Why Photo Paper is very special May 06, 2008 15:16 by John Sollars
A piece of paper is a rolled-out mix of wood pulp and cloth, right? And photo paper is wood pulp and cloth with varnish on top. Yet, printer and ink makers try and tell us ink and paper form a ‘technology system’ which governs the look and quality of our photos. Surely they’re just trying to justify the prices they charge?
Actually, having been around paper plants and into ink research labs, a lot of what they say is true. Inks and papers are developed together, so that a particular ink, which may work reasonably on a wide range of different papers, will work best on the paper it was designed to be used with.
- brighter colours
- less spread of ink dots
- faster drying time
- better light resistance
- better ozone resistance
Just about all the things you and I value in a photo.
Inks look as if they might have a fair amount of chemistry involved in making them, but it’s often harder to appreciate the role the paper plays. We’re used to reading and using all kinds of paper day-to-day, so the idea of one which is a composite of lots of different materials and built up in layers to try and get the best of all worlds may need some explanation.
Plain or copier paper used for office printing is generally made from straight wood pulp and is rated 80gsm. If you look at a sheet of plain paper under a magnifying glass, even under a low magnification loupe, you can see the minute fibres of which it’s made. Any liquid ink, such as most inkjet inks, is likely to soak in between and along these fibres.
It’s important to realise that inkjet inks are composed of two parts: the colourant and the carrier. The colourant is the particles of dye or pigment which make up the text or picture you’re printing, while the carrier is the liquid in which the colourant is suspended. These days the carrier is usually water, but it can be a solvent for specialist applications.
The key characteristics of dyes and pigments make them suitable for different uses. Dye molecules are normally around 2nm (2 x 10-9) in size, so small they can easily fall between the fibres of the paper and soak into it, along with the carrier. Pigments are around 50nm to 150nm and are more likely to sit on the surface of the paper. Because of their size, pigments are also more resistant to ultraviolet light attack, the main mechanism by which inks fade over time. So why don’t all ink makers use pigments?
Pigments don’t normally give such vibrant colours as dyes, and this has to be balanced against their robustness. Using pigments on plain paper, though, where most people will be printing black text, makes a lot of sense. That’s why many inkjet printer makers now include two black inks: a pigmented ink for printing black text on plain paper and a dye-based one for mixing with the coloured inks when printing photos. You can’t easily mix dye and pigment inks in the same print.
The main reason for applying coatings to papers is to improve the way the ink settles on them and to help it resist fading. It also reduces drying time, when printing images which take a lot of ink. Even so, the simplest of coatings still produces paper poorly suited for photographic images.
Many ‘inkjet’ papers have a simple coating which makes them better than photocopier stock for printing day-to-day text and graphics, because there’s less chance of the ink soaking into the paper fibres and giving a fuzzy or ‘feathered’ look. These papers often have improved brightness, too, which gives higher contrast with black ink.
To get the best out of an inkjet photo print, you need to increase the number of layers and introduce specialist coatings to handle particular aspects of the printing process. A modern photo paper may well have seven or more different layers to provide, among other things: a gloss finish, protection from UV and ozone, absorption of the carrier, improved drying time and resistance to sticking, one sheet on another.
Within the general category of photo papers, there are two distinct types: swellable and porous. Swellable coatings do actually swell. The surface coating gets thicker as the carrier and the ink soaks into it.
The idea is that the ink is sucked into the paper, which gives it extra protection from light and gas attack. As you might guess, if you took in the difference between dyes and pigments, swellable paper is best suited to dye-based inks, but even with dyes, you shouldn’t expect prints to be dry enough to handle safely until about an hour after printing. They can take over a day to dry completely.
Porous paper, on the other hand, works better with pigmented ink, as it’s designed to let the ink carrier soak into the paper and leave its payload of pigment on the surface. Because most of the liquid carrier soaks away fast into the porous sub-layers, images printed on porous paper are usually touch dry within a second or so and completely dry in a few hours.
Some photo papers also have deliberately rough back-coatings. This is so that when you’re printing several photographs in a batch, each new print to leave the printer doesn’t stick to the one below in the output tray, which may not have finished drying.
As with most technologies, things are not cut and dried and by varying the constituents of the different layers in a photo paper you can make a sheet which is supremely good at working with dye-based inks or one which is ideal for pigments.
In between, it’s possible to make a paper which is better than reasonable for both and this is why you’ll find papers which claim to be suitable for all the major brands of inkjet printer, even when some traditionally use dyes, while others rely on pigments. It doesn’t invalidate what the ink and paper makers tell you, but simply proves that compromise still has benefits.