~~~~~~~~ Excerpt from the Guide to Platinum Palladium Photographic Printmaking by Jeffrey D. Mathias ~~~~~~~~
No part of this document may be reproduced in whole or in part without the express written permission of the Author.
created August 1999, updated December 2000
 
Coating Efficiency Explanation
Measurment
Calculation

Coating efficiency is the percentage of chemistry that goes from the mixture into the paper.  Losses are typically from brush absorption or clinging to the wall of the mixing/pouring vessel.  Other losses can occur from mistakes and evaporation.  Mistakes can be avoided by careful technique; the others are discussed further.

The brush can suck up a lot of the mixture.  Soaking the brush and squeezing it out helps to prevent some absorption.  When dealing with a relatively small amount of mixture this loss can significantly lower the coating efficiency.  Several drops could be held by a larger brush; a smaller brush for smaller coating areas is a good idea.  It is recommended that the brush have a width of no more than 15% of the shortest side of the coating area (recommended brush sizes).  A rod should have a much lower loss than a brush, although some mixture does cling to the rod.

Mixture also clings to the vessel from which it was mixed and poured.  This amount becomes significant for a small amount of coating mixture.  One drop could easily cling, which would be 10% of a 10 drop solution.  It is recommended that the brush be used to wipe any remaining mixture from the vessel.

Because of these losses, it is recommended that coating efficiency be measured for both small and large area coatings of sizes typically used.

Evaporation can contribute a variable and uncontrollable loss of water from the mixture.  This does not cause a loss of active material in the coating, but it does play havoc with determining the efficiency.   It is recommended that the weighing (discussed below in the Measurement Procedure) be conducted at lower temperatures, without any cross ventilation, and as quick as possible.  For the measurement of efficiency, single coatings are a must.  Multiple coatings (with the quasi drying between layers) loose too much of the water to provide a useful and consistent weight measurement.

Evaporation can be more pronounced in an ambient of low relative humidity.  It is recommended that the coating efficiency be measured in ambient conditions of 50-70% RH with the paper stabilized at the ambient.  A change in RH of the paper is easily  detected by a changing weight.

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Measurment Procedure:
All weights are measured with scale capable of measuring and accurate to 0.01 grams.

  • Select a paper of interest and cut to slightly larger than 100 square inch (about 654 cm2).
  • Delineate a 100 square inch area on the paper.
  • Weigh the paper.
  • Weigh a shot glass.
  • Make up a typical coating mixture for an 8x10 area in the shot glass.
  • Weigh the shot glass with mixture.
  • Make sure scale is set to zero ballance (ready to weigh paper).
  • Apply the coating by the well practiced and consistent  technique to the paper within the marked area of 100 square inches using the appropriate tool.
  • The coated paper is immediately weighed (no drying).
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    Calculation of Coating Efficiency:
    The initial weight of the chemistry mixture is:
    [weight of shot glass with mixture]-[weight of shot glass] = [weight of mixture]
    It is assumed that the chemistry delivered to the paper is:
    [weight of post-coated paper]-[weight of pre-coated paper] = [weight of mixture coated]

    The coating efficiency is calculated as follows:

    [coating efficiency] = [weight of mixture coated] / [weight of mixture]

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