O.K. folks,
This is a plea for help after my first disastrous attempt at plate coating using a dummy emulsion
I cleaned a dozen old plates by immersing them in hot caustic soda.
My emulsion formula uses 15g of gelatine and it says that when complete the final quantity should be around 240ml. So, I made a dummy emulsion using 7g of gelatine in about 100ml of water. I added a little red food dye & also added a teaspoon of table salt, thinking this would simulate the salts in the real emulsion.
Duh! It was of course a daft idea as it behaves like an unwashed emulsion and the salt crystallises out on the surface. Thats what comes of trying to be too clever
I melted the gelatine in demineralised water at 40 degrees C with 4 ml of chrome Alum solution (which was 1g of Chrome Alum per 100ml).
I then tried pouring the plates. I wont go into the full gory details, other than to say it was a total disaster! I clearly have a lot to learn. I have had three attempts now and have solved some of my own problems, but here are the ones that I am still struggling with:
1. No matter how well I clean the plates, the emulsion doesnt flow evenly. Sometimes it decides to leave an islands where it just will not run. This can be overcome by pouring plenty of emulsion right in the middle of the area it doesnt like, but usually results in excess emulsion spilling over the edges and going everywhere.
Any ideas how to make the emulsion flow evenly over the whole plate?
2. What should the coating temperature be? I found that at 40 degrees the emulsion was very thin and watery and difficult to keep on the plate. It gave a very thin coating, possibly too thin at the corner opposite to where the excess was poured off. It easily spilled over the edges and went everywhere.
At 25 degrees it was like treacle. Much easier to handle, thicker coat, less waste, but much more likely to refuse to flow over the whole surface leaving lots of areas not covered and the side where the surplus is tipped into the bottle ends up three times thicker than the opposite corner.
3. Always, ALWAYS the corner where the excess is tipped off ends up with a drip that runs underneath and glues the plate to the slab. Any attempt to wipe it ends in tears as the plate is just resting on my fingertips
4. The original professional coatings on the plates were incredibly tough and difficult to remove. My coatings are extremely fragile. I was able to remove them completely by just placing in lukewarm water and gently rubbing. I do not believe they would stand up to normal processing. I thought that maybe the concentration of chrome alum was too low so I double the quantity to 8ml. This didnt seem to make the emulsion any harder and my last attempt may even have been softer. I could easily leave my fingerprint on the surface after 24 hours drying.
I am now concerned about the gelatine. It was bought from a photographic supplier as photographic grade gelatine but I do not know its bloom value.
Is there any way to tell?
The supplier said that it came from Kentmere who I emailed for the spec, but they didnt reply.
Could the gelatine be too soft? Or is the concentration of gelatine too low?
Or is there a problem with the amount of chrome alum?
Or are the additives in the food colouring to blame?
It contains glycerin and citric acid, although I only added a few drops.
Or maybe the table salt?
Im going to try just gelatine and water next at different concentrations.
Any answers or advice will be most gratefully received!
Steve
This is a plea for help after my first disastrous attempt at plate coating using a dummy emulsion
I cleaned a dozen old plates by immersing them in hot caustic soda.
My emulsion formula uses 15g of gelatine and it says that when complete the final quantity should be around 240ml. So, I made a dummy emulsion using 7g of gelatine in about 100ml of water. I added a little red food dye & also added a teaspoon of table salt, thinking this would simulate the salts in the real emulsion.
Duh! It was of course a daft idea as it behaves like an unwashed emulsion and the salt crystallises out on the surface. Thats what comes of trying to be too clever
I melted the gelatine in demineralised water at 40 degrees C with 4 ml of chrome Alum solution (which was 1g of Chrome Alum per 100ml).
I then tried pouring the plates. I wont go into the full gory details, other than to say it was a total disaster! I clearly have a lot to learn. I have had three attempts now and have solved some of my own problems, but here are the ones that I am still struggling with:
1. No matter how well I clean the plates, the emulsion doesnt flow evenly. Sometimes it decides to leave an islands where it just will not run. This can be overcome by pouring plenty of emulsion right in the middle of the area it doesnt like, but usually results in excess emulsion spilling over the edges and going everywhere.
Any ideas how to make the emulsion flow evenly over the whole plate?
2. What should the coating temperature be? I found that at 40 degrees the emulsion was very thin and watery and difficult to keep on the plate. It gave a very thin coating, possibly too thin at the corner opposite to where the excess was poured off. It easily spilled over the edges and went everywhere.
At 25 degrees it was like treacle. Much easier to handle, thicker coat, less waste, but much more likely to refuse to flow over the whole surface leaving lots of areas not covered and the side where the surplus is tipped into the bottle ends up three times thicker than the opposite corner.
3. Always, ALWAYS the corner where the excess is tipped off ends up with a drip that runs underneath and glues the plate to the slab. Any attempt to wipe it ends in tears as the plate is just resting on my fingertips
4. The original professional coatings on the plates were incredibly tough and difficult to remove. My coatings are extremely fragile. I was able to remove them completely by just placing in lukewarm water and gently rubbing. I do not believe they would stand up to normal processing. I thought that maybe the concentration of chrome alum was too low so I double the quantity to 8ml. This didnt seem to make the emulsion any harder and my last attempt may even have been softer. I could easily leave my fingerprint on the surface after 24 hours drying.
I am now concerned about the gelatine. It was bought from a photographic supplier as photographic grade gelatine but I do not know its bloom value.
Is there any way to tell?
The supplier said that it came from Kentmere who I emailed for the spec, but they didnt reply.
Could the gelatine be too soft? Or is the concentration of gelatine too low?
Or is there a problem with the amount of chrome alum?
Or are the additives in the food colouring to blame?
It contains glycerin and citric acid, although I only added a few drops.
Or maybe the table salt?
Im going to try just gelatine and water next at different concentrations.
Any answers or advice will be most gratefully received!
Steve
