This post is not meant to "bump heads" with Michael's techniques, but I was interested in what possible effects the Marshall's extender might have in the long run. I belong to a photo conservation mailing group, of which members are, at times, conservators or students of photo preservation. Here is an answer I received, and I'm posting it with the permission of the author.
_______________________________________________________________

To: photoconservation@yahoogroups.com
From: "Gawain Weaver" <gmw216@nyu.edu>
Date: Thu, 2 Dec 2004 22:02:31 -0500
Subject: RE: [photoconservation] Marshall's Extender


Hendriks and Dobrusskin looked at the effects of various
inpainting/retouching media on salted paper/albumen/silver gelatin.
Marshall's Extender showed yellowing of medium in all 3, bleaching of silver
in 2 of 3, and silvering out in silver gelatin only. Similar results were
found with linseed oil. In any case, it certainly cannot be recommended for
conservation purposes.

Klaus Hendriks and Sebastian Dobrusskin. "The conservation of painted
photographs" In ICOM Committee for Conservation, 9th triennial meeting,
Dresden, German Democratic Republic, 26-31 August 1990: preprints: 249-254


Gawain Weaver

Good catch, Ed.

Thanks Ed, I will file that away for future referance

Mike

I appreciate your diligent research on this. Just a couple of quick questions, what exactly does it mean to have "bleaching of silver" or "silvering out?" I am not familiar with these terms. The other thing is are these immediate reactions or things that occur over time and what is the severity of the damage?

I will edit my post on the other thread to note that it can cause damage and the severity to which the damage can occur when you get back with me on this.

Thanks again,

Michael

When I scan silvered prints, the silvering seems always to be in the deep shadow areas and returns as a cyan artifact. The cyan is pretty distinct from the shadow color and I've had good luck selecting it with Select:Color. From there I desaturate and darken it to blend it back in.

Chip

what exactly does it mean to have "bleaching of silver" or "silvering out?"

Hi Michael,

First of all, I'm glad to see you didn't take this thread as an attack on you. That says something about who you are.

Secondly, I am not a conservator, or even a student of photographic preservation. but I do have somewhat of an interest in the history of photography and preservation, and much of what I have learned has been forgotten. :oldman:

I am not 100% sure of this, but I think silvering is due to the fixer (hypo) not being completely removed from the print with washing. Fixer contains dissolved silver that was not developed or subjected to light, and it is that silver that becomes visible. We know what it looks like, and there are chemicals that can reduce or eliminate the silvering. But for our purposes, copying with a camera and polarizing lights, or using techniques in Photoshop will suffice. Those wanting to preserve the original photo should consult a conservator. It is always best not to use anything directly on a print, unless it is the last resort.

I think Marshall's oils are *generally* accepted as being good, and not regarded as being an agressive agent for deterioration. Marshall's oils have been used for probably 50 years or so, and I don't think effects are immediate. Regarding the severity of long term effects, I don't really know.

Glad to have you on board with us.

Ed

Thanks for getting back on this so quickly, and thank you for your kind words. I like to think that it is better to be proved wrong and correct the problem than to continue being wrong.

I am also not a conservator, I am involved in the buisiness of image recovery. I work primarily in situations where people need the image on the print more than they need the print itself. I will note on the other thread that this can cause long term damage and it should be used with great caution or not at all, you know, weighing the pros and cons. The person I learned this trick from is also in image recovery and restoration, and I will pass this information back to him.

Thanks again,

Michael