Clearing

Recommendations step-by-step Procedure Disposal of waste

Clearing removes all ferric and ferrous compounds and all metal salts leaving only metallic platinum or palladium imbedded in the substrate fibers.  Clearing platinum palladium prints completely has always been a concern.  Research has shown that the traditional methods of clearing were not complete.  Some discoveries have found that the addition of Oxalic Acid and EDTA (CAS: 60-00-4)  to the Ferric Oxalate sensitizer can dramatically improve clearing of any clearing agent (see the Clearing Study).  Other discoveries have included:

1. The traditional Hydrochloric acid bath may "bleach" the image (especially in the highlights) with most papers (especially the thicker ones requiring longer clearing times).
2. A pre-clear water bath for 2 minutes will dramatically shorten clearing times (in most cases by about half).
3. A new indicator helps to determine if clearing is complete.  (see Clearing Test)

Note: If the print dries at any time before clearing is complete, further clearing may not
          be successful.

Dick Arentz addressed the first item above by suggesting the use of Phosphoric acid clearing baths.
The author as well as several others independently discovered the pre-clear water bath.
The indicator of the third item was introduced by John Melanson.

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New Clearing Recommendations:

When using Ferric Oxalate Sensitizer, add 0.3% to 0.5% 0.04% to 0.1% EDTA (CAS: 60-00-4) and 3% to 4% Oxalic Acid.  (For example, for 100 ml of Ferric Oxalate solution, add 0.4 0.04 gram EDTA and 3.0 grams Oxalic Acid.)

NOTE (added 8/2001): Further study of the "bleeding" of metal during processing has indicated that too much EDTA seems the culprit.  It is now recommended that EDTA in the sensitizer be kept to a solution strength of between 0.04% and 0.1%.  The actual amount can vary with different papers, so the smallest amount of EDTA to add to the sensitizer to assist with clearing should be determined for each paper.

NOTE:  The same additives might also be added to Ammonium Ferric Oxalate sensitizer.  In general Ammonium Ferric Oxalate coatings will clear in about half the time of Ferric Oxalate coatings without the additives.  No study has yet been performed with additives to Ammonium Ferric Oxalate.
 

All clearing should be prefaced with a two minute water bath rinse.

 

All tap water should be filtered.  0.5 microns is a good filter size.  Particles in the water can scratch the print surface or deplete the clearing bath (as iron will).

Following these recommendations almost any clearing agent can be used including water, although times may vary as per the Clearing Test.  Refer to Chemicals and Preparing the Stock Solutions for various clearing Agents.
 

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Clearing procedure for all substrates using any clearing agent:

[When the sensitizer is mixed]  Add 0.4% EDTA (CAS: 60-00-4) and 3% Oxalic Acid to the sensitizer.

 

[After development]  Place print into a tray filled with tap water until most of the coating is dissolved (typically 2 minutes).  If the print floats, place image side down making sure there are no air bubbles on the print surface.

 

Lift the print from the rinse and drain (allow most liquid to drain from substrate).

 

Place print into first of three clearing baths.

 

After a third of the total clearing time with occasional agitation, lift the print from the bath and drain.  This is typically 3 to 5 minutes depending on the clearing agent used.

 

Place the print into the second clearing bath for the second third of the clearing time.

 

Lift the print from the bath and drain.

 

Place the print into the third clearing bath for the final third of the clearing time.

 

Lift the print from the bath and drain.

 

Place the print into a tray of filtered tap water for 5 minutes. (If acid clearing baths were used, some baking soda may be added to help neutralize the acidity.)

 

Lift the print from the rinse and drain.

 

The print is then washed in fresh, filtered tap water for another 5 minutes.

 

 

NOTE:  Washing is considered part of the clearing process.
 

The third clearing bath and second wash must be fresh.  When the third clearing bath shows slight coloration (in normal white room light), move it to be used as the second clearing bath.  Move the second clearing bath to be used a the first.  Properly dispose (see below) the first clearing bath when it shows a strong amount of coloration.

 

 

NOTE:  At the end of the work session, the second and third clearing baths may be stored in bottles and used as the first and second (respectively) clearing baths for next work session.

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Proper Disposal of Acid Clearing Baths

Spent clearing baths should be poured into a large bucket with some baking soda.  Use enough backing soda to give a pH of 7.  Water should then be added to dilute the waste.  Neutralized and diluted waste can then be poured down the drain with more water.
 

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