Blocking is the phenomenon of prints becoming adhered to each other front to back as in a stack, face-to-face as in a photo album or to smooth surfaces in contact with the prints such as glass in frames or plastic sleeves in enclosures. Ferrotyping is similar to blocking, but instead of bonding between prints or to adjacent materials, the softening of the print surface causes the gloss of the print to be degraded.
It has long been known that the gelatin layer used in traditional photographic prints can bond to glass in framing packages and many types of plastic sheeting used in storage enclosures. This happens when the glass transition temperature of the print’s gelatin layer is exceeded.
IPI performed the experiments described below to determine if digitally printed photographs were as likely to ferrotype or block as traditional silver-halide photographs. Preliminary testing at IPI indicated that 30oC (86oF) at 90%RH could replicate the blocking effects often seen with traditional photographic prints. These conditions are not designed for accelerated aging, but instead to simulate a worst-case scenario where a print may be stored, even briefly, in a hot, humid environment.
Sixteen different unprinted digital papers were tested from the following groups: inkjet specialty-photo (both porous and polymer), inkjet fine-art, dye sublimation (printed without image to include overcoat), chromogenic (processed without image), and coated digital press. Several examples from each type were tested when possible. Additionally, 10 different printed digital photographs were tested. The printed photograph samples consisted of uniform areas of mid-tone, process gray. All of the inkjet samples were printed on the same paper (plain white office). This was done to ensure that the paper did not contribute to, and thus confound, the colorant test results. This was not possible for the dye sublimation and chromogenic which were both printed with the manufacturer recommended glossy papers. The digital press prints were created using coated glossy stock. Several examples of each type were tested when possible.
The digital print materials were tested facing the following surfaces to evaluate their sensitivity to ferrotyping or blocking:
- Polyester sheeting
- Polypropylene sheeting (unprinted papers only)
- Polyvinyl chloride sheeting
- Soda-lime framing glass
- Print back
- Print face
The digital prints and test surfaces were incubated in direct contact and under a weight of 18gr/cm2 for seven days. At the end of the incubation period, the tests were taken out of the oven and allowed 24 hours to cool before being visually evaluated.
The traditional color photograph was the most sensitive to ferrotyping or blocking followed by the traditional black-and-white photograph. Almost all of the digital technologies were less sensitive to ferrotyping and blocking. Of the materials tested, the polymer coated inkjet papers were the only digital material to ferrotype or block. These inkjet papers were about as sensitive as the black-and-white traditional photograph. This makes sense because the polymer coating, like traditional photo coatings, is designed to swell with increasing moisture content. The traditional photographs and polymer inkjet prints were most sensitive to damage when stored face-to-face and secondly to glass.
The data for the printed papers showed generally the same trends as for the unprinted papers indicating that the colorants neither add to nor detract from a technology’s given propensity to ferrotype or block. The one difference was that PVC was the most likely to ferrotype or block to the surfaces of printed images as opposed to adjacent print faces or glass. PVC is already considered inappropriate for use in storing photographic images, and that recommendation can now clearly be extended to digital prints.
In general, the following conclusions were reached:
- When exposed to high humidity, digital prints are generally less likely to block or ferrotype than traditional silver-halide photographs.
- Of the digital prints, the polymer-coated inkjet photo paper was the most likely to block or ferrotype.
- Sensitive prints are at highest risk for blocking and ferrotyping when they are stored face-to-face as in a photo album. Storing prints in contact with glass presents the next highest risk.
- All prints are least likely to block when stored front-to-back as in stacks.