Appendix A: Thoughts about OBAs (fluorescing optical brighteners).

You can read a beautiful and very detailed analysis of OBA content in a large selection of inkjet papers by Ernst Dinkla. This comes about due to a debacle in the middle of the last century in which Kodak marketed some of the earliest RC papers, which were used by default world-wide, but in just a few years pictures printed on these papers started to degrade. Fluorescing optical brightening agents were identified as the main culprit and have been the bete noir (bete blanc?) of the print industry ever since.

However, much has changed since then, and paper mills have long since marketed pure white photo papers that have far better permanence capabilities. Whether better OBA chemicals have been identified or smaller amounts are now being used, I don't know. What I do know is that I see little correlation between OBA content and longevity in any of the above papers as measured by Wilhelm Imaging Research. Given a responsible manufacturer that uses acid-free materials, I'm seeing longevity as much more a consequence of the ink set used than of the paper used. Specifically, HP and Canon inks are testing much higher in longevity on papers with OBA content, than Epson inks on OBA-free papers.

But another issue related to OBAs is metamerism (hues that change depending on which type of light source the print is viewed in). In theory, the violets and blues of a typical RC print (lots of OBAs) would shift when viewed under tungsten vs halogen vs fluorescent vs daylight. I'm seeing no sign of that even with RC prints. And the smallish amount of OBAs present in the papers discussed here that have any should present little such problem.

If you are printing for your own enjoyment, my own take is that you can choose papers with whatever whiteness/brightness you like best. Unfortunately, if you are printing for sale and your customers include connoisseurs and gallery/museum types, you may very well meet up with the persistent prejudice that pure white papers are crass and inferior to the buff/cream colour of traditional, unbrightened stock. Papers like Hahnemühle Fine Art Pearl and Ilford Gold Fibre Silk are probably about as white a colour as will pass muster with those who are steeped in the traditional arts.

Appendix B: Inkjet paper characteristics

Substrate material. RC papers are made from paper pulp. Art papers are made from either 100% cellulose (lignin-free wood pulp) or cotton rag fibre.

Size. Photo paper is commonly available in rolls of a certain width, like 17 or 24 inches, or in cut rectangular sheets of certain standard sizes. In the metric world those sizes are typcially A4, A3, and A2. In North America sizes are the far less practical letter, 11x17, 13x19, and 17x22 inches.

Weight. This is specified in grams per square meter (GSM). This tells you the heft of a sheet of paper, but not much else. Numbers like 188 and 305 GSM are common.

Thickness. Measured in mils or millimetres. To convert: multiply mils by 0.0254. RC papers are usually 10.4 mils / 0.264 mm thick. Art papers are usually 3 or 4 mm thick.

Stiffness. If there is a measure for this I haven't seen it. In general cellulose papers are stiffer per a given thickness than rag papers. Stiffness is great to prevent creasing or sagging in a frame, but during printing it can be a show-stopper, which is why RC papers are very thin no matter how large the sheet size happens to be.

Opacity. Expressed as a percentage of total opacity, like 99%. It's always surprising to hold a thick sheet of paper up to a light and see that some light shines through. I only find opacity to be of importance when prints are bound in a book.

Coating. While we know that barium sulfide and calcium carbonate (as a white powder) are ingredients in many coatings, that's about all we know, since the formulae are competitively guarded secrets. In general, inkjet papers are coated with a chalky powder, containing at least calcium carbonate on the ink-receptive side to keep the ink from soaking into the substrate and thus reducing colour vividness. Non-matte papers have an additional clear coat over the powder. This serves to keep the powder and ink from rubbing off but also increases colour vividness in the same way that wetting a piece of wood makes the grain and colours stand out. The word micro-porous has been used to describe coatings by a variety of manufacturers for several years now.

Appropriate ink types. The nature of the coating, not the substrate, determines whether a paper is appropriate for dye inks and/or pigment inks, as well as ink sets containing a matte black vs. a photo black. Essentially, a paper with a clear coat is intended for photo black ink, while paper without a clear coat (matte finish) is intended for matte black ink. This is a distinction that only applies to pigment inks, not dyes.

Whiteness/colour. Expressed as a percentage of pure whiteness, like 99%. Pure cellulose or pure cotton rag has a buff/cream colour. Using a white powder coating helps to whiten a paper to a certain degree, but beyond that whitening chemicals called optical brightening agents (OBAs) must be added to the substrate when more brightness is desired.

Sheen. Measured as a percent of totally reflective gloss. Verbally expressed, an uncoated paper or a powder-only coated paper is matte (non-reflective) in finish. Clear coats can have a variety of sheens from near-matte to full gloss.

Texture. No common measure that I know of. Verbally, I'm reporting indentation separation/fineness, indentation depth, and pattern regularity..

Dmax. This is a technical term to indicate how black the darkest black achievable on a particular paper is. Problem is that it depends as much on the ink used as the paper itself, although of late pigment photo blacks seem to have caught up with each other and are now at least fairly close to dye black. Again, a glossy clear coat helps in achieving maximum blackness. For reference, a Dmax over 2 is considered a very good number.

DR (dynamic range): is the difference in the brightness range of colours that can be achieved between the paper's whiteness and its Dmax blackness. (If you know the brightness and Dmax of a paper you know the DR, so there's probably little point providing all three numbers.)

Gamut. This is the measure of how vivid/saturated each colour of the spectrum can be on a given paper. Again this is entirely dependent on the ink set used as well as the paper, but if a given paper has gamut limitations for one ink set it likely has them for all ink sets. Gamut numbers are quite complex. While gamut was a serious issue a few years back and remains a differentiator for matte papers, for clear coated papers the differences then to be small.

pH/Acidity. The pH of a paper needs to be absolutely non-acidic or the paper will simply eat itself over time. pH of 7 is neutral; less than 7 acidic, greater than 7 alkaline. Many or most of these papers also have a buffering compound in the coating to neutralize environmental acidity.

Longevity. Measured in years. This is another factor that varies with the ink set used as well as the paper itself. Paper can yellow with time, it can react to environmental pollutants like natural gas and ozone, it can react with the inks applied to it causing them to fade or discolour. But equally, dye inks have the advantage of really dark blacks but change colour over a much shorter time (up to a couple decades) than do pigment inks (up to a couple centuries). Measuring longevity is a black art about which you can read here; Wilhelm Imaging Research.