How “The Key to Griffin’s Canberra” was made
Now for a bit of an explanation about what type of object the ‘lost’ Griffin image (NAA: A710, 51 [Competitor number 29 Walter Burley Griffin - Perspective] View from summit of Mount Ainslie [Part D]) is.
As far as we can ascertain from studying and analysing the image, it is a FERROGALLIC print. This means it is a positive print (dark lines on a light background) made from an original drawing on translucent tracing paper (see below for more information). Ferrogallic prints were a handy process as they produced a positive print from a tracing without needing an intermediate negative step.
The ferrogallic process was developed in 1860 and was in use at the same time as the much more common (and recognisable) blueprint process. Both print types have iron as the basis of their images.
There were two different methods for making a ferrogallic print. One was used in the early stages of the development of the process, and a more advanced system came into existence after 1900; our print was produced in 1912*, so it is fairly safe to assume it is the later method. In this process gelatine or gum arabic is mixed with ferric (Fe3+) salts (usually iron chloride or iron sulphate) and an organic acid (tannic or gallic acid) and applied to a sheet of paper. This sensitized paper is exposed under a tracing to an ultraviolet (UV)-containing light source. The UV light reduces the ferric salts to a ferrous state (Fe2+) which is soluble in water.
After exposure, the paper is washed in water which releases the organic acids, creating an acidic development bath. The water soluble gelatine (or gum arabic) and ferrous salts are washed out of the print, while the acid bath coverts the ferric salts to a ferric gallo tannate pigment which stays behind in the paper and forms the image. This image is almost the same (chemically) as iron gall ink.
When the prints are first produced they have a black image on a white background, but as they age, the image fades and the paper darkens (thanks to the acidic bath mentioned above) and the contrast between image and paper becomes very low.
The acidic processing bath also contributes to making the support brittle. Paper is made up of cellulose molecules, which, simply put, are chains of glucose molecules joined together by an oxygen bridge. The residual acids in the paper support attack these bridges and cause the cellulose chain to break apart. As the chain breaks down, it loses flexibility. This decrease in flexibility manifests in the paper sheet as brittleness. The paper fibres can no longer bend and flex, but break instead.
Ferrogallic prints have never been known for their longevity. A life span of 30 years* has been predicted for them, and our print is now 100 years old. No wonder the poor thing is fragile!
*Kissel, E. and E. Vigneau, Architectural Photoreproductions: A Manual for Identification and Care, Oak Knoll Press, 1999, p46.