Tanning by Pyro in neutral or slightly acid environment?

[WAn]

Hello, everybody

 

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I finally had a chance to buy Pyrogallol and am going to test it soon. It would be helpful if you answer a couple of my questions:

 

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As far as I know, to be able to act as a developing agent the Pyrogallol needs an alkaline environment (pH > 7, even pH > 9), and in spite of staining it has a "tanning" effect in highlights.

 

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My first question is almost terminological: Is the "tanning" the same thing as "hardening"? Does it means the physical properties of gelatin get changed? Does a sort of texture appear on the negative?

 

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Q2: does the tanning (hardening?) take place in neutral or slightly acid environment (pH <= 7)?

 

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Thank you

[linas_kudzma]

Andrey,

 

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Pyro developers need alkaline conditions for tanning and stain

development. The sodium metaborate in the PMK formula, and sodium

carbonate in other pyro formulas provide the pH needed. Offhand I

don�t remember the exact pH required, but it is strongly alkaline.

See the "Book of Pyro" for details.

 

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Tanning does harden the gelatin and you very clearly see a texture on

the negative when it is still wet. The raised relief image is less

apparent when the negative dries. It is recommended to use a non-

hardening fixer because hardener is said to interfere with the

desirable stain formation. Acid stop bath is to be avoided also. I

sometimes use Kodak Rapid fix (without adding the hardener) and

despite the fact that this fixer is formulated slightly acidic, I

still get good stain. In fact, I avoid the alkaline after bath, which

gives me too much stain. I print Pd/Pt and very heavily stained

negatives have outrageously long print times. For silver printing

this may not be a problem because stain is what gives pyro negs their

magical properties.

 

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If you want all the details you really should get Gordon Hutchings

excellent �Book of Pyro�. Also consider trying Pyrocat-HD, a catechol

based staining/tanning developer which I have grown to like better

than pyrogallol based developers. For the Pyrocat formula see

www.unblinkingeye.com.

[dhananjay_n]

Pyrogallol is famous for its tanning/hardening properties - it was

originally used to tam leather. In fact, pyro based developers were

sometimes used to develop relief images for printing. Strictly

speaking, I think its the oxidation product of pyro which provides

the tanning action, whichi s why tanning is also proportional to

exposure. The tanning produces two wonderful qualities to negatives.

The first is a very unique adjacency effect. More importantly, the

tanning prevents the developer from penetrating into the depths of

the emulsion - this makes pyro/catechol developers surface developer,

therefore very, very sharp i.e., high acutance. The reason for this

is that developers that develop in the depths of the emulsion are

typically developing irradiated grains. That is, the exposing light

gets scattered within the emulsion. A developer developing in the

depths of the emulsion will thus reduce the acutance of the resultant

image. Surface acting developers will retain the highest acutance.

This will be typically most apparent in the highlights. Small local

contrast in the highlights will be preserved more accurately with a

tanning developer (which is why most developers formulated for long

luminance ranges tend to be based on tanning developing agents e.g.,

the famous Windisch formula). The reason has nohing to do with the

stain but with tanning that allows even small local contrast

differences in the highlights to be preserved - differences that

would be lost to the reduced acutance of developers that develop in

the depths of the emulsion. Irradiation in the emulsion is most

problematic with areas of high exposure - so long range scenes are

especially problematic and tend to be where tanning developers most

readily reveal their magic. Hutchings book will provide chapter and

verse on the properties of pyro. Cheers, DJ

[WAn]

"..nohing to do with the stain but with tanning that allows even

small local contrast differences in the highlights to be preserved" --

exactly this point is the peak of my interest now.

 

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Linas, DJ, thanks a lot!

[john_stockdale2]

As an aside, a good fixer to use is Agfa FX-Universal, sold primarily

for colour film but it's a good slightly alkaline general purpose

fixer. Non- hardening, of course. Diluted 1+5 or thereabouts it has

pH=7.3.

Here in Australia it's very reasonably priced.