Color Photography - Finlay Process
( Originally Published 1938 )
THE Finlay Color Process was named after Mr. Clare Finlay, to whose pioneer work in color photography all students owe a great deal. Like all other three-color processes, it is based on the theory of that great scientist Clerk Maxwell, who discovered in 1861 that all the colors in nature could be matched by the proper admixture of the three primary colors. It was on this principle that Mr. Finlay made a screen of geometric pattern comprising red, green and blue-violet squares in regular sequence.
With the aid of a compensating filter, a Finlay taking screen with a special Finlay panchromatic plate can produce a perfect picture in natural color with one exposure on a single plate. The Finlay color process is a practical method of producing color pictures on glass at high speed, thereby permitting the photographer to photograph objects in motion.
With this process the photographer can make as many color transparencies as he requires from the same negative, as well as black and white photographic prints. It is excellent for making lantern slides, the color rendering being as nearly perfect as it is possible to obtain and very brilliant when projected, taking very little more light to project than a black and white slide.
By means of the block-out screens, provided by the Finlay Company, it is posSible to separate the three primary colors in the single Finlay negative into three separate and distinct positives. It is also quite practical to secure a positive for the fourth color (black) when it is required. These separation color positives then become the basis for furnishing color plates for the printing press, or negatives can be printed from them either by contact or by projection for use in making color prints on paper ; such as, Three-Color Carbro, Eastman Wash-Off Relief positives, Chromatone positives, or thin base positives for dye-mordanting.
By the use of the block-out screens, sets of bromide prints or positives for the above processes can be made by projection direct from the original Finlay negative, thereby eliminating the step of making separation positives and negatives.
The Finlay negative, registered and bound or clamped together with the viewing screen, or the color transparency, can either be placed in a projection printer, or set up in front of an even illuminant and copied with a camera, using the contrast three-color filters, for making three-color separations for any of the subtractive color processes. This, however, involves the use of panchromatic plates and filters, and it is not as satisfactory as the block-out screen method.
Any ordinary camera and lens will execute the work, provided the plate holders are deep enough to admit both the panchromatic plate and taking screen, and have sufficient spring tension to keep them in contact. The best type of plate holder is the Finlay single holder, which has a spring back and affords even pressure, assuring perfect contact. A book-form holder will answer very satisfactorily. The regular Graflex plate holders can be used with 3 1/4 by 4 1/4 Graflex cameras, and the Graphic plate holders with the 3 1/4 by 4 1/4 view-type cameras. It is best, though, when using these plate holders, to load only one side of the holder. The Zeiss single metal color-plate holders and Kodak Recomar plate holders will take both the screen and panchromatic plate perfectly.
In the small sizes up to 5 by 7 the taking screen is about 3/64 of an inch thick and the panchromatic plate is also 3/64, so when combined the screen and plate are about 3/32 of an inch. In the 5 by 7 and 8 by 10 sizes the taking screen itself is about 1/8 of an inch thick.
The taking screen can be used repeatedly, provided the user exerciseS great care when handling it to avoid scratching or finger-marking the surface.
Before placing the panchromatic plate in contact with the taking screen, carefully polish the glass surface and brush off gently the emulsion side of the screen with a fine camel's-hair brush. Place the pan-chromatic plate in contact with the taking screen (emulsion to emulsion), then place both screen and plate in the plate holder, the glass side of the screen outwards, so that it faces the lens when in the camera. With the Finlay holder the taking screen may remain in the holder, avoiding unnecessary handling and being protected from scratching or finger-marking.
The taking screen, which is composed of hundreds of thousands of small squares of color (which are really minute tricolor filters), should be protected from light when not in use.
The Special Finlay-Eastman panchromatic plate (which is specially coated for the Finlay ProceSs and marketed under a joint label) is the only plate which should be used. These plates are best loaded in absolute darkness. A Series Three green safelight may be used, but work should be done at least six feet from the light.
FILTERS.-A compensating filter must be used on the lens when the exposure is made. The purpose of the filter is to compensate for the color values of different types of light.
For the Special Finlay-Eastman plate there are four types of filters:
No. 4508—For use when pictures are taken in sun-light.
No. 4525—For use when taking pictures on cloudy days or in the shade, and for white flame arc.
No. 4547—For use with Photoflash bulbs. No. 4600—For use with Mazda light.
How TO USE THE FILTERS.—The filter may be placed in three different positions:
(a) Immediately in front of the lens. This position, which is most convenient, should be used for a sealed filter, or it may be used for a gelatin filter mounted between two pieces of cardboard.
(b) Between the lens elements at the diaphragm plane. ThiS is the best position when using gelatin filters.
(c) Immediately behind the lens. This position should be used only with a sealed filter. Unless a sealed filter is used, there is danger in this position of destroying definition, especially with an extra long focus lens of large aperture.
Always protect your filter from light when not in use.
Focusing should always be done with the filter in position. For accurate work it is necessary to reverse the ground-glass before focusing to compensate for the thickness of the taking screen, which has placed the panchromatic plate 3/64 of an inch to the rear of the focal plane when using the small sizes and 1/8 of an inch for the 5 by 7 and 8 by 10 sizes.
If the worker does not choose to reverse the ground-glass, after focusing the picture the lens may be racked towards the glass 3/64 of an inch, or 1/8 of an inch, which is approximately the thickness of the taking screens.
The speed of the Finlay panchromatic plate when used in conjunction with the taking screen and compensating filter is approximately H. & D. 80. With the Weston exposure meter for daylight use a setting of two and read the meter direct. For Mazda light use a setting of two and multiply the resulting exposure by five.
As a general guide, for outdoor work in bright sun-light the exposure will average about 1/25 second at f : 8. There is no reason why exposures ranging from i/5 second to 1/50 second should not be made for street scenes and even 1/100 second for seascapes and yachting scenes, with the aperture at f : 4.5. With Photoflash bulbs a good exposure may be obtained with two bulbs No. 20 (medium size), five feet from the subject at f : 8, or one bulb four and a half feet with the lens at f : 5.6.
Many workers fail when making exposures because they are not observant. It is most important to observe the surroundings and the light before deciding on the exposure. The beginner should aim for soft lighting effects and avoid all fancy and hard lightings until he has had more experience.
When making portraits or still life subjects, it is very important that the worker observe the surroundings for strong reflected colors. If a sitter is placed close to a colored background, some color from the background will be reflected into the flesh tones, and while this reflected color may not be perceptible to the eye, the sensitive color plate will record it and perhaps throw the flesh tones off balance.
Do not mix different kinds of light when taking color photographs. If Mazda lights and daylight, or Mazda and arc lights are mixed, the resulting picture will be completely out of color balance.
When taking a photograph with a certain kind of light, use the compensating filter for that kind of light and exclude all other light from the room. This is very important.
White flame arcs and daylight can be mixed to a certain extent, and also Photo Blue bulbs and day-light, using the daylight filter on the lens.
Photoflood bulbs cannot be classed as Mazda light and are not a good source of light for Finlay color photography, for, being over-volted, they have much more blue in their spectrum value than the regular 115-volt Mazda lights burning on 115-volt current, and, owing to the short life of the bulb, the spectrum value is constantly changing.
It is important that the color registering scales at the ends of the taking screen be properly recorded by exposure on the negative. This must be done before the exposed negative has shifted position on the taking screen. When making still lifes, place white strips at each end of the subject so that they show on the groundglass 3/8 of an inch. The color scales will then be exposed at the same time the general exposure is taking place. When photographing exteriors or models when white strips cannot be used, expose the plate, disregarding the edges, then before inserting the regular slide in the holder use the special slide which is sup-plied by the Finlay Company. When this slide is inserted in the plate holder, light can reach the registering edges only. Then by pointing the camera at the sky or against a piece of white cardboard, an exposure is given for the edges.
The registering edges are used when making three-color separation positives from the Finlay negative by use of the block-out screens, and also when printing the positive color screens.
It is always better to overexpose than to under-expose.
It is impossible to base any calculations on exposure by theory in color work, as color varies in strength and manner according to the variation of light values, but we hope that the following table will be of some value in assisting you to produce a good color result.
As a general rule for outdoor work, the Finlay negative requires about six times as much exposure as regular par speed portrait film, and with Mazda light, about ten times as much exposure.
DAYLIGHT.—With Special Finlay-Eastman Plates use:
Filter No. 4508—when pictures are taken in sun-light.
Filter No. 4525—when taking pictures on cloudy days or in the shade, and with White Flame Arc. Exposures calculated at f : 8:
Sea and sky 1/25 to 1/50 sec.
The exposures as given are for bright sunlight between 11 A.M. and 3 P.M. (Summer or Daylight Saving Time) in May, June, July and August.
All these exposures, which have been based on practical experience, should ensure good color values in the negative.
MAZDA LIGHTING.—With Special Finlay-Eastman plates use Filter No. 4600.
Exposures calculated at f : 8 using 10 amperes of light:
Interiors 1 1/2 min.
PHOTOFLASH BULBS.—With Special Finlay-Eastman plates use Filter No. 4547:
1 bulb -4 1/2 ft. from subject f : 5.6
2 bulbs—6 ft. from subject f : 8
This filter can be used also for Photoflood bulbs with fair results.
DEVELOPING AND FIXING.—Any good developer may be used, provided it is a soft working one. We highly recommend the glycin formula, which is listed together with other formulas printed for your convenience. Be sure the temperature of the developer is not over 65° F. (18° C.), or the emulsion will swell, and when it is fixed the emulsion will shrink and possibly throw the negative dotS out of register.
Fix the negative in any regular acid hypo. A fixing bath which contains hardener made up of sodium sulphite, potassium alum and acetic acid is satisfactory.
Do not use chrome alum as a hardening agent, and always mix the hypo solution twelve hours before using.
Finlay negatives may be moderately intensified with Agfa intensifier, or may be slightly reduced with a weak solution of Farmer's reducer without destroying register or materially affecting color rendering, if carefully handled.
After washing, the negatives should be wiped very gently on both sides with a viscose sponge. This materially helps even drying. Do not force drying or use a fan. Let the plates dry slowly and naturally.
The negative should be what is commonly known as a "soft gradation" negative, fully timed, and developed for softness. A negative which will give a normal print on Azo paper No. 3 will generally give a perfect color transparency. At a glance the negative is an ordinary one, but when examined with a magnifying glass the image will be seen to be composed of a series of minute squares of varying density corresponding to the squares of the taking screen.
If negatives are too strong or contrasty, many of the delicate halftones, which should be recorded in separated stipples, overlap and merge into a mass, and the more delicate colors are lost or falsely recorded.
Underexposed negatives give imperfect color results, the shadows generally being too blue, with a bluish cast over the entire picture.
If the emulsion around the edges of the plate has frilled at all, the edgeS should be scraped to insure perfect contact between the negative and positive when printing.
Finlay transparencies can be made only by contact. Enlarging or reducing will change the size of the dots, so that they will no longer register with the viewing screen. There are two methods of making transparencies from a Finlay negative. One is by making a brilliant but not too dense positive, on a Finlay-Ilford, Hammer or Eastman Process plate. The positive is registered and bound with a viewing screen to give the Finlay color transparency. The other method is by using the Finlay positive color screen, which is a regular viewing screen coated with a positive emulsion.
PRINTING THE POSITIVES.—The negative is placed in the Finlay plate holder or an ordinary printing frame, and a positive plate is placed in contact (emulsion to emulsion). If a Finlay-Ilford positive plate is used, an exposure of twenty seconds six feet away from a 15-watt light will be found approximately correct for an average negative. While the positive plate is being exposed, it should be kept in a stationary position, facing the light squarely, not at an angle.
A very good way to print the positives if the operator has a projection printer is to throw a circle of light on the board (with no negative in the camera), and stop down the lens to f : 16 or f : 22, which gives convenient control of the light. Place the negative and the positive plate in the printing frame. The printing frame is then placed face up on the board of the projection printer in the center of the light circle to expose. This leaves both hands free to do any dodging that may be required.
The plate is developed in the usual manner with Eastman D-72 Elon-hydroquinone (see page 197), diluted one to four. Fix and wash thoroughly, and before placing to dry wipe both sides of the positive with a viScose sponge.
Before proceeding to register the viewing screen .with the positive, brush the emulsion side of the positive and the viewing screen carefully with a fine camel's-hair brush. Place them together (emulsion to emulsion) with viewing screen toward you, and sit well back from a light box. Slide the viewing screen over the positive in a circular motion until the cross plaid patterns become larger and larger and eventually disappear. Attach one clamp on each side to hold the positive and viewing screen in this approximately correct position. Look through the screen at various angles. If the color appears to be better at a certain angle, move the screen in that direction. A small, flat (ruler-shaped) piece of wood is used to tap the screen slightly in the direction of the correct color, changing the two clamps from side to top or vice versa to facilitate such movement.
When the correct colors are in view, the circles of the registration band will be green. While the viewing screen and the positive are held together on two sides by the clamps, bind the two opposite sides with lantern slide binding strip and allow the binding to set a few minutes; then remove one clamp at a time and complete the binding. The color transparency is now complete.
To prevent any shifting of either screen or positive after binding, it is well to see that all of the binding is securely fastened, including the extreme edges of screen and positive.
PRINTING THE POSITIVE COLOR SCREENS.--The positive color screens should be opened only in the dark-room, using a series OA safelight. A 50-watt bulb is substituted for the regular bulb. This will give plenty of light for registering. Dust both screen and negative with a camel's-hair brush. Place the emulsion side of the screen in contact with the negative and register the screen with the negative by means of registering edges at each end.
In the taking screen the registering edge is composed of a red outer band, a blue inner band, with green circles in between. Register the negative with the positive color screen so that these colors show in their complementary colors ; that is, the circles are red, the outer band green, and the inner band yellow. The registration must be exact or the transparency will be off-color.
When the two are registered, clip them on four sides with Acco Clamps No. 225 and examine the registering edges with a ten-times magnifier to see if registration is correct. Be sure that both edges are in register. A good test is t0 examine the red circles carefully with the magnifier to be sure they do not have any green in them. When the positive color screen is accurately registered with the negative, attach additional clamps. On the 5 by 7 size there should be one clamp at each end and two on each side; on the 8 by 10 size two clamps on each end and three on each side. Clean the back of the negative with a soft cloth before making the exposure. Place the clamped negative and color screen (negative up) on a flat board covered with black cloth or paper, directly under an electric light, and expose.
As a rough guide to exposure, an average negative will require ten to fifteen seconds, four feet from a 50-watt light. Avoid underexposure.
Take off the clamps and develop in the usual manner, Eastman D-72 diluted one to four. Development will require about three or four 'minutes. It is advisable to stick to this formula, as some developers have a tendency to change the colors. Overdevelopment is to be preferred to underdevelopment as the positive color screen can be easily reduced. Fix and wash thoroughly and, before placing to dry, wipe both sides of the screen with a viscose sponge. Positive color screens will show the correct color as soon as developed and fixed. At this point they will have parallax, which will disappear when dry. If the colors do not show correctly, you have not registered the negative and positive color screens correctly.
Local or complete reduction can be accomplished by careful use of Farmer's reducer (a weak solution of hypo and potassium ferricyanide).
Retouching and spotting can be done on the finished positive color screens with colors, dyes or pigments.
The finished positive color screen can be varnished with the same varnish as is given for Agfacolor Ultra plates or Lumiere Filmcolor, after which it may be bound with a sheet of flat, clear glass.
If for any reason you spoil a Finlay positive color screen, you can easily remove the silver emulsion: first dry in the usual way, then resoak for a few seconds in cold water, after which you can roll off the emulsion with the fingers. This will leave on the glass underneath a regular viewing screen which can be bound with a black and white positive.
In advertising, magazines and catalogs, natural color photography is being used more every day. For this purpose the Finlay process has found great favor with both the photographer and engraver.
To reproduce a Finlay color photograph the photographer furnishes the original Finlay negative, together with the color transparency, which is used as a color guide.
In the Finlay color process the colors of the spectrum are quite accurately reproduced by one exposure on a single plate. It is, of course, underStood that the Finlay taking screen has separated the colors in the negative into minute squares of varying gradation, which are similar in size to a 175-line halftone screen.
To separate these colors so that plates for the printing press can be made from the Finlay negative, the Finlay Company provides two screens, known as block-out screens. One screen is used to block out for both the red and the blue printer. This is made possible by shifting the screen to a different position on the negative. The other screen is used to block out for the yellow printer. These screens have a black and white dot formation, so that when a screen is registered with the original negative the unwanted colors are held back by the black dots of the block-out screen and only the color that is needed for that particular separation allowed to pass.
When a fourth color (black) is required, it is made without a block-out screen. A piece of plain glass the same thickness as the block-out screen is placed in front of the negative to compensate for the absence of the block-out screen.
NOTES FOR THE ENGRAVER.—In making color separation positives from the Finlay negative, a screen pat-tern results in the positives, and this screen pattern conflicts with the halftone screen in the making of halftone negatives. The Bassani apparatus is of great importance for eliminating the screen pattern in the color separation positives. The Bassani remains in the same position as for making highlight negatives, but a special back, which is obtained from the Bassani Company, is attached to the Bassani frame with four thumb knobs. Then the light shield, which fits in the same position as the engraver's standard plate holder, is attached. This change can be made easily and quickly. The micrometer displacement for the degree of rotation to eliminate this screen pattern is given on page 194, and is for use in making positives the actual size of the negative. When the positives are developed, it will be found that the Bassani has eliminated about 70 per cent of the screen in the red and blue printer positives, about 90 per cent in the black printer positive, and about 80 per cent in the yellow printer positive. No trouble will be encountered when making halftone screen negatives to any size from these positives.
Any lens that is used for photo-reproduction work can be used for making separation positives from a Finlay negative. The focal length of the lens should be eighteen inches to twenty inches.
The next step is to make a frame to hold the Finlay negative and the block-out screen. This frame can be made of wood about one inch thick. The outside dimensionS should be about sixteen inches high by twenty inches wide, and the bottom and top edges cut to fit the standard transparency holder. A horizontal opening is cut in the center of this frame, 7 3/4 inches by 10 1/8 inches for the 8 by 10 size, and 43A inches by 7/ inches for the 5 by 7 size. The top and bottom edges are recessed 3/16 inch by 3/16 inch to provide a support for the Finlay negative and block-out screen. As it is necessary to clamp the Finlay negative and the block-out screen in register, the four sides of the opening must be cut to provide room for the clamps. A cut-out 3 1/4 inches long by 2/ inches deep at the center on each side is large enough. Attach a leaf spring on each corner to hold the negative and the block-out screen in position.
The position of the frame in which the Finlay negative and the block-out screen are placed for photo-graphing should be equal to the distance between the illuminated white wall or white paper and the lens, when the camera is set for actual size.
If a transparency frame holder is not available, a base may be made to fit on your camera stand. The width of this base should be about ten inches, and a cut-out should be made at the bottom on each end to fit the camera stand. When the negative frame is attached to the base, care should be taken that the opening of the frame is centered with the lens.
Plates similar to Hammer Slow, Eastman Process or Eastman 33 will be satisfactory for making color separation positives from negatives of any degree of contrast. Positives for this purpose should be of medium density, not too clear in the highlights, and they must register with each other.
To make a set of separation positives from a Finlay negative, proceed in the following manner:
Place two registration marks between the circles on both ends of the negative. This can be done by making a perpendicular cross with pen and ink about three-eighths of an inch. These marks should be opaque, so that they will be white on the positives.
Before proceeding to register the block-out screen with the negative (emulsion to emulsion), brush the emulsion side of the negative and the block-out screen carefully with a fine camel's-hair brush, because any gritty matter adhering to the negative or the screen will scratch both.
To make the blue printer, use the block-out screen marked "Red and Blue." Register the screen with the negative so that the outer band of the registration edge at both ends appears black (dense), and the circles and inner band light. Mark this positive
When the blOCk-out screen is registered with the negative, clamp the two together firmly at the center on each side with an Acco Clamp No. 225. Now place the clamped negative and block-out screen in the frame (screen toward the lens), with diffused light behind the negative. This light should be adjusted so that suitable positives are obtained with about forty seconds exposure at stop f : 8 or f : 11, not smaller, for the blue, red and yellow positives. The Bassani degree of rotation for the blue, red and black printers is .0025.
Red Printer: Register the red and blue screen with the negative so that the circles at both ends appear black (dense), and the outer and inner bands light. Mark this positive "11."
Black Printer: The so-called black plate is made with the same focus without any block-out screen. A piece of plain glass of the same thickness as the block-out screen is placed in front of the negative (glass toward the lens) to compensate for the absence of the block-out screen, and the positive which is made from this requires about one-sixth the exposure necessary for the blue and red printers. Mark this positive "1111."
The Bassani degree of rotation for the yellow printer is changed to .002.
Yellow Printer: To make the yellow printer use the block-out screen marked "yellow." Register the block-out screen with the negative so that the inside band at both ends appears black (dense), and outside band and circles light. Mark this positive "1."
The foregoing instructions are for making positives actual size from a Finlay negative. If enlarged or reduced positives are required, a different degree of rotation will be necessary, and the degree which will eliminate the greatest amount of screen can be easily found by trial.
Enlarged or reduced screen negatives can be made from the positives of actual size and the small amount of screen left in the positives will cause no trouble.
Any clean-working developer can be used provided it gives positives of proper contrast. The following glycin formula has been tested and recommended to work satisfactorily for positives from Finlay negatives :
Hot water 125° F. (52° C.) 48 OZ. 1500 ccm
To develop, use:
Stock Solution one part
Develop about five or six minutes at 65° F. (18° C.).
Dissolve the chemicals in the order given, adding the carbonate in small quantities. Glycin is a slow-acting developer which keeps for a long time and yields positives perfectly free from stain.
NEGATIVE DEVELOPING FORMULAS.—Probably the most generally used developer for Finlay negatives is the glycin formula recommended above for positives. Other suitable developers follow.
Fine Grain D-76 Borax Tank or Tray Developer
Water at 125° F. (52' C.) 96 oz. 3000 ccm
Dissolve the chemicals in the, order given. Use without dilution. For tank use, develop twelve to four-teen minutes at 65° F. (18° C.). For tray use, decrease the time about zo per cent.
A faster working developer can be obtained by in-creasing the quantity of borax. By increasing the borax about ten times, from 116 gr. to 2 OZ., 290 gr. per gallon (from 8 g to 80 g per 4000 ccm), the development will be about one-half that of the regular D-76.
With use this developer becomes slightly muddy, due to the formation of a suspension of colloidal silver, and the tank usually becomes coated with a deposit of silver. Both of these are harmless and may be ignored.
For use with Fine Grain Developer D-76
Water (about 125° F.) (52° C.) 96 OZ. 3000 ccm
Use the replenisher without dilution and add to the tank to maintain the level of the solution. The life of D-76 is extended five to ten times by the use of this replenisher.
Warm water 80 oz. 2368 ccm
Dissolve the metol first, then add the sulphite slowly. For use take one part stock solution A, one part stock solution B, to five parts of water. Develop six minutes, temperature 65° F.
This developer also works very well with positives when used in conjunction with the Finlay-Ilford positive plates, provided the negative is not extremely thin. It is given in two solutions, so that the quantity of carbonate may be changed to meet the requirements.
D-72 DEVELOPER FOR FINLAY POSITIVE COLOR SCREEN
AND FINLAY POSITIVE PLATES
Water, 125° F. (52° C.) 16 oz. 500 ccm
For use, take four parts of water to one part stock solution.
This developer should be used for positive color screens and is excellent for all positive plates when printing from average negatives.
If negatives are too contrasty for this developer, use the metol developer greatly diluted and develop for ten minutes. The proportion of carbonate may be increased to advantage.
198 COLOR PROCESSES HYDROQUINONE CAUSTIC
Hot Water 125° F. (52°C) 20 oz.600 ccm
B. Caustic potash 2 OZ. 57 g
Caustic soda (sticks) will do equally well, same quantity.
For use, take equal parts of 4 and B. Development: Completed in one and a half minutes 65° F. (18° C.)
The above developer is only for use in making positives from negatives that are quite flat or thin.
Shake the bottles before use, otherwise the soda may sink to the bottom and no development be obtained. Rock the tray gently in alternate directions during development. Neglect of this precaution may give rise to markings and fog, as will overdevelopment at high temperatures. The developer may be used only once.
If negatives are too contrasty for these developers, use the metol developer greatly diluted and develop for ten minutes. The proportion of carbonate may be increased to advantage.
F-1 FIXING BATH
Water 64 oz. 2000 ccm
When thoroughly dissolved, add the entire quantity of the following hardening solution:
Water (about 125° F.) (52° C.) 5oz. 160 ccm
Dissolve the chemicals in the order named, following the instructions below for formula F-1a. Cool the hardener solution after mixing and add it slowly to the cool hypo solution, while stirring the hypo solution rapidly.
F—1a STOCK HARDENER
Dissolve in the order named:
Water (about 125° F.) (52° C.) 56 oz. 1700 ccm
Dissolve the sulphite completely before adding the acetic acid. After the sulphite acid solution has been mixed thoroughly, add the potassium alum with constant stirring. When the alum is dissolved entirely, add cold water to make up to the final volume. A fixing bath is quickly made by adding one part of this hardener to four parts of cool hypo solution, or thirty-two ounces (1000 ccm) of hardener to one gallon (4000 ccm) of cool hypo solution, containing two pounds of hypo to the gallon water.