Test for Clearing

Clearing times may be ascertained by this test.  It is recommended that this test be performed every time a new paper or coating chemistry is to be tried.  Also use this test before a new type of clearing agent is used.  It is highly recommended to first perform a fogging test as fogging could be mistaken for a lack of clearing.

Note: Safelight illumination must be used for all coating and processing during this test.
          Only turn on other lights after paper is in the wash water.

NOTE:  This clearing test procedure has been updated in September 1999
               with the addition of the use of an indicator of Potassium Ferricyanide
               after an exposure after the clearing procedure under evaluation.
               This indicator was designed and suggested by John Melanson.

               Caution: This indicator is extremely sensitive and may produce a false
                                 reading.   It is suggested that an uncoated control be used as
                                 a reference.  The solution strength of the indicator can be
                                 reduced to 1%.

The addition of this indicator has made this test much more sensitive to detecting an incomplete clearing.  John Melanson has demonstrated that several clearing solutions which had removed all traces of yellow and gray and appeared cleared, showed blue or gray values with this indicator.
 

PROCEDURE:

Place a strip of removable tape on the selected paper.

     Note:  Removable tape is handy for this, but use caution to not damage
                 or roughen the paper surface when removing.
 

Coat the paper with a very definite edge to the coating formed by the tape.  Do not let the coating mixture puddle.

 

Remove the tape.

 

Dry carefully so as not to push the mixture into a puddle at the edge.

     Note:  Puddling may cause a density at the edge of the puddle that
                typical clearing may not remove as quickly.
 

Mark the edge with a dotted pencil line(s).  The purpose is to locate the position of the edge after clearing.

 

Without any exposure, process the paper using the clearing procedure under evaluation, including any other processing typically given including development.

 

Dry (hair dryer with heat may use used).

 

Evaluation is made at this point under white light.

     Note:  Direct sunlight may be too intense to evaluate.
                Shaded sunlight is OK.

Look very closely along the edge delineated by the pencil line(s) for any differences.

Insufficient clearing will be seen as yellow, brown, or gray density in the coated area.
Yellow or brown may indicate a presence of  unreduced  ferric oxalate or metal salts.
Gray may indicate residual metal salts.
Examples of colors can be found in the Clearing Study.
 

Even the slightest difference will indicate incomplete clearing.  If not cleared, the indicator portion of the test can be omitted.  If there is no difference between the coated and uncoated areas, the indicator should be used.

INDICATOR PORTION OF TEST:

Expose for twice the typical exposure.

 

Place drops of 1% solution of Potassium Ferricyanide  K₃Fe(CN)₆ onto a clean Q-tip or similar applicator, then spread onto the coating so as to straddle the edge of the coating, moving only from uncoated to coated area.

    Note:  A puddle of a drop directly applied may allow activated indicator
                to move across the edge.
 

Wash for about 5-10 minutes to remover any yellow-orange base color of the Potassium Ferricyanide.

 

Dry (hair dryer with heat may use used).

 

Evaluation is made under white light.

     Note:  Direct sunlight may be too intense to evaluate.
                Shaded sunlight is OK.

Look very closely along the edge delineated by the pencil line(s) for any differences.

Insufficient clearing will be seen as blue or gray density in the coated area.
Blue indicates a presence of ferrous reduced from ferric oxalate by the light.
Gray is thought to be from residual metal salts.
Examples of colors can be found in the Clearing Study.
 

If there is no difference between the coated and uncoated areas, then clearing was successful.  Even the slightest difference will indicate incomplete clearing.

It is best to determine and use the least amount of time necessary to clear a paper.  However do remember that if the strength of the clearing agent is increased too high, then the paper may be adversely affected.  It is also suspected that weak acid clearing baths work best.
 

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