Sunday, February 5, 2012

Mix ratios

I get a lot of questions about mixing Hypercat, and I see a lot more regarding other two-part developers, so I thought I'd address the issue here.

One source of confusion is the convention photo chemical suppliers like Kodak adopted early on, of using different terminology interchangeably. Diluting D-76 stock with an equal volume of water is referred to as both 1:1, and 1+1, but only the latter is accurate. In a ratio, like 1:1, the first term refers to the solute, or the part being dissolved, and the second refers to the total solution volume, so in a 1:1 ratio, the total volume of the solution is equal to the solute, meaning it is the undiluted stock solution. This differs from a 1+1 terminology, where the first term refers to the stock solution, and the second refers to the solvent/water, so a 1+1 dilution is equal parts stock solution and water, and equal to a 1:2 dilution.

The X+Y convention is useful for very low ratios of dilution typical for the most popular developers like D-76, ID-11, Xtol, etc., where typical dilutions include Stock (1:1), 1+1 (1:2), and 1+2 (1:3), but not as useful for high ratios typical for developers like Rodinal (1+24, 1+49, 1+99), and even less so for two-part developers like PMK- Pyro (1+2+97). For these high dilution ratio developers, the general chemistry convention is more useful and allows for extrapolation to to other volumes and dilutions, but there remains ample opportunity for confusion.

To avoid confusion when working with two-part developers like Hypercat, a few fundamentals should be kept in mind; first, the roles of the two parts in the working solution. The part A contains the developing agent and a preservative/ stain controller. The ratio of Part A : total solution volume determines the dilution of the working solution. A more dilute solution will selectively exhaust more quickly during rest periods of development than a more concentrated solution will. The relationship between the developer concentration and the Agitation Frequency, or duration of the rest periods, will determine the extent of the edge and compensation effects, as well as the development time required to achieve a given contrast. Typical dilutions of the part A : total solution volume are in the range of 1:100- 1:300.

The part B contains only the alkali/accelerator and determines the pH of the working solution. Generally speaking, a higher pH developer will be a more active developer, producing higher contrast in a given set of development conditions, but there are other consequences resulting from developer pH. Too low pH can cause a decrease in film speed, very low contrast and stain formation, or below the activity threshold of the catechol, no development at all. Too high pH can cause chemical fogging/ general stain, very high contrast, coarse gradation, increased graininess, and slightly higher film speed.

For most uses, including silver printing or scanning, a working solution of Hypercat should contain about 5g/ liter of sodium or potassium carbonate, but the useful range begins around 3g/liter and extends up to 15g/ liter, or more. 3g/ liter is useful for developing high contrast materials like Tech Pan or other document films, and 15g/ liter is useful for negatives meant for printing in alt processes that require very high contrast.

To review, the ratio of the part A : total volume of working solution determines developer concentration, and the ratio of part B : total volume of working solution determines developer pH.

In practice, you might find 5g/ liter of carbonate works best for 90+% of everything you do, and so you might find it convenient to make up 4 liters of 5% solution. When you want to make a working solution, you simply measure out the 5% solution to your desired total solution volume, add the desired amount of part A, and stir.

Example: Making 300ml of Hypercat 1:300 w/5% carbonate/ liter-

  1. Fill graduate to 300ml with 5% solution
  2. Add 1ml Part A, stir
Making the same developer using a 20% stock solution of sodium carbonate would be as follows:

  1. Fill graduate to 200ml with water
  2. Add 7.5ml Part B, stir
  3. Add 1ml Part A, stir
  4. Top up to 300ml, stir
Both developers will be identical, but the first is certainly more convenient and less prone to errors in calculation or measurement. And making up the 5% solution isn't terribly critical, so long as you're consistent. If you always add 4 teaspoons of carbonate to 1 gallon of water, for instance, your results will be consistent, even though the resulting solution might not be exactly 5%. A carbonate solution is stable, so you don't need to worry about its shelf life, and mixing a new one only takes a few minutes. When using a ready-made carbonate solution, you can refer to your developer dilutions more simply, as in 1:100, 1:200, etc.

If, however, you find you regularly use solutions of varying carbonate concentration, a stock solution will undoubtedly be more convenient.

I hope this clarifies some of the issues related to using Hypercat and other two-part developers.