The possible chemistry of gum bichromate

The chemical mechanism of the gum process, even in its general outlines, is not well understood by most of its practitioners, resulting in many unfortunate and even laughable mistatements in gum texts and in discussions about the process.

However, it should be emphasized from the outset that it isn't necessary to know much about the technical aspects of the chemistry of the process to make excellent gum prints, as should be evident from the fact that people have been making gum prints for over 100 years without anyone understanding very much about how it works, and most of what they thought they knew being flat out wrong.

There's almost no literature available on the chemistry of dichromated gum arabic specifically. So while some of the information here, like the structure of gum arabic and the general behavior of irradiated dichromated colloids, for example, is fairly solid as far as it's known, the information about the crosslinking is less well established. I'll tell you what I've learned, and you can make up your own mind. The best way to come to a conclusion on this or any other scientific question is to read the original articles yourself rather than taking my word or anyone's word for what they say; references are provided at the bottom of the page.

 A brief review of the literature on dichromated colloids:

When I first got curious about the chemistry of the gum process, I inquired on an online forum if anyone could explain the chemistry to me, and was given this quote in return:

"In the presence of any organic matter the bichromate is decomposed by exposure to light into neutral chromate which is eliminated in the later washing and brown oxide or chromochromate mCr2O3 which by subsequent washing is decomposed into chromic acid CrO3 carried away by the water, and into GREEN OXIDE OF CHROMIUM Cr2O3 which combines with the gelatine and effects the TANNING action. In the course of this reaction a very small part of the gelatine is oxidized, resulting in products which are eliminated during washing. The insoluble is formed entirely of normal gelatine in combination with chromium oxide." (Eder,"1878" see reference note 10. The emphasis was added by the sender of the quote).

There is essentially nothing in this explanation that is consistent with what is understood about the process today, and it is curious that even at the end of the 20th century, it was still being offered as an explanation for the process.

As suggested in that ancient quote, conventional wisdom has long held that the process of making photographic prints is much like the process of tanning leather. I don't believe that analogy is a good one at all; if you're interested in exploring why that's the case, see my discussion of the reasons why I don't think leather tanning provides a good model for dichromated colloid processes.

So, if gum printing isn't like leather tanning, then what is it like? Chances are it's like the crosslinking of dichromated PVA (polyvinyl alcohol). Before I start, I should probably explain a little about the similarities and differences between PVA and gum arabic. PVA is a fairly simple polymer; the monomer is -CH2CH(OH)- that repeats itself in a long chain. The normal linkage involves hydroxyl groups on alternate carbons. However, 1% of the linkages place two hydroxyl groups on adjacent carbon atoms. These special sites are most likely to be the sites of oxidation of PVA. [Coker, ref. ]

There are different varieties of PVA depending on how long the chain is, the degree of hydrolysis, and so forth. One of the common ways PVA molecular weights are determined involves the oxidative cutting of the polymer at the sites mentioned above, using potassium periodate. [Coker]

Gum, on the other hand, is a rather complicated and variable arabinogalactan protein of somewhat indeterminate but huge molecular weight, as described in the section below. However, PVA can be printed much like gum in the gum printing process, which suggests that the mechanism may be similar.

Properties and structure of gum arabic:

Technically, gum arabic (acacia senegal) is classed in a group of substances called, oddly, arabinogalactan proteins. More descriptively, it is essentially a very complex polysaccharide, comprised mostly of galactose, arabinose, rhamnose, and glucuronic acid. There is also a very small amount of protein: 18 different amino acids have been identified in acacia senegal, although only four of them comprise more than 10% of the protein, and altogether these amino acids comprise only around 1-2% of the total gum; the other 98-99% is made of the aforementioned sugars.

Gum arabic readily dissolves in water to form highly concentrated solutions of relatively low viscosity, which is a consequence of the gum's highly branched very compact structure. Gum is heterogeneous in nature; at least three discrete components have been identified:

The first, which comprises the greatest part of the gum (~90%), has a molecular weight of about 250,000 and contains almost no amino acids. Analysis suggests that the structure of this component is globular and highly branched.

The second component, comprising around 10% of the total, has a molecular weight of 1,500,000 or so, contains about 10% protein, and is thought by most workers to have what they describe as a "wattle-blossom" structure, consisting of probably five globular lobes of carbohydrate, about 250,000 molecular weight each, which are attached to a common polypeptide chain. The predominant amino acids in this portion are hydroxyproline and serine.

The third component, comprising less than 1% of the total gum, contains 20-50% protein but is not degraded by proteolytic enzymes, suggesting that the protein is located deep in the center of the molecule, available neither to be attacked by enzymes nor to participate in crosslinking. The molecular weight of this component is about 200,000 and it is also highly compact. The predominant amino acids in this fraction are aspartic, serine, leucine, and glycine. [All information in this section gleaned from references listed under (1) below]

One set of researchers has described the structure of gum as being like a "twisted, hairy, rope" (reference 8) but most researchers find the description above, including the "wattle-blossom" structure, more plausible and more consistent with the bulk of empirical data (reference 11).

The active chromium species is HCrO4-. According to Sasaki, whatever dichromate or chromate compound is used (CrO3, (NH4)2CrO7, NH4CrO4, K2Cr2O7, or KCrO4) changes into Cr2O7 or a form very similar. In aqueous solution, Cr2O7-- exists in equilibrium with HCrO4- . At pH <5 the main species is HCrO4, at ph8 the main species is CrO4. The chromate ion CrO4 is not photoactive either in solution or in film according to most investigators.


What's the least amount of dichromate necessary to completely harden the gum?

How much dichromate is necessary to harden a given amount of gum completely is difficult to say; not that it's impossible to determine that but that it hasn't been determined, at least not to my knowledge. All I can tell you is what I know and what I think. What I think is that I want my gum hardened so hard that it can't be damaged by dampness or other possible assailants in the future. I live in a damp climate and am constantly reminded of what dampness can do to things, so maybe I think about that more than others might. But that's another reason why I prefer using saturated ammonium dichromate: when my gum prints are finished, the gum is so completely crosslinked that almost nothing can faze it, as I demonstrated above. I'm told that some gum printers prefer to print "soft," meaning that their gum is still somewhat soluble after development. They like the previous layer to "melt" a little when adding subsequent layers. I can see the appeal of that, in some way, but I can't imagine framing a print on which I knew that the gum was still partly soluble. But that's just me.

I do know that there is considerable excess hexavalent (unreduced) chromium left after my gum is completely hardened, so obviously not all the dichromate that I use goes into the process. But that's true even when dilute dichromate is used.

According to Kosar (4) "The tanning theory has never succeeded in explaining the most remarkable feature [of dichromated colloids]: that colloids containing only very minute amounts of dichromates can be completely hardened, providing that the exposure is sufficiently long." He speculates that perhaps this means that each chromium atom is used several times in a sort of optical sensitization mechanism. Whether that is a reasonable explanation I can't say; but my first instinct is to doubt it; perhaps a better explanation is that it takes a very small amount of chromium to crosslink a given amount of gum. According to Sasaki [] it takes 24 atoms of chromium to harden a molecule of gelatin of molecular weight of 40,000. I would like to know, but don't, how many atoms of chromium it takes to harden a molecule of gum, which would have a molecular weight of about 375,000. But the point is that it probably doesn't take much.



(1) P.A. Williams, O.H.M. Idris, and G.O. Phillips. "Structural analysis of Gum from Acacia Senegal (Gum Arabic)". In Cell and Developmental Biology of Arabinogalactan-Proteins. Edited by Eugene A. Nothnagel, Antony Bacic, and Adrienne E. Clarke. NY: Kluwer Academics, 2000.

P.A. Williams amd G.O Phillips. "Gum Arabic." In Handbook of Hydrocolloids, edited by G.O. Phillips and P.A. Williams. Cambridge (UK): Woodhead Publishing, 2000.

P.A. Williams and G.O. Phillips. "Gum Arabic: Production, Safety and Physiological Effects, Physicochemical Characterization, Functional Properties." In: Handbook of Dietary Fiber. edited by Susan Cho and Mark L. Dreher. NY: Marcel Drekker, date unspecified.

(Thanks to Pete Williams of Wales for making me aware of these helpful articles through his website, and sending me one of them through the mail.)

(2) B. Duncalf and A.S. Dunn. Journal of Applied Polymer Science, 1964, 8:1763

(3) Mannivannan, G. Changkakoti, R, and Roger A. Lessard. "Primary Photoprocesses of Cr(VI) in Real-Time Holographic Material: Dichromated PVA." Journal of Physical Chemistry, 1993, 97:7228-7233.

(4) Kosar, Jaromir. Light-Sensitive Systems: Chemistry and Application of Nonsilver Halide Photographic Processes.NY: Wiley, 1965.

(5) Udy, Marvin J. Chromium: Chemistry of Chromium and its Compounds. Volume 1. An American Chemical Society Monograph. NY: Reinhold, and London: Chapman & Hall, 1965.

(6) McLaughlin, G.D. Encyclopedia of the Shoe and Leather Industry. Chicago: Hide and Leather Publishing Corp, 1941.

(7) Galinsky, A. Biochem Journal 24: 1706-1715.

(8) Qui, W., Fong, C., and Lamport, D.T.A. Plant Physiology, 1991, 96:848-855.

(9) Shuttleworth,

(10) The reference that is given everywhere for this quote: (J.M. Eder. British Journal of Photography, 25:150, 1878) does not contain the quote nor does it offer a description of the chemistry of dichromated colloids. My efforts to track down the correct reference have been in vain.

(11) A. Bacic, G. Currie, P. Gilson, S.I. Mau, etc. "Structural Classes of Arabinogalactin-Proteins" In. Cell and Developmental Biology of Arabinogalactan-Proteins." Edited by Eugene A. Nothnagel, Antony Bacie and Adrienne Clarke. NY: Kluwer Academic, 2000.

All material copyright Katharine Thayer; all rights reserved.