Thursday, October 18, 2012

Spectral Analysis of Two Orotones

A positive image produced on a glass plate with a gold-tone pigment applied to the plate’s emulsion side over the developed image is known as an orotone, and is a photographic process that was most prominent during the late 19th century through the 1940’s, during a time when the aesthetic use of bronze power pigments in works of art and décor was prevalent.

Due to high cost, gold was never used as a backing pigment, and bronze pigment mixtures with copper and zinc metals were used instead.  The color tone of the pigmented backing material is dependent on the amounts of copper and zinc in the mixture, as more copper will exhibit a truer bronze tone, and the more zinc added will shift the color to a luminous yellow-gold tone.[1]  With their high degree of reflectivity, transparency and brilliant gold tone, orotones (also known as Curt-tones, Doretypes, and goldtones) seem to exude the appearance of a varnished painting rather than a photograph. 

The photographic collection at GWAC contains two orotones; one with a positive image on a glass plate, and the other image resides on a polymer film.  Using the Bruker Tracer III-V+ XRF spectrometer, both pigmented backing materials were analyzed in order to determine their elemental compositions.

“#4 Moonlight on the Lake” (stamped in red ink on the back of the frame containing the orotone):  The positive image is on a traditional glass plate and has a bright yellow sheen within the backing pigment.  Elements of zinc (Zn) and copper (Cu) were detected, and originate from the brass pigment.  Interestingly, arsenic (As) is also a detected element, and originates from the glass rather than from the pigment.  Silver (Ag) and strontium (Sr) elements are also included in the spectrum, and are components within the photographic image.


Cliffside Orotone:  This orotone consists of a positive image on a plastic film, rather than the traditional glass support.  Due to the incorporation of the polymer support material, this orotone must be more contemporary than “#4 Moonlight on the Lake”.   Similar to the previous orotone, Cu and Zn peaks are observed, confirming that the pigment on the orotone's reverse is also a brass alloy.  A silver peak from the image side is the only other element observed in the spectrum.  

[1] Richard Stenman, “Initial Investigation into Orotone Photographs”, Topics in Photographic Preservation, Vol. 14 (Photographic Materials Group, The American Institute for Conservation of Historic & Artistic Works), 2011, 268.

The Broad Tonal Range of Platinum Prints

Spectrum 1

Platinum prints are neutral black with some brown present in the mid-tones.  The sensitizer for platinum prints  consists of ferric oxalate and platinum salts.  Upon exposure to light, the ferric oxalate reduces to ferrous oxalate.  When the paper is submerged in the developer, ferrous oxalate reacts with the platinum salts.  The platinum salts reduce to metallic platinum and form the resulting image.  

Compared to other photographic processes, platinum is more resistant to deterioration because it is chemically less active.  The greatest concern facing platinum prints is the deterioration of the paper support (often a result of insufficient clearing or washing). Residual iron (Fe) degrade the cellulose in paper fibers and can lead to yellowing, foxing, decreased contrast, and increased fragility.  In Spectrum 1, the platinum (Pt) peaks decrease significantly from the areas of maximum (blue) to minimum (red) density.  Small peaks of nickel (Ni) and zinc (Zn) are also present.

In the spectra below, a gradual decrease of the platinum peaks can be observed.  Spectrum 2 compares maximum (red) to middle (blue) density.  Spectrum 3 compares middle (red) density to minimum (blue) density.

Spectrum 2

Spectrum 3

Saturday, October 13, 2012

Distinguishing a Carbon Tissue Print from a Woodburytype using X-Ray Fluorescence

Carbon tissue prints and Woodburytypes are two different photographic processes, with each comprised of pigments suspended in a hardened gelatin layer, and it is notoriously difficult to analytically discern one from the other.  Both photomechanical processes require the image to be transferred from a negative to a paper support, and as they lack any of the major metallic elements observed in photographic materials, are primarily pigment-based prints.

One difference that separates the two process types is that carbon tissue prints require their backing paper to be sensitized in a potassium dichromate solution before being exposed to the light through the negative, leaving the print with a dichromated gelatin layer.  An image created from the Woodburytype process does not retain a dichromated gelatin layer, as the final image is made from pouring a liquefied gelatin (not dichromated) and pigment mixture into a lead sheet mold of the image, which is then pressed into a sheet of varnished paper.   

Carbon Tissue Print – No. 94 Ivory Black, from a 32-variety Colour Chart of Autotype Carbon Tissues complied by The Autotype Company, London. (c. Late 19th century).  Red spectrum = Dmax, blue spectrum = Dmin.  A chromium (Cr) peak set is observed in the spectrum, with the Kα peak located at 5.4417 keV.  The presence of chromium in the XRF spectrum confirms the dichromated gelatin layer, an essential component of the carbon tissue print.

Woodburytype – Miss Attalie Claire, from “The Theatre”, February 1861.  Photographed by Alfred Ellis, Upper Baker Street, London.  Red spectrum = Dmax, blue spectrum = Dmin.  Unlike the carbon tissue print spectrum, the Woodburytype lacks the chromium peak set, and therefore does not have a dichromated gelatin layer.  Lead (Pb) peaks are also present in the spectrum, and stronger in signal than the paper support’s lead peaks.  The presence of lead could be from the Woodburytype photomechanical process, when the dimensional matrix of gelatin and pigment is pressed into a lead sheet mold, creating an intaglio impression of the image.