How I coat paper
Here is an example of how I coat paper sheets. The example is using Strathmore Smooth paper on an 11x14 sheet to achieve an 8x10 usable size.
The emulsion used actually is a dye made to simulate an AgBrI emulsion that is orthochromatic. This is just approximate, but how it would look coated at the levels used for film, not paper.
I did this so that you can see the coating on the paper.
They are, in sequence, tempering the emulsion and coating blade, charging the blade with emulsion as it rests on the paper, and inspecting the finished sheet for defects.
I have a set on film coataing and plate coating coming soon.
How thick can the emulsion layer be, using that coating blade? I am very interested in coating my own plates.
I use 5 mil (0.005 inches) gap for paper and up to 7 mil for film. The film is generally coated thicker. The blade allows coatings of up to 20 or 40 mil which is very thick and can be used for carbon tissue.
At 5 mil, you are coating roughly 12 ml per square foot.
Can the blade do a layer as thin as 4 microns (0.004 mm) thick? Would that be possible without many defects?
This is why...
The coated thickness has nothing to do with the dry thickness when you think about it.
If you coated water alone (if possible) and dried it down, there would be zero thickess if there was nothing in it regardless of the coated thickness. And so, we do not know what their coating thickness was, just the dry thickness.
Oh right, I see. I did not know it was the dried thickness, I assumed it was the wet thickness. Makes more sense now.
Thanks for the reply PE.
I have found in my coatings that 15% gelatin in water results in the dry thickness of about 15% of the wet thickness after drying :D. This is very approximate but should give you a general idea what happens in drying...
I have used 300 um (12 mil) gap when I have coated my film coatings but it certainly IS a bit thick, and can take 5 minutes to clear in fixer.
I think I've always been too high in gelatin compared to silver, because of dilution in washing and addition of gelatin afterwards. Maybe a bit too high coating temperature, too, leading to need for more gelatin.
Ron , where would one find the coating blade you are putting the liquid into??
I have them made to my specification. They can be purchased from the Photographers Formulary. I am making no profit on these as opposed to several comments here to the contrary.
They serve several purposes in coating paper:
1. Heavy weight to reduce lift and chatter in order to give a smooth coating.
2. High heat retention to keep the melted gelatin melted! This reduces the problems with varying coating plate temperatures.
3. Stainless Steel (308 or 316) to prevent contamination and corrosion.
The doctor blade (as it is properly called) was used in the trough coater shown here on APUG in one of the videos. It was used to even out coating nonuniformity. Another design is used for film, and yet another for plates. These latter two are not currently available.
Here you go Bob; http://stores.photoformulary.com/-st...ade/Detail.bok
Originally Posted by Bob Carnie