Preparing the Coating Solution

Components Mixing     Sensitizers     Metal salts     Contrast Agents Coverage Expectations Notation

There is a lot of art in the making of the coating solution. (The chemical solution is not an emulsion.)  Many choices are at hand to influence the look of the print.  Some general tendencies are described below, however one must try things for themselves to discover the nuances they prefer in a particular print.

Note: All coating and mixing must be done in safelight illumination.  It is important to check
           if the working illumination is safe by performing the Fogging Test.

Note: Any coating chemistry that spills on clothing and is illuminated by light will produce a
          stain that can only be removed with a pair a scissors.  Any coating chemistry that spills
          should not be used for important prints but should be recycled if practicable.

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COATING COMPONENTS:

SENSITIZER:  This must be included as it makes the coating light sensitive.
                          There are two major routes to take with the selection of the sensitizer.
                          Ferric Oxalate (FO) provides what is called a Developing Out Process (DOP).
                          Ammonium Ferric Oxalate (AFO) provides what is called a Printing Out
                          Process (POP).  Each of these processes has its own characteristics.
                          With DOP a developer is used; with POP the print goes straight into a water
                          bath and then clearing.

       Note: Only use a sensitizer solution that is known to be good and of the highest quality.

        FO             DOP - allows for a variety of developers or enhancing agents.
                          Provides for the best depth and substance.
                          In general, FO requires less metal (solutions may be weaker) than AFO.

        AFO          POP - no developer may be required.  However, use of Potassium Oxalate
                          developer will dramatically reduce contrast in the highlight values only.
                          Provides more neutral color at higher relative humidity.  Cool, bluish color
                          may be obtained with addition of some Pt double salt.
                          A 60% solution of AFO with the appropriate amount of metal can provide the
                          best depth and substance of FO.
                          In general, AFO must use more metal and higher strength solutions to
                          achieve results comparable to FO.
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METAL SOLUTIONS:  The metals are what form the final image.  The metal solutions are
                                        made from what are termed double salts of the noble metals platinum
                                        (Pt) and palladium (Pd).  These have the form X₂MCl₄ in which X
                                        represents a period I element (alkali metal) and M represents the
                                        nobel metal (Cl is the symbol for Chlorine).

          The following evaluates general relationships for the ratios of Platinum to Palladium
          solutions for DOP.  The strengths of the individual solutions are determined by the
          sensitizer and can be found in the section on Optimization or calculated from the
          Metal Solution Formula Calculator (requires JavaScript capable browser).

          100% Platinum
          80% Pt      There is not much noticeable difference between 80% and 100% platinum.
          50% Pt       More Pt will discriminate sharper detail; produce cleaner whites; more
                              contrast; a colder, blacker image;  exposes faster.  More than 50% is
                              rarely used.
          35% Pt
                               Maximum substance in image for most papers occurs at 25% to 35% Pt
          25% Pt
                               Good ratio for fabric is from 10% to 30% Pt
          10% Pt
                               Warmer, browner image; less contrast; exposes slower.
          100% Palladium 100% Pd gives slightly pinkish highlights, also will discriminate very
                                high values.  Can discriminate Zones IX, X, XI, and even XII.  Palladium
                                tends to cling to the paper fibers better making clearing more difficult.

           Note:  The platinum solutions for both DOP and POP should be warmed, but kept
                       below 140^oF.

           DOUBLE SALTS of PALLADIUM:  Several Pd salts have been used and can produce
                                                                        various results.  This are represented by X₂PdCl₄,
                                                                        where X is one of the alkali metals, lithium (Li),
                                                                        sodium (Na), or potassium (K).
                 Li       Achieves the most neutral color with POP (high RH).  Orangeish brown with
                            DOP.  Solutions stay dissolved at room temperature for both DOP and POP.
                            Holds the most moisture in the coating.

                 Na      Warm yellow, brown color with POP.  Yellowish brown color with DOP.
                            Na may react with the Pt salt at higher temperatures causing a precipitate.
                            Solution for POP must be heated in water bath to keep dissolved.

                  K       Slight warm color with POP.  Warm deep brown color with DOP.
                            Solutions must be heated in water bath to keep dissolved for both DOP
                            and POP.
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CONTRAST AGENT:  There are several agents that can be used to increase contrast either
                                        throughout the entire tonal range or within a local range.  These
                                        agents can be part of the coating or can be used in the subsequent
                                        processing as with those added to the developer.  When used in the
                                        coating, the are administered as a single drop of a certain
                                        percentage solution added to the coating mixture.  The contrast
                                        agent should always be the last ingredient added to the coating
                                        mixture.  Some unique effects may occur by mixing multiple contrast
                                        agents.  A few agents are discussed below.  Some general tendencies
                                        are as follows.

It is usually best to get the proper contrast in the negative.

Sometimes use of a contrast agent will prevent base fogging.

Less contrast agent may give more of a printout with DOP.

More contrast agent generally requires more exposure.

          Potassium Chlorate - This is the traditional contrast agent.  It tends to work mostly on
                                                the upper tone values.  Less gives a better quality print.
                                                However, sometimes some gives a better quality print than none.

                                      ½% to 1% (and for some papers 2%) is the maximum amount for
                                      an 8x10 with most papers before image degradation is noticed.
                                      Too much Potassium Chlorate will hurt the paper, degrade the
                                      image, or add a lot of grain.


          Hydrogen Peroxide - Hydrogen Peroxide wont degrade the paper or image like
                                               Potassium Chlorate.  However, the larger concentrations of
                                               Hydrogen Peroxide may cause the platinum salt solution to
                                               precipitate platinum or convert to K₂PtCl₆ (not very soluble in
                                               water).

          Potassium Dichromate or
          Ammonium Dichromate - Higher concentrations do not seem to harm the paper like
                                                      Potassium Chlorate.  Potassium Dichromate may be added
                                                      to the coating mixture (internal control) or to the developer
                                                      (external control), each producing some unique results.
                                                      Ammonium Dichromate is thought to provide for a true
                                                      contrast adjustment.  Ammonium Dichromate provides for
                                                      a more neutral color than other contrast agents for POP.

           Na₂PtCl₆ - Richard Sullivan has provided some interesting information on the use of this
                              Pt salt for contrast control.  His tests have shown it to provide true contrast
                              control without detrimental effects.   (technical paper at this link)
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MIXING THE COATING:

The basic mixing rules are these:

Components are added in the following order: sensitizer, metals, contrast agents.

Total amount of metallic salts is equal to the amount of sensitizer.

There is only one drop of a contrast agent.

Set out a shot glass.

Enter coating information in notebook (see notation below).

Count out drops of sensitizer (FO, AFO) from a dropper (or use ml from a pipette).

Calculate drops (or ml) of Pd and Pt solutions for the Pt/Pd ratio desired.

Add drops (or ml) of Pd  (Li, Na, K).

Add drops (or ml) of Pt.

Swish around to mix.

Add one drop of ?% solution contrast agent.

Swish around to mix and set aside for coating.

Note:  It has been observed that for most POP and some DOP situations, results
            (apparent in the print) are better when the contrast agent is added last.

Note:  It is good practice to mix each coating solution immediately prior to coating.
            Some mixed coating solutions have been observed to precipitate crystals if
            cooled or not used within 10 minutes.  Mixed coating solutions should not sit
            or be stored for more than an hour as the mixture can "sour" resulting in a
            noticeable loss of print quality or inconsistent results.
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COVERAGE:

Coverage determines the total amount of coating mixture to prepare for a print.  The coverage of a coating varies by substrate, coating procedure, and conditions and should be determined by experience and as explained in the section on Coating Coverage.  Typical coating coverage may vary for some selected substrates as follows.  Drops were measured to be 200 Drops = 10.0 ml  (error of +-0.05) (0.050 ml/drop).

Bienfang 360 (a thin paper) typically takes 24 drops total per 100 square inches.

Crane's Parchment Business Card Stock (a thick paper) typically takes


    28 to 30 drops total per 100 square inches.

Sea Isle fabric typically takes 90 drops total per 100 square inches.

Note:  These Coverages are for a temperature of 70^oF, higher temperatures
            may require more solution depending on the substrate.

Note:  An 8x10 image (roughly 8 ½ x 9 ½) with a 1/2 inch boarder on all sides
            has an area of 99.75 square inches.

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GENERAL EXPECTATIONS:
 

Too much chemistry wastes $ and may cause blotchy areas.

Too little chemistry will weaken the image.

Papers with more absorbency will require more chemistry.

Higher ambient temperature will require more chemistry on thicker papers.

In general fabric will require three times the chemistry as an equivalent area on paper.

Papers coat better when cooler (use a sonic mister rather than steam to humidify).


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KEEPING NOTES - NOTATION:

Always record the paper and coating chemistry in the notebook.
The following abbreviations can provide for quick and accurate notations.

General notation = [sensitizer]-[metal solutions]-[contrast agents]

sensitizer:
#FO%     =  Ferric Oxalate
#AFO%  =  Ammonium Ferric Oxalate
where
 # = amount in drops or ml
% = solution strength
Note:   If more than one source of FO is available, an additional letter
             can discern the difference, such as VFO, JFO, BFO, etc.
Pd solution:
#L%    =  Li₂PdCl₄
#N%   =  Na₂PdCl₄
#K%   =  K₂PdCl₄
where
#  = amount in drops or ml
% = solution strength (This is optional as the solution strength of the
metals is determined by the solution strength of the sensitizer.)
Pt solution:
#        =  K₂PtCl₄
or
#P%
where
#  = amount in drops or ml
P  = Pt salt (This is optional unless more than one Pt salt is used,
then notations can be used such as KP, NP, AP, etc.)
% = solution strength (This is optional as the solution strength of the
metals is determined by the solution strength of the sensitizer.)
contrast agents:
%PC = Potassium Chlorate
%PD = Potassium Dichromate
%AD = Ammonium Dichromate
%HP = Hydrogen Peroxide
where
% = solution strength


For example, an 8x10 coating for Bienfang 360 paper may be expressed as:

12BFO27-9K18-3KP23-1/4PD

or omitting the optional information:

12BFO27-9K-3-1/4PD

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