My first attempt... is in the fridge.

[Photo Engineer]

All photograde gelatin today in England, France and the US (AFAIK) is both inert and oxidized as well as being calcium free and neutral.

In the 40s, none of the above were true. One of the first operations with gelatin was to balance pH with Magnesium Oxide or the like, then the ripening was determined, and then it could be used. Today, I can pretty much take samples from Kodak, Gelita and Rousselot and get pretty much the same result with no special precautions.

From the first test, I can then tweak the emulsion formula and go on from there.

PE

 

[dwross]

Congratulations!! Jadedoto,

It's such fun, isn't it. I love how you're sharing your excitement. If you have the time and/or inclination, please share with us on hybridphoto.

Keep up the good work (actually, I prefer "Keep up the good play")

Denise Ross

 

[steven_e007]

All photograde gelatin today in England, France and the US (AFAIK) is both inert and oxidized as well as being calcium free and neutral.

PE

You mean all the stuff that we, as amateurs, can buy?

I have been told, by someone who would know, that some manufacturers still use active gelatines, with the sulphur compounds intact, in their emulsions. This means there are gelatine maunfacturers (in the UK certainly) that can provide active gelatines, but only in industrial quantities...
(I've been investigating )

Maybe one day there will be enough of us on this thread to club together and buy a few tons?

 

[Photo Engineer]

Steve;

There are some companies that do use active gelatins, I'm sure but for the most part the open market only supports the oxidized and inert photograde gelatins.

If you do get an active gelatin, you will have to test each batch for activity. It usually comes in 3 grades or weak, medium and strong, referring to their effects on sensitization. Each grade will vary from batch to batch.

Home experimenters will be having enough problems maintaining a constant speed and contrast without introducing another set of variables. It is much easier to maintain one stock of oxidized photograde gelatin and add a measured standard amount of hypo solution than it is working with the old gelatin.

PE

 

[steven_e007]

Steve;

It is much easier to maintain one stock of oxidized photograde gelatin and add a measured standard amount of hypo solution than it is working with the old gelatin.

PE

Ok, I'm convinced by that!

Is there a post on here somewhere that tells us how much hypo to add to the gelatine and at what stage?

Are we talking about plain sodium thiosulphate?

Are there other sulphur compounds that are used?

Thanks,


Steve

 

[steven_e007]

Just answering my own question...

PE,

I don't mind searching for answers, but this forum thread is now so big sometimes it takes a lot of digging to find the nuggets! No doubt this problem will be eased when your book comes out

I found this in one of your earlier posts:

"The only way to get speed is by chemical sensitization, or finishing. This involves the addition of any one of a variety of ingredients. The original was allyl thiourea, another was thiourea, and then finally they added sodium thiocyanate. Modern emulsions use either sodium thiosulfate or sodium thiosulfate plus a gold salt. It is done after the wash step, as excess halide represses this sensitization. This finishing step varies for every emulsion and sometimes for every batch..."

So, that answers my what to use question. Any good reason for choosing one over the other?
I have sodium thiocyanate and sodium thiosulphate (thiosulfate) in my darkroom.

I could still do with a hint about concentrations and how to add them.

Steve

 

[Photo Engineer]

Use sodium thiosulfate pentahydrate as a 0.1% solution in water.

The amount to use is dependant on grain size and can only say that the amount goes DOWN as grain size goes up. It can only be determined for a given emulsion by trial and error or by knowing average grain size.

The mid point on this scale is about 50 milligrams of hypo per mole of silver. I'm assuming you can figure this out. I use 100 mg / mole for my emulsion as it is quite finer in grain. I finish at 60 degrees for 60 minutes.

It took me about 10 experiments to arrive at this figure.

PE