Scratches
Introduction
Scratches occur on both sides of a film and are both the most common image defect on film and the most difficult to satisfactorily eliminate during restoration.
Until the late 1920's most projection prints were made from the original
negative and not from a duplicate negative, so once a serious scratch occurred
on the negative it was printed into every subsequent print [creating a dating
system for prints, a little like tree rings, that has never been fully
utilised!] In the early 30's and 40’s, duplicate negatives started to be
shipped overseas so that "release" projection prints could be made
locally. Any scratches printed into the duplicate negative from the original
were now an image, not a scratch, and were permanent.
Some scratches could not be treated until the introduction of "wet
gate" printing, and some emulsion damage cannot be treated even now,
unless digital electronic techniques are employed.
The back of a film is harder than the emulsion in most cases, but is more vulnerable and more likely to be scratched as the back of the film is more frequently in contact with hard surfaces, such as aperture plates, printer gates and sprocket drive rollers. Most films have uncoated base as the back but some were coated with a gelatine layer.
Base or "backing" scratches as they are often called, are treatable by several techniques, some of which depend on the chemistry of the base. Other treatments depend on the physical properties of the base. In all cases base scratches can be at least minimised whereas emulsion scratches are sometimes untreatable photographically.
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In this image you can see all sort of dirt and scratches, both on the base and on the emulsion |
Emulsion scratches
Varnishes and lacquers
Most film has on top of the emulsion a thin clear gelatine coat [the
supercoat] as the top layer. Scratches can be shallow and only cut into this layer,
which is more or less easily repaired, or the emulsion or emulsions below. The
worst emulsion scratches not only cut into the emulsion but also shatter the
hard gelatine layers so that pieces of emulsion are lost entirely and the clear
base below is exposed. Emulsion can also be lost by heavy abrasion so that the
scratch is a heavy scoring losing
emulsion over a wide strip of several millimetres.
Some film emulsions are harder than others and are less prone to scratching due to their manufacture, or in the case of some reversal colour films, or due to their processing.
In the 1930's to 40's a wide range of camera films were made for two and three colour synthesis that were double coated with a different emulsion on either side of the base. These suffered badly from emulsion scratching, as there was no film base side to be the contact with projector gates and sprocket rollers.
Treatment of emulsion scratches by re-washing
Immersing a film in water softens the emulsion, which swells. The swollen emulsion causes the edges of the cut gelatine to anneal and as the film dries, the edges stay together. Deep scratches can only be slightly improved, and if any emulsion is lost, the effect may be negligible. Nevertheless, the general effect of soaking film and redrying can be dramatic and a mass of tiny scratches vanishes to leave much cleaner appearing image.
Several manufacturers have made special small machines for this purpose. The principle of water washing is generally to keep the film in the water for as short a time as possible and wet only the top layer of emulsion. The principle of water soaking to anneal scratches is to soften and swell the emulsion as much as possible. In practice, laboratories with special machinery use a single compromise operation to wash and clean at the same time.
Film base scratches
Laboratory technicians have an entire vocabulary of terms for film scratches. Intermittent diagonal scratching can be called "rain", continuous parallel scratches are called "tramlines", short fine cross scratches are called "cinch marks", and so on. Some are treatable; most are reducible.
A scratch on a negative will be printed as an image and cannot be corrected by photographic means on the print once printed.
A base scratch on a print film will be projected as a black line and these are a feature of all old prints to a greater or lesser extent.
A base scratch on a negative will also project as a black line and therefore be seen on a projected print as a white line.
There are two common methods of eliminating scratches today. The film can be wet-gate printed, and this has become the almost universal method, or the scratch can be filled in with film base material.
The physical filling of a base scratch is called "polishing" although is not analogous in any way to the process of polishing as applied to metals or stones. However, in the past scratches film was sometimes lacquered to disguise a scratch.
Polishing
"Polishing" a film base is the process of softening the film base material with a solvent and pressing the soft plastic substrate onto a perfect smooth surface against which the film base hardens. In the pressure stage scratches, which are grooves in the surface are filled in with the softened substrate, and the base surface hardens to exactly the same surface characteristics as the smooth surface in contact with it. In many ways, it is a reconstruction of the original production method for film base.
Both acetate and nitrate film bases can be treated in almost exactly the same way. Acetone is a solvent for both, and the time taken for an acetone wet surface film to dry is only a few seconds. Film for treatment is surface wet with acetone, usually from a wet cloth or roller. The damp softened surface is immediately applied to the flat circumference of a rotating glass [occasionally metal] wheel. The base surface dries in contact with the polished surface and, in doing so, takes up the shape of the surface. The surface must be flat, smooth, polished and blemish free. The process can operate with a contact time of only 4-6 seconds, and a large 35 cm diameter wheel can polish at about 20m per minute.
Some really deep scratches may need two passes to fill but repeated polishing is rarely successful. Serious scratches are [or were] sometimes treated by a first pass using a matting wheel. The matting wheel was used first in place of the polishing wheel and had a finely ground glass [sanded] surface which created a fine matt film surface. In the case of some images the film was left in this state [there are recommendations for leaving intertitles and high contrast images at this stage. Most matted images were then given a polish with the polished wheel and this filled in even the most gross scratches, usually at the expense of some overall definition.
Polishing has always been treated as a "black art". A good operator was considered invaluable as he could rescue a damaged negative and the customer might never know about a laboratory induced scratch. There were many manufacturers of polishers - in Europe Arri and Debrie all made excellent units, in USA Carter was probably the last manufacturer [in the 1970's] and in England several patented systems from Davies and Doel used small wheels and longer drying periods. Since the late 1980's it is possible that almost no polishing has been done commercially, but the archive laboratory should never discard the equipment as small sections may still need to be polished from time to time.
Wet Printing Principles
More effective than the scratch treatments normally used, the technique of wet printing can provide clean copies from originals scratched on the base and to a lesser degree on the emulsion. The effect of a scratch is shown in figure A. Light transmitted by film is refracted and scattered by surface scratches and thus the scratch is seen as a dark line.
If the film surface - in figure B - is coated with the liquid of the same (or almost the same) refractive index as the film (dotted lines), light passes through it undeviated and a dark line is not seen.
Wet printing is based upon this phenomenon and depends for its success on the selection of a liquid with two qualities. Firstly, it must have the same or nearly the same refractive index as the film base (cellulose acetate has a refractive index of 1.490). Secondly, it must not contain water so that the liquid will not be absorbed by the film emulsion during printing, primarily so that the liquid can be dried off quickly before the film is wound up. Trichloroethylene (index 1.478) has the nearest optical properties to those of the film base and emulsion.
However, this liquid is too volatile for pre-wet printing so that the evaporation from the surface of the film, before reaching the exposure aperture, is too high. Trichloroethylene is going to be proscribed in most countries because of its toxicity. It also removes the plasticiser from the film base and after several printing passes makes film brittle.
Perchlorethylene (index 1.504) is less volatile and therefore preferred.