PD Chemistry Question


My apologies if I shouldn’t be posting this here (please advise if I shouldn’t.)

I just mixed dry chemicals into solutions to do some PiezoDN printing. Under the impression that the chemicals are only light sensitive when mixed together (ie. Palladium&Ferric Oxalate,) I mixed up my Ferric Oxalate solution with the florescent lights on in the darkroom.

The solution turned brown rather quickly, so I’m guessing that I’ve ruined it by exposure to UV in the fluorescents. Is this the case?

Have I also ruined the dry Ferric Oxalate crystals too?

I’ve ordered more of the dry crystals, just in case, but if I can salvage the remainder of the dry chemistry that has been exposed to the light, I’d count this a less than total loss.

Thanks in advance,



Your impression is wrong, as you have now proven. But now you know for sure! I’ve learned lot’s of things that way too.

Ferric oxalate is the light sensitive component of Pt/Pd chemistry, and as such is sensitive to any exposure to UV light. Normal incandescent room light is safe, most standard fluorescents are not (I proved this to B&S when testing a new batch of paper in their lab one day a year or so ago), not sure about LEDs. Pt and Pd salts, on the other hand, are not sensitive to UV exposure, even when mixed with FO. What’s that, you say! It is true, I say, at least for the traditional develop-out Pt/Pd process. UV exposure to the FO/Pd mixture causes the formation of a temporary iron image. In the developer, the iron is replaced by Pd and Pt.

I think your dry ferric oxalate powder will be ok, but you’ll have to try mixing a fresh solution with it to be sure.


We use LED drawing backlight panels as our overheads similar to this just larger: https://www.amazon.com/LitEnergy-Ultra-thin-Portable-Sketching-Animation/dp/B06Y21WQYQ/ref=sr_1_3?s=electronics&ie=UTF8&qid=1502133794&sr=1-3&keywords=LED+light+table

These work quite well and don’t emit UV although we keep them a bit away from the coating area and let the prints dry in the dark.






Keith, we also are in the habit of discarding our ferric oxalate solution by 90 days even though it is an amber bottle, etc. It is so cheap to make up and I suppose I do not want to waste a day of precious time learning when its expiration date has been exceeded.

But, I would love to know your experience with ferric oxalate. I got from David Michael Kennedy that I should first condition water with a 2 grms oxalic acid and then add the ferric. I mistakenly discovered that it could be mixed at a much higher temperature than I have read anywhere. Goes into solution within a few minutes - and I do not note any decrease in sensitivity or dMax. I just note that it mixed so quickly. I now use minimum 185ƒ and have on one occasion by accident allowed it to boil on our hot plate mixer. I got the impression that in the the days of olde, most practitioners would have been using a flame to heat the flask and boiling if not often the goal was the result. But excessive heat does not seem to harm the final product.

What are the actual practical warnings about mixing FO (if any), or it is some vast unchartered territory open to more exploration? And finally, you mention that the FO/Pd is not sensitive to light - so what then is the practicality of pre-mixing FO/Pd into a working solution that can be used over a few days or…? I meant to ask you the latter over paella earlier this summer but I took to drink instead… :wink:



I have an LED lightbox (viewing, not exposure) in my darkroom, but haven’t tested it for UV. It looks similar to the one in your link. I don’t have it on when coating though, because it is right next to the coating table. I’m still old-school with incandescents for ambient light in there. I know that will have to change at some point. There are UV filter sleeves for fluorescents - probably something similar for LEDs if necessary.

When the much anticipated new batch of Platine first arrived at B&S in January of 2016, I went down there to test it in their darkroom before buying several packages of uncut sheets. First thing I noticed when I walked into the darkroom was the fluorescent ceiling lights. I asked Dana what he used for working lights and he said that’s them. I asked if he ever had problems with fog, he said nope. So I got to work. First print (21-step Stouffer) badly fogged. I showed Dana, he shrugged. I found a clamp light with an incandescent bulb, and turned off the overhead fluorescents. Second print was fine. I showed Dana, and asked how what concentration Na2 he normally used for an 8x10 print. I don’t remember for certain, but he said it was either 5 or 10%. I told him I use 1.25%. In effect, he is using stronger Na2 to overcome the fog. I wonder how many others are doing that without realizing it. I don’t know if he has changed his practice.

I’m aware of the various additives, usually oxalic acid and/or edta, that some folks use when mixing ferric oxalate such as mentioned by DMK. I used to use them too. I had heard all of the claims about increased speed and Dmax, and just took it for granted that they were true since they came from people whose work I admired. At some point, I don’t remember exactly when, but it was either before I first moved to Taos in late 2004, or soon after I finally got settled in 2008, I did some comparative tests of my own and guess what … there was no significant and repeatable difference between plain ferric oxalate, ferric oxalate with oxalic acid, ferric oxalate with edta, or ferric oxalate with both oxalic acid and edta. Repeatable is the key word here. One round of tests might appear to suggest that one formulation is faster and/or darker than the others, but another round of tests come out a bit different, and after a couple more rounds and averaging the results, there was no discernible difference. Oh well. Other’s tests may differ, and if someone thinks their special formula is better than the others, that’s fine with me.

The temperature thing is another matter, as you have discovered, Jon. Something has changed in the ferric oxalate powder that B&S are making and selling. I don’t think they know what it is either. When I got started with Pt/Pd in the early '90s, FO powder went into solution fairly easily (within a couple of hours) when mixed with distilled water at 120°F. Sometime after they moved from LA to Fanta Se that changed, but I don’t remember exactly when. I had no darkroom and did no Pt/Pd printing from 2005-2008, and when I got started again in '08 I noticed that I sometimes had trouble getting the FO to dissolve completely. I tried heating the water up to 150 or so, which helped a bit sometimes. When I finally called B&S about it, Dana said to heat the water up to 180. Since it doesn’t stay there long once you take it off the hotplate, I have been heating to boil then letting it cool slightly before adding the FO powder. That seems to do the trick here. I had an accident mixing palladium solution once when I left a graduated cylinder in a water bath on the hotplate and forgot to turn the heat off. After the water evaporated the cylinder broke off of the base and I ended up with a palladium plated stainless steel beaker. Since then I don’t leave anything on the heat without my full attention.

As for premixing FO and Pd, I don’t recommend it. I tried that once, also a long time ago when i was printing 10x24 and 8x30 triptychs, and found that over the course of a day of printing, contrast decreased and fog began to appear. I also tried coating 6 sheets at the beginning of the session, but I due to limited sink space I could only expose and process one print per hour. Later prints were softer than earlier ones, and fog began to appear at the end of the session. On the other hand, if you are working small and want to be sure your mix is consistent, and can manage several prints at different stages of exposure and processing, making enough to use for several prints in an hour or so is quite sensible and not a problem at all. When I test papers with the 4x5 Stouffer 21-step, I always mix as if I’m printing 8x10 which usually is enough for 4 4x5s.

I’ve got something else to discuss with you about ferric oxalate, and one or 2 other things, but I think I’ll email or message you about that tomorrow.



Thanks to all for your responses. I’ve learned a great deal in the last 24 hours!

Please keep posting - I’d love to learn more!




Thanks, Keith.


Can I fact check one more assumption?

I assumed that the sodium vapor safe lights in my darkroom would be safe for the ferric oxalate. I mixed up the last of the dry powder (the stuff that had been exposed to florescent lights a few days ago. All looked well when I went home last night and left the solution on the magnetic mixer - to keep mixing it overnight (no heat, just agitation.) I left the safelights on so that the security staff at my school could do their rounds without turning on the white lights.

When I came in today the top half of the solution had turned dark, while the bottom half was still the original, greenish/whitish color. I’m guessing that the lights fogged the oxalate overnight though I’m not clear on why it would happen to the top half - with the mixer running. The nearest safelight hangs from the ceiling and would expose the glass beaker evenly, or so it seems.

So, do the traditional Osram low pressure sodium vapor lamps emit UV?

Any help would be appreciated.

ps - I have one of the LED lights that Walker linked to coming tomorrow, so I have a backup plan if the sodium lights are a bad idea.






Hi Mark,

Pictures would be helpful to be sure we are following you correctly. Also descriptions of the formulas and mixing procedures you are using including temperature. I know it’s probably the standard formula but just want to be sure.

I doubt the sodium vapor safelight is causing this. Here is a spectral output graph showing nothing close to the UV range.

[attachment file=28208]

If it looks like this after mixing overnight, it probably indicates a high level of bad FO (ferrous rather than ferric). This was from a jar that had been on my shelf for a few years. But it’s not clear from your description that what you have is the same.

[attachment file=28207]
But, this is also what you would likely get if you start with water that is not hot enough. One thing that absolutely does not work is to reheat the solution, so don’t bother trying. The portion that did go into solution is good, so you can decant that into another container if you want. The concentration will be a bit lower than if all the powder dissolved, which will give slightly higher contrast than normal. At least that was the result I got when this happened to me and I tested it. :wink:




One more thought - this is an upward pointing safelight like the Thomas Duplex, right, the light reflects off the ceiling? If it’s something else where the light is direct then my guess in my previous message might be wrong. I do have a vague memory of Thomas Duplex safelights in an alt process darkroom that I worked or taught in a long time ago. The only problems I’ve ever encountered have been from fluorescent ceiling lights.


Hi Keith,

Thanks for putting so much thought into this.

I can’t send a picture of the second, bad batch of Ferric Oxalate - I threw it out as soon as I saw it. But it did look just like the photo you sent in response. Maybe more volume of green on the bottom and less brown on the top, but the same idea.

The safelights are the Thomas variety - I’m attaching a photo. Nearest one is about 8-10 feet away. There are 5 in a 25X25 darkroom - if that matters.

I’m also attaching a photo of my mixing area.

I’m mixing 100 ml of distilled water with 29 grams of Ferric Oxalate powder - trying for 180 degrees F, though I haven’t quite gotten all the way there. In the photo, the mixing/heater is set to 145 Celsius - I was trying to get all the way to 180 F as a test for tomorrow’s mixing adventure. I started the heater off at 120C and inched my way up over the course of 2-3 hours or so. Each time I measured (with the instant-read digital in the photo) and saw that I wasn’t there yet, I’d move the heater up another 5-10C. After all that, I only made it to 178F.

I did try to reheat the batch that went bad. It never looked like anything other than pea soup - totally opaque, so I kept heating and mixing. Would you arrange the heating set-up differently? I don’t have access to a flame for direct heating. I could put the smaller beaker into a larger one with a water bath, to hold more of the heat against the inside beaker.

Anyway, thanks for putting so much thought into this little problem.





This problem most likely indicates that a portion of your powder has decayed from iron (III) or ferric oxalate to iron (II) or ferrous oxalate. Ferrous oxalate is poorly soluble in water; ferric oxalate is slightly soluble in water.

The brownish color of the dissolved FO in the beaker is what it should look like. Not all ferric oxalate will be that color, but the B&S powder will. How old is your ferric oxalate powder? I see you got it from ArtCraft. Dana at B&S told me recently that ArtCraft gets their FO from B&S (and B&S gets something, I forgot what, from ArtCraft). Is it possible that your FO is more than a couple years old? That is a problem I ran into a couple of years ago with a jar that was about 3 years old. A fresh supply from B&S solved the problem, but now I don’t keep more than I anticipate using in a year on hand.

180°F is 82°C, so it seems if it takes 2-3 hours to get to 178 there is maybe something wrong with the hotplate. My simple kitchen hot plate will boil 100ml of water in 5-10 minutes. A microwave oven is useful for this too.

I suppose it’s possible that being exposed to the sodium vapor light overnight could have caused this to happen, but it seems unlikely. I hadn’t thought about it before, but I think I always mix fresh solutions last thing and then turn the lights out.

But, I see another possible issue, and I guess that’s why I asked about the formula you are using. As you approach the saturation point it takes longer for the remaining solute to dissolve. The saturation point for ferric oxalate is around 33% if I remember right, though I can’t find a reference so I may be thinking of something else. I think you are using too much ferric oxalate powder. Where did you get the idea to use 29gm? Are you actually using 100ml of water, or is that the final volume? If the latter, which is the correct way to make your solutions, you are using too much powder.

Your problem could be due to one of these factors or a combination.

I suggest that you start with 25gm of (fresh) ferric oxalate powder and 80ml of distilled water. Heat the water to ~ 180°F or even boiling (I use a microwave), add the powder, put it on your stirrer for a little while, then just let it sit. Don’t continue heating it. All of the powder should dissolve within a couple hours. Then transfer to a graduated cylinder (for more accurate measuring) and add distilled water to bring the final volume to 100ml.

Hope this isn’t too much information. :wink:


Keith, just wondering about the reference to using a microwave to super heat ferric oxalate, or for that matter heating potassium oxalate developer. Does this practice render the microwave unsafe for cooking food, assuming the oven is clean?



NO! Use it just to heat the water before mixing.


And just to be obsessively and excessively clear, you should always add the powder to the already heated water. If you are adding the powder to room temp water and then heating it, that might - I’m not sure of this but it might - also cause problems.