BobWicks@aol.com
Date: 01/13/02-10:10:13 PM Z
In a message dated 1/13/02 10:09:03 PM Eastern Standard Time,
robert@RobertSchaller.net writes:
> Subj: Re: Dichromate and diazo
> Date: 1/13/02 10:09:03 PM Eastern Standard Time
> From: robert@RobertSchaller.net (robert)
> Reply-to: <A HREF="mailto:alt-photo-process-l@sask.usask.ca">alt-photo-process-l@sask.usask.ca</A>
> To: alt-photo-process-l@sask.usask.ca
>
>
>
>
> on 1/12/02 10:56 PM, Judy Seigel at jseigel@panix.com wrote:
>
> >
> >
> > On Sat, 12 Jan 2002, robert wrote:
> >
> >> on 1/12/02 9:42 AM, BobWicks@aol.com at BobWicks@aol.com wrote:
> >>
> >>> Dave:
> >>> Diazo salts is the most recent application to replace bichromate that I
> have
> >>> heard of. I do think the light sensitivity is slower. It is currently
> >>> being
> >>> used in the silk screen industry and it reacts in the same manner that
> >>> bichromates do on amino acid chains. As you may know bichromated salts
> make
> >
> > You may be mixing apples & bananas here... The diazo used for silk screen
> > is (I would assume) used for making the screen, not the final image, which
> > would be some kind of ink. It's my understanding that the diazo fades...
> > or at least the early books said that. I would also take the fact that
> > there's no entry for diazo in Chris James's *very* comprehensive book as
> > due to some reason besides ignorance. Diazo is I believe used as a direct
> > printing process, for commercial use where archival isn't an issue, but
> > not in silk screen.
> >
> > All that remains in the gum print is pigment & hardened gum arabic, said
> > to be as archival as the paper, as platinum, even, assuming the pigments
> > are archival.
> >
> > Judy
>
>
> There are some interesting points coming out here, and I am very interested
> to know how they all add up. Bob Wicks' claim that (as I understand it)
> Diazo can be used to photolytically cross-link gum in a similar same manner
> to the way in which dichromate cross-links gum is extremely intriguing,
> indeed, it would be the foundation of a new process, the gum-diazo print.
>
> Katherine Thayer's observation that
>
> > It is chains of complex polysaccharides, not amino acids, which
> > crosslink to harden, or insolubleize, the colloid.
>
> certainly accords well with the observation that gum arabic is mostly
> carbohydrate and very little protien -- I've never heard a gum suggested as
> a nutritional protien source, whereas albumin (egg-white), casein, and even
> gelatin are.
>
> My general question here is, what are these photo-cross linking mechanisms?
> Are they described anywhere, say, in the way that Michael Ware explains
> Cyanotype chemistry in his book (is this already out of print?)? Such a
> thorough explanation would be extremely helpful, and would be of great
> sevice in addressing questions like Katherine's:
>
> >Clarification:
> >I was speaking here of gum rather than of colloids in general, since the
> >discussion had been about gum printing. I don't know much about the
> >makeup of other colloids but it certainly seems possible from a
> >theoretical standpoint that amino acids could be involved in the
> >crosslinking in processes involving gelatin, casein, and albumen. And
> >maybe that's the key to the questions we've struggled with before, about
> >differences between gum and other processes. Just a thought...
>
> >Katharine Thayer
>
>
>
>
Hi Katharine Thayer:
I do not pretend to know everything there is to know about gum printing. I
have been doing this kind of work since 1954 and I am still learning. The
term Gum Printing is a very misused term. Even thought gum refers to gum
Arabic. As I stated previously, exposed emulsions that have most any organic
base sensitized with Bichromate will form amino acid chains where it is
exposed to light. Apparently there are some out there that dispute my
statement. Let me explain a little further. My very first experience with
this was accomplished with Knox Gelatin sensitized with Ammonium Bichromate.
Where light struck the emulsion, it became resistant to being soluble. This
same area also had a much higher melting point. I taught this at college
level and my students even did photographs by sensitizing the casein from
powdered milk. One of my colleagues Prof Barry Barnhardt did some original
research on this and wrote a paper on it which I tried to send to a group on
the Alternate Photo list, however I find that I goofed and it was
inadvertenly sent in XIF format. I don;t think most people use this so
tomorrow I will try to send it in JPG, which I think most can read.
I will add your name to the list of recipients tomorrow. Also this week I
will send out some material on Diazo from a company in Schaumberg, IL. The
group can argue it all they want, but results give the proof. Now I will say
this, the application was in reference to silk screen photo stencils. If
some in this group would like documentation that is published, I refer them
to The Gum Bichromate Book by David Scopic, available from Light Impressions,
Rochester, NY. He gives a rather straight forward explanation of how
bichromate colloids work. Put this together with the Barnhardt article that
I will snd out tomorrow, should give everyone so very good insights on this
process.
Bob Wicks, Professor Emeritus, Art & Photography
Harrisburg Area Community College
This archive was generated by hypermail 2b30 : 02/15/02-11:47:41 AM Z CST