Photo Engineer
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The book is about done and I am going through it trying to collect (and recollect) all of the questions about hand coating and make sure that they are anwered in the book. I have gone through my e-mail and pm questions as well as those on-line to come up with some comments. I thought I might post some of the preliminary work here. Some of the information on film and plate coating will appear elsewhere.
Firstoff, the blade I use is exactly that used at Kodak in our research labs. The basic difference is that the end cap (doctor blade) is adjustable, but at Kodak we got a set with spot welded doctor blades at fixed gaps. We could not alter them. We usually had a set of up to 4 of these for paper and 4 for film.
I started at home by using a puddle pusher wrapped with tape, because two turns of tape was just about exactly 5 mils. Although I made some good coatings, it was messy, used a lot of emulsion and had a lot of defects. I usually could not do better than 4x5. Also, the tape wore out rapidly on paper due to abrasion. This didn't depend much on the type of tape, but more on the paper itself.
Then I went to the extent of building a well out of a leading glass device taped to the puddle pusher. It was better, but still the tape wore out and the seals on the bonded items eventually began to fail due to the temperature and solvents used. I could have probably gone to a better glue, but by then the prototype stainless steel coating blade arrived and I moved on.
As a historical note here, when I finally got the metal blades made, the shop told me they would be milled to 0.001" (0.0254 mm) tolerance. About 1/2 of the blades failed to be within specs, so the good ones were used by me or sold at the original price while the defective blades were re-milled and are now sold at a higher price to reflect the need for reworking. Since the milling accuracy problem was found to be beyond their control, I had to use a different shop. I am now no longer offering the blades and will sell what I have from the remilled stock. The blades work in widths from 4" to 16" producing good coatings in that range. Kodak used blades up to 41" wide for production in the early years of the last century. The paper or film was drawn by a stationary blade as the emulsion was pumped down onto the support in an even stream, or was picked up from a trough. (see the Kodak film here on APUG for an example of that) This note is included to answer the questions raised about the increase in price.
So, I have basically 3 types of blade now. A paper blade, a film blade and a plate blade. This is in answer to the questions regarding the types I have worked with.
Now, in answer to the remaining questions, here are the keys to coating paper, using these blades, and it is intended to cover all of the questions posed by those wriiting me. If I have left any out, now is the time to ask!
1. Use a hot press paper or baryta. If hot press, use 100# or higher, if baryta use DW. Cold press paper or thinner paper can be used but can wrinkle during coating. You need a surfactant and this may be different for each emulsion and for each type of paper. Baryta is more difficult to coat on due to the relative hardness and the sizing.
2. Coat paper cold. Paper when wetted will expand rapidly. Hot paper with hot emulsion expands differentially and will wrinkle, especially at the interface between wet and dry at the edges and as you draw the blade across the paper. (film and plates are best coated hot, but for film it is not as important as it is for plates)
3. Cut the paper just a tad overwidth. If you cut wide sheets of paper and coat a narrow strip down the center, the stresses on the paper are increased manyfold and lead to wrinkles. Narrow edges of about 1/2" or so are best. Even here there may be wrinkles, but they do not appear until the coating is fully dry, or at least dry enough that the emulsion does not run.
4. Draw the blade at a constant rate. The rate depends on viscosity and to determine the whole thing - practice in the light with dyed gelatin at the concentration that you intend to coat. I have coated many dozens of sheets of dyed gelatin just to get my draw correct. This is the art part of coating!
5. Make sure that you let the sheet rest on the cold coating surface for a few moments. The paper should be held reasonably flat at the top and bototm by a small piece of tape or a weight. You will see that the paper surface changes from very glossy to a dull matte. This is when you can move it. I use tape at the top and my finger at the bottom.
6. If you get puddles or runs, the emulsion is too hot, too thin or the paper is too thin or too warm. Cooling everything down will help. However, if the paper is too thin, you may not be able to fix this problem unless you use thicker paper. I have coated on regular 20# typing paper and it will work if you can stand the mess. I've also used a 90# cold press paper that worked well but fell apart during processing.
In the final analysis, if the paper buckles or wrinkles, something is wrong. It can be any of the above such as thin paper, hot paper, wide edges or others but truly, something is wrong. I have coated from 68F to 75F ambient in my darkroom with the emulsion at 100 - 110F and the gelatin at about 8% with no problem. I have coated from 4x5 to 16x20 with good results, keeping all of the above in mind.
Remember, this applies only to Fibre Based paper. This does not apply to Resin Coated paper.
I would also like to mention that local water supply and air pressure may affect the coating properties on both film and paper. Since you must rinse the blades after every use and dry them, the drying towel(s) are also important. I have used some paper towels that had a bad effect on coating. These types of paper towel seem to contain a skin conditioner. Also, any oil or grease will adversely affect coating. All of my blades are washed in detergent and then distilled water before I ship them or use them.
Your hands can leave defects on coatings due to oils in the skin. Only touch your paper at the edges.
PE
Firstoff, the blade I use is exactly that used at Kodak in our research labs. The basic difference is that the end cap (doctor blade) is adjustable, but at Kodak we got a set with spot welded doctor blades at fixed gaps. We could not alter them. We usually had a set of up to 4 of these for paper and 4 for film.
I started at home by using a puddle pusher wrapped with tape, because two turns of tape was just about exactly 5 mils. Although I made some good coatings, it was messy, used a lot of emulsion and had a lot of defects. I usually could not do better than 4x5. Also, the tape wore out rapidly on paper due to abrasion. This didn't depend much on the type of tape, but more on the paper itself.
Then I went to the extent of building a well out of a leading glass device taped to the puddle pusher. It was better, but still the tape wore out and the seals on the bonded items eventually began to fail due to the temperature and solvents used. I could have probably gone to a better glue, but by then the prototype stainless steel coating blade arrived and I moved on.
As a historical note here, when I finally got the metal blades made, the shop told me they would be milled to 0.001" (0.0254 mm) tolerance. About 1/2 of the blades failed to be within specs, so the good ones were used by me or sold at the original price while the defective blades were re-milled and are now sold at a higher price to reflect the need for reworking. Since the milling accuracy problem was found to be beyond their control, I had to use a different shop. I am now no longer offering the blades and will sell what I have from the remilled stock. The blades work in widths from 4" to 16" producing good coatings in that range. Kodak used blades up to 41" wide for production in the early years of the last century. The paper or film was drawn by a stationary blade as the emulsion was pumped down onto the support in an even stream, or was picked up from a trough. (see the Kodak film here on APUG for an example of that) This note is included to answer the questions raised about the increase in price.
So, I have basically 3 types of blade now. A paper blade, a film blade and a plate blade. This is in answer to the questions regarding the types I have worked with.
Now, in answer to the remaining questions, here are the keys to coating paper, using these blades, and it is intended to cover all of the questions posed by those wriiting me. If I have left any out, now is the time to ask!
1. Use a hot press paper or baryta. If hot press, use 100# or higher, if baryta use DW. Cold press paper or thinner paper can be used but can wrinkle during coating. You need a surfactant and this may be different for each emulsion and for each type of paper. Baryta is more difficult to coat on due to the relative hardness and the sizing.
2. Coat paper cold. Paper when wetted will expand rapidly. Hot paper with hot emulsion expands differentially and will wrinkle, especially at the interface between wet and dry at the edges and as you draw the blade across the paper. (film and plates are best coated hot, but for film it is not as important as it is for plates)
3. Cut the paper just a tad overwidth. If you cut wide sheets of paper and coat a narrow strip down the center, the stresses on the paper are increased manyfold and lead to wrinkles. Narrow edges of about 1/2" or so are best. Even here there may be wrinkles, but they do not appear until the coating is fully dry, or at least dry enough that the emulsion does not run.
4. Draw the blade at a constant rate. The rate depends on viscosity and to determine the whole thing - practice in the light with dyed gelatin at the concentration that you intend to coat. I have coated many dozens of sheets of dyed gelatin just to get my draw correct. This is the art part of coating!
5. Make sure that you let the sheet rest on the cold coating surface for a few moments. The paper should be held reasonably flat at the top and bototm by a small piece of tape or a weight. You will see that the paper surface changes from very glossy to a dull matte. This is when you can move it. I use tape at the top and my finger at the bottom.
6. If you get puddles or runs, the emulsion is too hot, too thin or the paper is too thin or too warm. Cooling everything down will help. However, if the paper is too thin, you may not be able to fix this problem unless you use thicker paper. I have coated on regular 20# typing paper and it will work if you can stand the mess. I've also used a 90# cold press paper that worked well but fell apart during processing.
In the final analysis, if the paper buckles or wrinkles, something is wrong. It can be any of the above such as thin paper, hot paper, wide edges or others but truly, something is wrong. I have coated from 68F to 75F ambient in my darkroom with the emulsion at 100 - 110F and the gelatin at about 8% with no problem. I have coated from 4x5 to 16x20 with good results, keeping all of the above in mind.
Remember, this applies only to Fibre Based paper. This does not apply to Resin Coated paper.
I would also like to mention that local water supply and air pressure may affect the coating properties on both film and paper. Since you must rinse the blades after every use and dry them, the drying towel(s) are also important. I have used some paper towels that had a bad effect on coating. These types of paper towel seem to contain a skin conditioner. Also, any oil or grease will adversely affect coating. All of my blades are washed in detergent and then distilled water before I ship them or use them.
Your hands can leave defects on coatings due to oils in the skin. Only touch your paper at the edges.
PE
