Re: pigment

Jack, Katharine,
I went back to my general chemistry textbook and I quote here
"From Greenwood, “Chemistry of the Elements”

Addition of K4[Fe(II)(CN)]6 to aqueous Fe(III) salts produces intensly blue
precipitate, Prussian blue. The X-ray powder and Mosbauer spectrum of this
are the same as those of Turnbull’s blue which is produced by the converse
addition of K3[Fe(III)(CN)6] to aqueous Fe(II). By varying the conditions
and proportions of the reactants, a whole range of these blue materials can
be produced of varying compositions with some, which are actually colloidal,
described as soluble Prussian blue. They have found application as pigments
in the manufacture of inks and paints since their discovery in 1704 and, in
1840, their formation on sensitized paper was utilized in the production of
blueprints. It appears that all these materials have the same basic
structure."

As far as what is relevant to cyanotype, it is made by reacting potassium
ferricanide K3[Fe(CN)6 with with Fe(II) salts that are a result of light
reduction of ferric ammonium citrate. Cyanotype is then Turnbull's blue.
That is apparently true only if you know how it was made, once made it can
not be distinguished from Prussian blue, as both pigments are the same in
chemist's eye.

The quote might also shed some light on pigment stability. Cyanotype is made
from same, well defined chemicals since its inception and most likely
results in a similar pigment composition every time (various post-treatments
might alter it a little). Cyanotype seems to be quite stable. On the other
hand Prussian blue can be made from a variety of starting materials,
different conditions, etc and the resulting pigment can be quite variable.
It is quite reasonable to expect that the stability of the pigment (Prussian
blue) will vary from manufacturer to manufacturer or even with the same
manufacturer from batch to batch if they are not made in exactly the same
manner.

Word of advice, use thalo blue for your tricolor gums, test your prussian
blue if you intend to use it and have fun with it all.
My 3c for the day.

Marek, Houston

>From: Jack Fulton <jefulton1@comcast.net>
>Reply-To: alt-photo-process-l@sask.usask.ca
>To: alt-photo-process-l@sask.usask.ca
>Subject: Re: pigment
>Date: Mon, 21 Nov 2005 21:15:53 -0800
>
>Yes, the 'blue' of the cyanotype is Prussian Blue. The cool thing about it
>is that it was the first true blue that was what one might call
>inexpensive. It came around in the early 1700's, so Herschel knew all
>about it when developing the cyanotype.
>
>One interesting anecdote for today's climate of terrorism is that Prussian
>Blue and Potassium Iodide can be ingested to aid in the removal of
>radioactive materials from the body. Both of these are chemicals of our
>medium. The FDA approval of Prussion Blue is either coming up or was
>recently approved. The U.S. is stockpiling capsules of it.
>Jack Fulton
>
>
>
>On Nov 21, 2005, at 8:49 PM, Katharine Thayer wrote:
>
>>
>>
>>
>>On Nov 21, 2005, at 6:55 PM, Dave Rose wrote:
>>
>>>Isn't Prussian Blue the same pigment/chemical that forms cyanotype
>>>prints?
>>
>>Is it? I guess I've never heard or thought about what the final product
>>is that forms a cyanotype print, but Prussian blue is hydrous ferric
>>ferrocyanide or feriammonium ferrocyanide, is that what it is?
>>
>>To my eye, Prussian blue pigment has a greener cast to it than most
>>cyanotypes I've seen, or than pthalo, which to my eye has probably the
>>purest cyan hue for tricolor of the blue pigments available, followed by
>>ultramarine. And to me it's a duller blue than either pthalo or
>>ultramarine, and for those two reasons I didn't suggest it for tricolor.
>>But I can't say for sure that it's not good for tricolor, because I've
>>never tried it. I guess I was also influenced by someone who wrote the
>>other day that he had tried Prussian for tricolor and it didn't work well
>>at all.
>>
>>I feel another test
>>kt
>>
>
Received on Tue Nov 22 16:34:19 2005