A new type of photo processing uses no silver and requires no messy chemical baths. The technology could prove cheaper and more environmentally friendly than today's silver-based standard. And with its speed and high resolution, the process might be ideal for printing pictures from digital cameras.
Standard photographic film and paper is covered with crystals containing positively charged silver ions. Light causes some of these ions to absorb electrons to form neutral silver atoms. When the film is washed in a "developer," these atoms trigger a chemical reaction that creates many more neutral silver atoms. This snowballing reaction colors dyes in the film, forming an image. The image is then projected onto a piece of photographic paper, and a similar chemical process produces the final photograph. During both film and paper processing, the silver must be washed away before the reaction goes too far, and that requires more messy and expensive chemicals.
A much simpler method has now been developed by John Marshall, Stephen Telfer, and colleagues from Polaroid Corp. in Waltham, Massachusetts, Their two-layer photographic material contains no silver and requires only a mild toasting. In the first layer, light breaks down a compound known as an iodonium salt to produce tiny amounts of a strong acid. The material is then heated, which releases a chemical from the second layer. Combined these make much more of a weaker acid, which then colors a dye that makes the image. The process even turns itself off: The heat also releases a base from the second layer, which moves in to neutralize the iodonium salt. The chemists stacked six layers to make full-color images, as they report online in Science  today.
The process isn't nearly sensitive enough to light to replace camera film, Telfer says, but it produces sharp images and should develop faster than an inkjet prints. So it might make an excellent coating for photographic paper, he says, especially for printing pictures from digital cameras. The technology should also cut costs, he says, because the developing requires fewer chemicals and less expensive machinery.
The technology might also have industrial applications, such as making the templates to print newspapers, says Douglas Neckers, a photochemist at Bowling Green State University in Ohio. Researchers have been looking for practical alternatives to silver-based technology for decades, but the Polaroid team has employed some particularly clever chemistry, Neckers says: "This brings to the table some new ways of thinking about this problem."