Brian R Pritchard Motion Picture and Film Archive Consultant
THE LUBRICATION OF MOTION PICTURE FILM
J. I. CRABTREE and C. E. IVES
Reprinted from TRANSACTIONS OF THE SOCIETY OF MOTION PICTURE ENGINEERS
Vol. XI, Number 31, 1927, pp. 522-541
WHEN freshly developed or so-called “green” motion picture film is passed through a projector, there is a tendency for an incrustation to accumulate on the aperture plate or tension springs which retards the free passage of the film through the machine. Chemical analysis has shown that this incrustation consists largely of gelatin with more or less silver, dirt, and oil, but it contains usually only a trace of the metal or alloy of which the gate is composed.
The effect of the incrustation is to increase the friction between the metal parts of the gate and the gelatin coated surface of the film. This causes excessive strains on the edges of the perforations at the pull-down sprocket which ultimately results in torn perforations and therefore a diminished projection life of the film.
It is possible to reduce considerably the tendency for the formation of the gate incrustation by suitable lubrication of the film surface. This is accomplished usually by the application to the edge of the film of a thin line of paraffin wax which melts under the heat of the projector and forms an effective lubricant. However, the wax tends to wander over the picture area if applied in excess, and particularly in the case of sound record films this is very objectionable.
It is the object of this paper to discuss the various methods of lubrication employed to date and to indicate a new method which is equally satisfactory for sound record and ordinary motion picture films.
Factors Affecting the Ease of Passage of Motion Picture Film through a Projector
The facility with which the film passes under the pressure springs in the projector gate depends on:
1. The physical condition of the gelatin coating of the film.
2. The conditions to which the film is subjected in the projector.
1. If motion picture film is examined under a microscope by reflected light, it is seen that the gelatin surface, even in the region which is relatively free from silver, is covered with innumerable extrusions (see Fig. 1, magnification 540). The roughness of the surface is much greater in the vicinity of the silver image (see Fig. 2, magnification 790) and if the latter is toned with iron or uranium the roughness is still greater (see Fig. 3, magnification 790). This is as
Fig. 1. Clear area of film. Fig. 2 Area in region of silver image Fig 3. Silver image toned with iron ferrocyanide.
Photomicrographs showing appearance of surface of motion picture film by reflected light.
would be expected, because the toning process intensifies the image by virtue of the deposition of iron or uranium ferrocyanide around the silver grains composing the image and thus enlarges them.
It is possible to smooth the film surface either by grinding away or burnishing down the minute projections or by filling up the crater-like depressions. The effect of burnishing and of filling up the depressions with wax and then burnishing or polishing is strikingly shown in Figs. 4, 5, and 6. Fig. 4 shows the surface of untreated film (magnification 540). Fig. 5 shows the same film after burnishing and Fig. 6 after applying and burnishing.
Fig. 4. Untreated film. Fig. 5. Burnished. Fig. 6. Waxed and burnished.
Photomicrographs showing effect of burnishing, and coating the film surface with wax and then burnishing.
Tests have shown that the act of burnishing or polishing the film surface without the application of a lubricant such as wax or oil does not appreciably facilitate the passage of the film through the projector gate. It is well known, however, that film which has been projected once or twice has a much less tendency to produce an incrustation on the gate than “green” film, and this is usually attributed to the burnishing or polishing action of the aperture plate or pressure springs on the gelatin coating of the film. The burnishing effect produced by projecting the film in a Simplex projector ten times is very slight, as shown in Fig. 7 (magnification 540). This is a photo micrograph of the film surface in the region between the perforations. The lower half of the figure shows a portion of the film surface which was in contact with the aperture plate. The burnishing effect on the film surface is negligible.
Fig. 7. Showing partial burnishing effect on film during projection.
It is considered that traces of oil which are transferred to the film surface during the first projection are chiefly responsible for the increased ease of passage of the film on subsequent projection.
It is obvious also that the moisture content and degree of hardening of the gelatin coating are important factors which determine the rate of formation of the incrustation in the gate. If the gelatin coating of the film contains an excess of moisture, it tends to soften and become “tacky” much more readily in the hot projector gate than is the case with dry film. This tendency of the gelatin coating to soften under the action of heat can be diminished by hardening during processing. However, excessive hardening tends to increase the brittleness of the film and is not to be recommended.
2. Apart from the condition of the film, the following factors relating to the conditions existing in the projector also determine the extent of the formation of the gate incrustation.
A. The tension of the gate springs. This should be of the order of 8 ounces for each spring or a total of 16 ounces. The spring tensions should be adjusted individually at intervals by attaching a spring balance to the upper end of a narrow film strip placed at one side of the gate and increasing or decreasing the gate tension until the film just commences to travel upwards when the spring registers eight ounces with an upward pull.
In a like manner the tension with full width film should be adjusted to sixteen ounces.
B. The nature and smoothness of the gate surfaces. The nature of the gate material in contact with the film surface, providing it is of sufficient hardness, is of less importance than its degree of smoothness. Satisfactory materials are cast iron or stainless steel, either plain or chromium plated. Corrosion should be carefully guarded against and any gelatin incrustation removed with a wood or bone scraper so as not to scratch the polished surface.
C. The temperature existing at the gate. As explained above, the tendency of the gelatin to incrust on the gate springs in the case of freshly processed film increases with temperature. Any means of reducing gate temperature, such as the use of heat absorbing glass, a blast of air impinging on the gate, or suitable radiating fins on the gate, is desirable.
Methods of Facilitating the Passage of Motion Picture Film through the Projector
Even though a projector is in good mechanical condition and the above requirements are fulfilled, there is invariably a tendency for a gate incrustation to form with green” film. Numerous methods of treating the film to offset this have been suggested from time to time as follows:
1. By Edge Burnishing the Gelatin Surface. It was considered that if the burnishing effect of the gate springs on the gelatin coating of the film could be simulated by a preliminary treatment, the difficulty caused by incrustation might be diminished. Accordingly, a machine was constructed for burnishing the edges of the film (see Fig. 8) consisting of a highly polished undercut roller (R1) working against the edges of the film and revolving above an idler roller (R2) The film was fed between rollers R1 and R2 by means of a gearing so arranged that the film advanced through a distance of 1/80th the circumference of the roller R1 for every revolution of the latter. It was possible to adjust the pressure on the burnishing roller by means of adjustable screws.
With undeveloped film an appreciable degree of burnishing was observed to take place, but with film fixed in a hardening bath the effect was very slight. Moreover, in view of the slight buckling of the film around the perforations, it was necessary to exert consider able pressure on the burnishing roller in order to flatten out the film so as to insure perfect contact.
Fig. 8. Film burnishing machine.
As a result of this pressure, a consider able amount of heat was developed, so much so that after the passage of a few feet of film, the gelatin coating commenced to grind away and particles of gelatin accumulated on the burnishing roller, stopping the machine. In order to prevent this it was necessary to apply a thin film of grease or oil to the burnishing roller which reduced the friction and prevented the grinding away of the gelatin.
In order to determine the precise effect of the burnishing apart from the effect of the grease, a roll of film burnished with the aid of automobile grease was passed through the projector, and a similar roll was merely treated along the edges with the grease. Since no difference was observed between the projection life of the two films, it was concluded that a mere application of oil or grease was just as effective as burnishing.
Experiments were made also with a heat burnisher consisting of a highly burnished roller working above a second roller and between which the film was passed with the gelatin surface in contact with the burnished metal. If the roller was heated to a temperature at which a drop of water sizzled on the metal, a satisfactory degree of burnishing was effected although this treatment did not materially prolong the life of the film on projection.
2. By Edge Lubrication. Before the advent of more refined machinery for the purpose, it was customary to apply a layer of wax to the edge of the film during rewinding by passing the film face downwards over two blocks or candles of hard wax separated by a distance equal to the width of a picture frame. Although effective from a lubrication standpoint, with such an apparatus it is difficult to control the quantity of wax applied because this depends on the temperature of the wax, the pressure applied, and the rate of travel of the film. Usually the tendency is to apply too much wax, which then encroaches on the picture area and causes dark spots or patches on the screen. An excess of wax is also apt to cause projector trouble as explained below.
A suitable machine for applying a thin line of wax along each edge of the film surface and between the perforations has been described by J. G. Jones. This consists essentially of two parallel thin steel discs separated by a distance of l_3/32ff rotating in a vertical plane. The discs dip into a bath of molten paraffin wax and apply the wax to the film at their upper edge. The quantity of wax applied is controlled by the thickness of the discs, the temperature of the molten war, and the rate of travel of the film.
Precautions to be Observed when Edge Waxing
The above method of lubrication is entirely satisfactory providing the wax is applied correctly, and no better lubricant than paraffin wax is known to date. However, if the temperature of the molten wax is not sufficiently high during application, too much wax is applied by the discs, and this does not solidify sufficiently before the film is rewound. This causes the wax to cement the edges of the film convolutions so that on rewinding, particles of wax are torn away from the film and these tend to encroach on the picture area, causing spots and blotches on the screen. This is harmful, particularly in the case of film with an edge sound record.
Another very serious danger resulting from the application of an excess of wax arises if the projector is threaded while hot with newly waxed film. As the projector cools, the wax solidifies and holds the film so tightly that on starting the projector, the intermittent sprocket may tear out the perforations instead of pulling the film down through the gate. Since the fire shutter opens immediately when the projector starts, more or less film is apt to be burned up if the film does not start to move down promptly past the aperture.
A series of practical tests were made in this connection to determine the exact conditions under which candle edge waxing or Eastman edge waxing tends to cause the above trouble.
Film was first waxed with a waxer of the candle type which normally applies an excessive quantity of wax. After focusing the light ray from a 30 ampere reflector arc on the aperture opening of a Simplex projector for 30 minutes, this projector became heated to a temperature which would normally exist after the projection of a reel of film. Film waxed in the above manner was then threaded in the heated projector and left to cool for forty minutes. After cooling for such a period the projector had attained room temperature and any wax in the gate had hardened. On starting the projector, the intermittent sprocket tore through the perforations leaving the film stationary in the gate. Upon examination of the samples, it was found that the wax had softened and collected in the perforations and had cemented the film to the film tracks and the pressure springs.
The possibility of this difficulty occurring when film was waxed with the Eastman waxer was then determined. The projector was cleaned thoroughly, and a one thousand foot reel of film waxed with the Eastman waxer was projected in the normal way. Immediately after projection the projector was threaded with a length of unwaxed film and allowed to cool. When the projector was started the film pulled down through the gate with no difficulty. Several thousand foot rolls were then waxed with the Eastman waxer and projected in thousand foot units, each reel being run through the projector only once. After the projection of each reel the projector was threaded with unwaxed film allowed to cool, and then started. No trouble was experienced until seven thousand feet had been projected, when enough wax had collected to hold the film from being properly drawn through the gate. After the projector had been cleaned, it was possible again to project several thousand feet of waxed film before enough wax collected to cement the film, but after each 8,000 or 10,000 feet, the trouble was almost sure to occur.
Tests were then made to determine the quantity of wax which could be put on the film before it could be classed as waxed film, which would cause trouble by sticking in the projector. Several strips of film were prepared by waxing on the Eastman waxer, once, twice, three times, etc. In this way film coated with a known quantity of wax was obtained. The projector was then thoroughly cleaned, heated for thirty minutes, and threaded first with film which had been waxed once and allowed to cool. This procedure was followed with the film waxed twice, three times, etc., successively until indications of sticking in the gate were discovered. Repeated tests showed that trouble was not likely to occur unless the film was waxed for five or six times and therefore contain five or six times the quantity of wax normally applied by the Eastman waxer.
The above experiments serve to emphasize the importance of applying the correct quantity of wax to the film and of removing at very frequent intervals any wax which accumulates on the projector gate.
Lubrication of the Entire Gelatin Surface of Motion Picture Film
At the outset it was considered that by coating the entire gelatin surface of motion picture film with a thin layer of a suitable lubricant, many of the objections to edge lubrication would be overcome. Also, if the coating could be made impermeable to oil, trouble from oil spots would be eliminated likewise.
The idea of lubricating the entire gelatin coating of the film is by no means new. A large number of patents have been granted for particular lubricating formulas which include the use of tallow, lard, spermaceti, stearic acid, sodium stearate in methanol, oil of turpentine, olive oil, cotton seed oil, linseed oil, petrolatum, a suspension of gypsum in methanol, beeswax, and paraffin wax.
Before the commencement of the experiments described below, the Dworsky Film Mfg. Co. was supplying a film buffing machine shown in Fig. 9. This consists essentially of a series of four or five cloth buffing wheels similar to those used for polishing electro-plated metals, which buffers rotate at a high speed in contact with the gelatin surface of the film. The film is pulled through the machine by means of two rubber covered rollers of the laundry wringer type, the machine being entirely sprocketless. (The lower application roller attachment was not originally fitted to this machine). Usually a little Tripoli (polishing powder) was applied to the buffers to produce more rapid polishing of the film surface.
Although it might be expected that this buffing treatment would tend to scratch the gelatin surface, it was not found to be the ease.
Projection life tests made with buffed and unbuffed film indicated that the buffing treatment was of questionable value. However, the machine appeared to be readily adaptable for the application of lubricants to the entire film surface, and the following experiments were therefore made.
Fig. 9. Film waxing and polishing machine.
1. Machine oil or Russian mineral oil was applied to the entire gelatin surface and then buffed in the above manner. Projection tests indicated that film so treated had a projection life comparable with that of edge waxed film, although after storing in the rolled up condition for two or three days, the film developed oil spots. Attempts were made, therefore, to find a solid lubricant which would be impervious to the effect of oil.
2. Waxes were next applied to the film surface by holding a piece of solid wax against the first buffing wheel, which in turn applied the wax to the film. The remaining buffers then spread out the wax more evenly and imparted a high gloss to the film surface which resembled that of highly polished footwear.
Projection tests with film waxed in this manner with various waxes indicated that there is a wide difference in the lubricating quality of different waxes. Data regarding this will be given later. Oil treatment tests after waxing indicated that a surface coating of almost any wax over the gelatin surface of the film will materially reduce the propensity of the film to show oil spots on the screen.
Mechanical Methods of Applying Wax to the Film.
A. It was soon apparent that the above method of application of the wax was entirely impracticable and that a mechanical method of application was required. The application roller method of applying a solution of various waxes in suitable solvents was tried and this was ultimately entirely satisfactory.
The first arrangements of application rollers is shown in Fig. 10. The wax solution is contained in tank T, in which rotates a small flangeless aluminum roller R1 covered with felt. Roller R1 bears against roller R2 which is covered with silk plush. The film runs face downwards against roller R2 and rotates it, and in turn this roller rotates the lower roller R1 which is immersed to a depth of about ¼ inch in the wax solution. By adjusting the distance between the rollers R1 and R2, roller R2 acts as a wringer and squeegees the excess wax solution from roller R2 so that the quantity of liquid applied by the plush coating of roller R1 can be regulated.
This method of application had the objection that the plush did not apply the wax solution sufficiently evenly, and it was not possible to control the quantity of wax applied with sufficient precision to insure against the• wax solution passing through the perforations on to the base side of the film.
B. An entirely satisfactory mechanism for applying the wax solution is shown in Fig. 11. The film passes gelatin side downward over the polished aluminum roller R2 (about 2½ inches diameter), which dips in the wax solution at room temperature in tank T to a depth of about ¼ inch
Fig. 10. Application roller for applying wax solution to film surface. Fig. 11 Application roller for applying wax solution.
. The excess wax solution is removed from the surface of the roller by means of a “doctor” S consisting of a sheet of thick paper or ordinary motion picture film. This leaves an extremely thin layer of wax solution on the roller which is applied to the film
Fig. 12. Improved type of waxing and polishing machine.
surface at P. The friction between the roller R1 and the film is sufficient to drive the roller R1 , without danger of slippage. This friction can be increased by lowering the idler roller R2 in relation to roller P.
The latest type of Dworsky buffing machine is shown in Fig. 12. This is shown fitted with application rollers as first developed by the authors. It is suggested that the single application roller shown in Fig. 11 be fitted to the new type machine.
Although at first sight the waxing machine appears somewhat complicated, it is very simple to operate, (does not get out of adjustment, and requires little or no attention other than changing of the reels. The level of the wax solution is maintained constant by means of an inverted bottle or can be fitted with a tube dipping into the tank of liquid. The depth of the liquid is then at all times equal to the distance between the tube orifice and the bottom of the tank which holds the liquid. The film travels at the rate of 6 to 12 inches per second, the 6-inch speed giving a more desirable polish to the film surface. A roll of 1000 feet is, therefore, waxed in from 15 to 30 minutes.
About 2 ounces of liquid are required per 1000 feet of film providing the exposed surface of liquid in the tank is covered as far as possible to prevent evaporation.
After waxing, it is desirable to rewind the film while passing it between the folds of plush so as to remove occasional particles of lint which tend to adhere to the film after polishing.
It might be considered that the film would ignite from the heat developed by friction perchance the film should remain stationary in contact with the rotating buffers. Tests indicated that the film did not fire after remaining stationary in contact with the buffers for 30 minutes.
Choices of Waxes and Solvents
The following waxes were tested: beeswax, cantol wax, candelilla, carnauba, Japanese, Johnson’s floor wax, montan, hard paraffin, and Simoniz wax.
The choice of suitable solvents is somewhat limited because, as pointed out in a previous paper, many solvents have a tendency to attack the silver image and are therefore unsuitable. The three solvents—benzene, gasoline, and carbon tetrachloride—were used in the preliminary tests. Since carbon tetrachloride is non-inflam and when pure has no harmful effect on the film, this solvent was used exclusively in the later tests.
Properties of Motion Picture Film With a Coating of Wax over the Entire Gelatin Surface
1. The Projection Life as Compared with Edge Waxed Film
Comparative measurements were made on the projection life of the various samples of waxed film as follows: The ends of a 6-foot length of each sample of film were spliced together so as to form a loop, and this was run continuously through a Powers projector maintained as nearly as possible under standardized conditions. The gate spring tensions were checked at regular intervals, and the machine otherwise maintained in first class condition. If any incrustation tended to form in the gate, this was indicated by a distinctive noise and the incrustation was at once removed. The number of times which the film passed through the machine was recorded by a counting device, and projection of the film was continued until perforations became torn to such an extent that the film would no longer pass successfully through the machine.
Assuming a basis of 100% projection life for normally processed film which was not treated in any way before projection, the results of tests with films lubricated over the entire surface with various waxes were as follows:
Nature of Wax or Oil Solution Projection Life
in Carbon Tetrachloride
Plain film (untreated) Projection Life 100%
Montan 1% 77
Montan 5% 94
Carnauba 5% 125
Johnson’s floor wax 6% 185
Paraffin 1% (M. P. 130°—140°F.) 310
Paraffin 2 parts }
} 2% 380
Carnauba 1 part }
Paraffin 2 parts }
} 5% 480
Carnauba 1 part }
Paraffin 5% 850
Eastman edge waxed film 950
The above results indicate that certain waxes, such as montan, can produce a negative lubricating effect and that the lubrication produced by all the waxes increases with the quantity applied.
Of the waxes tested, paraffin wax was the best lubricant and at a concentration of 5% was as satisfactory as Eastman edge waxing. However, a coating of pure paraffin wax was relatively soft and tended to show finger marks. Experiments were therefore made with a mixture of a hard wax (carnauba) and paraffin wax in the proportion of two parts of paraffin and one part of carnauba. Such a mixture gave a harder coating with a high gloss, but the projection life was about 40% less than that of plain paraffin. However, in practice the projection life of film is usually determined by factors other than the point at which the perforations break down. Film is frequently rejected on account of bad scratches before this point is reached, and it is therefore considered that the projection life of film treated with the carnauba-paraffin mixture is satisfactory.
2. The Tendency of Surface Waxed Film to Deposit an Excess of Wax in the Projector Gate
As mentioned previously, in the case of edge waxed film, there is a tendency for an excess of wax to accumulate in the gate so that on threading a warm projector and allowing it to cool, the wax cements the film to the gate so that it will not pass down through the gate on starting the machine.
Tests were accordingly made with film, surface waxed with a 5% solution of a mixture of two parts carnauba wax, and one part paraffin wax. Twelve 1000-foot reels were projected in succession through a Simplex projector without disturbing the gate. At the completion of the run, a piece of unwaxed film was threaded in the ma chine and the machine allowed to stand for one hour so as to cool thoroughly. On starting the cooled machine, the film was pulled down satisfactorily through the gate, showing that no serious quantity of wax had accumulated as a result of the projection of the twelve reels. Examination of the gate showed the presence of negligible traces of wax, but there was present a slight amount of “fluff” which was presumably deposited on the film from the buffing wheels.
In this connection the heated reels after projection were allowed to cool thoroughly and then rewound. No tendency for the convolutions to stick together was observed, and the possibility of this happening is somewhat remote because the surface coating of wax applied to the film is extremely thin.
3. Effect of Surface Waxing on the Propensity of Film To Show Oil Spots on Projection
It is well known that when film accumulates oil in the projector, the effect of the oil is usually visible on the screen as patches of lesser density than the surrounding portion which is free from oil. The effect of clean oil is to fill up the tiny surface craters, thus reducing light scatter which results in an increased transparency of the film. In the case of dirty oil, or when dirt is applied to clean film treated with clean oil, the oil spotting is greatly exaggerated and such dirty oil produces dark spots.
A study of the surface structure of motion picture film (see Fig. 2) explains why it is difficult to remove oil by mere wiping. The oil sinks into the innumerable craters present on the surface and can only be removed by treatment with suitable solvents.
At the outset it was considered that a waxing treatment might insulate the gelatin surface from the oil and reduce the propensity for oil spots to show on the screen. This was tested as follows:
Film toned with a uranium toner was used for the test because such toned film has a maximum propensity to show oil spots, presumably because of the extremely pitted nature of the film surface. A reel of film was assembled consisting of fifty foot strips treated as follows:
Sample Nature of Film
No. 1 Plain uranium tone
No. 2 Waxed with 5% paraffin in carbon tetrachloride
No. 3 Waxed with 5% solution of 2 parts paraffin and 1 part carnauba
No. 4 Waxed with 1% carnauba and overcoated with 5% paraffin
No. 5 Waxed with 1.5% candelilla and overcoated with 5% paraffin
Preliminary tests to determine the rate of solution of cold waxes and mineral oil indicated that carnauba and candelilla waxes were more impermeable to oil than paraffin, so that in the above double coated tests the object of the first coating of carnauba or candelilla wax was to protect the film against oil, while the coating of paraffin was to secure maximum lubrication.
A good grade of light machine oil was splashed in liberal quantities on all the samples while the film was being wound from one reel to another. The film roll was then rewound twice, during which time the oil was smeared over the surface with a cloth. After this treatment the film surface presented a mottled appearance. The reel was then projected immediately and subsequently three times each day.
After the first day objectionable oil spots appeared on the unwaxed sample. At the end of one week no oil spots were visible on any of the waxed samples. It was concluded therefore that paraffin wax although miscible with oil prevents oil spots. In order to prevent oil spots it is apparently merely necessary to fill up the crater-like depressions on the surface of the film.
A roll of toned film treated with candelilla wax and then splashed with oil was prepared over a year ago and projected at frequent intervals up to the present time. No oil spots have appeared on this film to date.
4. The Tendency of Surface Waxed Film to Accumulate Dirt and Develop Scratches on Projection
Sufficient data have not as yet been secured to determine the effect of the surface coating on the propensity of the film to ac cumulate dirt and develop scratches on projection in comparison with untreated film. A projection test was made by applying dirt to the projector gate and by throwing the film on a dirty floor and then projecting. No appreciable difference in the quantity of scratches or dirt accumulated on the film was noticed between surface waxed and edge waxed film.
Data in this connection are being secured by circulating reels, half of which are surface waxed and one half edge waxed, through various exchanges.
5. The Tendency of Surface Waxed Film to Retain Moisture
It is well known that if the gelatin coating of motion picture film is deprived of its moisture content, the film tends to become brittle. The chief cause of brittleness of projected film is the loss of moisture as a result of repeated baking of the film in the hot projector gate.
It was considered that possibly the surface coating of wax might retard the evaporation of moisture from the gelatin, and this was tested by first humidifying a strip of film for one hour in an atmosphere at 90% relative humidity, surface coating one-half of the strip with wax, and then placing the waxed and unwaxed strips in a desiccator overnight. No difference in brittleness of the two dried out film samples was noticed. Apparently the wax coating on the film surface is so thin that it does not appreciably retard the rate of evaporation of moisture from the film.
Summary and Practical Recommendations
The projection life of motion picture film can be prolonged considerably by coating either the edges in the region of the perforations or the entire gelatin surface of the film with a thin film of wax. Edge waxing as now practiced by use of the Eastman edge waxing machine is an efficient means of lubrication providing it is done correctly, but if the molten wax is not heated sufficiently during application, there is a tendency to apply too much wax to the film.
This causes an excess of wax to accumulate in the projector gate so that if the freshly waxed film is threaded in a warm projector which is then allowed to cool, the wax solidifies and holds the film so tightly that on starting the projector the film remains stationary in the gate and, in the case of most projectors, then catches fire.
An excess of wax on the film also causes the convolutions of the film to adhere together when the film roll cools after projection, and particles of wax torn from the film during rewinding tend to settle on the picture area causing spots and unevenness on the screen. Edge waxing is also impossible in the case of film with an edge sound record.
By coating the entire surface of the film with an extremely thin coating of a suitable wax, or mixture of waxes and then buffing or polishing many objections to edge waxing are overcome. This may be done efficiently by applying a 2% solution of a mixture of carnauba wax and paraffin wax dissolved in carbon tetrachloride, by means of a suitable machine, which buffs the film surface to a high gloss after application of the wax. The exact proportion of carnauba and paraffin waxes is a matter of choice. A high proportion of carnauba gives a hard highly polished coating, while a high proportion of paraffin gives a softer coating with less gloss but with a greater lubricating value. The following formula containing equal parts of carnauba and paraffin gives a sufficiently hard coating with satisfactory lubricating qualities:
Carnauba wax (M. P. 175°—185°F.) 10 grams 150 grains
Hard paraffin wax (M. P. 130°—140°F.) 10 grams 150 grains
Carbon tetrachloride to 1000 cc. 32 oz.
With this method of application it is practically impossible to apply a dangerous excess of wax to the film, so that the above difficulties caused by the application of an excess of wax are eliminated.
A film surface waxed in the above manner has also a minimum propensity to show oil spots on projection even when a liberal quantity of machine oil is applied to the film in the projector.
Practical tests have shown that the projection life of surface waxed and buffed film, as determined by the point of complete breakdown of the perforations, is not quite as great as that of edge waxed film. However, in practice, film is rejected usually for other reasons before the complete breakdown of the perforations, so that the projection life of surface waxed film is considered satisfactory.
The authors are indebted to F. J. Closser and L. E. Muehler, who assisted in the experimental work, and to R. N. Titus, who made the photomicrographs.