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Photo Engineer
04-06-2008, 08:03 PM
Ron,

That was my point.

I agree with you on the numbers, but I can't say I know of any papers that have that much silver... 5000 mg would seem very high except in special cases....

Ray

Ray;

Current papers use about 100 - 200 mg/sq ft.

Older papers used about 200-500 mg/sq ft. These are the so called silver rich emulsions of a byegone era. They are not superior in any sense, but rather suffer from a large number of grains that are dead. To approximate mg/sq meter multiply by 10 (actually about 9) to go from mg/sq ft to mg / sq meter.

My coatings use from 125 - 250 mg/sq ft, but I've gone as high as 500. It is merely another variation on a theme......

The dead grains come about by poor precipitation or poor chemical sensitization or both. Or, other factors which inhibit development such as Iodide at the wrong place at the wrong time in the wrong concentration.

PE

David A. Goldfarb
04-06-2008, 08:53 PM
Line-by-line seems like a very slow method of coating. If the emulsion at one end of the sheet sets before the head comes back in coating the next line, I'd imagine it would produce an uneven emulsion layer. If one is making traditional gelatin silver prints, at least one shouldn't have to worry about "pizza wheel" marks and such.

David A. Goldfarb
04-06-2008, 09:00 PM
Now here's another idea--How about mounting a coating blade on the exit side of the rollers of a most-likely-soon-to-be-useless Polaroid 8x10" processor? Based on what I saw in Ron's workshop, the speed of the paper travel seems about right. It would be necessary to add a stage to support the paper under the blade and a trough for runoff, but it might be do-able, and there's a tray to catch the coated sheet.

Photo Engineer
04-06-2008, 09:09 PM
David;

The coating blade I used was adapted for hand use and then applied to actual production equipment at Kodak Park nearly 100 years ago. It is still used in some plants. The blade is fixed, the paper moves under it, and emulsion is poured in at a rate to be sufficient to keep the blade full. Coating speed of this type of equipment is about 10 - 100 ft / min.

PE

David A. Goldfarb
04-06-2008, 09:18 PM
It takes about one second for the 8x10" Polaroid sheet to pass through the rollers, so that's on the order of 60-80 ft/min. That seems only a bit faster than we were coating by hand with the blade in the workshop.

jnanian
04-06-2008, 09:34 PM
some printers have what is called a continual inking system ( a large tank with tubes to feed the printer, instead of the cartridges )
i wonder if this might be adapted for such a system to hold liquid emulsion, instead of the tiny 1.5ml tanks.

Photo Engineer
04-06-2008, 09:41 PM
Yes it could, but insulated jackets would be needed to keep the emulsion hot, and the large reservoirs must also be kept hot. This is how it is done in production. The hopper on a coating machine is the 1.5 ml tank and the feed is coming from stainless tanks that are jacketed and heated. The lines going to the hopper are kept at the same temperature otherwise the gelatin will set up in the lines or the viscosity will go out of control.

PE