darkroom-based photography

The gelatin coating hurdle

How do I make smooth, flat, thin, even and bubble-free gelatin coatings on water-colour paper? Anwering this is crucial if I’m to make good Bromoil, Oil, Carbon Transfer and hand-made silver gelatin emulsion prints …

I’ve tried a number of different ways and have now settled on one approach. There have been quite a few problems on the way, so this post mentions these.

I decided to standardise my process on one paper: Rives BFK. It is a strong (280gsm), white rag cotton paper, easily accessible here in the UK and not as expensive as some other brands. For Oils and Bromoil, the paper needs to be soaked for 15 minutes in 3% Oxalic acid to remove the internal alkaline buffer, as this will interfere with the process at a later stage. I have not tested this; I have simply taken it to be a fact from the literature that I have read.

Gelatin

I chose a hard bloom and decided to spend extra for purity. So I chose photo-grade BelliniFoto 250 bloom gelatin. No doubt there are cheaper alternatives around which I will need to investigate at some point.

Quantity and Concentration

The first thing to work out was the amount of gelatin required to coat a 5×7 inch rectangle of paper. Here I made a few mistakes.

I was aiming for an 8% solution of gelatin. For a 35 square inch coating of the required thickness I worked on 0.6 mls for each square inch. This gave me 21 mls of solution. However allowances need to be made for the addition of isopropyl alcohol, a hardener and Tween 20. So I made a 10% solution of gelatin to allow for further dilution and I rounded the volume required to 25mls to make measurement easier.

I weighed out 2.5 grams of gelatin and added 25mls of distilled water. Theoretically this would give me a gelatin thickness of 0.1cms which seems about right 1. I then allowed the gelatin to bloom for 1 hour. I then stirred the mixture. At this stage it looked like this:

Gelatin stirreed just after the bloom stage

As can be seen, it’s full of tiny bubbles. The way to reduce the bubbles is to add a small amount of isopropyl alcohol and melt the gelatin in a warm bath (about 40C.) Here is the result after an hour’s stand:

Melted gelatin after alcohol treatment

Many of the bubbles have gone. However, in arriving at this stage I made two mistakes:

  • I stirred the neat gelatin too vigorously and therefore introduced many bubbles.

  • I also added a drop of Tween 20, thinking that this would help in spreading the gelatin on the paper. However, it just introduced more bubbles.

I also added a drop of hardener.

Preparing for Coating

There are various ways to coat paper. The method I chose was to wet-coat. This ensures that the paper is completely flat so that an even coating becomes more likely.

A few things that took me a few attempts to learn. The coating table needs to be completely flat and horizontal. The gelatin remains fluid even after it appears to be a fairly rigid structure. So, any uneveness or tilt will mean that that the gelatin will flow into pockets or to a downhill side of the paper. This seems very obvious, but it pays to be fastidious.

My coating surface is steel and so I am able to use magnets to keep the 5×7 containing mask in place and pressed tightly to the steel bed to minimise seepage of the gelatin into the borders. The following picture shows this. The magnets that I use are the magnet-bars that are used for hanging tools on a wall. They are powerful and push the containing-mask firmly down onto the steel coating surface.

the containing mask
The magnets pushing down the mask.

As you can see in the above picture, I also tape the margins of the picture area with 3M Blue masking tape. I do this because any leakage of gelatin into the border area will interfere at a later stage by sticking to the pegs that keep the paper flat. 3M Blue tape comes off easily, but keeps the borders clean.

The paper needs to be married flat to the steel coating surface. I found the best way to do this was to pre-soak the paper for 3 minutes than squeegee the paper onto the steel surface. To protect the paper from damage, particularly for a toothed paper, I used a 5×7 mylar sheet between the squeegee and the paper.

Coating

I learned to coat the paper when the gelatin was still fairly liquid. Of course, too liquid isn’t good, but it’s better than too jelly-like. So I experimented with various barriers to contain the gelatin within the perimeter of my 5×7 photo-area. In the end I found that an impermeable 5×7 mask made from carbon fibre worked well.

One mistake I made was to have the gelatin and paper at different temperatures. If the gelatin is close to becoming jelly-like, pouring it onto a colder paper solidifies the gelatin so that it does not flow evenly. So I learned that, when pre-soaking the paper, I had to finish the pre-soak in warm water.

I used a comb to move the gelatin around the paper. Most of the distribution is effected by the way your pour the gelatin onto the paper, but you need to coax it into the corners. A comb does not push too much gelatin in one go. I saw the idea of using a comb on a You-Tube video.

Drying

I even made mistakes with the drying phase. I cut the gelatin free from the mask-container with a scalpel, which worked well, and then pegged the sides onto wooden battens to prevent the paper from curling. But the battens stuck to the paper, despite pre-waxing them. On removal of the battens, some tears in the paper margins were unavoidable. I will need to sort this out.

Gelatin on paper.

In my next post I will take the process a stage further, sensitizing the gelatin, exposing the paper, then inking the image with oil paint.

  1. volume of a cuboid is l x w x h ; 5 inches = 12.7 cms; 7 inches = 17.78 cms; h=25/(12.7 x 17.78) = 0.11cms

2 Comments

  1. Liz kemp

    Thank you for this excellent article. I have tried coating papers with little success mainly because the coating was too uneven but the quality of the blacks was amazing, so rich and beautiful I shall certainly follow the instructions especially as I love the different gorgeous papers A winter project indeed

    • Sidewayseye

      Thanks Liz – a long process but I think it will repay in the end.

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