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Ray Rogers
10-19-2009, 01:27 PM
To get an even coat, I put all my plates in the oven and heated them to about 120, just hot enough to where they are almost uncomfortable to hold, then keep them in a stack as I pour, so that the plates behind the ones I'm coating smooth out the temperature across the top plates and keep my fingers from causing thick spots.

How are you guys (it's not just TA) putting glass plates in ovens?
(Are you using some kind of rack?)

How are you stacking the plates without worry of scratches,
& keeping the surface level while moving them to their setting areas?

totalamateur
10-19-2009, 05:33 PM
I just set them on the rack, which is clean. They are never in the oven with the emulsion on them, I just heat them up before taking them to the dark room to coat, hence the need for multiple plates (or a ceramic tile, now that Kirk mentions it) to keep the heat in.

The oven isn't for drying, it's so the emulsion doesn't set up when it hits a cool plate, making the coating uneven. My kitchen isn't light - tight, so I can't really put coated plates in the oven.

I've been lazy so far as getting a level surface, but my next plan is to put a chunk of plexi or plywood on top of big balls of modeling clay, one on each corner, and using bubble levels while squishing the balls of clay to make a level surface on my workbench.

I loke the idea of coating the edges of the plate in wax of some other substance to keep the emulsion from running off the side unexpectedly. Maybe a strip of masking tape perpendicular to the edges would do the trick. that could help with a larger warmed plate, since you would have a little more time to work the emulsion around the surface before it set-up.

totalamateur
10-20-2009, 01:04 AM
Well, I poured a batch of plates tonight and found that you do not want them to be all that hot, The emulsion was too thin, and didn't coat very well until everything cooled down a bit. So, my description of about 120 degrees was quite incorrect.

Photo Engineer
10-20-2009, 09:34 AM
The optimum that I have found over the years is to have everything at about 110 deg F with about 5 - 10% gelatin.

PE

keithwms
10-20-2009, 09:49 AM
When I want a *really* uniform, bubble-free coating of photoresist on a plate I put it in a spin coater. I have a coater that can handle plates up to 9". Now, I wonder if something like a potter's wheel might be able to handle really big plates. My spin coater has a vacuum that holds the plate down firmly; with a potter's wheel I guess you'd have to glue it, or maybe design some suction cup or something, or make a holder for the plate that can be screwed onto a platten. Anyway, for emulsion it'd not take much speed. For my photoresist we have to get up to 1000+ rpm, but that's for submicron films. For ordinary films with good uniformity and if you don't care too much about the thickness, I am thinking that you'd not need more than ~20 rpm. Say, how about a good old record player....

Spinning would also save a lot of emulsion, you can make a thing that catches what flies off and recycle it.

dwross
10-20-2009, 12:25 PM
I find that even 110F is too warm for coating. I need to keep my emulsion in the 34C-36C/~95F range for optimum coating, with the glass plates at about 70F. Since I'm quite sure Ron's results are spot-on for his emulsions, it just goes to show that people need to determine their own best conditions. Don't assume too quickly that your emulsion or coating technique is flawed. A very simple tweak of your workflow may be all that's needed to get great results.

Keith: A spin technique may work well for collodion. If I understand that process, coating thickness doesn't matter so much. Not so with silver gelatin. Too-thick coating won't develop evenly and/or will be too dense to print, and too-thin will result in flat negatives (not wasted, fortunately, if you are willing to go the Photoshop => digital route). As for catching emulsion that flies off, that too presents problems. I think the flying emulsion would cool and set up almost immediately unless you were able to collect it in a heated container and return it the original batch for the immediate coating of your next plate(s). Still, it's an intriguing idea, even if it's a bit more complex (and messy!) than required for the home darkroom emulsionmaker. There are probably any number of engineering wizards here on APUG who could have a lot of fun with the concept.

d

keithwms
10-20-2009, 02:07 PM
Keith: A spin technique may work well for collodion. If I understand that process, coating thickness doesn't matter so much. Not so with silver gelatin. Too-thick coating won't develop evenly and/or will be too dense to print, and too-thin will result in flat negatives

The really good thing about spinning is that you can control the thickness via the spin rate. I think if the viscosity of the emulsion is reproducible then it should be possible to measure spin curves that tell you what rpm and time to use to achieve whatever thickness you desire.... from thin to ultrathin. Thicker is actually harder; to get really thick films we usually just spin more than once. But I am thinking that in this case you want ~microns of thickness and that should be easy.

About reclaiming the fly-off, yeah that may be wishful thinking ;) But anyway one of the great advantages of spinning is that you can use minute amounts of material to coat the surface very uniformly. A drop of standard viscosity photoresist easily coats a 6" wafer, for example.

I'll give it a try if/when I have time.

Ray Rogers
10-20-2009, 02:21 PM
I just set them on the rack, which is clean.

That is what I don't follow...
I bake bread in my oven; I have different trays, metal, ceramic and glass, but I would not think of putting glass plates down on any of them...

I thought you might be putting them in a rack as when drying, but the ovens here are much much smaller than where you folks are... so it would be kind of crowded... not to mention being sented wth Pizza or Gingerbread. :)

Photo Engineer
10-20-2009, 02:28 PM
Ray;

The clean uncoated plates are warmed in the oven before coating.

They are coated warm and then chilled. When chilled they set up and can then be placed in a cool dark drying rack!

PE

Ray Rogers
10-20-2009, 02:34 PM
About reclaiming the fly-off, yeah that may be wishful thinking ;)

Well, if you have a lot of material and you really want to, you don't actually need to pour it... Just scoop it up in what ever condition it is in and put it away safely for the next day of coating.

But this probably won't work very well if, as you mentioned above, you are coating by the "drop". :)

Ray

(thanks for the pdf btw!)

Ray Rogers
10-20-2009, 02:54 PM
Ray;

The clean uncoated plates are warmed in the oven before coating.

They are coated warm and then chilled. When chilled they set up and can then be placed in a cool dark drying rack!

PE

I understand that they are still un coated, but what I am thnking about is spoiling the glass surface...

So after all the effort of cleaning the plates to remove all traces of dust, dirt oils and grease... you just (gently) drop them onto the ktchen oven rack... flat I presume... for a short heating?

Well, if it works it works- but it seems a bit odd to clean something so well and then put them face down in the kitchen oven.

How long do you heat them for?

Ray

totalamateur
10-20-2009, 03:25 PM
OK, I suppose if you use your oven more often than I do, a nice piece of silpat or possibly one of those dollar store aluminum turkey tins would suffice to lay the glass in when heating.

I turn on my oven to warm, and let it heat to about 150, then throw the plates in for about 5 - 10 minutes. I do not measure the temperature precisely, but more by feel (gloved hands, of course)

Photo Engineer
10-20-2009, 04:15 PM
So, stack them up and discard the top and bottom plates. The rest are protected from the oven environment except for the heat.

PE

wildbillbugman
10-20-2009, 06:57 PM
Say, how about a good old record player....


A table-top centrifuge with a rheostat for exact rpm controle would be very effective for spin coating. I bought both the centrifuge and a table top rheostat for future use. Have not tried it yet. I need to figure out how to secure the glass plate while maintaning ballance.
My research on table-top spincoaters indicate that I could buy a brand new Lexus for less $.
Bill

keithwms
10-20-2009, 09:23 PM
I need to figure out how to secure the glass plate while maintaning ballance.

How about a circular plate made of 1/8" or 1/4" aluminum or similar, fitted with clips or ridges or a recess to hold the plate. The attachment to the rotary shaft would be via the underlying plate. Thus no attachments to the plate itself are necessary.

Actually I don't think you need variable speed control, you just need reproducible speed. So a record player may be just fine. The thickness of the resulting film could be controlled by time and the viscosity of the fluid.

wildbillbugman
10-21-2009, 12:24 AM
But the viscosity would have to be very low for record player rpm. The aluminum plate idea is a good one. It would fit right into the lip at the top of the centrifuge.
Bill

Kirk Keyes
10-21-2009, 11:48 AM
As far as heating plates goes, Denise Ross uses a small food drier heater that is placed into a little plywood drying cabinet that she has in her darkroom. It's about a 1 or so square, and a few feet tall, and the food drier sits in the bottom. She then has shelves (or was it slots that the glass slips into) for the sheets of glass. She put them in and lets them get up to temp before she coats. It's a nice setup and takes just a little space from her darkroom.

Hopefully, Denise will swing by here and correct any errors I made in the description.

Photo Engineer
10-21-2009, 11:57 AM
I turn my hotplate on low and cover it with about 4 layers of paper towels. I place one 4x5 sheet of glass on at a time and the glass reaches about 100 F. The hotplate has an indicator lamp to tell when the heat is going above a certain value and I can control it easily from the front dial.

This is a high-end hotplate. I don't suggest doing this with a unit with poor indication of overtemp or poor temp control.

PE

Kirk Keyes
10-21-2009, 04:40 PM
This is a high-end hotplate. I don't suggest doing this with a unit with poor indication of overtemp or poor temp control.

Yeah, the paper towels might catch fire with a cheap hot plate that doesn't have good heat distribution!

Kirk Keyes
10-21-2009, 04:42 PM
I have an Infrared Thermometer that I bought at Harbor Freight for about $20. It has a laser pointer to show you where you are aiming it and a digital readout to tell you the temperature of the surface that you are measuring. It might be a handy device for figuring out it your plates are at the right temperature.