44th Annual Meeting – Photographic Materials Session, May 16, "Separation Anxieties: Freeing photos adhered to glazing or to each other" by Barbara Lemmen and Emma Lowe

Title image
This talk was split into two sections, beginning with a presentation by Emma Lowe examining the nature of the adhesive bond formed between blocked photographic prints (prints stuck together in a block) and glazing. This was then followed by Barbara Lemmen providing an overview of existing treatments used by photo conservators to tackle this issue.
The Experiment
Lowe’s research aimed to assess the nature of a blocked bond, and determine the factors that affect the bond e.g. surface finish, glass coatings, age of the bond.
She tested artificially aged samples of both glossy and matt prints adhered to three different types of glass including glass with no coating, glass with UV surface coating, and glass with integral optical coating. Half the samples were aged for 80 days, the others for 150 days at 30°C (86°F) and 50% RH.

Blocked glossy and matt DOP samples
Blocked glossy and matte DOP samples

Experiment Results
At 80 days, it was found that all the samples could be popped off the glass. At 150 days, 75% of the samples tore upon removal; glossy prints were even more likely to tear than matte samples.
XRF analysis of the gelatin layer pre and post-experiment showed that there was migration of the elements in and out of gelatin. At 80 days, there was a slight increase in Si in matte samples. At 150 days, there were changes in elemental composition. FTIR showed compositional changes in gelatin and glass pre and post experiment. SEM/EDS on cross sections of the artificially blocked samples showed the migration of elements across the blocked bond. Changes in elemental composition were seen within the gelatin colloid.
In short, Lowe’s experiment determined the following points:

  • The initial adhesion between matte finished prints to glass is stronger, but aging leads to a stronger bond between glossy finished prints and uncoated glass.
  • UV coating on glass acts as a barrier to adhesion
  • There is a migration of materials between 80 – 150 days; electrostatic attraction between the glass and print converts to a covalent bond, explaining the increased bond strength. At 80 days 100% of samples separated without damage, at 150 days 75% of samples tore from the glass.

Treatment Overview
Lemmen then presented a variety of techniques used for the separation of photographs from glazing or blocked prints. For prints on glass (depending on the sensitivities of the object), the introduction of heat or moisture allows the gelatin to swell; this can be done via local humidification, the targeted application of aqueous solutions, steam or immersion (less common). Mechanical methods include removing the object from glass with a blade, or breaking the glass using a glass cutter.  Dry heat or freezing with dry ice can also be used as a form of separation.
Blocked prints can be peeled apart mechanically, or they can be swelled with moisture. While trying to peel the sections apart, the local application of aqueous solutions to the adhered areas can aid separation. Overall humidification can be done on fiber base only. Splitting the RC prior to immersion was also discussed. 

Immersing RC prints
Immersing RC prints

what works reliably
Treatment options for fiber base and RC prints

In conclusion, it was recommended to prioritize treatment of prints adhered to glass. In the occasion where prints need to be framed without a window mat or spacer, use UV filtering glass face-in to reduce the possibility of adhesion.