Archival Properties of Varnishes used in Wetplate

[mdarnton]

In my field, a "sandarac" varnish would be sandarac only. If that's not the meaning here, I wish a better and more accurate term could be used for this discussion, a recipe, even.

A couple of observations from my own field:

-Modern (often acrylic) varnishes have fared very poorly in the art restoration field, losing clarity due to microfracturess, and their inability to be reversed after their faults were discovered, resulting in a relatively serious art disaster. There are recent varnishes that claim to avoid problems and are regarded as safe. . . . just as acrylics were when they were originally used, before their reputation went bad. I wouldn't touch unproven synthetics with a ten-foot pole for at least 50 years, until the actual long-term results are known. A lot of restorers are jumping on that bandwagon, but I"m not one of them.

-Sandarac may be neutral and it's an important component in reversible varnishes, but used alone it has a strong tendency to abrade in really ugly ways.

-Shellac alone proves to be one of the most durable things out there, but when ozidized over time becomes impossible to remove except by abrasion . . . but it doesn't develop optical defects. It is, however, forever.

--Shellac and sandarac together, about 50:50, has been one of the mainstays of violin restoration for almost a century because of it's durability and reversibility. Since it's reversed with alcohol, carefully, that might be a problem in photo applications from what I'm reading here.

I don't pretend to know how any of this applies to picture varnish, but it's worth knowing, and so I thought I'd insert it.

[Mark Sawyer]

For what it's worth, the University of Buffalo did a Pyrolysis gas chromatography-mass spectrometry
(py-GC-MS) analysis of the varnishes used on 221 historic tintypes, and found the photographers were using all sorts of different formulae.

"Each detected resin appears in historical recipes, but just 24% of the samples have varnish layer constituents consistent with published tintype varnish recipes. Forty-four percent of the tintypes have varnish constituents consistent with formulations recommended for other collodion images, but the varnishes of the remaining samples have no direct literature equivalents. The preponderance of shellac- and Pinaceae-based varnishes suggests that these correspond to inexpensive commercial varnishes, but tintypists may have developed their own preferred mixtures or simply used what was at hand"

"Conclusions

This study represents the first large-scale analysis on the manufacture of tintypes and provides a unique glimpse into the working habits of photographers of the nineteenth and early twentieth centuries. The poor correlation between recommended tintype varnishes and the materials detected on the historical samples serves as a warning to art historians, conservators, and conservation scientists that practices and processes described in the contemporary literature were not necessarily followed by the artists of the day."

http://static1.1.sqspcdn.com/static/...GqxyU20FvfI%3D

[bob carnie]

Just photoshop differences, I think -- I see no fading or surface defects.

[goamules]

Correct, I used two different cameras, even different formats, between the 2009 and 2020 shots. Differences in white balance and all, and not very scientific, I should have scanned the image. But in hand, with reflections and all, seems to be a good way to demonstrate my hypothesis. It's a black and white process, don't worry about the color cast, the plate hasn't changed one bit in 11 years.

[paulbarden]

"Each detected resin appears in historical recipes, but just 24% of the samples have varnish layer constituents consistent with published tintype varnish recipes. Forty-four percent of the tintypes have varnish constituents consistent with formulations recommended for other collodion images, but the varnishes of the remaining samples have no direct literature equivalents. The preponderance of shellac- and Pinaceae-based varnishes suggests that these correspond to inexpensive commercial varnishes, but tintypists may have developed their own preferred mixtures or simply used what was at hand"

Fascinating! Thank you for that information. I'm not at all surprised.