Before and after conservation treatment, cellulose acetate x-ray from the Joint POW/MIA Accounting Command
Vinegar syndrome, or the process of cellulose acetate deterioration, is an unavoidable issue for any collection of acetate negatives, either presently or in future decades. Before deterioration occurs or early in the deterioration process, preventative measures can be taken to preserve negatives including cold or cool storage, adequate ventilation, and appropriate acid free enclosures – all of which can greatly extend the life of a negative collection. Steps can also be taken to preserve the negatives through digitization and duplication before the deterioration begins to cause image loss.
Bubbling or “orange peel pattern” deterioration in acetate negative.
Crystalline deposits between layers of deteriorating acetate negative.
Very characteristic channeling of an acetate negative in advanced stage of deterioration.
However, once advanced deterioration such as bubbling, channeling, and crystalline deposits begin to form between the layers of the negative, more drastic measures must be taken to retrieve a legible image. In the 1980s, Doug Munson developed the first practical solution for recovering the emulsion layer (or pellicle) of cellulose acetate negatives. Though it was already known that the gelatin emulsion could be removed from the deteriorating acetate film base, he eliminated the problems of fragility and dimensional stability of the emulsion layer by rigorously controlling the amount of water in the emulsion recovery routine, a process often referred to as “stripping”. Because CAW already ran an accurate and highly calibrated negative duplicating facility, they were in a unique position to combine the stripping process with the creation of an extremely accurate duplicate negative on archival polyester film. After recovering and duplicating 793 negatives from the Walker Evans Archive in the collection of the Metropolitan Museum of Art, Doug presented his technique at the Photographic Materials Group meeting in 1997. In 2000, Doug trained Kijoo Kim in the process. Though the basic technique of stripping the emulsion layer from the base has not changed significantly since CAW began providing this service 30 years ago, over the last 13 years Kijoo has refined the technique while recovering over 11,000 negatives. Emily Phoenix joined the acetate stripping team in 2010 and has recovered over 1,500 negatives.
Diagram of the layers of a film negative.
The ability to separate the image emulsion layer from a deteriorated acetate film base relies on the presence of cellulose nitrate subbing layers between the emulsion, support, and anti-curl layers of the negative. The initial separation of the pellicle from its deteriorated acetate base can be accomplished by dissolving away these nitrate layers in a non-aqueous solvent combination. Subsequent solvent baths perform three tasks. They clean the pellicle of residual cellulose nitrate retained from the first stripping bath, they allow a progression from dangerous solvent chemicals to the less toxic ethanol bath, and they introduce a controlled amount of water to the pellicle to allow it to relax and be temporarily flattened, without evidence of its previous furrowing.
The next stage of the process is to flatten the pellicle, while still wet, between Mylar and glass. If the pellicle is undamaged, this is usually simple to do. Unfortunately, in many cases the emulsion is cracked or broken and must be carefully reassembled. After the pellicle has been arranged, and reassembled if necessary, it is scanned and then dried in an archival paper sleeve. We do not attach the pellicle to a new support since it is quite stable on its own, and if it is needed in the future it can be safely handled by an experienced photograph conservator. From the digital file we produce an archival film negative using LVT film recorders and process the film in-house in order to carefully control the development, fixing, and washing of the negative to meet the highest archival standards. This duplicate negative serves all practical purposes an institution may have, and the original pellicle is stored safely away from regular handling.
Kijoo Kim closing the cracks between the many pieces of a badly deteriorated 14×17″ x-ray. X-rays are particularly difficult to recover because they have not one but two emulsion layers. The process is like piecing together a puzzle. . . which happens to be mixed up in the same box with a different puzzle of the same image.
In published material on the subject of acetate negative deterioration, channeling is frequently referred to as the final stage of deterioration. This is not the case. A pellicle may be complete and undamaged even though channeling has occurred, and recovering these pellicles is a relatively simple procedure resulting in a complete image with no trace of deterioration remaining. However, the deterioration process does not end there. As the acetate base becomes increasingly brittle and breaks into pieces, it may force the emulsion layer to break along with it, resulting in cracks and pieces of the emulsion becoming separated from the rest of the negative. Many years of practice have allowed us to become skilled at saving even very badly broken negatives, but we can only reassemble pieces that we have, and all too often the fragile pieces of emulsion are lost or destroyed before they are sent for recovery.
The orange spots on this acetate negative were most likely caused by nitric acid from cellulose nitrate negatives stored nearby. Nitric acid compromises the emulsion layer, and is one of the main causes (along with water damage) of a pellicle breaking down during the stripping process.
We have heard concerns about the potential of the stripping process to damage negatives. While there is risk involved, it is slight compared to the risk of taking no action. Of the tens of thousands of negatives that we have recovered through this process, in only four cases was the emulsion compromised beyond recovery, and we were forced to use the “before” image for the duplicate negative. In all of these cases we suspected that deterioration factors beyond those posed by the negative itself were at play – most likely nitrate negative contamination or excessive water/mold damage. However, we have observed that every year we receive a higher proportion of negatives which have deteriorated past the channeling stage and are beginning to crack and break apart, as well as an increase in the number of negatives with indelible stains caused by the deterioration processes. As more collections of negatives advance from a stage of deterioration where they are merely channeling or bubbling to a stage where they are breaking and crumbling, we fear that the number of negatives with image loss will, regrettably, rise. Though we work steadily to increase our skill to meet the demands that the future may bring, it is our hope that collections are preserved before such labor and skill are required. We encourage all institutions to make a preservation plan for their negative collections, both before deterioration occurs and once it sets in. Though it may be daunting, the steps that we take in the present will preserve valuable images and information for future generations.
Close up of before and after the recovery process demonstrating the removal of crystalline deposits.