The chemistry of kallitypes

[tnp651]

I'm trying to understand the chemistry of the process with a practical goal: to determine when it is no longer sensitive to light. I've read Mike Ware's treatise <http://www.mikeware.co.uk/mikeware/Iron-based_Processes.html> and learned the following:
1. The ferric oxalate in the sensitizer reacts with UV light to form ferrous oxalate in proportion to exposure. I surmise that the changed parts of the image are no longer light-sensitive, but the less-exposed parts remain so.
The rest is my speculation:
2. Ferrous oxalate quickly oxidizes back to ferric oxalate, so the silver nitrate in the sensitizer combines with it to form a more stable compound. Silver nitrate is itself light-sensitive, so safelights remain on.
3. Silver itself is subject to oxidation, so toning is necessary to replace it and any residual ferrous oxalate with a noble metallic substitute. Traces of silver nitrate remain in the print but they are not very light-sensitive.
4. Fixation removes all remaining silver, producing (with washing) an archival permanent print.

So: safelights only for development, brief incandescent inspection during toning, lights on once it's in the fix. Have I got it right?

 

[Andrew O'Neill]

I work entirely under a yellow bug light unless gold toning. After the print has gone through the citric acid clearing bath and rinsing, I flip on the room light (one of those 40W compact fluorescent bulbs), then tone. The light stays on for the rest of the process.

 

[tnp651]

I work entirely under a yellow bug light unless gold toning. After the print has gone through the citric acid clearing bath and rinsing, I flip on the room light (one of those 40W compact fluorescent bulbs), then tone. The light stays on for the rest of the process.

Thanks, Andrew.
Tom

 

[Jim Noel]

silver nitrate is not light sensitive unless in contact with an organic substance, or other chemicals.

 

[TheToadMen]

Kallitype is done with UV light sources. Lamps like LED don't emit UV, so you can work under dimmed "normal light" conditions. As long as you watch out for continuous UV light coming through windows, etc. you'll be fine.

 

[nmp]

2. Ferrous oxalate quickly oxidizes back to ferric oxalate, so the silver nitrate in the sensitizer combines with it to form a more stable compound. Silver nitrate is itself light-sensitive, so safelights remain on.
3. Silver itself is subject to oxidation, so toning is necessary to replace it and any residual ferrous oxalate with a noble metallic substitute. Traces of silver nitrate remain in the print but they are not very light-sensitive.

2. Ferric reduces to Ferrous on exposure. But the subsequent exchange with silver nitrate does not occur until it comes in contact with the developer as the ferrous oxalate needs to be dissolved in order to be able to react.

3. Any remaining ferrous oxalate after development can oxidize the Ag metal affecting the print longevity, hence the need to replace the latter with something that has a greater resistance to oxalate induced oxidation via toning: i.e. Au, Pt, Pd. However that does not do anything to the residual ferrous oxalate.


A very nice practical guide on Kallitype by Sandy king is here:

http://sandykingphotography.com/resources/technical-writing/the-kallitype-process

 

[tnp651]

Kallitype is done with UV light sources. Lamps like LED don't emit UV, so you can work under dimmed "normal light" conditions. As long as you watch out for continuous UV light coming through windows, etc. you'll be fine.

In terms of UV output, compact fluorescents probably emit enough UV to be worrisome, incandescents emit less, and LEDs emit almost none. Here's a test of each type:


From the responses, I gather that I'm safe to inspect the toning under intense (but brief) incandescent light. Thanks to all who responded.
Tom

 

[TheToadMen]

To avoid UV light: use LED lamps or common, ordinary(old type) light bulbs, don't use energy saving lamps, CFL, halogen lamps, tubes.

BTW: there are tube lamps now made with LED that are UV safe! Perfect for our alt-photo business so we can work continuously with normal lights on.

 

[tnp651]

2. Ferric reduces to Ferrous on exposure. But the subsequent exchange with silver nitrate does not occur until it comes in contact with the developer as the ferrous oxalate needs to be dissolved in order to be able to react.

3. Any remaining ferrous oxalate after development can oxidize the Ag metal affecting the print longevity, hence the need to replace the latter with something that has a greater resistance to oxalate induced oxidation via toning: i.e. Au, Pt, Pd. However that does not do anything to the residual ferrous oxalate.

A very nice practical guide on Kallitype by Sandy king is here:

http://sandykingphotography.com/resources/technical-writing/the-kallitype-process

Sandy's article is helpful but does not explain how silver nitrate reacts with ferrous oxalate. Am I right that there's a reaction between silver and iron which interferes with the reduction of ferrous to ferric? The clearing step presumably removes most of the ferrous oxalate but leaves the silver. Toning replaces both silver and iron? Fixing removes residual silver. Am I getting close?

 

[nmp]

Sandy's article is helpful but does not explain how silver nitrate reacts with ferrous oxalate. Am I right that there's a reaction between silver and iron which interferes with the reduction of ferrous to ferric? The clearing step presumably removes most of the ferrous oxalate but leaves the silver. Toning replaces both silver and iron? Fixing removes residual silver. Am I getting close?

I get the feeling that you are giving too much credit to the reverse reaction of ferrous to ferric in the ambient. Yes, there is a tendency for that to happen but I don't believe it is instantaneous. It is important only if you are going to leave the exposed paper around for extended time without developing (which normally you wouldn't do,) in which case there is a loss of the latent image that was formed during exposure. So the purpose of silver nitrate is not primarily to counter oxidation of ferrous oxalate - rather it is there to be reduced by the latter to give silver metal which forms the basis of the image.

So the reactions can be summarized as follows:
________________

On exposure: Fe(3) oxalate + UV ----> Fe(2) oxalate + CO2 ----> Latent Image

Fe(2) is now a reducing agent which can immediately react with the silver nitrate present in the sensitizer, but its reactivity is limited in the solid state. That's why a "solvent" is needed for this reaction. This is the function of the developer molecule like sodium citrate or potassium oxalate.

Fe(2) oxalate + developer molecule ----> complex (soluble)

This complex will immediately seek out a neighboring silver nitrate:

complex + AgNo3 ----> Ag + bunch of other things including Ferric oxalate

Now your image is formed.

Continued contact with the developer solution will remove most of the soluble ferric compounds as well as soluble unreduced AgNo3. The rest will come out in the acidic clearing baths.

Silver thus obtained particularly in presence of trace amounts of iron may not have the greatest stability, hence it is strongly recommended that it be replaced with more stable metals such as Au, Pt, and Pd in a toning step.

There may still be some AgNo3 left in the print so follow with a fixing step with hypo as well.
_______________

Hope this helps. (or was it too much?)

 

[tnp651]

Thank you, Niranjan, for a complete response. I feel the need to know what everything is doing in this more-complicated process (compared to traditional silver printing).
Tom

 

[nmp]

Thank you, Niranjan, for a complete response. I feel the need to know what everything is doing in this more-complicated process (compared to traditional silver printing).
Tom

You are welcome, Tom. It is good to know a little bit about the underlying chemistry/science behind these processes so one is able to appreciate the rationale of various steps and when (not if) needed, be able to trouble-shoot problems that are bound to come up. Kallitype is known to be a poor person's Pt-Pd. If done right, The end result can be just as beautiful and archival when it is toned with Pt and/or Pd. Based on the your early work, you seem to be well on your way. Looking forward to seeing more.

:Niranjan.

 

[iakustov]

Can someone help me with two simple things:
1) As far as I get the sodium citrate commonly available is trisodium citrate with pH of alkali. Can you tell how much citric acid is to be added to 20% of sodium citrate solution to make it work well as developer for kallitypes?
2) How long should the working solution of the developer normally last?

 

[nmp]

Can someone help me with two simple things:
1) As far as I get the sodium citrate commonly available is trisodium citrate with pH of alkali. Can you tell how much citric acid is to be added to 20% of sodium citrate solution to make it work well as developer for kallitypes?
2) How long should the working solution of the developer normally last?

For 1):

Assuming you have trisodium citrate dihydrate and citric acid monohydrate and you want to make monosodium citrate:

Need to add 2 moles of citric to 1 mole of trisodium -

2 x 210 g of citric = 420 g
1 x 294 g of trisodium = 294 g

For 20% citric acid: 420/0,2 = 2100 g

So the 20% citric to trisod ratio will be 2100/294 = 7.14

 

[iakustov]

For 1):

Assuming you have trisodium citrate dihydrate and citric acid monohydrate and you want to make monosodium citrate:

Need to add 2 moles of citric to 1 mole of trisodium -

2 x 210 g of citric = 420 g
1 x 294 g of trisodium = 294 g

For 20% citric acid: 420/0,2 = 2100 g

So the 20% citric to trisod ratio will be 2100/294 = 7.14

It looks like I have pentahydrate(?) trisodium citrate (Na3C6H5O7*5,5H2O). As I understand, I would need to multiply by 1.2 for dihydrate.

 

[nmp]

It looks like I have pentahydrate(?) trisodium citrate (Na3C6H5O7*5,5H2O). As I understand, I would need to multiply by 1.2 for dihydrate.

For pentahydrate trisodium citrate, replace 294 (molar mass of di) in the above calculation with 348 (molar mass of penta.) You don't need to do any other multiplication as the molar mass includes the hydrate.

 

[iakustov]

For pentahydrate trisodium citrate, replace 294 (molar mass of di) in the above calculation with 348 (molar mass of penta.) You don't need to do any other multiplication as the molar mass includes the hydrate.

Sorry to ask again, just to make it clear:
2 moles of dihydrate citric acid to 1 mole of pentahydrate trisodium citrate ratio is 1.2.
So, should I add 200 g of trisodium into 500 ml of water then add 240 g of citric acid and water to make 1 litre of 20% (mono)sodium citrate solution?

Or should I just use 200 g of trisodium in 600 ml of water and add citric acid until pH drops to 6, then water to 1 l?

 

[nmp]

Sorry to ask again, just to make it clear:
2 moles of dihydrate citric acid to 1 mole of pentahydrate trisodium citrate ratio is 1.2.
So, should I add 200 g of trisodium into 500 ml of water then add 240 g of citric acid and water to make 1 litre of 20% (mono)sodium citrate solution?

Or should I just use 200 g of trisodium in 600 ml of water and add citric acid until pH drops to 6, then water to 1 l?

200g tri-citrate to 240g citric is right, but you end up with 370g of mono-citrate. So if you make 1 liter, it will result in a 37% and not 20% solution. If you use 200 for the monosodium citrate (for 20% solution) in the table in this link, you will get the other amounts:

https://www.webqc.org/balance.php?reaction=C6H8O7*H2O+Na3C6H5O7*5(H2O)=NaC6H7O7+H2O

The second method probably does not give you the exact monosodium solution but it does solve the problem of not wanting alkaline developer.

P.S. In my earlier response, I confused the issue a bit by thinking you had 20% citric acid available to use, which after I read your original post, it was clearly not. Looks like you are trying to make it from scratch now so I will not try to correct that.

 

[iakustov]

Thanks for your help. From the calculation there should be 108 g of trisodium pentahydrate and 130 g of citric acid monohydrate to make 200 g of monosodium citrate.
I will prepare this amount and check the pH to make sure then.

 

[nmp]

Thanks for your help. From the calculation there should be 108 g of trisodium pentahydrate and 130 g of citric acid monohydrate to make 200 g of monosodium citrate.
I will prepare this amount and check the pH to make sure then.

That sounds right. Hope it works out for you. I love the website I linked to. All you need is the correct chemicals involved and their structure and it does the rest.