The table below tries to show the approximate chronology of the Paterson 'Acu' range of developers.
Clicking on the names takes you to a place on the web page (below) which shows more detail about each product. The dates within the table header should not be taken to imply the date a product first appeared (though the date for the original Acutol is believed correct). In general the dates are merely dates for which information is available.

Information on other 'Acu' products, not developers, can be viewed here.

INSTRUCTIONS FOR USE. Acutol is a concentrated liquid developer which has unique qualities in producing the maximum possible sharpness, definition and resolution obtainable on slow and medium speed films, together with a fineness of grain and full tonal scale which represents a new standard in "high definition" developers. Acutol is not a "Beutler" type developer, but like other developers designed for acu-tance Acutol has as part of its action the function of increasing "edge" sharpness by producing adjacency effects which heighten the contrast at the edge of fine detail. Normally this is done by using a dilute developer which necessitates a long development time, and sometimes restricts keeping qualities. In Acutol, chemical means are employed to produce and artificially enhance edge effects. This allows the developer to have markedly shorter development times and improved keeping qualities. Apart from its adjacency effect mechanism, Acutol has a further action which ensures maximum definition on those types of film which do not readily respond to this mechanism. Consequently its special advantages are obtained no matter with what make of film it is used. This second action is achieved in part by a system of "buffering" which also results in the exceptional gradation and minimising of grain given by the developer. Acutol is an unpublished formula invented by the leading independent authority on the development process, G. W. Crawley, whose work on high definition development is well known. This formula is available only in the form of Paterson Ac EXPOSURE. Acutol gives a slight increase in film speed, and it is recommended that films should be exposed at a meter setting ½-stop higher than the manufacturer's rated speed for minimum exposure. WORKING SOLUTION-Dilute 1+6. 1 part of the concentrated liquid is diluted with 6 parts of water. In order to find the amount of concentrated liquid needed to make up a given quantity of developer, divide the quantity required by 7.

ACUTOL FX-14 DEVELOPER
(now 1:10 dilution, not 1:6 as original)
Acutol gives maximum definition on films ISO 5-200 by increasing the apparent sharpness at the edges of fine detail.
It achieves this effect chemically, so allowing normal agitation and giving negatives of excellent gradation. There is only the slightest compression of the middle tones, more than compensated for by the brilliance of detail within the picture. The edge effect is most marked on high acutance films, which are generally 35mm films of speed rating ISO 5-200. As these slower films usually have quite high contrast, Acutol suits them very well and will produce impressive results on subjects containing textures.
Films faster than ISO 200 and most roll films do not give the extreme sharpness of which this developer is capable. Acutol is a medium fine grain developer and while it will give good results with fast films and roll films it is normally preferable to use Aculux and so ensure the finest grain and longest tonal range.
Acutol gives a ½stop speed increase, useful with the slower films and is well suited to pictorial photography.
Dilution 1 + 10

Acuspecial FX-21 had appeared by autumn 1969

A new type of soft working 'surface' developer invented by Geoffrey Crawley. "It is designed to give a special result with modern high performance slow and medium speed miniature films 5-200 ASA but may also be used with higher speed films. Acuspecial produces a negative of 'engraving' type sharpness and definition, coupled with extremely fine grain. The emphasis on this type of definition is obtained with a minimum reduction in continuous tonal gradation, and this is offset by exceptional separation of tones on fine detail. The disadvantages of streaking and inconsistency of action, usual to surface developers, have been overcome. There is an effective film speed increase of a half-stop.

Acuspecial does not replace Acutol which continues to give the optimum balance of tonal gradation, sharpness and definition. But the 'engraving' type negative given by the new developer, and the control of contrast which its soft working action provides, make it an important new tool for the knowledgeable photographer in the miniature and sub-miniature fields. Acuspecial is available as a highly concentrated liquid which is normally diluted 1:29 for use. Full instructions are supplied. Recommended prices: 55ml (2fl.oz) bottle 5/- 250ml (8.8fl.oz) bottle 17/6."

Paterson's brochure for 1973 / 1974 shows no new additions to the range, though some of the packaging has changed.

Back row; Acufix (medium), Acustop, Acutol, Acuprint, Acuspeed.
Acutol-S and FX-18 (small tins), cleaning solution, Acufix (small)
Acustop, Acutol-S (large tin)
Front row; Anti-Static Wetting Agent, Acuspecial

FX-18 Developer. A standard P.Q. Borax developer conferring advantages when used instead of the more conventional M.Q. Borax developers. A single powder for rapid preparation. Gives full emulsion speed, fine grain, good image sharpness and has a long working life.

There is a new 'Pro' range, called Pro-Print and Pro-Fix, in 2.5litre and 5litre containers only.

PRO-PRINT DEVELOPER FX-34
Paterson Pro-Print Developer is a rapid working, positive developer suitable for all black and white enlarging papers, producing a neutral image.
It is characterized by having a high capacity, 1 litre of working strength solution being capable of developing between 3 and 4 square metres of paper, equivalent to 60-80 20.3x25.4cm (8x10in) prints, depending on the proportion of heavily to lightly exposed areas on the prints.
Development should be carried out ideally at a temperature of 20°C. At that temperature a correctly exposed print on resin coated paper will give image appearance within 10-15seconds and development will be complete in 45-60 seconds. In the case of fibre base papers, the image will appear in 20-25 seconds and development will be complete in 1½minutes. Longer development times will not affect print quality, though excessive immersion will give rise to veiled highlights.
The developer is supplied in the form of a concentrated solution, which is diluted in the ratio of 1 part of the developer to 9 parts of water.

By 1998, Pro-Print is one of Paterson's economy range Phototec Scales Brand, and the bottle (1litre and 3litres) bears the simple name 'Rapid Print Developer'.

The Scales Brand trademark suggests it may have originally been part of the Johnsons of Hendon formulas purchased by Photo Technology Ltd in 1976.

It was the subject of a Mini Test by William Cheung:
Practical Photography magazine, January 1994.

"If you use variable-contrast papers you get the best results by using a dedicated print developer. Paterson's latest formula, Acugrade, is suitable for all variable-contrast papers.
To use, dilute Acugrade 1+9 with water. I tried it with Kentmere Kenthene VC paper and Agfa Multicontrast Premium. To be honest, until I tried Acugrade my results with the Kentmere paper were disappointing, even in a variable-contrast developer. Acugrade, however, gave solid blacks, clean whites and plenty of sparkle to my prints. It performed equally well with Premium and the resulting prints are brilliant.
The image appears fairly slowly to start with, but the build-up is rapid and a fully-toned print is achieved in one minute. An extra 15 seconds or so helped to give more highlight detail. Dish life is comparable with rival developers and print capacity is also good.
Acugrade's an excellent developer and is a welcome addition to Paterson's strong existing line-up of photo chemicals. Try it."

The Acugrade packaging had changed by 1998. The pack alongside contains 2 x 300ml bottles.

Modern variable contrast papers are characterized by their high printing speed and short development times. Acugrade developer has been especially formulated to exploit both of these features whilst generating high maximum density and providing better separation of shadow detail at any grade setting than other developers. The good rich blacks, truly neutral well defined middle tones and clean sparkling whites which Acugrade generates on the best of the variable contrast papers makes this the ideal standard print developer for today's materials.

• Produces good rich blacks, neutral mid-tones and clean sparkling whites.
• At 20°C the image appears at 5 to 10 seconds, full development at 45 to 60 seconds.
• Also suitable for fibre based papers (development times x2).

Dilutions 1:9 or 1:14

Aculux 2 can be used for all black and white films, regardless of speed or grain group. It is a one shot, fine grain developer, now reformulated to give a more compact grain structure, even better tonal gradation and improved highlight and shadow detail. When used with modern hi-tech films such as Paterson Acupan 200 it will produce almost grain-free negative of exceptional quality.
With films in the medium speed group, Aculux 2 produces very fine grain and long tonal range negatives capable of a high degree of enlargement.
With modern fast films such as Acupan 800 which, by its nature, has a more coarse grain structure, Aculux 2 keeps grain to an absolute minimum but still provides a wide tonal range.

• New formulation, gives a more compact grain structure with improved highlight and shadow detail
• Almost grain-free negatives with slow, fine-grain films
• Gives film speed increase of 1/3 of an f-stop.

Acutol is a medium fine grain, high acutance developer. Acutance is the term used to describe sharpness of the image - particularly the edges of fine detail. The 'edge effect' of Acutol is most marked on slow, fine, conventional grain films up to ISO 125. As these are usually of high contrast, the compensating effect available with Acutol is also beneficial.
Normal dilution is 1+10 which gives only slight compensation, 1 + 15 and 1+20 dilutions give the opportunity to select a degree of compensation to suit the film and subject. Very high contrast films such as Kodak Technical Pan can be used for full tonal range subjects by developing at 1+20.

• Medium grain, high acutance developer
• Gives a speed increase of 2/3rds of an f-stop or EV
• Choice of dilutions for controling negative contrast.

FX-39 is a one-shot high definition developer designed to allow a new standard of sharpness and definition to be reached. It has been designed to exploit the properties of films using advanced silver halide grain technology such as those in the Paterson Acupan Kodak T-Max, Ilford Delta, Agfa APX and Fuji ranges.
As the development times of many traditional films are short compared to these emulsions, FX-39 can be diluted to 1 + 14 or even higher for convenience, accuracy and great economy without any loss of quality. Calculating the new time could not be easier -just multiply the 1 + 9 time by 1.5X. At 1 + 19 diluted (2X normal development time) FX-39 gives a valuable compensating effect with extra shadow detail and the ability to cope with high contrast subjects.

• Formulated to give optimum results with tabular grain films.
• Gives the film manufacturers full speed rating.
• Doubling of the ISO setting possible with extended development.
• Also well suited to conventional grain films up to 200 ISO.

Varispeed makes the most of the faster, modern B+W films, including their ability to be "push processed".
Films having an ISO speed of 400, can be exposed at 800, 1600, or even 3200 and still record shadow detail, without the increase in contrast that can occur with other developers. The full tonal range is maintained throughout the densities and pushed films have an image quality almost as high as those exposed at their ISO rating.
Most effective on ISO 400 films, Varispeed is by no means restricted to them; although there is less latitude for the developer to utilise on ISO 100 films, at least one stop increase can be achieved.
Varispeed should not be confused with earlier 'push processing' developers which yielded poor results at the standard rating. It yields excellent results when no speed increase is needed.

• Suitable for any b&w film type or speed
• Films can be exposed at up to 8 times their ISO rating
• Maintains full shadow detail
• "Rescues" underexposed films

Universal developer is suitable for both films and papers can be very economical for the beginner or those on a budget.
Negatives developed in Universal will be crisp in detail, bright in contrast and with low fog level. They print easily to show a wide range of tones with a neutral image colour. At 1+4. dilution it is also useful as a high contrast developer for copying or other special darkroom effects on film or paper.

• Suitable for films or papers.
• Ideal as a high contrast developer for special effects.

P-Tec Unitol Fine-Grain developer
Unitol, the well-known developer, from the long discontinued Johnson's chemical range. This developer gives good results and a minimum of grain with most of today's black and white films.

• Can be used to "push" films by up to two stops with a minimum loss of quality.
  

For Microsoft Word format technical documents fully describing the current Paterson chemistry, visit the Paterson website.

Separate documents are available for Aculux 3 and FX-39 film developers, for Acugrade FX-31 paper developer
and Acufix Rapid Fixer.

Aculux 2 is still listed here.

ACUSTOP EXTENDED LIFE STOP BATH
An acid stop bath between developer and fixer ensures accurate control of image quality by ending development at a precise time. This is important when modern developers with short development times are in use. The stop bath also extends the life and properties of the fixer by preventing the carry-over of harmful alkalis into the fixer. Such a stop bath is therefore justified not only in terms of more consistent image quality but also in terms of costs saved on fixer. By preserving the fixer there is less danger of inadequate fixing and subsequent lack of permanency of the image.
Acustop contains an indicator and the solution changes colour when it is exhausted. The stop bath is yellow when fresh and maybe used repeatedly until it starts to turn to purple, when it should be discarded. The difference in colour is visible under safelight conditions.
Dilution 1 + 30