Note that they say "black-and-white prints" meaning either RC or fiber prints.

Having said this, if asked what I would chose for the longest permanence under possible display conditions, I would recommend a fiber base paper over RC. At the very least, there seems to be a benefit to having a porous support. It is thought that air pollutants entering a fiber base photograph are able to escape to the paper support and away from the emulsion, making the photograph a little less susceptible to oxidation damage.

In an RC base, migration of pollutants through the polyethylene is very slow so the pollutants tend to stay in the emulsion where they can attack the silver. Limited data during the development of the ANSI peroxide fuming test seems to support the data, but the paper samples were also affected by the antioxidant technology used in each paper, which can vary from product to product even from the same company and, of course, this technology is proprietary. So it remains a theory, but is supported by much of the research community involved with black-and-white photographic papers.

In addition, there is still some question about the titanium dioxide in the base. It is still considered to be the best whitener for RC paper, but in the presence of UV light, it forms a singlet oxygen which is responsible for both the cracking and many of the silver deterioration problems of the 1970s and 80s. The industry has addressed this problem by adding anti-oxidants or peroxide scavengers. UV absorbers are also said to have been used. UV absorbers will eliminate the benefits of optical brighteners, but the brighteners in themselves will also act as a UV protectant.

This approach has helped the problem greatly, and in general, there is little concern about the cracking problems now, although eventually the preservatives will fail. Even a Twinkie won't last forever. It is possible that with infinite money the industry could make papers even more stable. However, the industry, or at least a few of the companies, have determined that people just aren't willing to pay much of a premium for greater stability than is currently available. It makes no sense to spend a lot of money on research for a product that almost no one will buy.

Ultimately, the only way to protect a photograph on either support that will be on display is to treat it with gold, platinum, selenium, or sulfur.