Revisiting Chemical Reconditioning Of Cellulose Acetate Motion Picture Films For Improved Digital Reformatting

John Baty, Diana Little, and La Verne Lopes
Electronic Media Review, Volume Six: 2019-2020

ABSTRACT

Cellulose acetate film, which has been a primary motion picture medium since the early 20th century, can be subject to brittleness and loss of dimensional stability due to polymer degradation and plasticizer loss. Although contemporary film scanners using advanced gate and roller technologies have significantly improved the quality of film preservation reformatting and increased the number of films that can undergo the process, many assets have physical deformations that preclude an acceptable scan or are too brittle to be scanned at all. These cultural artifacts are in danger of being lost.

In the era of analog reprinting, chemical reconditioning processes were employed primarily for deshrinking but also to restore a portion of the original strength and flatness, which makes these methods good starting points to consider for digital reformatting.  While shrouded as trade secrets, researchers characterize these processes as depositing some combination of solvent and plasticizer vapors at ambient or slightly negative pressure.  We present evaluations of referenced solvent/plasticizer treatments followed by objective and subjective brittleness and dimensional stability tests to determine their utility in contemporary digital reformatting.

Two proof-of-concept studies used six film reels (all 16 mm, various stock types) randomly selected from a study collection of 116 films that had been previously stored outdoors in Pittsburgh, Pennsylvania for several years. We exposed first test lengths, followed by the whole reels, to conditions commonly associated with reconditioning (a combination of water, acetone, camphor, and methyl phthalate vapors, –5 in. Hg, overnight). Measuring thickness and weight before and after treatment, and performing the destructive mechanical MIT Folding Endurance Test on treated samples versus untreated controls, we observed a statistically significant weight increase in all test lengths after treatment, suggesting solvent and/or plasticizer uptake. We also observed a significant increase in both MIT Folding Endurance and thickness in the same two of six samples (both Kodak, ca. 1940), establishing that this chemical pretreatment can improve an objective material strength criterion. The test reels were evaluated by trained reformatting production staff (single blind) using a survey of production-relevant criteria (targeting brittleness and dimensional stability) before and after treatment.  Thereafter, the reels were evaluated following two cycles of re-equilibration in ambient air (1 week) to assess reversibility problems.  Despite the study collection’s storage history, five of six reels were consistently deemed “not brittle.”  The one that was deemed brittle experienced the greatest improvement following treatment, suggesting that the next study should select from the most brittle films in the study collection. Overall, results suggested poorer reformatting properties following re-equilibration to the ambient environment compared with the initial condition, warning of possible reversibility issues and the need to evaluate the various solvents and plasticizers separately. Evaluators also reported inconsistency of properties within reels of treated films, suggesting that the whole-reel treatment design needs improvement.

A third study employed test lengths from films that broke at least five times on initial inspection (four 16 mm and four 8 mm, various stock types).  All films were treated in separate water, acetone, dimethyl phthalate, and triphenyl phosphate vapor environments (–5 in. Hg, 1 week) and evaluated subjectively (flatness, suppleness) and objectively (weight, MIT Folding Endurance).   We concluded that chemical reconditioning’s ability to objectively improve the strength of motion picture films extended to the most brittle films, where the need is greatest. Water vapor was the single most effective solvent/plasticizer for strengthening and dimensional stabilization.  Acetone vapor, while most readily taken up of all the solvents/plasticizers tested, worsened flatness and suppleness and did not consistently improve mechanical strength.  These results suggest that although acetone was an important tool in chemical deshrinking, it may be less important in chemical reconditioning to improve contemporary digital reformatting.

In future work we will continue to work on the most brittle films, developing a more uniform treatment of whole reels while monitoring image quality to ensure optimum reformatting.

CONTACT INFORMATION

Diana Little
Head of Film Preservation
The MediaPreserve, a division of Preservation Technologies, LP
111 Thomson Park Dr.
Cranberry Township, PA 16066-6424
724-779-2111
little@themediapreserve.com

La Verne Lopes
Senior QC Technician
Preservation Technologies, LP
111 Thomson Park Dr.
Cranberry Township, PA 16066-6424

John Baty
Technology Manager
Preservation Technologies, LP
111 Thomson Park Dr.
Cranberry Township, PA 16066-6424
USA
724-779-2111
baty@ptlp.com