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?

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.)

Thanks

-- Carl Bowser (bowser@geolgy.wisc.edu), November 26, 2001

Answers

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.

-- Ed Buffaloe (edb@unblinkingeye.com), November 26, 2001.

From Richard Knoppow on USENET:

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.

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.

From Doug Nishimura:

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 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 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.

-- Tim Brown (brownt@flash.net), November 27, 2001.