Bill Winkler shows us how to coat glass for platinum and palladium printing.
When I saw, for the first time, a backlit 8×10 color transparency, I immediately lost all interest in reflective prints. This was in the mid 1990s and I had been taking courses in photography for only about 1 year. I consider myself to be fortunate in that I started my studies in photography just prior to the “Digital Revolution”, and therefore I received a good foundation in the exposure and processing of film.
Since then, I have experimented with many types of historic photographic processes on many transparent media. From Physautotypes on glass to inkjet transparencies on clear plastic. Because Platinum and Palladium prints are, zone per zone, more transparent than are silver and pigment based processes, I have settled on Pt/Pd/Au printing for Black and White prints on glass. For color I use a variation of the gum bichromate process over Pt/Pd.
I am only now beginning to do serious work using the Carbon Transfer process on glass.
Preparation of the Clear Coating
- Gama-aminopropyltriethoxy silane (Silane A1100)
- 99.9% Propyl alcohol
- 100%hydrolyzed polyvinyl alcohol (Elvanol 71-30)
- 50-100%propyl alcohol/water
- Gum Tragacanth
- Arrowroot Starch
- Ethylenediamine tetra-acetic acid (E.D.T.A.)
The most important part of this procedure is the reaction of amine groups on the aminosilane molecule with a small portion of the hydroxyl groups on the polyvinyl alcohol. This leaves the polymer with some active silane groups which will bond tenaciously to clean glass. However, too much silane will reduce the water solubility of the polymer. If all of the hydroxyl groups on the polymer are reacted with the aminosilane molecule, the polymer will become totally insoluble in water, and therefore useless for the adsorption of the Pt/Pd solutions. The following composition for the clear-coat reflects careful experiments that I have done to balance water solubility of the final coating against adhesion to glass.
Arrowroot starch is for absorption of the Pt/Pd/Au/ferric oxalate or pigment suspension.
Gum tragacanth is for integrity through developing and clearing.
E.D.T.A. is for even absorption/adsorption of the precious metal solutions
Prepare the Coating:
Water will destroy silane functionality. It is important to use clean and dry containers, especially in the first step. After the poly vinyl alcohol has been heated with the silane/isopropyl alcohol mixture, water is no longer a problem.
Silane A1100, 5% in 99.9% isopropyl alcohol——————–200.0 parts by weight
100% hydrolyzed polyvinyl alcohol (Elvanol 71-30)—————–35.0 parts by weight
Let stand at ambient temperature for at least 10 minutes
Under agitation using a hot plate with a magnetic stir bar or overhead mixer, bring this mixture to 60C.
Continue agitation at 60C for 30 minutes. Nothing will be dissolved. A keen eye will notice a slight color shift to pink.
While agitating add: Gum tragacanth……………….5.0 parts by weight
Arrowroot Starch……………….5.0 parts by weight
E.D.T.A…………………….0.2 parts by weight
Distilled water……………… 400.0 parts by volume
Mix at 60C until the Polyvinyl alcohol is dissolved and the others solids evenly dispersed.
Allow the mixture to cool to around ambient temperature, and then add 50-100% isopropyl alcohol. The polymer will begin to precipitate. Continue adding isopropyl alcohol until precipitation stops.
Discard liquid portion and drain solids. Flatten the soft solids to hasten evaporation of isopropyl alcohol.
Place solids outside or in a well ventilated area and allow to dry out.
When dried out, the solids will have the consistency of a hard rubber. Wearing gloves, cut the solids into small pieces. Use stainless steel scissors.
Cover the solids with 400parts distilled water. Let it soak for at least an hour. Then place on a hotplate and, with agitation, bring to 60C until uniform. Allow to cool.
Filter in gradations, from course cloth to coffee filter paper. More water may be added to facilitate filtration. The final coating will be very slightly cloudy and a light yellow in color. % solids will vary from 5% to 20%, depending on the amount of water used to dissolve and filter the dried solids.
Determine percent solids
It is useful to know the concentration of solid in the final coating:
- Weigh an oven proof small container
- Weigh 10.0g of coating into the container
- Bake the sample at 100-150C until dry
- Let the container cool
- Weigh the container
- (5) – (1) X (10) = Percent solids
Preparation of Glass
If a glass plate is expected to sustain any waterborne coating it must be able to hold a vertical sheet of water. And it should do so until the water sheet evaporates. Any breaking of the sheet, especially beading, is indicative of future failure of the photographic image at some point during processing.
I have arrived at the following glass cleaning procedure through years of working with various types of imaging on glass.
If it seems like “overkill”, remember that failure of the coating during sensitization, exposure, processing or clearing is much more frustrating than having to invest time in glass cleaning.
- Determine which side of the glass is “up”. Most sheet glass is float glass. This means that it was made by pouring molten glass onto molten tin. It will have a side that was in contact with the tin, the “down” side and a slightly rougher “up” side. These can be told apart by rubbing a wet cloth over each side. The “up” side will feel and sound rougher. I always coat the “up” side.
- Scour with a brush and “Barkeepers Friend”. I use this cleanser because it contains oxalic acid.
- Rinse under running water
- Spray on and scrub “Lyme-A-Way”, or any cleanser containing phosphoric acid. This removes inorganic contaminates such as iron from the glass
- Rinse under running water
- Spray on and scrub “Clorox Clean-Up”. This removes organic contaminates. It also may produce a layer of sodium silicate, an adhesion promoter
- Rinse under running water
- Soak in a solution of hot water and 2oz. ‘Oxi-Clean” (or equivalent) per gallon of water. 10 minutes
- Rinse in distilled or filtered water
- Dry at least 100C or higher
- Test one plate for beading or breaking
Although not absolutely necessary, it is beneficial to add a cross liking hardener to the coating. This should be done just prior to coating. Either Formaldehyde or glyoxal may be used. I find that formaldehyde yields a harder coating. But glyoxal is less irritating to the nostrils. In either case, concentration should be 0.5% of the solids in the coating. For example’
100.0g coating = 7.0g solids (for example) X 0.005 / 0.37(the concentration of formaldehyde in Formalin) = 0.09g Formalin
The amount of formaldehyde or glyoxal is not very critical. In the above case, the weight may be rounded off to 0.1g
Virtually any coating method may be used. Brushing the coating on is clean and easy. I pour the coating onto the glass, then drain and dry the plate in a vertical position. If a thicker coating is desired (not at all necessary) drying can be done horizontally.
If a hardener has been added, the coating will dry in about an hour. But holding overnight or baking is advisable. If no hardener is used, baking is required. I bake all of my plates at 150C for 1 hour. Heating much over 150C will turn the coating brown. The coated and dried plates should be visually indistinguishable from uncoated glass.
SENSITIZING: Platinum / Palladium / Gold
Any method used to produce “Standard” platinum/palladium prints; Ziatypes; or Aurotypes may be used with the clear-coat on glass. I prefer to use Ammonium Palladium Chloride with Ammonium Ferric Oxalate, standard commercial solution..Ammonium Palladium Chloride is:
- Palladium Chloride-2.3g
- Ammonium Chloride-1.6g
- Distilled Water- to make 25ml
It is not the intention of this article to teach Pt/Pd printing. Mixing of solutions, application, developing and clearing are the same on clear coated glass as for paper.
Exposure, in general, is longer on clear coated glass than Pt/Pd on paper. Test should be done to determine optimum exposures under your specific conditions.
Alternative Coating / Sensitizing Method
Sometimes I combine the clear coating with the Pt/Pd/FAO solution (1 part clear-coat +: 1 part combined Pt/Pd / AFO) Then coat this mixture directly onto the cleaned glass.. In this case a hardener is required, and baking should not exceed 75C.
Pigment on glass
Pigmented images may be printed over the Pt/Pd/Au images discussed above, or a freshly clear coated and dried plate of glass may be used. I have worked out the following method for printing color separation negatives over Pt/Pd/Au images on glass.
- A saturated solution of ammonium dichromate or potassium dichromate
- Enough water to wet a brush
The brand of pigment concentrates used is important. I have not tested every brand of pigment concentrate. But I know that some brands will not react with the dichromates without an additional dose of Gum Arabic. Therefore, I always use Winsor & Newton brand Watercolors. For color-separation work, I use:
- Phthalo Turquoise – For Cyan
- Quinacridone Magenta
- Transparent Yellow
- Neutral Tint – for Black
- Chinese White
Starting with a dry coated plate, with clear-coat only, or with a cleared, dry Pt/Pd/Au image, Dip a dry foam brush into the saturated dichromate solution. Paint on an even coating of this solution. It will dry very quickly. Paint on a second coating of dichromate and let it dry.
Put a bead of pure pigment concentrate in the middle of the image. Then, with a wet foam brush, work the pigment into the dichromate until you have an even coating. Keep adding just enough water to keep the pigment flowing until it is even. It will dry quickly.
Exposure will depend on the pigment used and your light source. There is no way to avoid testing.
“Developing” is carried out in warm water. Just like with “normal” Gum bichromate printing.
A1100 Aminosilane, 5% in “Ultra-Pure” Isopropyl alcohol: hisglassworks.com
Poly vinyl alcohol, 100% hydrolyzed: chemistrystore.com
Arrowroot Starch, Gum Tragacanth, 50 – 100% isopropyl alcohol, and Watercolor Pigments are available from many sources online or in stores.