Hardeners, how do they work?

I have need for information about hardners, the purpose they

serve, conditions under which they should be used (and not),

how the process of hardening works, and by what criteria

hardeners are measured. Is a hardened emulsion one with a

lower water content? How do various hardening agents actually

work to produce a hardened emulsion?

<p>

If you could I'd appreciate pointing me any technical information

about how the hardening process works (chemistry, rate of

hardening effects, proper use, alternate hardening processes,

etc.)

<p>

Thanks

I have a very brief description of gelatine hardening on my page of

hardener formulas. The URL is too long to post here, but you can go

to the bottom of my <a

href=http://unblinkingeye.com/Articles/articles.html> Articles</a>

page and click on "Hardening Solutions for Film and Paper."

<p>

Essentially, gelatine hardening prevents the emulsion from swelling

and increases its melting point. My information comes

from "Fundamentals of Photographic Theory" by James and Higgins. I

have read somewhere that a hardened gelatine layer doesn't

necessarily have less water in it, just stronger bonds between the

long strands of protein molecules, but I don't have a reference for

that bit of information and can't guarantee its accuracy.

From Richard Knoppow on USENET:<br><br>

The hardener hardens the gelatin improving its ability to cope with

rough handling during washing. The usual hardening agent in fixing

baths is Potassium Aluminum Sulfate, ususally called "Alum". The

hardening action reduces the amount the gelatin can swell when it

absorbs water and increases the temperature at which it

melts.<br><br>

Most modern film emulsions are very effectively hardened in

manufacture and don't really need the additional hardening in the

fixing bath.

The presense of Alum tends to bond hypo and its reaction products to

the emulsion so it slows down washing. The use of a wash aid like

Kodak Hypo Clearing Agent breaks the bond and allows much faster

washing. For film, the washing becomes about as efficient as if no

hardener had been used.

It is often recommended that fixer without hardener be used for

fiber-based paper or for paper of any type which is to be toned. It

seems that even when a wash aid is used the hardener may affect the

color produced by the toning.

\

Packaged powdered fixing baths all contain hardener. Most packaged

liquid fixers have the hardener in a separate container so it can be

added or not as you choose. Ilford sells a liquid fixer for paper

without hardener.<br><br>

From Doug Nishimura:<br><br>

Among other things they found that the pH of the emulsion after

fixing had little to do with washing rate. It had been thought that

the low pH left by hardening fixers slowed down washing.

Theory says that pH should have an effect, but I'm willing to bet

that it's so trivial

that its contribution is lost. Gelatin is a protein and is therefore

is a long chain

of amino acids strung together. There are thus two major groups

hanging off the sides

of the chain -- amino groups (-NH2) and carboxyl groups (-COOH). If

we had sodium

carbonate and we tossed it into water, the carbonate ion would react

with acids and

therefore we would call it an alkaline. The -NH2 groups on gelatin

can behave the same

way and will react with acids to form -NH3 groups with a + charge.

The -COOH groups

can similarly behave as an acid to form -COO (with a minus charge).

So if we put pure

gelatin into water, there is a competition between the -NH2 and -COOH

groups (one

behaving as an acid and the other as an alkaline.) Ultimately pure

gelatin in pure

water behaves as an alkaline. If we start adding alkaline, more of

the -COOH groups

react and if we add acid, the -NH2 groups react so gelatin can behave

as both an acid

and alkaline. (Chemists call it an <amphoteric> substance.) If we put

pure gelatin

into pure water and we want the gelatin in solution to be neutral

(carry no positive

or negative charges anywhere), the water would have to be slightly

acidic. The acidity

or alkalinity required is referred to as the <isoelectric point.>

When I was studying

photographic gelatin at the National Archives of Canada, the

literature said that the

isoelectric point was around a pH of 5.1. I don't remember which

paper that came from,

but I find pH 4.7 cited more frequently. What are the consequences?

If the gelatin was

in a solution that was more acidic than pH 4.7, there would be -NH3

(+) groups and

since like charges repel each other, the gelatin mass expands or

swells. The more

acidic the solution, the greater the number of -NH3(+) groups and the

greater the

swelling (although eventually there will be other chemical reactions

going on that

cause the gelatin to solubilize.) Similarly, if the solution was more

alkaline than pH

4.7 we would form -COO(-) groups and the more alkaline the solution,

the more -COO(-)

groups there would be and the greater the swelling. (I know this

first hand since I've

made dozens of gelatin swelling measurements to test these

properties.) This effect is

used for hardening. Metal hardeners such as aluminum (3+), chromium

(3+), and iron

(3+) harden by bonding to the negatively charged -COO (-) groups so

these are used in

acid solution. Aldehyde-based hardeners (such as formaldehyde) harden

by bonding with

the positively charged -NH3(+) groups so aldehyde hardeners are used

in alkaline

solution. (Compare and acid hardening fixing bath that uses alum

(aluminum) versus

Kodak SH-1 hardener that uses formaldehyde mixed with alkaline sodium

carbonate.)

Chemicals pass through the gelatin more and more easily the more

swollen it is so in

theory, washing should be most difficult around pH 4.7 and get easier

as we get more

acidic or more alkaline (assuming that there were absolutely no other

mechanisms going

on.) This effect, of course will be swamped by the greater effect of

the ion exchange

effect of course especially since gelatin alone washes pretty easily.

(Compare how

easily an emulsion on plastic such as RC base washes versus fiber

base paper.) Film is

complicated further by other things (and if it was coated on paper

instead of plastic,

it would be even tougher to wash.)

The "salting" effect of sulfite washing aids should also occur to

some degree with

carbonate baths as well. (Even sodium chloride with it's single

negative charge will

compete to some degree although the effect is pretty small.) Ion

exchange resins

behave in a similar manner (those of you with demineralizers or

deionization columns

for water.) The carbonate will have a bigger effect than chloride by

this mechanism,

although appears to be much weaker than sulfite.

You can see that that if an acid was needed to get a trivalent metal

hardener to work,

competition with an alkaline should help to remove it. (As Richard

described.)

Similarly, and acid should help to remove an aldehyde hardener

(although the bonding

is different so it's not exactly analogous to the acidic removal of

metal hardening.)

Based on all of the above stuff, you might expect that chromium

washes out easier than

aluminum hardening. This was confirmed in washing studies done in the

1980s at the

National Archives of Canada. (They also found that formaldehyde

hardener was harder to

wash out than aluminum.) The manufacturers also knew about this

although it doesn't

appear that they really talk about it anywhere.