In almost any discussion of inks for letterpress printing, strong opinions will be expressed in favor of one or the other name brands of inks, and even more in whether one should use oil-based, rubber-based, or acrylic inks. The industry is fortunate that such choices are available for particular types of work and to match various modes of working. The following discussion will deal not with the name brands, but rather with the various types of ink used in letterpress printing today.
Components of a Printing Ink
Traditional textbooks in printing technology divided the components of printing inks into three parts: vehicle, pigment and binder. In this discussion one more component will be added to the mix, additives.
The vehicle of an ink is the component which allows the ink to flow, and allows ink to spread and transfer from roller to roller and from roller to plate (in the case of letterpress, whatever raised surface is used as an image carrier). Traditionally vegetable oils have been used as the vehicle in inks. Linseed oil, the expressed oil from the flax seed has had the longest history in this role. With the advent of petroleum refining, mineral distillates have reduced the role of linseed oils in inks, but most conventional inks for letterpress use have at least some linseed or other vegetable oil in the vehicle component. Soya bean oils have been used for some time as an alternative for linseed oil, as they share similar characteristics. Whatever is used as a vehicle must have some drying characteristics which can be altered or controlled to provide the proper “curing” of the ink on the various surfaces upon which printed images must be applied.
The major role of pigment is that of color and opacity in a printing ink. Combinations of various pigments may be used to provide the vast range of colors which can be printed. White and black pigments are used in colored inks to provide varying tints (lighter) and shades (darker) of the base pigments.
Pigments used in printing inks have changed through the years due to new technologies, availability of material, and increasing knowledge of their effect on people and the environment. One pigment which is still widely used in printing inks is carbon black. The source of this pigment has also changed over the years, but has always been the product of burning something to create a black substance or “soot”.
Pigments are chosen for their color appearance, but also for various properties they possess for resistance to ultraviolet rays, heat, solvents, water, etc. You will notice that inks can vary a good deal in cost for different colors as the pigments which color them vary greatly in the raw materials cost. Black is generally the least expensive of the ink colors as carbon black is a inexpensive colorant. Some materials which could prove useful as pigments are not suitable as they may react to the resins and varnishes used in inks, and the color can be greatly affected by contact with those other chemical components.
One example of a pigment which lost favor in recent history is chrome yellow. The process used in its production was hazardous to the employees involved as well as to the general environment. Chrome yellow is a lead chromate, and lead as a pigment component has been gradually excised from the printer’s palette as it has from the house painter’s. Lead also was used extensively in other colors to lighten them, and has been primarily replaced with titanium dioxide and zinc compounds.
Resins used in printing inks serve primarily as “binders”. This means that they serve to bind together the other components, and allow them all to come together to make a film which lies on top of the paper. The resins also serve to provide an adherence to the material upon which the inks are printed. The type of resin used is used quite often to describe the type of ink, and provides the basis of many discussions and expression of opinion as to what is the best ink type for letterpress printers to use. In the three most typically used inks, the resins used are; alkyd (oil-based), rubber and acrylic. There are other types of resin which can be used for specific characteristics, but these three mentioned are the most frequently appearing in letterpress formulations and in the offset-litho inks used by letterpress printers.
Additives and modifiers are used in printing inks to “dial in” the characteristics of the ink to make it more suitable for the job at hand. For instance wax can be added to increase the rub resistance of an ink used for packaging applications. Most inks for letterpress printing come ready-to-print without need for additives to be added. Some printers do, however, like to “tweak” their ink for various reasons, and a discussion of the additives which can be added can be valuable to the printer for troubleshooting ink problems on the press.
There are three major additives which can assist the printer in tailoring ink to the stock or to the nature of the image being printed. The most often used of the additives is a drying agent. This most generally reacts with the oxygen in the air to crosslink the resins used in the ink, and helps to form a tough layer of ink on the surface of the substrate. It generally takes very little of this additive to do a good job. Too much can actually inhibit the crosslink action. The driers used most often are salts of cobalt and manganese.
Another oft-used additive by letterpress printers is magnesium carbonate (chalk). The chalk changes the “length” of the ink and is used to modify the varnish to make it “shorter”. This means that it assists with combating the tendency of the ink to “string out” as the ink film is separated, and allows the ink to be applied easily by the rollers to the very surface of the image and upon printing, to allow the ink film to break once the print is made and the plate begins to back away from the substrate, forming a more distinct edge to the image. Magnesium carbonate can also be used to add opacity to an ink, although addition of opaque white ink is more generally used for this application and is much better suited for that purpose.
Gel Reducer is a product which reduces the “tack” of an ink. It finds use when printing on very soft paper stock which tends to “pick”. Picking is when the paper fibers are pulled away as the plate or type pulls away from the paper stock. The ink is tacky enough to separate fibers from the paper and these fibers can get mixed in with the ink or will tend to fill small open areas in the image, requiring frequent cleaning during a run.
What Type of Ink Will Work Best?
Most modern letterpress printers utilize inks which have been put on the market primarily as inks for offset lithography. There are some specialized suppliers who have tailored an ink line for letterpress printing, but the majority of modern letterpress printers use offset litho inks for their production. This is perfectly fine as the ink composition and, for the most part, the physical specs for the ink vary little from what would be specified for letterpress printing inks.
In general, the inks designed for small offset presses and duplicators work well and have the required body and tack for use in letterpress without modification. Some of the inks designed for high-speed web and sheet-fed offset litho presses have less body and lower tack than would be ideal for letterpress. That is not to say these high-speed inks would not work, they just may require a bit more modification by the press operator to work as well. One of the advantages of these high-speed inks, however, is that the pigments are generally more finely ground and the ratio of pigment to vehicle may be higher.
There are some inks which are designed for other applications which, in certain instances can be used for letterpress work. Each would have characteristics which fit its expected use, but make it difficult to use for all letterpress applications.
One of these inks is water-based linoleum block printing or relief ink. While it is never wise to generalize, most of these inks lack the body required for distribution in the more automated inking systems utilized in platen and cylinder letterpresses. They were truly designed for hand inking fairly broad image areas and slower speed operation where the paper is peeled from the surface of a plate or image slowly and not separated in one quick pull. The inks were designed for use in home and school situations where cleanup must be done with water and soap cleanup, and the inks work passably well for classroom use. If water based materials are used, make certain the rollers on the press are rubber or synthetic rubber as conventional glue composition rollers will swell when exposed that directly to water, and will become useless.
The same lack of body or stiffness of ink is found in even oil-based inks designed for linoleum block printing and many a beginning letterpress printer tries to utilize these inks because they are readily available in art supply stores in small-sized tubes. It would seem this would be an economical way to purchase inks for letterpress, but the nature of the ink is such that only mediocre results will be obtained when printing from type or plates with any amount of fine detail.
Some screen-printing inks have found use in letterpress operations for printing on textiles as they were designed to print on textiles with some degree of permanence and wash-ability. Once again, this ink was not designed to be distributed in the manner of letterpress inks, and, while it may work, it will not give the same degree of quality that an ink designed for more automated printing will allow.
It is critical when using any ink not designed for letterpress or offset litho, to make certain to do a thorough job of cleaning the inks off the press and rollers, and to put something on exposed metal parts (in the case of water-based inks) which will keep them from rusting between uses. A clean oil evenly spread will do a good job of coating open metal parts.
As has been mentioned above, drying agents may be added to inks to speed the curing of the ink on various types of substrates. Some ink types are naturally prone to particular drying characteristics if additional drying agents are not added.
Rubber-based inks for the most part are formulated to be slow drying and “drying” more by absorption into the paper substrate more than drying on the surface of it. This feature of the rubber-based inks make it the ink of choice for many schools as a press may stay inked up for proofing all day and not have an issue with drying on the rollers. Some printers leave a rubber-based ink on the press overnight or for extended periods without clean-up, and enjoy the use of these inks on account of their forgiving nature. Leaving ink in the press overnight leaves it open to collect dust and particulate matter from the air, and even though the ink stays “open”, it does change in character somewhat when allowed exposure to the air for a long time, and some of the vehicle in the ink can evaporate, making the ink stiffer than its original condition. Even after almost drying on the press, the rubber-based inks remain easy to remove from the press rollers with standard press wash or solvents.
Oil-based inks, on the other hand, have varying drying characteristics dependent on the amount and type of driers used in the ink formulation. Even without added driers, the alkyds used as binders are naturally drying oils (linseed, soya, etc.) which, given enough time, will dry on the substrate to a hard finish. Driers can be added to enable the drying of these inks on non-porous surfaces like plastics and clear-coated paper. Because they are naturally drying, it is important to clean the press after use, and certainly not leaving the rollers and press inked up overnight. Special sprays are available from ink suppliers to spray on the ink drum or plate and rollers to enable leaving the press inked up for a longer period, but such sprays may slightly change the ink character when first rolled up after spraying until more ink is added.
If a piece is designed to be later run through a laser printer, a letterhead, for instance, a hard drying oil-based ink should be used as rubber-based inks can be picked up by the imaging drum on the copier or printer and ruin subsequent copies or imaged sheets.
Instructors tend to the slow drying oil and rubber-based inks to eliminate the worry about students leaving the press inked up and the subsequent hard work of cleaning with stronger solvents, or more effort. Many other printers choose to use oil-based inks on account of their better drying characteristics and performance on the press. Certainly there is not one ink which will satisfy all types of printers, or work with all types of substrates.
Ink is one of those things that each individual printer must experience on her own, and in her own time to see what suits both the printer’s style and the quality of the finished printed piece.
- Printing Ink Technology, E.A. Apps, New York: 1959, Chemical Publishing Co.
- Printing Inks: Their Chemistry and Technology, Carleton Ellis, New York: 1940, Reinhold Publishing Co.
- Rubber Base Plus Colors M.S.D.S, Isandia, NY: 2006, Van Son Holland Ink Corp. of America
- CML-Oil Base Plus Colors M.S.D.S., Islandia NY: 2003, Van Son Holland Ink Corp. of America