Hardeners, how do they work?

Discussion in 'Black and White' started by carl_bowser, Nov 26, 2001.

  1. 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
     
  2. 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 Articles page and click on "Hardening Solutions for Film and Paper."
    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.
     
  3. 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.
     

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