Shrink-Swell & Fixers

[miha]

Hi everybody,

Using alkaline fixer should lead to shorter fixing times, which naturally leads to short washing times. The amount of soaked fix is thought to be lower because the paper is immersed for less time so the fixer can't penetrate deep into the paper. All seem logical...well, almost. The logic behind is that alkaline solutions cause swelling which helps with washing. But doesn't swelling already start during the fixing stage? Meaning that although there is less time for the fixer to penetrate, the gelatine already begins to swell, becoming softer so the alkaline fixer penetrates deeper especially if the acid stop bath is omitted from the process - which is often (but not always) advised.

I believe that the gelatine should shrink in a stop bath and stay this way during fixing in an acid fixer so that there is as little penetration as possible. It's only after fixing the the paper should be immersed in an alkaline solution (HCA) which swells the gelatine and promotes efficient washing.

What do you think?

p.s. I'm aware that fixer penetrates several media (gelatine, baryta, pulp) independently, and that not all are pH dependant.

 

[Photo Engineer]

Your basic approach is faulty.

The gelatin swells and allows the chemicals to diffuse in and out more freely thus making fixing and washing more rapid.

In acidic conditions, the gelatin is indeed less swollen, but the diffusion rate of chemicals is thus slower by a large factor.

And, BTW, this is why Ammonium fixers are faster than Sodium fixers. The Ammonium complexes of AgX are much smalller and diffuse out more rapidlly. Alkaline Ammonium fixers are thus very very rapid.

Another approach is to make an acid fix with a swelling agent.

PE

 

[miha]

Thanks PE. But doesn't fixing (or any other processing stage) take place on the very surface, under the supercoat layer, so there is no need for the fixer to 'enter' the gelatine too deep in the first place?

 

[Photo Engineer]

No. The emulsion is found throughout the gelatin in the light sensitive layer, and every processing solution must diffuse to the bottom of that gelatin layer to carry out the assigned function such as development, stop and fixation. In the case of the fix step, the fixer chemistry must diffuse inward and then it must diffuse outward while carrying the extra Silver Halide salts.

In the case of FB papers, the paper itself is totally wetted with the chemistry in water in each step. Baryta has an affinity for Hypo, and thus the wash step with Baryta papers becomes much more difficult and critical.

PE

 

[miha]

No. The emulsion is found throughout the gelatin in the light sensitive layer, and every processing solution must diffuse to the bottom of that gelatin layer to carry out the assigned function such as development, stop and fixation. In the case of the fix step, the fixer chemistry must diffuse inward and then it must diffuse outward while carrying the extra Silver Halide salts.

I think I understand - thoroughly swelled gelatine which allows the fixer to enter and exit it fast is preferable because it's necessary for the fixer to penetrate it wholly and do it fast both ways.

In the case of FB papers, the paper itself is totally wetted with the chemistry in water in each step. Baryta has an affinity for Hypo, and thus the wash step with Baryta papers becomes much more difficult and critical.

PE

Is baryta also pH sensitive? If it's not the role of alkaline vs acid fixer diminishes. Or am I wrong?

 

[Photo Engineer]

Baryta, AFAIK, is insensitive to pH effects in this case BUT, Baryta is coated in gelatin which is sensitive as I pointed out. So, there may be effects, but probably not directly related Baryta vs pH.

PE

 

[miha]

If baryta has an affinity to hypo thus it's baryta that it's most difficult to wash thoroughly. if this is the case, then gelatine which of course benefits from alkaline fixer should be our second concern - it's probably properly washed out long before the baryta is. Regardless of the fixer type.

 

[Photo Engineer]

Again, not the case.

Since Baryta is in gelatin in FB papers, then the Baryta layer swells more in alkaline fix or with swelling agents present and so the wash again is more efficient.

PE

 

[miha]

Not that I would doubt you PE, but I always thought that the baryta layer sits under the gelatine as illustrated on the second page of the link below:
http://www.kentmere.co.uk/Webfiles/20093161412372020.pdf

I didn't know that baryta swells nor that it swells more in alkaline fix.

 

[Photo Engineer]

Baryta is Barium Sulfate. It is made as a finely divided powder and when mixed with water makes a thick paste. It is used in medicine for taking high contrast X-Rays of the intestines. However, it will not coat as is. Therefore, this paste is mixed with a fairly substantial quantity of gelatin in order that it then becomes coatable. This gelatin/water/Baryta mix is coated on FB paper and then subjected to pressure and heat to get a smooth surface.

This gelatin/baryta mix swells and shrinks during the process.

So, you have /gelatin overcoat/gelatin + emulsion/gelatin + baryta/paper/ And everything in this happy family swells when wet and shrinks when dry. If the proportion of gelatin to Baryta is incorrect, small flakes of the Baryta comes off at the edges of the paper in a process called skiving, and during coating, bad FB paper causes severe dust in the coating room.

There, more about Baryta than you ever wanted to know.

I did leave out the fact that Baryta is hardened just like the emulsion layer(s) and it contains spreading agents, antioxidants, dyes and etc.. It can get very complex.

PE

 

[miha]

Thanks very much for clarifying PE - I'm always eager to learn new things I know fixing is a complex process, as you often highlight.