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Sep 1, 1996 12:00 AM
Conductivity, one of the most misunderstood terms in lithographic printing, continues to confuse printers trying to use this control tool.
To help ease the confusion, the recent Web Offset Assn. conference provided a series of informal roundtables titled Printers Tech Talk. These popular sessions offered a forum in which printers could discuss problems with or ask questions of other execs or suppliers. Indeed, many good responses were provided. In addition, experts discussed the performances of several new products, as well as shops' experiences with the items.
During one conversation, a printer mentioned he had good luck running a fountain solution with a conductivity of 1500; another printer replied you could not run anything less than 2500, at least according to his plate supplier. It was evident that both printers attributed the lithographic properties of their systems to a conductivity number, when in fact nothing could be further from the truth.
Let's get back to basics. When Senefelder invented lithography by writing with a greasy crayon on a limestone block, he had to keep the non-image areas wet with water so the ink would transfer only onto the greasy image area and not onto non-image areas. He etched the limestone with acid to make it even more water receptive and, at the same time, discovered that by coating it with a solution of gum arabic it would retain its water receptivity even longer. That technology, with few modifications, lasts to this day.
It so happens that to keep gum arabic in solution and impart its water-loving properties to the plate's non-image areas, the gum needs to be in an acidic solution between 3.5 pH and 4.5 pH. That essentially is why we have acidic fountain solutions. When metal plates were introduced, first zinc and later aluminum, the surfaces still needed the protection of the gum and acid to remove any oxidation that may occur during downtime, which could cause scumming (printing in the non-image areas).
Then, some changes occurred. First, aluminum plates were anodized. That is, a thin layer of controlled oxidation was applied to the plate surface. This tactic decreased the plate's tendency to oxidize during press stops, so crews no longer needed to "gum" up plates feverishly during brief downtimes.
Gum arabic became expensive. Therefore, to protect the plate, vendors introduced alternative gums or colloids that would form a film once the solution was dried. These synthetic gums merely protected the plate; they did not impart the same hydrophilic properties gum arabic did. To compensate, the industry began using lower pH ranges or more acidic fountain solutions.
At the same time, firms introduced buffering agents to prevent wide swings in pH due to impurities that could enter fountain solution, such as paper dust, etc. The problem, then, was how to determine if the correct amount of solution had been added for optimum press performance.
Let's throw one more wrinkle into the equation - alkaline fountain solutions. These products were introduced into newspaper printing to overcome algae growth problems associated with groundwood newsprint. You could not control with pH the amount of concentrate added with these solutions as you could with acid solutions. Enter conductivity.
Conductivity is the measurement of how easily a current passes through a solid or, in the case of fountain solutions, a liquid. Resistance to conductivity is measured in ohms; how easily a medium conducts, therefore, is expressed as the opposite, or mhos. In Europe, the term Siemens is used in place of mho and, since these increments are very small, they are known as micromhos or microSiemens.
Pure, distilled water will not conduct electricity. It is only when impurities are added that the passage of an electrical current is allowed. The greater the number of impurities, the more conductive the water becomes. That is why hard water has a higher conductivity reading than one with lower totally dissolved solids (TDS), the latter term being the way water hardness is defined.
Suppose, for example, medium hard water registers a conductivity of 300. Now, add 2 oz. of fountain solution to a gallon of that water and take another reading - it may register 900. Add another 2 oz., and the reading might now be 1500. So, we can say that 4 oz. of fountain solution will produce a reading of 1200 over the water reading.
The latter is important since water hardness varies in different parts of the country. It is up to printers to determine if the level of fountain solution discussed in the previous paragraph will print satisfactorily. If it does, then 1500 becomes the shop's standard for printing. From that point on, the printer can monitor conductivity to ensure it remains at the desired level. Remember, any type of contamination will affect conductivity, whether it be paper dust, emulsified ink or other foreign substance, so conductivity is yet another way of monitoring the solution.
However, the number only applies to that grade of water and that particular fountain solution; change either and you must re-establish the conductivity level for the new conditions. That is why both printers were right in the earlier discussion - each reading (1500 and 2500) was correct for their particular conditions. Does that mean we can forget about pH altogether?
Not really. We still want to know how acidic our fountain solution is, whether we are using gum arabic or not. Too acidic and we risk counter-etching the plate, too little and the plate may become sensitized, particularly during a long run. Remember, conductivity only tells us "something" is in the water; it gives no clue as to what it is, nor will it indicate if the "something" is in any way useful to the lithographic process.
One further point. If, in the unlikely event you still use isopropyl alcohol in your continuous dampening system, remember that alcohol does not conduct electricity. Consequently, your readings will be entirely different if alcohol is present. In this case, establish the level of concentrate required before adding alcohol, then take another reading after its addition. Continue to monitor it at that level, bearing in mind you are not getting a true reading.
To sum up, conductivity is a useful tool to monitor the level of a particular fountain solution in a particular source of water. You then must determine what level is correct for your running conditions. The arbitrary use of conductivity readings means nothing if press performance has not been established. And water will vary from one source to the next so its conductivity also must be considered. Finally, keep in mind that any number of minerals dissolved in water will affect conductivity level; they don't all help the lithographic process.
TERRY SCARLETT, Contributing editor and president of Burntwood Industries, Inc., a consulting company specializing in inks and coatings