Coating Coverage

Definition Example Some Coverage values

The potential quality of a print is directly associated with uniformly placing a desired amount of metal into a desired area on a particular substrate.  Accuracy and consistency of the coating technique is of critical importance.  It is highly recommended that all coating adhere to the following.

Only Optimized solutions should be used.

The coating must fully and accurately be kept within a defined area.

Brushing or spreading technique must be practiced and consistent.

The coating efficiency should be known for each technique, and reevaluated if any change of bushing or spreading occurs.

The volume per drop must be known and consistent, if droppers are used to make the coating mixture.  The droppers must be of the same type and have the same drop size.  Accurate pipettes may be used instead of droppers.

If one must error, a denser coating only wastes material, while a weak coating adversely affects quality.

Definition:
Coating Coverage is expressed as cm²/ml and defined as:

[Coverage] = [area of coating in cm²]  * [coating efficiency] / [ml per drop] /
[number of drops of sensitizer at specified solution concentration]

where,

[coating efficiency] is the percentage of chemistry that goes from the mixture into the paper.  Refer to the section on Coating Efficiency for its description and measuring procedure.

[ml per drop] is calculated by counting the number of drops (#) it takes to fill a graduated cylinder to 10 ml, then calculating (10/# = ml/drop) (This term is eliminated if pipettes are used and solution measurements are made in ml instead of drops.)

[number of drops of sensitizer at specified solution concentration] is determined by either of the following options:

• A) experience and consistency of coating and performing the optimization study  in Verification of Optimized Formulas, except finding the optimum number of drops (or volume) of the specified sensitizer solution for the given area (as opposed to determining the solution strength).  This is accomplished by varying the amount of given coating mixture for a given area.

 
• B) experience and consistency of coating as suggested in this Guide (Chapter 7), and using the recommended value for a particular paper.  This choice relies on measurements made by the author, whereas option A requires the performance of the optimization study.

Because diluted mixtures may be made (perhaps for multiple coatings), the total number of mixture drops (or volume) is not an accurate reference.  The appropriate reference is the number of drops (or volume) of sensitizer solution of a given concentration, which is identical to the number of drops of metal solution.  The purpose of coating is to get a known amount of active chemicals evenly distributed throughout the coated area.  Coverage must indicate that amount.

Coverage can vary by paper and should be determined for each paper.  Knowing the Coverage will permit the easy transition from one paper to another without the worry of insufficient or wasted chemistry.  Coverage can also vary with coating size as a variation of the Coating Efficiency.  Coverage should be calculated for the various coating sizes which are commonly used with the specific paper .

Top of Page

Example to calculate drops needed using the Coverage:
An 8x10 image with with half inch boarders (100 inches² or 645 cm²), using a dropper having 0.05 ml/drop, on a paper with a Coverage of 829 cm²/ml and a coating efficiency of 0.90 (90%), the number of drops of sensitizer would be calculated as follows.

drops = [area coated] / [ml per drop] / [Coverage] * [efficiency] = 645 / 0.05  / 830 * 0.90  = 14

A coating mixture for this 8x10 would consist of 14 drops of sensitizer, 14 drops of the appropriate metal solution(s), an optional drop of contrast agent, and any optional dilution water (as when Quasi Multi-Coating).  The coating would be evenly spread onto an area of 645 cm².

Top of Page

Coverage Values for some substrates:
The values below are for the purpose of demonstration.  The Coverage of a coating may vary by coating technique and should be determined as outlined above including determination of the coating efficiency.  Coating coverage may decrease at higher temperatures (especially greater than 75^oF) for some papers.  The following substrates have had Coverage measured to be as follows.
 

		for an 8x10			for a 4x5
Substrate	type	coating efficiency	Coverage	drops sensitizer	coating efficiency	Coverage	drops sensitizer
Bienfang 360	thin paper	.92	76.5	24	.81	73.7	6
CP  (Crane's)	thick paper	.92	65.5	28	.81	63.2	7
Sea Isle fabric	fine fabric	.85	28.3	60	.78	28.4	15

Note:  These Coverage values are for 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.  A 4x5 image (roughly 3 3/4 x 4 3/4) with a 1/2 inch
           boarder on all sides has an area of 27.31 square inches.

Top of Page
 

Table of Contents       Home