Copyright © 2016 Richard Eugene Puckett All Rights Reserved
Patent Pending Richard Eugene Puckett
There are three ways to print an image with a noble metal:
The element rhodium is hardly attacked by any acids; aqua regia only slightly affects it. On the other hand, finely divided palladium, as is found in a photographic print, readily dissolves in hydrochloric acid with a solution strength above 0.5%. Think about that: rhodium is impervious to hydrochloric acid and palladium dissolves in hydrochloric acid. So given a print that is mostly rhodium with some palladium what will happen when that print is immersed in a solution of hydrochloric acid above 0.5% strength? Yes, that's right: the palladium will be dissolved, leaving only rhodium. This formula is a subtractive process, with a surfeit of palladium added to the requisite amount of rhodium solely to induce full reduction of the rhodium to image-forming metal, and that unnecessary palladium subsequently dissolved in a bath of hydrochloric acid. Use half as much again rhodium chloride as you would for any other metal salt (platinum, gold, palladium) for a print of any given size. Note: you can minimize the volume of rhodium salt used by increasing the solution strength. Rhodium chloride hydrate readily dissolves at 20% solution strength.
At one point in 2016, I prepared a rhodiotype consisting of roughly 75% rhodium and 25% palladium. I cleared one half of the print in a weak acid bath (<0.3% hydrochloric), water wash, and T-EDTA and sodium carbonate baths. I cleared the other side as specified below, though in a roughly 3% hydrochloric acid bath. XRF spectrometry performed by a local assayist in Austin, Texas, revealed that the side cleared normally, in very weak acid, was 28% palladium; the side cleared in 3% hydrochloric acid was 12% palladium. (The remainder, on both halves, was mostly rhodium with a small amount of iron and near trace amounts of odd metals such as tin -- probably from the brush ferrule.)
The results, then, of the XRF spectrometry suggested that about 53% of the palladium was removed by the acid. In that case, the print below, which began as roughly 23% palladium (and 77% rhodium), ended up as about 10% palladium, 90% rhodium. Entirely as much a Rhodiotype as a typical Platinum print made with 9 drops of 20% platinum and 2 drops of 10% iridium is a Platinotype...
Below: A print composed of 16 drops of 10% rhodium and 3 drops of 15% palladium. The image measures 6" by 8"; the 16 drops of rhodium more than suffice to form an image. The 3 drops of 15% palladium serve to precipitate the reduction of the rhodium chloride to image-forming elemental rhodium. The palladium can be removed with an acid bath.
Note the brown tone in the overbrushed area. The telltale sign of Rhodium. Note: The only modification to these images was to rotate the first and third to a vertical orientation for display. I made no changes to color, density, saturation, granularity, etc. They are as they were photographed.
Here the print has just been removed from the contact frame.
The print soaking in a bath of 2.25% hyrdrochloric acid -- to dissolve [most of] the palladium:
The washed print:
If you perceive the lightening of the final image relative to the freshly exposed print, your eyes are not deceived. The freshly exposed print is darkened by the surfeit of palladium; the acid-bathed print is lightened by the dissolution of the palladium.
I offer this formula to the public largely to protect my intellectual property from a person who has created a reputation for himself as a claimer of processes (and of at least one tool for alt process photography) which were not of his invention and for which he conceded slight acknowledgment only after called out for his omission. That individual will not lay claim to my rhodiotype.
1. Prepare a 10% solution to 20% solution or a saturation solution of one of the following:
* Rhodium(III) Chloride Hydrate
* Ammonium Rhodium(III) Chloride Hydrate
* Sodium Rhodium(III) Chloride Hydrate
* Potassium Rhodium(III) Chloride Hydrate
2. Prepare a solution of 10% to 15% tetrachloropalladate (one of sodium palladium chloride, potassium palladium chloride, or lithium palladium chloride). NOTE: a solution of platinum chloride (that is, ammonium platinum chloride, potassium platinum chloride, lithium platinum chloride, or other double platinum chloride) can be prepared and used in lieu of the palladium. In such a case, the final step, dissolving of the added metal used to reduce the rhodium, is not carried out. The rhodiotype so prepared is a mixture of rhodium and platinum -- a rhodiotype with platinum. Be aware that should you choose to omit the step for dissolving the palladium you will obtain a rhodiotype with palladium. Finally, if your goal is a rhodiotype with palladium and also platinum, simply add both to the rhodium -- and do not immerse the print in an acid bath to dissolve palladium.
2. Prepare a solution of 10 ml of 40% ammonium ferric oxalate, sodium ferric oxalate, lithium ferric oxalate, cesium ferric oxalate, or guanidine ferric oxalate.
4. Prepare a solution of 2% vitamin C (ascorbic acid, ascorbate, or sodium ascorbate)
5. Count 8 drops of the 2% vitamin C solution into the 10ml of 40% double ferric oxalate. Shake the bottle containing the double ferric oxalate with 2% C added vigorously for 30 seconds. (Caution: this step must be performed correctly; otherwise, the ferrous iron produced will simply precipitate from the solution.)
6. Count into a shot glass or similar container approximately an equal number of drops of the modified double ferric oxalate and of the rhodium chloride equal to half as much again as would be normally required for a photographic print of a given size (e.g., assuming an 8x10 print requires 16 drops each of 10% rhodium salt and of oxalate, use 24 drops of each.). This obviously does not apply if you double the solution strength of the rhodium to 20%. In such a case you would count out 12 drops of rhodium chloride and 12 of the modified double ferric oxalate.
7. Count into the same shot glass a number of drops of the modified double ferric oxalate and of the palladium chloride to constitute 25% of the total volume of oxalate and of metal salt in the sensitizer (e.g., add 2 drops of 10% palladium if you prepared the sensitizer in step 6 with 6 drops of 10% rhodium.).
8. Swirl the solution in the shot glass to mix, and pour out and brush the sensitizer onto a sheet of dry rag cotton paper, such as Bergger Cot 320, Hahnemuhle Platinum, Arches Platine Hot Pressed or Revere Platinum, among others). Note: if you prepared a 10% solution of rhodium chloride, you may need to apply half the sensitizer, dry the paper, as in the next step, and then apply the remaining volume and sensitizer and repeat step 9. If you prepared a 20% solution of the metal salt only one coat will be needed.
9. Place the sensitized sheet of paper in a dark, dry place to dry until dry to touch or for 5 minutes after the coated surface of the paper has ceased to glisten with moisture when viewed obliquely against a light.
10. Mount the paper, a sheet of 2 mil mylar or acetate, and on top of both the negative to printed in a contact print frame.
11. Expose the frame to a strong ultraviolet light source, such as sunlight.
12. Drop the frame back to examine the progress of the print out process. In a zone with high UV index, the print out will likely occur very quickly, in a matter of a few minutes at most. Tip: Overbrushing the area covered by the negative leaves a border you can readily observe growing darker as the print out proceeds.
13. When the printed out image is approximately 1/2 stop to 1 stop darker than desired, remove the paper from the frame and immerse it in a tray of hydrochloric (muriatic) acid diluted to a strength of at least 2% acid.
14. Agitate the print gently and observe the lightening of the image. When the image is approximately 1/3 stop lighter than desired, immediately remove it from the acid and immerse it in an alkaline solution such as 10% sodium carbonate (washing soda).
14. Gently agitate the print in the solution for 5 minutes.
15. Wash the print for one hour in running water.