Gum arabic is the sap of the acacia plant. The plants are cut so that they will weep sap; the sap oozes out and dries in the air into clumps called "tears" which are harvested and sold, mostly for use in the food industry, but also for the likes of us gum printers. Two countries that export a lot of gum arabic are Sudan and Chad. The main differences between gums as we buy them are (1) the variety of acacia, acacia senegal being the most desirable by most accounts (2) the "sort" that the gum is purchased from. The "sorts" are equivalent to grades of purity. The higher the sort, the more expensive and purer the gum. The lower the sort, the cheaper, and also the more bark, branches, dirt, etc that will be mixed in with the gum arabic. (3) Also the color and quality of the gum is different from year to year, depending on the climate. My understanding is that the worse the conditions for the tree (drought, bad soil, etc) the better the gum.
As far as I can determine, there are standards of purity for food grade gum arabic, but no standards of purity for gum arabic used in printing-related applications. In searching for sources for good gum, I've found that some distributors of liquid gum believe that the label "14 degree Baume" is a standard of quality and that all gum arabic designated thus is the same purity and the same material. This of course is not accurate; this label simply designates a certain specific gravity. For all I know there may be crankcase oils that are 14 degree Baume, but that doesn't mean I'd want to print gum with them.
Pure gum acacia ranges in color from a light amber to a deep amber, is slightly cloudy and translucent when liquid, and in small amounts dries hard and transparent:
My favorite gum, which I can't get any more, is memorialized in this little disk. When I've been too tired after a printing session to clean up before I leave the studio, next morning there would be one of these little disks in the bottom of my gum-measuring vial. A squeeze of the plastic vial pops the disk out clean; it's an easy way to clean the vial. The disk of dried gum is as hard as clear plastic, and tapping it with a fingernail makes a hard sound like tapping hard plastic. It stays hard and dry even in quite humid air, but if you drop it in cold water, it dissolves completely within half an hour or less. You can see from the picture that where the gum is thinnest (the middle of the vial bottom is raised, so the gum is thinnest in that area) the hardened gum appears colorless, as it would in a finished gum print.
This is the substance that I know as gum arabic and prefer to use in my work; one of the side benefits of gum printing is working with this beautiful and fascinating substance.
There is another substance that has very different appearance and characteristics, that also goes under the label "gum arabic" sometimes also called "lithographer's gum" or "graphic arts gum" although not every gum that is so designated is of this type. Whether the difference is the "sort" or the variety of acacia, or whether (as I read somewhere but cannot verify) this gum is not technically gum acacia but from some other tree, I don't know, but this gum is very dark, heavy with sediment, and very gummy and viscous. Leaving some of this gum in the bottom of a vial overnight yields not a clear disk of dried gum arabic, but a black gummy mess that is almost impossible to clean out of the vial. Some gums of this type print as well, or almost as well, (depending on printer preference) as the pure gum acacia, and are cheaper, but I don't like them. For one thing, I mix colors by eye so I need to be able to see the color of the gum-pigment mixture as I mix it. A darker gum prevents seeing the colors accurately. But also, I just prefer the transparent amber quality of the pure gum acacia from an aesthetic viewpoint.; I like the way it looks and feels and behaves.
My tests indicate that as a rule, the gums of type 1 above print with longer, more nicely-gradated scales, while the gums of type 2 print with fewer, more widely separated steps. Since I like printing with very subtle gradations, particularly in the midtones and highlights, the prints made from type 2 are too high-contrast and too jumpy in tone for my taste, but they might just be someone else's cup of tea.
The best way, perhaps the only way, to ensure a consistent supply of good quality gum is to buy enough high quality gum powder from one batch to last your entire lifetime, and mix it yourself. While I prefer the convenience of letting someone else mix the gum, add the preservative, and ensure that the liquid is of the correct specific gravity, I do insist that the gum I buy be mixed directly by my supplier from powder, so that I know that there's nothing in the liquid but pure gum arabic and preservative.
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.
References:
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.
Qui, W., Fong, C., and Lamport, D.T.A. Plant Physiology, 1991, 96:848-855.
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.
Copyright Katharine Thayer; all rights reserved.