Printing and Design Tips: October 2022, Issue #255

Ceramic Decals for Decorating Pottery

What is the definition of printing? Or, put another way, what does printing not include? Keep this question in mind.

Earlier this week my fiancee showed me two ceramic pieces that were decorated with color and a gloss coating. The first is a female torso with lettuce leaves in strategic places and the words "Flathead Lake Lookout, Montana" written in black ink. The background color is a light pink flesh tone.

Based on my understanding from my fiancee, the background flesh color was probably added with a brush and applied to the unglazed ceramic, as were the brown and green pigments in the lettuce leaves covering the body. After a gloss coating of glaze (derived from the Middle English word for glass) was applied, the ceramic piece went into the kiln (at least once) to be fired at high heat.

While this is not strictly speaking a printing operation, since presumably the decoration of the piece was done individually with a brush, the kind of pigment used for the paint is also used in the actual printed decoration of glass and pottery. More specifically, frit-based inks (which include glass particles as well as pigment) are first applied to the surface of the ceramic. When these are heated to high temperatures in a kiln, the glass melts and becomes an actual part of the ceramic product.

This is very different from UV ceramic printing, which just applies UV ink to the surface of a ceramic piece. UV inks when cured will adhere to a non-absorbant surface like glass. However, since it does not actually bond with the glass, UV ink is more likely to eventually scratch off. But since digital (or regular) ceramic inks are actually fused to the glass or ceramic substrate, the surface decoration is a lot more durable.

Now these ceramic inks can presumably be applied in a number of ways, including ink jet (digital ceramic printing) and potentially (if not yet in all cases) from DTS (direct-to-shape) inkjet printers using ceramic (i.e., frit-based) inks. These direct-to-shape inkjet printers can print on irregular objects in part because the print heads never actually touch the substrate. Because of this the ink can get into all the nooks and crannies of an object. Plus, you have a lot more printable space than you would have with a clear label or shrink sleeve.

To take this one step further, my fiancee gave me a ceramic plate emblazoned with an image of a piece of cheese and the word "cheddar." What makes the piece of cheese interesting is the level of detail in the image of a French map on the printed wax shell encasing the image of the cheese. With a 12-power printer’s loupe I can see that the image is not only detailed but also a halftone.

This sample is actually printed, in the traditional sense. Based on my fiancee’s description (she is a sculptor, so she knows whereof she speaks), the halftone of the cheese image was transferred from a carrier sheet to the plate using a ceramic decal.

Based on my research, it seems that printed decals, which transfer an image to a ceramic piece that can then be fired to fuse the image to the clay, employ two methods of positioning the frit-based ceramic ink on the ceramic substrate. In both cases the backing sheet is first decorated ("wrong-reading," such that the image will be "right-reading" once it has been transferred to the ceramic piece). Interestingly enough, one way to actually print such a decal is with special inkjet equipment. (Unlike most inkjet work, this is a "dirty" process, leaving residue and powder, so you would want to dedicate such an inkjet printer to only printing digital, frit-based ceramic inks.)

So the two application techniques are as follows. In the first, the decal is first wet down. The wet decal is then placed against the ceramic prior to firing and then slid off, leaving the pigments of the printed image on the ceramic substrate (right-reading, since they have been reversed in the application process). Then the piece is fired in the kiln, and the transferred image becomes part of the ceramic.

The second way is to use a decal that is attached to the unfired ceramic but not completely removed. Once the clay product goes into the kiln, the thin decal backing sheet is burned away in the intense heat of the process.

So to answer the initial question at the opening of this article, what makes this particular operation a printing process is the way that by using a transfer sheet (as with a number of current processes such as dye sublimation printing) you can apply a very detailed image (like the print of a piece of cheese with an intricate French map on it) from one object (the transfer sheet) to another (the ceramic plate). And then by doing all of this using pigment containing embedded flecks of glass, you can produce a durable, decorated ceramic piece that actually has the coloration not "on," but permanently "in," the ceramic substrate.

Paper Weight, Caliper, and Bulk

To return to traditional ink on paper, here are a few terms that you should know to communicate your paper needs to your printer.

Paper Basis Weight and Basic Size

Different kinds of paper are weighed (and specified) at specific dimensions. For instance, 500 sheets, a ream, of 100# cover stock weighs 100 pounds at its basic size of 20" x 26", and 500 sheets of 100# text stock weighs 100 pounds at its basic size of 25" x 38". This is why 100# cover stock seems so much thicker than 100# text stock.

This is irrespective of the other sizes in which a printer can buy the same printing paper, such as 28" x 40" text stock (for a larger press). If you specify paper weight on a standard based on paper produced at a specific basic size, you can communicate with your printer articulately and precisely.

Or can you?

Paper Caliper

Coated sheets such as 100# gloss text will seem to be much thinner than uncoated sheets of the same weight. In fact, if you compare two coated or uncoated text or cover sheets of the same basis weight, you will find that they often do not exactly match in thickness. Why? Because some sheets are more intensely compressed between the stacks of metal calender rollers during the paper making process. This calendering process yields a harder paper surface but also a thinner press sheet.

So what can you do?

You can often specify paper in points rather than by basis weight. For instance, you can buy a 10 pt or 12 pt C1S (coated one side) cover stock for a perfect-bound textbook cover, and you will find that it will be 10 or 12 points when you check it with a micrometer. This is called its "caliper" as opposed to its weight. It is the actual measurement of paper thickness.

Paper Bulk

Another related term is "bulk." A high-bulk press sheet is very thick relative to its weight. For instance, a 100# uncoated text sheet will have a high bulk compared to a 100# gloss coated text sheet (due to its calendering). This would make it ideal for a paperback book containing all text and no photos or tint screens.

You can also specify 7pt stock for a business reply postcard that will meet the Post Office’s thickness standard for their automated mailing system. In this case you will know it will not be thinner than 7 pt regardless of its basis weight.

With all of this in mind, what’s the best way to avoid confusion when specifying paper for your print job? Give your printer a paper sample (printed or unprinted) to show him exactly what you want. Don’t rely solely on numbers (paper weight, opacity, brightness, whiteness, etc.) taken from the internet or a paper sample book.

[Steven Waxman is a printing consultant. He teaches corporations how to save money buying printing, brokers printing services, and teaches prepress techniques. Steven has been in the printing industry for thirty-three years working as a writer, editor, print buyer, photographer, graphic designer, art director, and production manager.]