A conservation treatment to remove residual iron from platinum prints

Background

Platinum printing was a popular photographic process from the late 1880s until 1916 when, due to the rise in the cost of platinum metal, silver-based processes became more widely used.

In a platinum print, unlike most other photographic processes, the image is suspended in the surface fibres of the paper rather than supported on the print paper in an emulsion layer. Due to the unreactivity of the metal platinum images tend to be very stable. As a result the majority of the deterioration seen in such prints is usually associated with the supports, which are often yellowed and brittle, rather than the actual image. Residual iron deposited in the print paper due to inadequate processing may be one contributing factor to cellulose degradation and yellowing in platinum prints. Such staining is also aesthetically displeasing. An article by Helen Burgess (1991) on the use of chelating agents to remove metallic stains from paper artefacts was used for the basis of this research.

The project

The aim of this project was to develop a practical conservation treatment for the removal of residual iron from platinum prints using the chelating agent ethylene diamine tetra acetic acid (EDTA) in conjunction with sodium dithionite (Na₂S₂O₄) solution. Sodium dithionite, as well as enhancing the effectiveness of the EDTA, acts as a mild brightener. It was hoped that this treatment would reduce the yellowing in platinum prints and stabilise the print support by preventing further cellulose degradation.

The effectiveness of treatment was to be monitored using energy dispersive X-ray fluorescence (EDXRF) analysis and chromameter colour difference readings. EDXRF analysis would quantify the amount of iron removed from the prints and the colour difference readings, taken before and after treatment, would demonstrate if there was any significant change in the image during treatment.

The objects

Fig 1. Lady Portrait print after treatment (click image for larger version)

As the proposed treatment was experimental it was considered unethical to use photographic prints from the Museum collection as initial samples. An exploratory investigation into the preparation of improperly processed platinum prints proved unsuccessful, as a result four low value fin-de-siecle platinum prints were purchased to use as experimental material. The prints were chosen because they showed visible 'foxing', yellowing or staining. The objects also had to be physically stable to withstand the treatment procedure. The four prints were titled Family Portrait, Lady Portrait, Signed Lady Portrait and Woodland Scene.

Prior to analysis the prints were removed from their mount boards which were preserved for remounting after the images had been treated. This allowed the unhindered analysis of the images and the support paper. Treatment of the mounted objects would have reduced the effectiveness of the proposed treatment.

Initial analysis and treatment of the objects

Melinex protective sleeves were made for each of the prints and holes, 1 cm in diameter, were punched in the sleeves to allow specific areas of the prints to be analysed by XRF and a chromameter. The sleeves protected the prints during analysis and allowed the areas analysed to be located rapidly and accurately.

EDXRF analysis established that all the objects were platinum prints. Three of the four prints contained residual iron. The Woodland Scene was found to contain no residual iron. It was concluded that the staining visible on this print may have been due to a surface coating of Catechu, a vegetable toner. The print paper also contained paper fillers such as calcium and zinc.

Fig 2. XRF spectrum of iron levels in Lady Portrait print before treatment (click image for larger version)

The four prints were initially treated using the solutions given by Helen Burgess (1991). A 0.1 Molar solution was made using the EDTA disodium salt. The pH was adjusted to 6.5. To this sodium dithionite was added to produce a 2% w/v dithionite solution. The objects were immersed, face up in the solution for 20 minutes. The prints were supported on custom made Melinex cradles which enabled easy manipulation of the prints with the minimum of handling during treatment. This initial treatment showed little improvement in any of the prints and so the prints were treated for longer periods of time, up to 22 hours, in an 8% dithionite-EDTA solution with an adjusted pH of 8.5. The backing boards were also treated as they were found to contain high levels of residual iron.

Results and further analysis

Fig 3. XRF spectrum of iron levels in Lady Portrait print after treatment (click image for larger version)

On drying the appearance of the prints had dramatically improved, both the 'foxing' and yellowing in all the prints was reduced. EDXRF analysis showed that approximately 80% of the residual iron had been removed from the prints. Unfortunately the treatment had also removed all the fillers from the print paper. Colour difference readings of areas of the images before and after treatment showed that whilst the image areas were not affected the yellowed areas of the print had been altered significantly by the treatment.

Conclusions

The choice of stable objects was important as this allowed lengthy aqueous treatments to be undertaken. Although the yellowing and 'foxing' in the prints was dramatically reduced as a result of the treatment, it is not possible to say how long the bleaching effect of the sodium dithionite would last. EDXRF analysis demonstrated the effectiveness of the treatment in the removal of residual iron from the platinum prints treated.

References

Burgess, H D: The Use of Chelating Agents in Conservation Treatments, The Paper Conservator (15) 1991 p.36-44
Gent, M: An Investigation of a Practical Treatment for removing Iron Stains in Platinum Prints, Final year Research Project, RCA/V&A Conservation Course, 1993
Rees, J E: An Investigation of Photographic Processes in Prints of the Late Nineteenth and Early Twentieth Century with Emphasis on the Platinum Image, MPhil Thesis, RCA/V&A Conservation Course, 1993