Watkins Factor: developing

Time of first appearance as estimator of developing time for that emulsion in that developer.

In re-working information about Deep/Thick vs Thin emulsions… da-net posers made me do it.

Deeper dive into Time-Temperature of photography.

  • Watkins
  • Sheppard and Meyer
  • Clerc, section 400, the influence of temperature
  • Clerc, section 380 & 381, watkins factors, & combined factors

How Does It Work?

Developing is a process of saturating the emulsion; surrounding the exposed material with fresh developer, then moving exhausted developer. This main process is agitation. The key part of the emulsion is the gelatin. Think of the emulsion as having depth, as well as surface. This emulsion, whether film or paper, has to absorb the fresh developer– this takes time. An induction time. How fast the exposed silver begins developing depends upon the strength (developing power) of the developer. It is affected by temperature, emulsion, etc, but the main factor was the developing agent(s) used.

The watkins method is mainly of use with emulsions suited to development by inspection. It is of great use in making lith negatives, or, somewhat, in making lith-prints. Sadly, in a developer containing two developing agents, the temperature coefficient loses significance when temperature varies much from 20 C.

Developing AgentC
Ferrous oxalate1.7
Hydroquinone2.2 to 2.8

Some developing agents change activity with temperature — hydroquinone loses activity at low temperature. (Jacobsen, p.67)

Temperature coefficients vary slightly with emulsions. Concentration of developer has little effect.

The temperature coefficient of a developer is the increase in speed of development for a 10F increase. A TC of 2 means that a developer works twice as quickly at 75F as it does at 65F — typical Coefficients from the mid-century were: 1.8 for MQ developer(D76), for fine-grain developers (D23), and 2.3 for super-fine grain developer.

Note: pyro and amidol do vary by amount of grains per ounce. Bromide alters the pyro factor.

Compensating Timers

Temperature control is better than a compensating timer. Keeping agitation in line, changes of developer is more important to critical processing than is time-temperature duration. Build tray rockers, insulated, water-jacket trays with lids instead.

But, if you are interested search: Tundra. DLG. ProcessMaster.


Kodak RA-4 Paper

The digital RA-4 paper notes. Endura in 2013 included printing in darkroom. 2019, no such advise. See the calibration routines for setting your digital printer. I don’t know that any of these papers are sold in 2023. The world, especially in non-western countries, is rapidly moving from wet-lab to dry-lab imaging.

The “analog resurgence” is a thing in major metropolitan areas, but a thing for 35mm film. Along with this bounce has come a demand for scanning; a demand so strong that premiums are being paid for scavenged “scan heads” of minilab equipment.

Of note should be the filtration recommended for Kodak Endura Premier paper. A white-light starting pack of 40M+50Y, or a Tricolor filter set of 25,99,47B … Don’t forget — still recommended are IR and UV filters (B).

From the datasheets you could build upon your Rosetta of process procedures.

Profiles provided by Kodak give you an indication of what products have been made, but not what is current product.

Kodak’s LED Digital Color Printer model 20O/R can be controlled from within Photoshop. Yes, Kodak made LED printers, no matter what the Forgotten of Foradum say. It also suggests that calibration of LED printer is possible for at least one Kodak RA-4 paper.

Of course, ZBE’s Chromira is a long standing LED printer used by many US West-Coast labs.