RE: Coating prints with polyurethane wood finish

Dmax increased by 1 stop means the darkest part of the image is 0.3 unit
darker for the coated specimen, when compared to the uncoated one (same
coating solution, same paper, exposure timing the same, processed the
same day).

The polyurethane I'm using is water based - can provide the CAS numbers
of the ingredients later.

Thanks,
Loris.

-----Original Message-----
From: Ryuji Suzuki [mailto:rs@silvergrain.org]
Sent: 02 Şubat 2006 Perşembe 08:04
To: alt-photo-process-l@sask.usask.ca
Subject: Re: Coating prints with polyurethane wood finish

What do you mean by "Dmax increased by 1 stop" in the ziatype test
print? Did you mean that 1 stop less exposure to reach Dmax, or you
found 0.3 unit higher Dmax?

Anyway, I see two potential problems in using polyurethane. One is the
durability of polyurethane itself. Another is how effective it is in
providing a barrier against gaseous oxidizing agents.

I am no expert in this area but I think only some water based
polyurethane is recommended for archival artwork on paper.

The second issue can be tested by peroxide fuming test. I don't really
see a better way to test for this. I've been using this to test efficacy
of my toning solutions for silver gelatin prints.

Regarding pollutants coming from the back of the paper, I bet some
papers are better than others. I'd expect paper with generous amount of
hydrophobic internal sizing material would provide better barrier.

Ryuji

From: Loris Medici <mail@loris.medici.name>
Subject: Coating prints with polyurethane wood finish
Date: Tue, 31 Jan 2006 20:13:37 +0200

> http://duke.usask.ca/~holtsg/photo/current/0273.htm
>
> One more thought
> Coating the the front of a colloidal silver image (Kallitype, Vandyke,
> Argyrotype) with polyurethane may protect it from pollutants also -
> since it seals the front of the paper. So maybe one would not need to
> tone (therefore alter the image color) the print in order to increase
> its permanence(?). What do you think?
>
> TIA,
> Loris.
>
> P.S. I know the pollutants may attack from the back of the paper, but
> I think the effect may not be as strong(?).
>
Received on Thu Feb 2 00:17:42 2006