Shrinkage

Here you can see two superimposed strips of film. The black one is a piece of modern, unshrunk film, the other belongs to a shrunk piece of nitrate. You can see that the two rows of perforations do not match. This is due to the shrinkage of the nitrate original film.

The extent to which a particular film has shrunk can be measured in several ways.

It is possible to judge the approximate shrinkage by comparing with a length of new film known to be unshrunk. A simple method is to compare 100 frames of fresh modern film [about 1.90m of 35mm film or 0.76m of 16mm] with 100 frames of archive film. The percentage shrinkage is roughly equal to the shortage in frames.

How to measure it

There are several film shrinkage meters or "pitch" gauges that have been made from time to time. Pitch is the distance from the base of the leading perforation to the base of the next perforation.

A "stick" gauge is a portable bar with two pins to locate perforations. The results are rather approximate.

The Maurer type instrument consists of a channel in which the film lies. A plate is sometimes laid on top of the film to retain it in the channel. At one end is a fixed pin onto which a perforation is placed. At the other end is a moveable pin attached to a light spring and a dial gauge. The moveable pin is fitted to the 100th perforation[or 39th 16mm frame, or any other standard] from the fixed pin and the dial gauge shows how much the pin needed to move and the shrinkage of the film. A standard rule, sometimes called a "setting bar", is used to calibrate the zero position..

Restoration House Inc. in Canada made a pitch gauge that measured 1 foot [or 40 16mm frames, or 16 35mm frames] on a precision made steel bed..They also produced a Mylar 35mm film strip, which was dimensionally very stable, with a fixed pitch for use in calibrating pitch gauges.

Most gauges are marked in inch units or centimetres or are also marked to show "percentage shrunk" directly.

Current Standard pitches

Film gauge	Inches	Mm
Standard 8	0.1500	3.810
Super 8	0.1667	4.234
9.5mm	0.2977	7.56
16mm long pitch	0.3000	7.60
16mm short pitch	0.2994	7.605
35mm long pitch	0.1870	4.75
35mm short pitch	0.1866	4.74

Short pitch perforations are intended for negative films and all films that need to be printed on a contact printer. Long pitch perforations are used on print films. The distinction was not universal until about 1960, when almost all manufacturers fell in with the practice. Prior to 1960 most 35mm negative and print stocks were all long pitch.

Effects of shrinkage

The degree of shrinkage is an essential piece of information in the subsequent stages of restoring the film, and will determine which type of printing is possible. Printers are designed to handle unshrunk film with the above standard pitches, unless they have been modified to have shorter distances between the perforations.

In general there is no way to guess the degree of shrinkage by simply looking at film or by knowing the age or origin of the film and the factors that effect shrinkage are very complex. Some film after 70 years may have only shrunk by 0.7%, but 4% is possible in a few cases and still the film may be supple and intact.

Such film is still printable provided the printer can cope with this degree of shrinkage. Some film emulsion shrinks at a similar rate to the base, but in other cases the emulsion shrinks to a greater or lesser extent resulting in creases or fragmentation of the emulsion, seen under a lens as a "crazing" or reticulated pattern.

How to avoid them

Quite frequently film of the same origin, making up one roll, is all more or less uniformly shrunk and this makes printing decisions much easier. Some film shrinks irregularly in sections, and this can be the most difficult to print, needing different treatment for different short sections. Film like this may need to be aligned in a manual printer frame by frame to print successfully. Just before nitrate film decay sets in, it seems that those areas that are about to decay shrink more than the rest of the film but usually a roll of film showing varying degrees of shrinkage is a mixture of different film types or origins joined together. Nitrate leader film is well known to shrink more than the nitrate image film joined to it. Nitrate leader shrinkage can be as great as 5%, whereas acetate film shrinkage rarely exceeds 1.5 or 2%.

De-shrinking

If shrunken film is placed in an enclosed atmosphere of acetone, glycerol and water vapour for several days or weeks both nitrate and acetate slowly expand to approach their original dimensions. The effect is not permanent and the film shrinks back, sometimes quite quickly. The process can be speeded up at increased temperatures up to about 30C or at reduced pressures [when the solvents are a larger proportion of the atmosphere]

Several commercial systems have been marketed to "deshrink" film base.

An effective system is Redimension from Restoration House Inc. in Canada. This involves placing the roll of film over an open tray of solvents and plasticisers [such as camphor and dimethyl phthalate] in a closed metal container or in a reduced pressure container in a warm place [about 30C] . Over a period of days or weeks [depending on temperature primarily] the film re-extends and shrinkage of up to 2.0% can be fully corrected for both acetate and nitrate film. The process can be accelerated by using a reduced pressure chamber to increase the vapour concentration in the surrounding atmosphere.
Deshrinking is sufficient to enable seriously shrunken film to be printed on an unmodified printer. The effect is not never permanent and shrinkage restarts after a few weeks but tests done in Canada showed that Redimensioned film did not return to the serious level of shrinkage it started from within a measurable time. Soho Images in London was the only European laboratory licensed to use the process, and now use a similar technique, but it is also used by the Swedish Film
Archive[1996].

It is not known what the long term effects of using Redimension [or any deshrinking process] are but some Redimensioned film is now over 20 years since it was treated and still seems to be in good condition. Some archives are understandably wary of using any deshrinking processes, like Redimension, licensed to users without publishing the formulations of the chemistry involved. For this reason a number of laboratories have devised their own formulations which are published and thus more acceptable to archives.

Today these processes are not much used as a routine for shrunken film as it takes far too long and takes up a lot of valuable space. Also variable pitch pull-down optical printers like the Debrie TAI and capstan drive rotary printers like the BHP Modular can handle severely shrunken film as a routine.

A number of commercial archive film specialist laboratories still use a similar but more time consuming process of leaving film for some weeks in a high humidity. It seems that the film base takes up some water and extends. The process is used to even out irregular shrinkage rather than extend shrunk film fully. The prime value of Redimension [and its related cousins including Rehumid] is that it can produce a printable piece of film from the most extreme material. Crisp and crunchy, shrivelled film that shatters and simply can't be unwound can be treated in the roll until it can be unwound. Most film handled by Redimension or itís related processes today cannot be safely handled any other way and would have been destroyed as unprintable.