Ferricyanide Toning of Cuprotypes - Double Your Pleasure

.

A
.

  • 5
  • 1
  • 218
Promethea Moth

D
Promethea Moth

  • 2
  • 1
  • 177
On The Nest

D
On The Nest

  • 3
  • 1
  • 183
Reception area - Spain

A
Reception area - Spain

  • 3
  • 3
  • 309

Recent Classifieds

Forum statistics

Threads
189,622
Messages
2,644,450
Members
97,312
Latest member
NB25
Recent bookmarks
0

nmp

Member
Joined
Jan 20, 2005
Messages
1,872
Location
Maryland USA
Format
35mm
I inadvertently discovered that my 2% K ferricyanide solution that I have been re-using as a toner for cuprotypes contained ferrocyanide as well. Then it dawned on me, of course ferrocyanide is a by product of the reduction of ferricyanide. For example, in case of blue toning of silver gelatin, ferricyanide bleaches (oxidizes) silver, forming silver ferrocyanide and potassium ferrocyanide. The latter forms Prussian blue promptly with ferric ammonium citrate that is present in the toner. In that case, the silver ferrocyanide may be fixed out (via bromination) or be redeveloped or sulfide-toned to end up with additive toning rather than replacement toning – boosting the density of the print and also shifting the color to green.

In the case of cuprotypes, ferrocyanide toning is based on the same principle but follows a slightly different path. Here, copper(i) is oxidized to copper(ii) and copper(ii) combines with ferrocyanide, the now reduced form of ferricyanide, to make Hatchett’s brown. Potassium ferrocyanide is also a by-product. However, unlike blue toning, it simply goes into the solution, accumulating in the toner. So what if we were able to capture this ferrocyanide and get it to make more copper ferrocyanide? By my chemical balancing, for every 4 ferricyanide and 4 Cu(i), we get 1 copper ferrocyanide and 2 potassium ferrocyanide molecules.

4K3Fe(CN)6 + 4Cu+1 → Cu4[Fe(CN)6]2 (s) + 4K+ + 2K4Fe(CN)6

So if we add CuSO4 to the toner:

2K4Fe(CN)6 + 4CuSO4 → Cu4[Fe(CN)6]2 (s) – another molecule of Hatchett’s Brown.

Of course, CuSO4 will form cupric ferricyanide instantly in the toner – a kind of muddy brown compound. So we borrow the technique used in copper toning of silver gelatin – using a citrate to bind up the copper so it can co-exist with ferricyanide. I found the typical copper toner (like the one from Photographer’s Formulary) too slow for cuprotypes (more so if it was to be brush or rod-coated which I prefer) so I modified it to a more concentrated single-use version.

Next the recipe and pictures.

:Niranjan.
 
OP
OP

nmp

Member
Joined
Jan 20, 2005
Messages
1,872
Location
Maryland USA
Format
35mm
.....continued from Post #1.


Modified Copper Toner

Solution A:
CuSO4.5H2O 20%, 1 part
Sodium Citrate 20%, 4 parts

Solution B:
Potassium Ferricyanide 20%, 1 part

Add solution B to A slowly while constantly stirring.

This toner is designed for one-shot use using a brush or glass rod. Apply on the print, leave it for 5-10 minutes with occasional additional passes to keep the liquid from drying up, adding more solution if needed.

Pictures

Paper: Canson XL

1. Initial cuprous/thiosulfate Image:

Before Toning.jpg

2. Comparison of straight ferricyanide (fresh, not re-used) vs copper toner:

After Toning.jpg

Looks like the modified copper toner does give a nice boost (OK may be not double) to the red intensity. The hue itself is quite different – much more reddish than the more brownish tone of the ferricyanide only toner. The contrast seems to have increased as well. Slightly more work required to make up the toner (A and B to be added only before toning) but it's worth the effort if you are looking to intensify your print.

:Niranjan.
 
Last edited:
OP
OP

nmp

Member
Joined
Jan 20, 2005
Messages
1,872
Location
Maryland USA
Format
35mm
An update:

Since these posts, Jim Patterson over at the altphotolist pointed out that ferricyanide in the copper toner would also be reduced by thiosulfate in the image, producing ferrocyanide which then in presence of copper sulfate would form copper ferrocyanide as well. So there are two potential sources of extra copper ferrocyanide in the case when a copper toner is used, not just the one I described in Post #1. Luckily I was not completely wrong (whew!) - the first mechanism is also operative even when there is no thiosulfate present as in the case of bromo-cuprotype where the bromide is the counterion to Cu(i).

There is a repercussion to this parallel reaction - which is depletion of thiosulfate from the image which is required for the next toning step with an acid. There, the acid dissociates thiosulfate into sulfurous compounds which then convert a copper ferrocyanide to copper sulfide - as per the current understanding of why the image darkens significantly on acidic treatment. So if a good portion of thiosulfate was used up in boosting the copper ferrocyanide level in the step before, we should expect less of a dramatic change with acid. That is indeed the case - there is a boost in overall density but the tone does not change to near neutral as it is in the case when straight ferricyanide is used for toning. In spite of that, the overall Dmax is still much higher in the copper toned print. Here is the final comparison:


2023-01-04-0001&2+.jpg

A = ORIGINAL CUPROUS/THIO IMAGE
B= STRAIGHT FERRICYANIDE TONING
B'= COPPER TONER
C=STRAIGHT FERRICYANIDE + 1% SULFONIC ACID
C'= COPPER TONER + 1% SULFONIC ACID


Full (COPPER TONER + 1% SULFONIC ACID) image:

Final Print.jpg

The image still retains the reddish quality of copper ferrocyanide or Hatchett's brown.


:Niranjan.
 
Last edited:
Photrio.com contains affiliate links to products. We may receive a commission for purchases made through these links.
To read our full affiliate disclosure statement please click Here.

PHOTRIO PARTNERS EQUALLY FUNDING OUR COMMUNITY:



Ilford ADOX Freestyle Photographic Stearman Press Weldon Color Lab Blue Moon Camera & Machine
Top Bottom