Photo Engineer
Subscriber
I have been reading several threads lately that discuss replenishment and several older threads stick in my mind that covered steady state seasoning and replenishment.
It seems that there is a great misunderstanding over the situation with both seasoned developers and with the steady state of seasoned developers.
First off, let me state that I fully understand that by many definitions, steady state excludes startup and shutdown of a given process. Now, understand this. If, in photography, steady state did not include startup and shutdown, a lot of customers films would be ruined every time a photo lab charged their machine(s) with new developers. The startup in a photo process must yield the same results as the seasoned one as closely as is possible.
In the case of D-76, it is achieved by D-76 as startup and D-76R as replenisher. Each of these have different formulations and used separately would yield different results. Used properly, they will give you a constant result from startup to shutdown thus giving the effect of a steady state condition. The tank developer does not resemble either the fresh developer or the replenisher (see below).
In other words, fresh D-76 and replenished seasoned D-76 should give close to the same results. I know that there are some differences, but this is due to unavoidable leaching of development byproducts and chemistry into the developer by the wide range of films being put through the process and which cannot be perfectly compensated for due to the variety of films one may use.
In the final analysis, the steady state may yield the same characteristic curve, sharpness, speed and grain as the fresh developer, but it does it by a slightly different chemical composition. So, for example, the final seasoned D-76 in the tank contains Iodide and Bromide and neither the startup developer nor the replenisher did. But, the seasoned developer also has a different blend of HQ, Metol and alkali. This balance is changed to reflect not a steady state in chemical analysis necessarily, but rather, as closely as possible, it represents a steady state in the films characteristics when processed in the ever changing developer. In other words, the films steady state remains constant from startup through equilibrium and no customer film is lost even though the chemistry in the tank is undergoing a change from startup to steady state or equilibrium.
Even some excellent engineers and chemists seem to miss this point. Since it is not a generally published topic, nor is the condition easy to analyze for, it is often misinterpreted or overlooked. I thought I might shed some light on it.
So, steady state can refer to the condition of the developer solution composition, or to the state of the results obtained from the process, or both. In photography it mainly refers to the results obtained. And, in so doing, it must encompass the startup and continuous running or we would be ruining a lot of customer film in photo labs. I think that you can see that what I have described above is obvious.
A photofinisher cannot run blank film through his process to season a fresh tank of developer. It is not cost effective. The developer must be right at startup. The same goes for us when we mix up new developer. We must not be required to season that developer. It must be at a steady state at startup to yield the proper results.
I hope I have clarified things for some people.
PE
It seems that there is a great misunderstanding over the situation with both seasoned developers and with the steady state of seasoned developers.
First off, let me state that I fully understand that by many definitions, steady state excludes startup and shutdown of a given process. Now, understand this. If, in photography, steady state did not include startup and shutdown, a lot of customers films would be ruined every time a photo lab charged their machine(s) with new developers. The startup in a photo process must yield the same results as the seasoned one as closely as is possible.
In the case of D-76, it is achieved by D-76 as startup and D-76R as replenisher. Each of these have different formulations and used separately would yield different results. Used properly, they will give you a constant result from startup to shutdown thus giving the effect of a steady state condition. The tank developer does not resemble either the fresh developer or the replenisher (see below).
In other words, fresh D-76 and replenished seasoned D-76 should give close to the same results. I know that there are some differences, but this is due to unavoidable leaching of development byproducts and chemistry into the developer by the wide range of films being put through the process and which cannot be perfectly compensated for due to the variety of films one may use.
In the final analysis, the steady state may yield the same characteristic curve, sharpness, speed and grain as the fresh developer, but it does it by a slightly different chemical composition. So, for example, the final seasoned D-76 in the tank contains Iodide and Bromide and neither the startup developer nor the replenisher did. But, the seasoned developer also has a different blend of HQ, Metol and alkali. This balance is changed to reflect not a steady state in chemical analysis necessarily, but rather, as closely as possible, it represents a steady state in the films characteristics when processed in the ever changing developer. In other words, the films steady state remains constant from startup through equilibrium and no customer film is lost even though the chemistry in the tank is undergoing a change from startup to steady state or equilibrium.
Even some excellent engineers and chemists seem to miss this point. Since it is not a generally published topic, nor is the condition easy to analyze for, it is often misinterpreted or overlooked. I thought I might shed some light on it.
So, steady state can refer to the condition of the developer solution composition, or to the state of the results obtained from the process, or both. In photography it mainly refers to the results obtained. And, in so doing, it must encompass the startup and continuous running or we would be ruining a lot of customer film in photo labs. I think that you can see that what I have described above is obvious.
A photofinisher cannot run blank film through his process to season a fresh tank of developer. It is not cost effective. The developer must be right at startup. The same goes for us when we mix up new developer. We must not be required to season that developer. It must be at a steady state at startup to yield the proper results.
I hope I have clarified things for some people.
PE