Gum arabic desensitizes the non-image areas of a lithographic plate: it adsorbs onto the grained aluminum oxide surface and holds a water film there, so those areas take water and reject ink. That single job is why gum has stayed in fountain solutions, plate gums and plate etches for as long as offset lithography has existed.
The problem lithography has to solve
Relief and intaglio printing separate image from non-image by height. Lithography does not. Image and non-image areas sit on one surface at the same level, and the press has to keep ink on one and off the other using chemistry alone.
The image areas are the oleophilic coating left after imaging: they wet with ink and repel water. The non-image areas are the bare plate, grained and anodized to give an aluminum oxide surface with real surface area on it. Aluminum oxide is hydrophilic to begin with, and that on its own is not enough. Ink is a persuasive material, the plate runs at speed against a rubber blanket, and a bare oxide surface will start taking ink in the open areas within a short run.
What the gum does about it
Gum arabic is a highly branched polysaccharide, an arabinogalactan whose chains carry acidic sugar residues, with a minor protein component bound into the structure. The acidic groups are what make it an anionic polyelectrolyte, and they are what stick.
Put the gum in acidic water against an aluminum oxide surface and the acidic groups adsorb onto the metal oxide. The rest of the branched molecule projects out into the solution, where the hydroxyl-rich polysaccharide is strongly hydrophilic and carries a water layer with it. The result on the plate is a molecular film: bound at the oxide, water-loving on the outside, and thin enough that it changes nothing about the plate geometry.
Ink meeting that film meets water, not aluminum. The film is what turns a merely hydrophilic surface into one that actively rejects ink through a print run.
It is also consumed. Every revolution the plate cylinder passes the blanket, the ink train and the dampening rollers, and every revolution some of the film goes with them. Gum in the fountain solution is what replaces it, continuously, for as long as the press runs.
The second job, off the press
A plate coming off a press with no protection loses its desensitized surface. Gum applied at the end of a run, buffed down to a thin dried film, holds that surface through storage and lets the plate come back and run without a fresh treatment.
That is the same chemistry in a different vehicle, which is why gum arabic turns up across a pressroom line: in the fountain concentrate, in plate gums and finishers, in plate etches used to bring back an area that has started taking ink, and in gum-containing rinse steps after a solvent wash.
Where the gum sits against everything else
A conventional fountain concentrate is a small formulation with distinct jobs in it, and mixing them up is where trouble starts.
| Component | Job | What happens without it |
|---|---|---|
| Gum arabic | Forms and renews the hydrophilic film on the non-image areas | The open areas start taking ink as the run goes on |
| Acid and buffer, commonly citric acid | Holds the pH the system is designed around against drift from paper, ink and make-up water | pH wanders, and the gum, the ink and the plate all behave differently at the ends of the range |
| Corrosion inhibitor, commonly a benzotriazole | Films the copper and brass the circulating solution touches | Yellow-metal attack in the recirculation and dampening hardware |
| Wetting agents and solvents | Lower surface tension so the solution spreads in a thin even film | The solution beads and dampening becomes uneven across the plate |
| Preservative | Controls growth in a water-based system carrying a natural polysaccharide | The concentrate spoils in the tank |
The gum and the acid are frequently confused for one another because both are in every conventional concentrate and both are talked about as pH-side chemistry. They are separate. A 25 percent solution of gum arabic Type 4880 or 4915 sits at pH 4 to 5 on its own, which makes it compatible with an acid system, and it still needs the acid and its buffer to hold the operating pH.
Diagnosing which one is at fault
Ink appearing in the open areas points at the desensitizing film: gum level, gum quality, or dampening that is not reaching the plate. A weak or dropping-out image points the other way, at gum where it should not be. Corrosion in the hardware is the inhibitor's problem, not the gum's.
Request a SampleThe failure modes
Scumming. Ink taking in the non-image areas, printing as a dirty background. The desensitized film has broken down or was never fully established, and the fixes run through fountain solution feed, gum concentration and plate condition.
Tinting. A faint even ink haze across the open areas, distinct from scumming because the ink got there through the water rather than directly. Ink has emulsified into the fountain solution and is being carried back onto the plate. This is an ink and water balance problem that the solution chemistry either aggravates or damps.
Blinding. The image area losing its ink receptivity so the image prints weak or drops out. Gum that reached the image areas and dried there is a classic cause, most often from a plate gum applied heavily and not buffed down before the run.
Roller stripping. Ink rollers refusing to carry a proper ink film because their surface has taken water instead. Gum deposits building up on the roller surface are one of the routes to it, and it is one of the reasons a fountain concentrate is a balance rather than a maximum.
All four are formulation-sensitive, and none of them is fixed by adding more gum.
Kibbled, spray-dried, instantized
The three forms of gum arabic put the same molecule on the plate. What separates them is how the material gets into solution.
- Kibbled. Raw gum broken into lumps, never taken into solution. It has to be ground and hydrated, and it carries the insoluble matter that came in with the crop.
- Spray-dried. The gum is dissolved, clarified and dried, so the insolubles are removed upstream. The powder is fine, which makes it prone to lumping if it is added to water too fast.
- Instantized. The dried particles are agglomerated into clusters, so water penetrates rather than sealing the surface. The specification wording is fast soluble in cold water, forming a colloidal solution.
Gum arabic Types 4880 and 4915 are both purified agglomerated spray-dried powders, country of origin Germany. What changes when a plant moves between these forms is covered in moving a fountain solution off kibbled gum arabic.
One row on the specification is worth reading before you compare two grades. Specific optical rotation identifies the botanical species: dextrorotatory is Acacia seyal, levorotatory is Acacia senegal. Seyal gives a thinner solution than senegal at equal gum content, so it stays pumpable at a higher concentration.
Frequently asked
What does desensitizing mean in lithography?
Desensitizing means making the non-image areas of the plate reject ink. Image and non-image areas sit on one surface at the same level, so the separation is chemical: the image areas are ink-receptive and the open areas are made water-receptive. Gum arabic is the material that makes the open areas hold water.
How does gum arabic bond to the plate?
Gum arabic is an anionic polysaccharide carrying acidic sugar residues. Those groups adsorb onto the grained, anodized aluminum oxide of the plate surface, and the rest of the branched molecule projects into the solution where it is strongly hydrophilic. The layer is thin and tenacious, and the dampening system renews it on every revolution.
Can too much gum cause a problem?
Yes. Gum that reaches the image areas and dries there makes them reject ink as well, which shows up as blinding: the image prints weak or drops out. Over-gumming is most common where a plate gum has been applied heavily and not buffed down before the run.
Does gum arabic replace the acid in a fountain solution?
No. They do different jobs. The acid sets the pH the system runs at, and the gum forms the hydrophilic layer on the non-image areas. A 25 percent solution of gum arabic Types 4880 or 4915 sits at pH 4 to 5 on its own, which is compatible with a conventional acid concentrate rather than a substitute for its buffer.
Does the form of the gum change the mechanism?
No. Kibbled, spray-dried and instantized gum arabic put the same molecule on the plate. The form determines how the material gets into solution, not what the dissolved gum does once it reaches the plate surface.
Where to start
P.A.T. Products supplies gum arabic Types 4880 and 4915 into lithographic fountain solutions and pressroom chemistry, along with sodium benzotriazole 40% solution for the yellow metals and citric acid for the pH. Type 4880 is the grade to open on.
Send the grades and volumes you buy today. We will match them, send samples with the specifications, and say where our values differ from the ones you are working to.
See gum arabic for fountain solutions